test(sandbox): keep the SID helper from colliding with #886

A profile that sets denyRead drops WRITE_RESTRICTED, and the token that
replaces it carried the World SID. Every principal carries Everyone, so the
restricted-SID check passed for free on any path whose DACL grants Everyone
write, and the workspace write jail fell back to the caller's own permissions,
which is the boundary the token exists to be stricter than. No privilege, no
symlink and no race: an Everyone-writable directory was enough, and share roots
opened Everyone:F and loose installer trees supply them. #865 closed this for
the WRITE_RESTRICTED token and left this half open.

It could not simply be dropped. Without WRITE_RESTRICTED the restricted-SID
check covers reads as well, and default Windows DACLs grant BUILTIN\Users
rather than anything in the list, so a token without Everyone cannot open
cmd.exe and dies at launch with a bare access denial. Confirmed by
impersonating both token shapes against C:\Windows\System32\cmd.exe.

So the read capability takes its place. BuildWindowsACLPlan already grants that
SID on every read root and denies it on every denyRead path under exactly the
same condition, so the read allowance and the restriction become one decision,
and it names only what setup granted rather than every principal on the
machine. The principal path already worked this way.

Regressions drive a real restricted token through a real impersonated write:
an Everyone-only DACL is refused, a directory the capability grants is written,
and the token's own restricted-SID list is read back from the token. A separate
test pins the plan and the token to the same answer, because the two halves are
decided in different files off the same field.

Closes #869

internal/cli/sandbox.go

@@ -22,7 +22,7 @@ const permissionProfileScopeNote = "permissionProfile is derived from this proce
 
 func runSandbox(args []string, stdout io.Writer, stderr io.Writer, deps appDeps) int {
 	if len(args) == 0 {
-		return writeExecUsageError(stderr, "sandbox subcommand required. Use `zero sandbox policy` or `zero sandbox grants list`.")
+		return writeExecUsageError(stderr, "sandbox subcommand required. Use `zero sandbox policy`, `zero sandbox exec`, or `zero sandbox grants list`.")
 	}
 	switch args[0] {
 	case "-h", "--help", "help":
@@ -36,6 +36,8 @@ func runSandbox(args []string, stdout io.Writer, stderr io.Writer, deps appDeps)
 		return runSandboxSetup(args[1:], stdout, stderr, deps)
 	case "check":
 		return runSandboxCheck(args[1:], stdout, stderr, deps)
+	case "exec":
+		return runSandboxExec(args[1:], stdout, stderr, deps)
 	case "grants":
 		return runSandboxGrants(args[1:], stdout, stderr, deps)
 	default:
@@ -174,10 +176,17 @@ func runSandboxSetup(args []string, stdout io.Writer, stderr io.Writer, deps app
 	if !setupHelper.Available() {
 		return writeAppError(stderr, "Windows sandbox setup helper is not available", exitProvider)
 	}
+	// Resolved here, in the shell the user typed `zero sandbox setup` into, and
+	// carried in the args. The helper may be launched elevated, and an elevated
+	// process does not inherit this shell's environment. Stated explicitly rather
+	// than left nil (which resolves the same way) because this is the call site
+	// the opt-in is about.
+	principalOptIn := zeroSandbox.WindowsSandboxPrincipalOptIn(nil)
 	setupArgs, err := zeroSandbox.BuildWindowsSandboxSetupArgs(zeroSandbox.WindowsSandboxSetupArgsOptions{
 		CommandCWD:        workspaceRoot,
 		WorkspaceRoots:    []string{workspaceRoot},
 		PermissionProfile: profile,
+		PrincipalOptIn:    &principalOptIn,
 	})
 	if err != nil {
 		return writeAppError(stderr, err.Error(), exitCrash)
@@ -646,6 +655,7 @@ Commands:
   policy      Inspect active sandbox policy and platform backend
   setup       Run native platform sandbox setup
   check       Evaluate the sandbox decision for a hypothetical tool action
+  exec        Run one command through the real sandbox
   grants      Manage persistent sandbox grants
 
 `)

internal/cli/sandbox_exec.go

@@ -0,0 +1,204 @@
+package cli
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"os"
+	"os/exec"
+	"strings"
+
+	"github.com/Gitlawb/zero/internal/config"
+	zeroSandbox "github.com/Gitlawb/zero/internal/sandbox"
+)
+
+// runSandboxExec runs ONE command through the real sandbox and exits with its
+// status.
+//
+// This exists because until now the sandbox could only be exercised through a
+// full agent turn with a model in the loop. `zero sandbox policy` reports what
+// the posture would be and `zero sandbox check` evaluates a hypothetical
+// decision, but nothing actually ran a command and let you look at what
+// happened on disk afterwards. The practical result is that enforcement is
+// covered almost entirely by tests asserting the shape of an ACL plan, and
+// almost not at all by tests asserting a write was refused.
+//
+// A plan can be perfectly correct and never reach the filesystem. That is not
+// hypothetical here: the .git rename guard was emitted correctly by the planner
+// and silently skipped by the applier, and four tests covering the plan all
+// passed while the ACE was absent from disk. Something that runs the real
+// binary and then stats the file is the only thing that catches that class.
+//
+// Deliberately NOT a debug curiosity: it takes the same path a shell tool
+// takes, through SandboxManager.BuildCommandPlan, so what it proves is what
+// users get. It prints the resolved backend and enforcement level to stderr
+// before running, so a harness can assert the sandbox was actually engaged
+// rather than quietly downgraded.
+func runSandboxExec(args []string, stdout io.Writer, stderr io.Writer, deps appDeps) int {
+	command, err := parseSandboxExecArgs(args)
+	if err != nil {
+		if errors.Is(err, errSandboxExecHelp) {
+			if writeErr := writeSandboxExecHelp(stdout); writeErr != nil {
+				return exitCrash
+			}
+			return exitSuccess
+		}
+		return writeExecUsageError(stderr, err.Error())
+	}
+
+	workspaceRoot, err := resolveWorkspaceRoot("", deps)
+	if err != nil {
+		return writeExecUsageError(stderr, err.Error())
+	}
+	resolved, err := deps.resolveConfig(workspaceRoot, config.Overrides{})
+	if err != nil {
+		return writeAppError(stderr, err.Error(), exitProvider)
+	}
+	policy := applyConfiguredSandboxPolicy(zeroSandbox.DefaultPolicy(), resolved.Sandbox)
+
+	scope, err := zeroSandbox.NewScope(workspaceRoot, resolved.Sandbox.AdditionalWriteRoots)
+	if err != nil {
+		return writeAppError(stderr, fmt.Sprintf("resolve sandbox write roots: %v", err), exitCrash)
+	}
+
+	// Through the ENGINE, not a SandboxManager built here.
+	//
+	// The engine is what a real tool call goes through, and it does more than
+	// hand the manager a request: it resolves the permission profile, calls
+	// prepareSandboxRuntime, and folds the runtime state into that profile
+	// before planning. Building a manager directly skipped all of it, so the
+	// command ran without the runtime write root, and cache or temp writes could
+	// pass or fail differently from the sandboxed command this exists to imitate.
+	// A harness that exercises the wrong path is worse than no harness, because
+	// its result still reads as evidence.
+	engine := zeroSandbox.NewEngine(zeroSandbox.EngineOptions{
+		WorkspaceRoot: workspaceRoot,
+		Policy:        policy,
+		Scope:         scope,
+		Backend:       deps.selectSandboxBackend(zeroSandbox.BackendOptions{}),
+		// The same scrub list a real tool call gets. Without it the engine keeps an
+		// empty set, and only the hardcoded names plus the provider catalog's own
+		// AuthEnvVars are removed from the child's environment: a key named by
+		// `apiKeyEnv` in the user's config would be scrubbed for every sandboxed
+		// tool call and handed to this one. Reproducing the production environment
+		// is the entire point of the command, and a credential is the last part of
+		// it that may differ.
+		SensitiveEnvKeys: providerSensitiveEnvKeys(resolved),
+	})
+	plan, err := engine.BuildCommandPlan(zeroSandbox.CommandSpec{
+		Name: command[0],
+		Args: command[1:],
+		Dir:  workspaceRoot,
+		Env:  os.Environ(),
+	})
+	if err != nil {
+		return writeAppError(stderr, fmt.Sprintf("build sandbox command plan: %v", err), exitCrash)
+	}
+	// The plan owns resources beyond its construction: on Linux it allocates a
+	// policy-report file and registers the removal here, so without this every
+	// invocation leaves a /tmp/zero-sandbox-report-* behind.
+	defer plan.Cleanup()
+
+	// Printed before the command runs and on stderr, so it survives a command
+	// that writes to stdout and stays greppable by a test harness. A downgrade
+	// is reported loudly for the same reason: a smoke test that passes because
+	// the sandbox quietly stood down is worse than no smoke test.
+	fmt.Fprintf(stderr, "sandbox: backend=%s enforcement=%s wrapped=%t workspace=%s\n",
+		plan.Backend.Name, plan.EnforcementLevel, plan.Wrapped, plan.WorkspaceRoot)
+	if strings.TrimSpace(plan.DowngradeReason) != "" {
+		fmt.Fprintf(stderr, "sandbox: DOWNGRADED: %s\n", plan.DowngradeReason)
+	}
+
+	return runSandboxPlannedCommand(plan, stdout, stderr)
+}
+
+func runSandboxPlannedCommand(plan zeroSandbox.CommandPlan, stdout io.Writer, stderr io.Writer) int {
+	process := exec.Command(plan.Name, plan.Args...)
+	process.Dir = plan.Dir
+	if process.Dir == "" {
+		process.Dir = plan.WorkspaceRoot
+	}
+	if len(plan.Env) > 0 {
+		process.Env = plan.Env
+	}
+	process.Stdin = os.Stdin
+	process.Stdout = stdout
+	process.Stderr = stderr
+
+	if err := process.Run(); err != nil {
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) {
+			// The command's own status, not ours. A harness asserting "the write
+			// was refused" needs the refusal's exit code, not a wrapper's.
+			// A SIGNALED CHILD HAS NO EXIT CODE TO REPORT. ExitCode() answers -1
+			// there, and os.Exit truncates that to 255, so a child that took SIGTERM
+			// became indistinguishable from one that chose to exit 255. Fold the
+			// signal into the conventional 128+n a shell would report, which needs
+			// the ProcessState rather than the integer.
+			if status, signaled := signaledExitStatus(exitErr.ProcessState); signaled {
+				return status
+			}
+			return exitErr.ExitCode()
+		}
+		fmt.Fprintf(stderr, "sandbox exec: %v\n", err)
+		return exitCrash
+	}
+	return exitSuccess
+}
+
+var errSandboxExecHelp = errors.New("help requested")
+
+// parseSandboxExecArgs takes everything after `--` as the command, so the
+// command's own flags are never mistaken for ours.
+func parseSandboxExecArgs(args []string) ([]string, error) {
+	if len(args) == 0 {
+		return nil, errors.New("usage: zero sandbox exec -- <command> [args...]")
+	}
+	// Only the FIRST token is ours to interpret. Everything from the second
+	// onwards belongs to the child, help flags included.
+	//
+	// Written as a straight-line decision on args[0] rather than a loop, because
+	// a loop here is a lie. An earlier version scanned for `--` "up front" and
+	// said so in its comment, but every branch of its body returned, so it only
+	// ever examined index 0 and the promised scan did not exist. The rule below
+	// is what that code actually implemented, and it is the rule we want: it is
+	// the wrapper's own prefix that can ask for the wrapper's help, and the
+	// prefix is at most one token long.
+	switch args[0] {
+	case "--":
+		// Everything after the separator is the command, verbatim, including
+		// `--help`. That is the documented contract: `zero sandbox exec -- cmd
+		// --help` must run cmd's help, not ours.
+		command := args[1:]
+		if len(command) == 0 {
+			return nil, errors.New("usage: zero sandbox exec -- <command> [args...]")
+		}
+		return command, nil
+	case "-h", "--help", "help":
+		return nil, errSandboxExecHelp
+	}
+	// The separator-less form, tolerated for interactive use: the first token is
+	// the command, so nothing after it is ours to read. Without this,
+	// `zero sandbox exec mycmd --help` printed OUR help and never ran mycmd,
+	// which is the same contract break as reading past `--`. The help text still
+	// shows the separator, because anything with a leading dash needs it.
+	return args, nil
+}
+
+func writeSandboxExecHelp(w io.Writer) error {
+	_, err := fmt.Fprint(w, `Usage:
+  zero sandbox exec -- <command> [args...]
+
+Runs one command through the real sandbox and exits with its status.
+
+Everything after the -- separator is the command, so its own flags are not
+parsed as Zero's. The resolved backend and enforcement level are written to
+stderr before the command runs, and a downgrade is reported there explicitly.
+
+Examples:
+  zero sandbox exec -- cmd /c echo hello
+  zero sandbox exec -- powershell -Command "Set-Content out.txt x"
+
+`)
+	return err
+}

internal/cli/sandbox_exec_signal_other.go

@@ -0,0 +1,28 @@
+//go:build !windows
+
+package cli
+
+import (
+	"os"
+	"syscall"
+)
+
+// signaledExitStatus reports the conventional shell status for a child that was
+// terminated by a signal, which exec.ExitError cannot represent.
+//
+// ExitCode() returns -1 for a signaled child, and the top level hands that to
+// os.Exit, which truncates it to 255. A child that takes SIGTERM is then
+// indistinguishable from one that exited 255 of its own accord, which breaks the
+// documented status contract for a command whose whole job is to report the
+// child's own status faithfully. 128+signal is what a shell reports and what a
+// harness comparing statuses expects.
+func signaledExitStatus(state *os.ProcessState) (int, bool) {
+	if state == nil {
+		return 0, false
+	}
+	status, ok := state.Sys().(syscall.WaitStatus)
+	if !ok || !status.Signaled() {
+		return 0, false
+	}
+	return 128 + int(status.Signal()), true
+}

internal/cli/sandbox_exec_signal_other_test.go

@@ -0,0 +1,65 @@
+//go:build !windows
+
+package cli
+
+import (
+	"errors"
+	"os/exec"
+	"syscall"
+	"testing"
+)
+
+// A SIGNALED CHILD MUST NOT LOOK LIKE ONE THAT CHOSE TO EXIT 255.
+//
+// `sandbox exec` exists to report the child's own status faithfully, and
+// exec.ExitError cannot represent signal termination: ExitCode() answers -1, and
+// the top level hands that to os.Exit, which truncates it to 255. So a child
+// killed by SIGTERM was indistinguishable from an ordinary exit of 255, and a
+// harness comparing statuses could not tell a refusal from a kill.
+//
+// Driven with a real subprocess and a real signal rather than a synthetic
+// WaitStatus, because the mapping is only correct if the OS agrees with it.
+func TestSignaledChildReportsTheConventionalStatus(t *testing.T) {
+	for _, testCase := range []struct {
+		name   string
+		script string
+		want   int
+	}{
+		{name: "SIGTERM", script: "kill -TERM $$; sleep 5", want: 128 + int(syscall.SIGTERM)},
+		{name: "SIGINT", script: "kill -INT $$; sleep 5", want: 128 + int(syscall.SIGINT)},
+	} {
+		t.Run(testCase.name, func(t *testing.T) {
+			err := exec.Command("/bin/sh", "-c", testCase.script).Run()
+			var exitErr *exec.ExitError
+			if !errors.As(err, &exitErr) {
+				t.Fatalf("SETUP INVALID: the child did not end with an ExitError: %v", err)
+			}
+			// The behaviour being corrected: the integer alone cannot say this.
+			if code := exitErr.ExitCode(); code != -1 {
+				t.Fatalf("SETUP INVALID: a signaled child reported exit code %d, expected -1 on this platform", code)
+			}
+			status, signaled := signaledExitStatus(exitErr.ProcessState)
+			if !signaled {
+				t.Fatal("a signaled child was not recognised as signaled, so it would be reported as exit 255")
+			}
+			if status != testCase.want {
+				t.Fatalf("status = %d, want %d (128 + signal), which is what a shell reports", status, testCase.want)
+			}
+		})
+	}
+}
+
+// An ordinary non-zero exit is untouched: it has a real code and must keep it.
+func TestOrdinaryExitIsNotTreatedAsSignaled(t *testing.T) {
+	err := exec.Command("/bin/sh", "-c", "exit 3").Run()
+	var exitErr *exec.ExitError
+	if !errors.As(err, &exitErr) {
+		t.Fatalf("SETUP INVALID: the child did not end with an ExitError: %v", err)
+	}
+	if _, signaled := signaledExitStatus(exitErr.ProcessState); signaled {
+		t.Fatal("an ordinary exit was reported as signaled, which would rewrite its status")
+	}
+	if code := exitErr.ExitCode(); code != 3 {
+		t.Fatalf("exit code = %d, want the child's own 3", code)
+	}
+}

internal/cli/sandbox_exec_signal_windows.go

@@ -0,0 +1,12 @@
+//go:build windows
+
+package cli
+
+import "os"
+
+// signaledExitStatus has no Windows counterpart: a process there ends with an
+// exit code, and there is no signal to fold into one. Windows keeps the exit
+// code exec.ExitError already reports.
+func signaledExitStatus(*os.ProcessState) (int, bool) {
+	return 0, false
+}

internal/cli/sandbox_exec_test.go

@@ -0,0 +1,108 @@
+package cli
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+// parseSandboxExecArgs sits on the boundary between Zero's flags and a child
+// command's, and it had no tests at all: not for the reported defect, not even
+// for the happy case. The contract it has to keep is that everything after the
+// separator belongs to the child, help flags included, so the wrapper can never
+// answer for a command it was supposed to be running.
+func TestParseSandboxExecArgs(t *testing.T) {
+	for _, testCase := range []struct {
+		name    string
+		args    []string
+		command []string
+		help    bool
+		usage   bool
+	}{
+		{
+			// The reported defect: --help after the separator is the CHILD's.
+			name:    "help after the separator belongs to the child",
+			args:    []string{"--", "cmd", "--help"},
+			command: []string{"cmd", "--help"},
+		},
+		{
+			name:    "a second separator is the child's too",
+			args:    []string{"--", "cmd", "--", "inner"},
+			command: []string{"cmd", "--", "inner"},
+		},
+		{
+			name:    "a flag-looking command survives the separator",
+			args:    []string{"--", "--weird-binary"},
+			command: []string{"--weird-binary"},
+		},
+		{
+			// The separator-less form. The first token is the command, so its own
+			// flags are not ours to read either.
+			name:    "help after a separatorless command belongs to the child",
+			args:    []string{"cmd", "--help"},
+			command: []string{"cmd", "--help"},
+		},
+		{
+			// A separator arriving AFTER the command has already started is part of
+			// the child's argv, not a delimiter we get a second go at.
+			name:    "a separator after the command is the child's argument",
+			args:    []string{"ls", "--", "foo"},
+			command: []string{"ls", "--", "foo"},
+		},
+		{name: "bare help", args: []string{"--help"}, help: true},
+		{name: "short help", args: []string{"-h"}, help: true},
+		{name: "help subcommand", args: []string{"help"}, help: true},
+		{name: "help in the prefix wins", args: []string{"--help", "--", "cmd"}, help: true},
+		{name: "no arguments", args: nil, usage: true},
+		{name: "separator with nothing after it", args: []string{"--"}, usage: true},
+	} {
+		t.Run(testCase.name, func(t *testing.T) {
+			command, err := parseSandboxExecArgs(testCase.args)
+			switch {
+			case testCase.help:
+				if !errors.Is(err, errSandboxExecHelp) {
+					t.Fatalf("parseSandboxExecArgs(%q) = %q, %v; want the help sentinel", testCase.args, command, err)
+				}
+			case testCase.usage:
+				if err == nil || errors.Is(err, errSandboxExecHelp) {
+					t.Fatalf("parseSandboxExecArgs(%q) = %q, %v; want a usage error", testCase.args, command, err)
+				}
+				if !strings.Contains(err.Error(), "usage:") {
+					t.Errorf("a usage error must show the usage, got %q", err.Error())
+				}
+			default:
+				if err != nil {
+					t.Fatalf("parseSandboxExecArgs(%q) returned %v", testCase.args, err)
+				}
+				if strings.Join(command, "\x00") != strings.Join(testCase.command, "\x00") {
+					t.Errorf("parseSandboxExecArgs(%q) = %q, want %q", testCase.args, command, testCase.command)
+				}
+			}
+		})
+	}
+}
+
+// The wrapper must never consume a token it then fails to hand over. Whatever it
+// returns as the command has to be a trailing slice of what it was given, with
+// at most a leading separator removed, or an argument has gone missing on its
+// way to the child.
+func TestParseSandboxExecArgsNeverDropsAChildArgument(t *testing.T) {
+	for _, args := range [][]string{
+		{"--", "cmd", "--help", "-x", "--"},
+		{"cmd", "--help"},
+		{"ls", "--", "foo"},
+		{"--", "--weird-binary", "arg"},
+	} {
+		command, err := parseSandboxExecArgs(args)
+		if err != nil {
+			t.Fatalf("parseSandboxExecArgs(%q) returned %v", args, err)
+		}
+		if len(command) == 0 {
+			t.Fatalf("parseSandboxExecArgs(%q) returned an empty command", args)
+		}
+		suffix := strings.Join(args[len(args)-len(command):], "\x00")
+		if strings.Join(command, "\x00") != suffix {
+			t.Errorf("parseSandboxExecArgs(%q) = %q, which is not a trailing slice of the input; an argument was dropped or reordered", args, command)
+		}
+	}
+}

internal/doctor/hardening.go

@@ -100,10 +100,17 @@ func windowsSandboxSetupCheck(goos string, backend sandbox.Backend, workspaceRoo
 	}
 	profile := sandbox.PermissionProfileFromPolicy(workspaceRoot, doctorSandboxPolicy(sandboxConfig), scope)
 	setupConfig := sandbox.WindowsSandboxSetupConfig{
-		SandboxHome:       sandboxHome,
-		CommandCWD:        workspaceRoot,
-		WorkspaceRoots:    []string{workspaceRoot},
-		PermissionProfile: profile,
+		SandboxHome:    sandboxHome,
+		CommandCWD:     workspaceRoot,
+		WorkspaceRoots: []string{workspaceRoot},
+		// Same augmentation the setup args and the command plan apply, so doctor
+		// fingerprints what a real command fingerprints. Safe here: doctor runs in
+		// the operator's shell, not behind the sandbox's TEMP redirection.
+		PermissionProfile: sandbox.WindowsSandboxProfileWithRuntimeRoots(profile, []string{workspaceRoot}),
+		// Same opt-in a command would resolve, so doctor reports the principal
+		// mismatch as out-of-date setup instead of passing a check the next command
+		// will fail.
+		PrincipalOptIn: sandbox.WindowsSandboxPrincipalOptIn(nil),
 	}
 	if err := sandbox.ValidateWindowsSandboxSetupMarker(setupConfig); err != nil {
 		result := check("sandbox.backend", "Sandbox backend", StatusWarn, fmt.Sprintf("Native sandbox backend %s is installed, but Windows sandbox setup is missing or out of date: %v.", backend.Name, err), map[string]any{
@@ -115,6 +122,53 @@ func windowsSandboxSetupCheck(goos string, backend sandbox.Backend, workspaceRoo
 		})
 		return &result
 	}
+	// Setup is valid and the opt-in is set, so report which principal a command
+	// will actually run as.
+	//
+	// This used to warn that the principal stood down under the DEFAULT
+	// network-deny policy, which was true while network denial could only be
+	// expressed on the restricted token: a principal token cannot carry the
+	// synthetic offline-marker SID the WFP filters matched. Dual-role provisioning
+	// removed that limit by giving each workspace an offline account whose real
+	// group membership the same filters match, so deny mode now runs on a
+	// principal exactly as allow mode does. Leaving the warning would have doctor
+	// telling operators their reads are unconfined while they are confined.
+	//
+	// Naming the role rather than just saying "active" is the useful part. The two
+	// accounts differ only in that membership, so an operator debugging network
+	// behaviour needs to know which one this workspace picks.
+	if setupConfig.PrincipalOptIn {
+		// Asked of the shared rule rather than re-derived here, so doctor and the
+		// runtime cannot disagree about which account a command runs as. Two
+		// copies drifting is what produced the wrong report this replaces.
+		role := sandbox.WindowsSandboxPrincipalRoleForNetwork(profile.Network.Mode)
+		// SETUP IS CURRENT IS NOT THE SAME AS THE PRINCIPAL IS LIVE, and this used
+		// to claim the second from the first. Marker validation compares serialized
+		// plans and hashes; it names no account. Delete the selected account or its
+		// secret, or drop the offline account out of ZeroSandboxOffline, and the
+		// marker still validates while the runtime either falls back to the weaker
+		// restricted-token path or fails outright. Doctor asserting read
+		// confinement in that state is worse than saying nothing, because it is the
+		// surface an operator checks precisely when they are unsure.
+		//
+		// Verifying liveness needs the account, a usable secret and (for offline)
+		// the group membership, all Windows-only live queries this package cannot
+		// make. So the claim is narrowed to what the marker actually proves, and
+		// the role is still named because that part IS derived rather than assumed
+		// and is what an operator debugging network behaviour needs.
+		result := check("sandbox.principal", "Sandbox principal", StatusPass,
+			fmt.Sprintf("Sandbox principal setup is current; a command in this workspace selects the %s principal. Setup state does not confirm the account is usable.", role), map[string]any{
+				"backend":  string(backend.Name),
+				"platform": goos,
+				"optIn":    true,
+				// Deliberately not "active": nothing here checked the account, the
+				// secret or the group membership.
+				"setupCurrent": true,
+				"role":         role,
+				"networkMode":  string(profile.Network.Mode),
+			})
+		return &result
+	}
 	return nil
 }
 

internal/doctor/hardening_principal_windows_test.go

@@ -0,0 +1,72 @@
+package doctor
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/Gitlawb/zero/internal/config"
+	"github.com/Gitlawb/zero/internal/sandbox"
+)
+
+// A VALID MARKER IS NOT A LIVE PRINCIPAL. Marker validation compares serialized
+// plans and hashes and names no account, so it stays valid after the selected
+// account is deleted, its secret is removed, or the offline account is dropped
+// out of ZeroSandboxOffline. In each of those states the runtime falls back to
+// the restricted-token path with no read confinement, or fails outright.
+//
+// Doctor is the surface an operator checks precisely when they are unsure, so
+// claiming read confinement it has not verified is worse than reporting less.
+// This pins the narrower claim: setup is current, the role is named because it
+// is derived rather than assumed, and nothing asserts the account is usable.
+func TestPrincipalCheckDoesNotClaimAnUnverifiedPrincipalIsActive(t *testing.T) {
+	sandboxHome := t.TempDir()
+	workspaceRoot := t.TempDir()
+	t.Setenv("ZERO_WINDOWS_SANDBOX_HOME", sandboxHome)
+	t.Setenv("ZERO_WINDOWS_SANDBOX_IDENTITY", "1")
+
+	sandboxConfig := config.SandboxConfig{}
+	scope, err := sandbox.NewScope(workspaceRoot, sandboxConfig.AdditionalWriteRoots)
+	if err != nil {
+		t.Fatalf("NewScope: %v", err)
+	}
+	profile := sandbox.PermissionProfileFromPolicy(workspaceRoot, doctorSandboxPolicy(sandboxConfig), scope)
+
+	// Mirrors what windowsSandboxSetupCheck derives, so the marker it writes is
+	// the one the check will validate against. Nothing here provisions an
+	// account, which is the whole point: the marker is valid and the principal
+	// does not exist.
+	if _, err := sandbox.WriteWindowsSandboxSetupMarker(sandbox.WindowsSandboxSetupConfig{
+		SandboxHome:       sandboxHome,
+		CommandCWD:        workspaceRoot,
+		WorkspaceRoots:    []string{workspaceRoot},
+		PermissionProfile: sandbox.WindowsSandboxProfileWithRuntimeRoots(profile, []string{workspaceRoot}),
+		PrincipalOptIn:    true,
+	}); err != nil {
+		t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
+	}
+
+	result := windowsSandboxSetupCheck("windows", sandbox.Backend{Name: sandbox.BackendWindowsRestrictedToken}, workspaceRoot, sandboxConfig)
+	if result == nil {
+		t.Fatal("expected a principal check once the marker validates and the opt-in is set")
+	}
+	if result.ID != "sandbox.principal" {
+		t.Fatalf("check = %q (%s): the marker did not validate, so this test is not exercising the principal branch", result.ID, result.Message)
+	}
+
+	lowered := strings.ToLower(result.Message)
+	for _, claim := range []string{"is active", "commands run as"} {
+		if strings.Contains(lowered, claim) {
+			t.Errorf("doctor asserts %q about a principal it never checked: %q", claim, result.Message)
+		}
+	}
+	if active, ok := result.Details["active"]; ok {
+		t.Errorf("details carry active=%v, which nothing here verified", active)
+	}
+
+	// The role is still worth reporting: it is derived from the network mode by
+	// the shared rule rather than assumed, and it is what an operator debugging
+	// network behaviour needs.
+	if role, ok := result.Details["role"]; !ok || strings.TrimSpace(role.(string)) == "" {
+		t.Errorf("the selected role should still be reported: %#v", result.Details)
+	}
+}

internal/peermsg/private_dir_other.go

@@ -10,7 +10,7 @@ import (
 	"strings"
 )
 
-func ensurePrivateDir(path string) error {
+func EnsurePrivateDir(path string) error {
 	abs, err := filepath.Abs(path)
 	if err != nil {
 		return err

internal/peermsg/private_dir_unix.go

@@ -12,7 +12,7 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func ensurePrivateDir(path string) error {
+func EnsurePrivateDir(path string) error {
 	abs, err := filepath.Abs(path)
 	if err != nil {
 		return err

internal/peermsg/private_dir_unix_test.go

@@ -18,7 +18,7 @@ func TestEnsurePrivateDirRejectsSymlink(t *testing.T) {
 	if err := os.Symlink(target, link); err != nil {
 		t.Fatal(err)
 	}
-	if err := ensurePrivateDir(link); err == nil {
+	if err := EnsurePrivateDir(link); err == nil {
 		t.Fatal("expected symlink runtime directory to be rejected")
 	}
 }
@@ -33,7 +33,7 @@ func TestEnsurePrivateDirRejectsSymlinkedParent(t *testing.T) {
 	if err := os.Symlink(target, link); err != nil {
 		t.Fatal(err)
 	}
-	if err := ensurePrivateDir(filepath.Join(link, "peers")); err == nil {
+	if err := EnsurePrivateDir(filepath.Join(link, "peers")); err == nil {
 		t.Fatal("expected symlinked parent to be rejected")
 	}
 }

internal/peermsg/private_dir_windows.go

@@ -12,7 +12,7 @@ import (
 	"golang.org/x/sys/windows"
 )
 
-func ensurePrivateDir(path string) (resultErr error) {
+func EnsurePrivateDir(path string) (resultErr error) {
 	abs, err := filepath.Abs(path)
 	if err != nil {
 		return err

internal/peermsg/private_dir_windows_test.go

@@ -54,7 +54,7 @@ func TestEnsurePrivateDirAppliesOwnerOnlyProtectedDACL(t *testing.T) {
 	if !windowsDirectoryDACLContains(t, path, worldSID) {
 		t.Fatal("test setup did not grant the broad Everyone ACE")
 	}
-	if err := ensurePrivateDir(path); err != nil {
+	if err := EnsurePrivateDir(path); err != nil {
 		t.Fatal(err)
 	}
 	descriptor, err := windows.GetNamedSecurityInfo(
@@ -149,7 +149,7 @@ func windowsDirectoryDACLContains(t *testing.T, path string, wanted *windows.SID
 
 func TestSecurePrivateDirectoryRejectsOwnerMismatch(t *testing.T) {
 	path := filepath.Join(t.TempDir(), "private")
-	if err := ensurePrivateDir(path); err != nil {
+	if err := EnsurePrivateDir(path); err != nil {
 		t.Fatal(err)
 	}
 	handle, err := openWindowsDirectory(path, windows.FILE_LIST_DIRECTORY|windows.FILE_TRAVERSE|windows.SYNCHRONIZE|windows.READ_CONTROL|windows.WRITE_DAC)
@@ -172,7 +172,7 @@ func TestSecurePrivateDirectoryRejectsOwnerMismatch(t *testing.T) {
 
 func TestSecurePrivateDirectoryReportsDACLWriteFailure(t *testing.T) {
 	path := filepath.Join(t.TempDir(), "private")
-	if err := ensurePrivateDir(path); err != nil {
+	if err := EnsurePrivateDir(path); err != nil {
 		t.Fatal(err)
 	}
 	handle, err := openWindowsDirectory(path, windows.FILE_LIST_DIRECTORY|windows.FILE_TRAVERSE|windows.SYNCHRONIZE|windows.READ_CONTROL)
@@ -206,7 +206,7 @@ func TestEnsurePrivateDirRejectsWindowsReparseParent(t *testing.T) {
 	if err := os.Symlink(target, link); err != nil {
 		t.Skipf("cannot create Windows directory symlink: %v", err)
 	}
-	if err := ensurePrivateDir(filepath.Join(link, "peers")); err == nil {
+	if err := EnsurePrivateDir(filepath.Join(link, "peers")); err == nil {
 		t.Fatal("expected reparse-point parent to be rejected")
 	}
 }
@@ -217,7 +217,7 @@ func TestEnsurePrivateDirRejectsWindowsFileComponent(t *testing.T) {
 	if err := os.WriteFile(file, []byte("not a directory"), 0o600); err != nil {
 		t.Fatal(err)
 	}
-	if err := ensurePrivateDir(filepath.Join(file, "peers")); err == nil {
+	if err := EnsurePrivateDir(filepath.Join(file, "peers")); err == nil {
 		t.Fatal("expected file path component to be rejected")
 	}
 }

internal/peermsg/service.go

@@ -158,7 +158,7 @@ func New(options Options) (*Service, error) {
 }
 
 // canonicalRuntimePath normalizes aliases in the existing prefix. It does not
-// establish a security boundary; ensurePrivateDir validates that boundary when
+// establish a security boundary; EnsurePrivateDir validates that boundary when
 // the service starts.
 func canonicalRuntimePath(path string) (string, error) {
 	missing := make([]string, 0, 4)
@@ -219,10 +219,10 @@ func (service *Service) Start(handler Handler) error {
 	if service.closed {
 		return errors.New("peer messaging: service is closed")
 	}
-	if err := ensurePrivateDir(service.root); err != nil {
+	if err := EnsurePrivateDir(service.root); err != nil {
 		return fmt.Errorf("peer messaging: create runtime directory: %w", err)
 	}
-	if err := ensurePrivateDir(service.registryDir()); err != nil {
+	if err := EnsurePrivateDir(service.registryDir()); err != nil {
 		return fmt.Errorf("peer messaging: create registry: %w", err)
 	}
 	endpoint, err := service.transport.Endpoint(service.root, service.nonce, service.pid)

internal/peermsg/transport_unix.go

@@ -44,7 +44,7 @@ func canonicalPrivateDir(path string) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if err := ensurePrivateDir(canonical); err != nil {
+	if err := EnsurePrivateDir(canonical); err != nil {
 		return "", err
 	}
 	return canonical, nil

internal/sandbox/profile.go

@@ -96,15 +96,104 @@ var protectedMetadataNames = []string{".git", ".zero", ".agents"}
 // gitMetadataWriteCarveouts below.
 var sandboxFullyProtectedMetadataNames = []string{".zero", ".agents"}
 
+// sandboxRenameProtectedMetadataName is the metadata directory that cannot be
+// fully write-protected but must still not be REPLACEABLE.
+//
+// It is deliberately not in the list above. That list denies write, and git has
+// to write index, objects and refs. But the carveouts guarding it are attached
+// to .git/config and .git/hooks as objects, so a principal that renames .git and
+// recreates it gets fresh paths inheriting the workspace allow with no denies,
+// which restores credential.helper and core.hooksPath. The Windows ACL plan
+// therefore denies DELETE on this directory alone, uninherited.
+const sandboxRenameProtectedMetadataName = ".git"
+
 // gitMetadataWriteCarveouts returns the .git subpaths that stay write-denied
 // under the OS-level sandbox even though the rest of .git is writable to git
 // subprocesses. Nonexistent paths are harmless no-ops in every backend's
 // enforcement (seatbelt regex, bwrap ro-bind, Windows ACL deny entry).
 func gitMetadataWriteCarveouts(root string) []string {
-	return []string{
-		filepath.Join(root, ".git", "hooks"),
-		filepath.Join(root, ".git", "config"),
+	specs := gitMetadataWriteCarveoutSpecs(root)
+	out := make([]string, 0, len(specs))
+	for _, spec := range specs {
+		out = append(out, spec.Path)
 	}
+	return out
+}
+
+// gitMetadataCarveout is a write-denied .git path together with the shape git
+// expects it to have. The shape matters to exactly one backend: the Windows ACL
+// plan creates a missing carveout so the deny ACE is in place before git first
+// runs, and creating .git/config as a directory makes `git init` fail outright.
+// Every other backend only ever names the path, so it can ignore IsFile.
+type gitMetadataCarveout struct {
+	Path   string
+	IsFile bool
+}
+
+// gitMetadataWriteCarveoutSpecs is the single source of truth for the carveout
+// set. gitMetadataWriteCarveouts derives its list from this so a path can never
+// be added in one place and have its shape forgotten in the other.
+func gitMetadataWriteCarveoutSpecs(root string) []gitMetadataCarveout {
+	gitPath := filepath.Join(root, ".git")
+	// A LINKED WORKTREE OR SUBMODULE HAS .git AS A FILE, NOT A DIRECTORY.
+	//
+	// It holds a `gitdir:` pointer to the real control directory, which lives
+	// outside this write root. Naming .git/config and .git/hooks there asks the
+	// Windows plan to materialize them by descending through .git as a directory,
+	// which cannot open a child beneath a regular file, so opted-in elevated setup
+	// aborts and the sandbox is unusable in any worktree. Zero's own development
+	// worktrees are exactly this shape.
+	//
+	// Deny the pointer file itself instead. It is the right protection rather than
+	// a lesser one: a principal that can rewrite `gitdir:` repoints the whole
+	// repository at a control directory it chooses, which subsumes editing config
+	// or dropping a hook. The real control directory is outside the write root, so
+	// the principal has no inherited access to it and needs no carveout there.
+	//
+	// Stat, not a lexical guess. Whether .git is a file is a property of the
+	// checkout, and the two layouts want different ACEs. A missing .git (git has
+	// not run yet) keeps the directory-shaped carveouts, which is what makes them
+	// materialize before git first runs.
+	if info, err := os.Lstat(gitPath); err == nil && !info.IsDir() {
+		return []gitMetadataCarveout{{Path: gitPath, IsFile: true}}
+	}
+	return []gitMetadataCarveout{
+		{Path: filepath.Join(root, ".git", "hooks")},
+		{Path: filepath.Join(root, ".git", "config"), IsFile: true},
+	}
+}
+
+// gitMetadataCarveoutSuffixBase is a sentinel root used only to recover the
+// trailing segments of the carveout specs. It is never touched on disk.
+const gitMetadataCarveoutSuffixBase = string(filepath.Separator) + "zero-carveout-base"
+
+// gitMetadataCarveoutIsFile reports whether path names a carveout git expects
+// to be a file.
+//
+// It matches on the trailing segments rather than on a whole reconstructed
+// path. The subpaths reaching the ACL plan are already normalized — resolved
+// through EvalSymlinks where that succeeds — while a rebuilt spec path cannot
+// be, because .git/config does not exist yet at setup and resolution falls back
+// to a plain Clean. On a host where two spellings of the same path differ (an
+// 8.3 short name, different casing) a whole-path equality check silently misses
+// and the carveout is created as a directory again, which is the original bug
+// reintroduced quietly. The suffix cannot drift from the spec list because it
+// is derived from it.
+func gitMetadataCarveoutIsFile(path string) bool {
+	candidate := strings.ToLower(filepath.Clean(strings.TrimSpace(path)))
+	if candidate == "" {
+		return false
+	}
+	for _, spec := range gitMetadataWriteCarveoutSpecs(gitMetadataCarveoutSuffixBase) {
+		if !spec.IsFile {
+			continue
+		}
+		suffix := strings.ToLower(strings.TrimPrefix(spec.Path, gitMetadataCarveoutSuffixBase))
+		if suffix != "" && strings.HasSuffix(candidate, suffix) {
+			return true
+		}
+	}
+	return false
 }
 
 func PermissionProfileFromPolicy(workspaceRoot string, policy Policy, scope *Scope) PermissionProfile {

internal/sandbox/profile_gitfile_test.go

@@ -0,0 +1,79 @@
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// A LINKED WORKTREE HAS .git AS A FILE.
+//
+// The Windows principal plan materializes every carveout, and it does so by
+// descending through .git as a directory. Against a `gitdir:` pointer file that
+// walk cannot open a child, so opted-in elevated setup aborts and the sandbox
+// cannot be used in a worktree at all. Zero's own development worktrees have
+// exactly this layout, so this is the common case for anyone working on the
+// sandbox itself.
+func TestGitCarveoutsHandleAWorktreeGitfile(t *testing.T) {
+	root := t.TempDir()
+	gitFile := filepath.Join(root, ".git")
+	// The real shape git writes for a linked worktree.
+	if err := os.WriteFile(gitFile, []byte("gitdir: "+filepath.Join(root, "..", "main", ".git", "worktrees", "wt")+"\n"), 0o600); err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+
+	specs := gitMetadataWriteCarveoutSpecs(root)
+	if len(specs) != 1 {
+		t.Fatalf("carveout specs = %#v, want only the .git pointer file", specs)
+	}
+	if filepath.Clean(specs[0].Path) != filepath.Clean(gitFile) {
+		t.Fatalf("carveout path = %s, want the .git pointer file %s", specs[0].Path, gitFile)
+	}
+	if !specs[0].IsFile {
+		t.Fatal("the .git pointer carveout is not marked as a file, so materialization would create a directory where git needs a file")
+	}
+	// Nothing may name a path BENEATH the pointer file, which is what forces the
+	// directory descent that aborts setup.
+	for _, path := range gitMetadataWriteCarveouts(root) {
+		rest := strings.TrimPrefix(filepath.Clean(path), filepath.Clean(gitFile))
+		if rest != "" {
+			t.Errorf("carveout %s descends through the .git pointer file, which cannot have children", path)
+		}
+	}
+}
+
+// The ordinary repository layout must keep the config and hooks carveouts. They
+// are the whole reason the set exists: without them a principal reinstates
+// credential.helper or core.hooksPath through inherited workspace write access.
+func TestGitCarveoutsKeepConfigAndHooksForARealGitDir(t *testing.T) {
+	root := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(root, ".git"), 0o700); err != nil {
+		t.Fatalf("MkdirAll: %v", err)
+	}
+	paths := gitMetadataWriteCarveouts(root)
+	for _, want := range []string{filepath.Join(root, ".git", "hooks"), filepath.Join(root, ".git", "config")} {
+		found := false
+		for _, got := range paths {
+			if filepath.Clean(got) == filepath.Clean(want) {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("carveout %s missing for an ordinary .git directory: %#v", want, paths)
+		}
+	}
+}
+
+// A workspace where git has never run keeps the directory-shaped carveouts. That
+// is what lets the Windows plan create them before git first runs, so the deny
+// ACEs are already in place rather than being applied to paths git will create
+// later with inherited write access.
+func TestGitCarveoutsKeepConfigAndHooksWhenGitIsAbsent(t *testing.T) {
+	root := t.TempDir()
+	paths := gitMetadataWriteCarveouts(root)
+	if len(paths) != 2 {
+		t.Fatalf("carveouts = %#v, want the config and hooks pair when .git is absent", paths)
+	}
+}

internal/sandbox/runner_windows_integration_test.go

@@ -352,6 +352,15 @@ func realSmokeExecutable(t *testing.T, envKey string, fallbackName string) strin
 
 func runWindowsRealSmokeSetup(t *testing.T, setupExe string, options WindowsSandboxSetupArgsOptions) {
 	t.Helper()
+	// options.PrincipalOptIn is deliberately left nil by both call sites, which
+	// makes BuildWindowsSandboxSetupArgs resolve the opt-in from this process's
+	// environment — the same value the command half resolves, since the smoke
+	// WindowsSandboxCommandArgsOptions carries no explicit entry either. Do not
+	// "fix" this by setting it to false: anyone running this suite with
+	// ZERO_WINDOWS_SANDBOX_IDENTITY=1 (the only way to exercise the principal
+	// backend) would then serialize `--sandbox-principal 0`, disagree with the
+	// command half, and fail every command at marker validation instead of
+	// testing the sandbox.
 	args, err := BuildWindowsSandboxSetupArgs(options)
 	if err != nil {
 		t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)

internal/sandbox/runtime_fallback_anchor_other.go

@@ -0,0 +1,33 @@
+//go:build !windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+)
+
+// physicalTempDir returns os.TempDir with every symlink in its ancestry
+// resolved, so the fallback anchor's parent chain is physical and only the
+// anchor itself is a new component.
+//
+// peermsg.EnsurePrivateDir walks from the root with O_NOFOLLOW and refuses any
+// component that is a link. That is right for the owned tail and wrong for the
+// ancestors above it: on macOS os.TempDir lives under /var, which is a symlink
+// to /private/var, so the first version of this fix refused every fallback on
+// every Mac with "refusing non-directory or symlink runtime path component
+// var". The constraint jatmn stated for #901 applies here identically:
+// redirected cache and TEMP locations above the owned tail are the operator's
+// business and must keep working; the restriction is on what Zero owns.
+//
+// EvalSymlinks is the correct resolver off Windows, where the only reparse
+// shape is a symlink and it traverses them.
+func physicalTempDir() (string, error) {
+	return physicalDir(os.TempDir())
+}
+
+// physicalDir resolves any directory the same way, for the other places that
+// have to hand EnsurePrivateDir a physical parent.
+func physicalDir(path string) (string, error) {
+	return filepath.EvalSymlinks(path)
+}

internal/sandbox/runtime_fallback_anchor_windows.go

@@ -0,0 +1,63 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"fmt"
+	"os"
+
+	"golang.org/x/sys/windows"
+)
+
+// physicalTempDir returns os.TempDir as the object it actually names, so the
+// fallback anchor's parent chain is physical and only the anchor itself is a
+// new component.
+//
+// peermsg.EnsurePrivateDir walks from the volume root with OBJ_DONT_REPARSE and
+// refuses any component that is a reparse point. That is right for the owned
+// tail and wrong for the ancestors above it: a redirected %LOCALAPPDATA% is an
+// ordinary Windows configuration, and TEMP sits beneath it. The constraint
+// jatmn stated for #901 applies here identically: redirected cache and TEMP
+// locations above the owned tail must keep working; the restriction is on what
+// Zero owns.
+//
+// GetFinalPathNameByHandle rather than filepath.EvalSymlinks, because
+// EvalSymlinks does not traverse a junction on Windows: it returns the
+// junction's own path and the walker then refuses it. The handle answers where
+// the directory actually is. Same recipe as verifyWindowsACLTargetNotRedirected,
+// which is why the flag constants and the prefix trim are shared.
+func physicalTempDir() (string, error) {
+	return physicalDir(os.TempDir())
+}
+
+// physicalDir resolves any directory through a handle, for the other places
+// that have to hand EnsurePrivateDir a physical parent. GetFinalPathNameByHandle
+// rather than EvalSymlinks, because EvalSymlinks does not traverse a junction.
+func physicalDir(path string) (string, error) {
+	utf16Path, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return "", fmt.Errorf("encode directory %s: %w", path, err)
+	}
+	handle, err := windows.CreateFile(
+		utf16Path,
+		windows.FILE_READ_ATTRIBUTES|windows.SYNCHRONIZE,
+		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_FLAG_BACKUP_SEMANTICS,
+		0,
+	)
+	if err != nil {
+		return "", fmt.Errorf("open directory %s: %w", path, err)
+	}
+	defer windows.CloseHandle(handle)
+	buffer := make([]uint16, windows.MAX_LONG_PATH)
+	n, err := windows.GetFinalPathNameByHandle(handle, &buffer[0], uint32(len(buffer)), windowsFileNameNormalized|windowsVolumeNameDOS)
+	if err != nil {
+		return "", fmt.Errorf("resolve directory %s: %w", path, err)
+	}
+	if int(n) < len(buffer) {
+		buffer = buffer[:n]
+	}
+	return trimWindowsExtendedPathPrefix(windows.UTF16ToString(buffer)), nil
+}

internal/sandbox/runtime_fallback_anchor_windows_test.go

@@ -0,0 +1,155 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// DETERMINISM IS NOT OWNERSHIP.
+//
+// The fallback runtime root moved from a fresh os.MkdirTemp parent to a
+// predictable name so that elevated setup and the later command agree on it.
+// That is required. But a predictable path beneath the shared system temp is
+// not a private allocation: whatever another local user, or a stray earlier
+// process, has already placed at that name is what preparation then leases,
+// creates, chmods and cleans through. The anchor beneath which the tree lives
+// must therefore be proven to be this user's private directory before anything
+// is built under it, on BOTH paths that can hand back a fallback root.
+//
+// Junctions need no privilege on Windows, so the "attacker-precreated parent"
+// jatmn described is modelled as a junction at the anchor pointing into a
+// directory the test owns and watches. Either trigger must refuse, and the
+// watched directory must stay empty.
+
+// fallbackAnchorFixture redirects TEMP so the anchor lands inside a directory
+// this test owns, plants a junction at the anchor into a watched target, and
+// returns the target. The junction is the shape that defeats a pathname walk:
+// it is not a symlink to os.ModeSymlink, and EvalSymlinks will not traverse it.
+func fallbackAnchorFixture(t *testing.T) (target string) {
+	t.Helper()
+	tempRoot := t.TempDir()
+	target = t.TempDir()
+	t.Setenv("TMP", tempRoot)
+	t.Setenv("TEMP", tempRoot)
+
+	// The anchor's parent is the PHYSICAL temp dir, so compare against the
+	// resolved form of the redirect rather than its spelling: t.TempDir can
+	// come back as an 8.3 short name or in a different case from what the
+	// handle reports, and a prefix check on the raw string would fail for a
+	// correct anchor.
+	anchor := fallbackRuntimeAnchor()
+	resolvedTempRoot, err := physicalTempDir()
+	if err != nil {
+		t.Fatalf("SETUP INVALID: cannot resolve the redirected TEMP %s: %v", tempRoot, err)
+	}
+	if !pathWithinRoot(resolvedTempRoot, anchor) {
+		t.Fatalf("SETUP INVALID: anchor %s is not under the redirected TEMP %s (resolved %s)", anchor, tempRoot, resolvedTempRoot)
+	}
+	if out, err := exec.Command("cmd", "/c", "mklink", "/J", anchor, target).CombinedOutput(); err != nil {
+		t.Fatalf("mklink /J: %v\n%s", err, out)
+	}
+	return target
+}
+
+func assertNothingUnder(t *testing.T, target string) {
+	t.Helper()
+	entries, err := os.ReadDir(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(entries) != 0 {
+		names := make([]string, 0, len(entries))
+		for _, entry := range entries {
+			names = append(names, entry.Name())
+		}
+		t.Errorf("preparation built %v beneath the junction target, which is somebody else's directory", names)
+	}
+}
+
+// Trigger one: the user cache resolves INSIDE the workspace, so
+// sandboxRuntimeRootFor itself returns the fallback root and the first lease
+// attempt is already against it.
+func TestFallbackAnchorIsRefusedWhenTheCacheLandsInsideTheWorkspace(t *testing.T) {
+	target := fallbackAnchorFixture(t)
+	workspace := t.TempDir()
+
+	previous := sandboxUserCacheDir
+	t.Cleanup(func() { sandboxUserCacheDir = previous })
+	sandboxUserCacheDir = func() (string, error) { return filepath.Join(workspace, ".cache"), nil }
+
+	_, release, err := prepareSandboxRuntime(workspace)
+	if err == nil {
+		release()
+		t.Fatal("preparation went through a junction at the fallback anchor and reported success")
+	}
+	if !strings.Contains(err.Error(), "fallback anchor") {
+		t.Errorf("the refusal does not name the anchor, so the operator cannot act on it: %v", err)
+	}
+	assertNothingUnder(t, target)
+}
+
+// Trigger two: the cache root cannot be leased (a FILE sits where the cache
+// runtime tree would go), so preparation falls back explicitly.
+func TestFallbackAnchorIsRefusedWhenTheCacheRootCannotBeLeased(t *testing.T) {
+	target := fallbackAnchorFixture(t)
+	workspace := t.TempDir()
+
+	// Make the cache-derived root unleasable at its ROOT: the user cache is a
+	// regular file, so nothing beneath it can be created and the first lease
+	// fails, which is the path that asks for the fallback explicitly. The
+	// first draft planted the file at the derived leaf instead, and the
+	// preparation failed on that mkdir before ever reaching the fallback
+	// branch, so the trigger under test never fired.
+	cache := filepath.Join(t.TempDir(), "cache-is-a-file")
+	if err := os.WriteFile(cache, []byte("not a directory"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+
+	previous := sandboxUserCacheDir
+	t.Cleanup(func() { sandboxUserCacheDir = previous })
+	sandboxUserCacheDir = func() (string, error) { return cache, nil }
+
+	_, release, err := prepareSandboxRuntime(workspace)
+	if err == nil {
+		release()
+		t.Fatal("preparation fell back through a junction at the anchor and reported success")
+	}
+	if !strings.Contains(err.Error(), "fallback anchor") {
+		t.Errorf("the refusal does not name the anchor: %v", err)
+	}
+	assertNothingUnder(t, target)
+}
+
+// And the honest control: with nothing planted, the fallback prepares, the
+// anchor is a real directory this test can see, and the runtime tree sits
+// beneath it.
+func TestFallbackAnchorIsCreatedPrivatelyWhenNothingIsInTheWay(t *testing.T) {
+	tempRoot := t.TempDir()
+	t.Setenv("TMP", tempRoot)
+	t.Setenv("TEMP", tempRoot)
+	workspace := t.TempDir()
+
+	previous := sandboxUserCacheDir
+	t.Cleanup(func() { sandboxUserCacheDir = previous })
+	sandboxUserCacheDir = func() (string, error) { return filepath.Join(workspace, ".cache"), nil }
+
+	runtimeState, release, err := prepareSandboxRuntime(workspace)
+	if err != nil {
+		t.Fatalf("a clean fallback was refused: %v", err)
+	}
+	defer release()
+
+	anchor := fallbackRuntimeAnchor()
+	info, err := os.Lstat(anchor)
+	if err != nil || !info.IsDir() {
+		t.Fatalf("the anchor was not created as a directory: err=%v", err)
+	}
+	if !pathWithinRoot(anchor, runtimeState.Root) {
+		t.Errorf("runtime root %s is not beneath the anchor %s", runtimeState.Root, anchor)
+	}
+}

internal/sandbox/runtime_fallback_test.go

@@ -0,0 +1,85 @@
+package sandbox
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// SETUP AND EVERY COMMAND RUNNER MUST DERIVE THE SAME FALLBACK ROOT.
+//
+// They are separate processes. The fallback used to call os.MkdirTemp and cache
+// the answer in a process-global map, so elevated setup granted the sandbox
+// principal write access to the directory IT made, and each later
+// __windows-command-runner made a different one and pointed TMP, GOCACHE and npm
+// there. Those directories are created by the calling user and carry no ACE for
+// the principal, so ordinary cache writes failed with a bare ACCESS_DENIED.
+//
+// A repeated call is what a second process looks like from here: no shared
+// state, same inputs. The old implementation could not pass this, because the
+// only thing that made repeat calls agree was the in-process map.
+func TestFallbackRuntimeRootIsTheSameForEveryProcess(t *testing.T) {
+	workspace := filepath.Join(t.TempDir(), "ws")
+
+	first, err := fallbackSandboxRuntimeRoot(workspace)
+	if err != nil {
+		t.Fatalf("fallbackSandboxRuntimeRoot: %v", err)
+	}
+	second, err := fallbackSandboxRuntimeRoot(workspace)
+	if err != nil {
+		t.Fatalf("fallbackSandboxRuntimeRoot (second process): %v", err)
+	}
+	if first != second {
+		t.Fatalf("two processes derived different runtime roots, so setup grants one and commands write to another:\n  %s\n  %s", first, second)
+	}
+	if first == "" {
+		t.Fatal("no runtime root derived")
+	}
+}
+
+// Different workspaces must not share a runtime tree, or one workspace's
+// principal would hold an ACE on another's caches.
+func TestFallbackRuntimeRootIsPerWorkspace(t *testing.T) {
+	base := t.TempDir()
+	first, err := fallbackSandboxRuntimeRoot(filepath.Join(base, "alpha"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	second, err := fallbackSandboxRuntimeRoot(filepath.Join(base, "beta"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if first == second {
+		t.Fatalf("two workspaces share the runtime root %q", first)
+	}
+}
+
+// It must NAME the tree without creating it. Teardown asks for this path on its
+// way out, and materializing a directory there would leave one behind on every
+// teardown.
+func TestFallbackRuntimeRootCreatesNothing(t *testing.T) {
+	workspace := filepath.Join(t.TempDir(), "ws")
+	root, err := fallbackSandboxRuntimeRoot(workspace)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pathExists(root) {
+		t.Errorf("naming the runtime root created %s", root)
+	}
+}
+
+// The root must land outside the workspace, which is the entire reason this
+// branch exists.
+func TestFallbackRuntimeRootStaysOutsideTheWorkspace(t *testing.T) {
+	workspace := filepath.Join(t.TempDir(), "ws")
+	root, err := fallbackSandboxRuntimeRoot(workspace)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if pathWithinRoot(workspace, root) {
+		t.Errorf("runtime root %q is inside workspace %q", root, workspace)
+	}
+	if !strings.Contains(root, "zero") {
+		t.Errorf("runtime root %q does not look like a zero-owned tree", root)
+	}
+}

internal/sandbox/runtime_state.go

@@ -7,10 +7,14 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime"
 	"sort"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
+
+	"github.com/Gitlawb/zero/internal/peermsg"
 )
 
 var sandboxUserCacheDir = os.UserCacheDir
@@ -21,11 +25,6 @@ const (
 	sandboxRuntimeMaxRoots = 64
 )
 
-var fallbackSandboxRuntimes = struct {
-	sync.Mutex
-	roots map[string]string
-}{roots: make(map[string]string)}
-
 type SandboxRuntime struct {
 	Root  string `json:"root,omitempty"`
 	Cache string `json:"cache,omitempty"`
@@ -33,8 +32,37 @@ type SandboxRuntime struct {
 	Temp  string `json:"temp,omitempty"`
 }
 
+// sandboxRuntimeRootFor derives the per-workspace runtime root. It is separated
+// from prepareSandboxRuntime because the elevated Windows setup path needs the
+// same answer WITHOUT taking a lease or creating anything: a sandbox principal
+// is a separate account with no inherited rights under the user cache, so setup
+// has to grant it write access to this tree before any command runs.
+//
+// Both callers must agree exactly. If they ever drift, setup grants the ACE on
+// one directory while commands write to another, and the failure is a bare
+// ACCESS_DENIED from npm or go build with nothing pointing at the sandbox.
+func sandboxRuntimeRootFor(workspaceRoot string, cacheRoot string) (string, error) {
+	if root, ok := deterministicSandboxRuntimeRoot(workspaceRoot, cacheRoot); ok {
+		return root, nil
+	}
+	return fallbackSandboxRuntimeRoot(workspaceRoot)
+}
+
+// deterministicSandboxRuntimeRoot returns the cache-derived runtime root and
+// whether it is usable, meaning it lands outside the workspace.
+//
+// Neither this nor the fallback creates anything now, so a caller that only
+// needs to NAME the tree can safely go through sandboxRuntimeRootFor and get the
+// answer commands will actually use. This remains separate for callers that need
+// to distinguish the cache-derived root from the temp-derived one.
+func deterministicSandboxRuntimeRoot(workspaceRoot string, cacheRoot string) (string, bool) {
+	digest := sha256.Sum256([]byte(workspaceRoot))
+	root := filepath.Join(cacheRoot, "zero", "runtime", "v1", hex.EncodeToString(digest[:8]))
+	return root, !pathWithinRoot(workspaceRoot, root)
+}
+
 func prepareSandboxRuntime(workspaceRoot string) (SandboxRuntime, func(), error) {
-	workspaceRoot = filepath.Clean(strings.TrimSpace(workspaceRoot))
+	workspaceRoot = canonicalSandboxWorkspaceRoot(workspaceRoot)
 	if workspaceRoot == "" || workspaceRoot == "." {
 		return SandboxRuntime{}, nil, errors.New("sandbox runtime requires a workspace root")
 	}
@@ -42,17 +70,30 @@ func prepareSandboxRuntime(workspaceRoot string) (SandboxRuntime, func(), error)
 	if err != nil {
 		return SandboxRuntime{}, nil, fmt.Errorf("resolve user cache directory: %w", err)
 	}
-	cacheRoot = filepath.Clean(strings.TrimSpace(cacheRoot))
+	// Canonicalized the SAME way as the workspace root, because
+	// sandboxRuntimeRootFor compares the two: it falls back to a private temp
+	// tree when the derived runtime root would land inside the workspace.
+	// Normalizing only one side made that comparison run on two different
+	// spellings of the same path — /var vs /private/var on macOS, an 8.3 short
+	// name vs its long form on Windows — so the containment check missed and the
+	// fallback never fired.
+	cacheRoot = canonicalSandboxWorkspaceRoot(cacheRoot)
 	if cacheRoot == "" || cacheRoot == "." {
 		return SandboxRuntime{}, nil, errors.New("user cache directory is unavailable")
 	}
-	digest := sha256.Sum256([]byte(workspaceRoot))
-	root := filepath.Join(cacheRoot, "zero", "runtime", "v1", hex.EncodeToString(digest[:8]))
-	if pathWithinRoot(workspaceRoot, root) {
-		root, err = fallbackSandboxRuntimeRoot(workspaceRoot)
-		if err != nil {
-			return SandboxRuntime{}, nil, err
-		}
+	root, err := sandboxRuntimeRootFor(workspaceRoot, cacheRoot)
+	if err != nil {
+		return SandboxRuntime{}, nil, err
+	}
+	// THE ANCHOR IS PROVEN BEFORE THE TREE IS BUILT, on both paths that can
+	// hand back a fallback root: sandboxRuntimeRootFor returns one itself when
+	// the cache lands inside the workspace, and the branch below asks for one
+	// when the cache root cannot be leased. A deterministic name under the
+	// shared temp is only safe beneath a directory this user owns; if the anchor
+	// is a link, or exists and belongs to someone else, this refuses rather than
+	// leasing, creating and chmod-ing through it.
+	if err := ensureFallbackRuntimeAnchor(root); err != nil {
+		return SandboxRuntime{}, nil, err
 	}
 	lease, err := prepareSandboxRuntimeLease(root)
 	if err != nil {
@@ -60,6 +101,9 @@ func prepareSandboxRuntime(workspaceRoot string) (SandboxRuntime, func(), error)
 		if err != nil {
 			return SandboxRuntime{}, nil, err
 		}
+		if err := ensureFallbackRuntimeAnchor(root); err != nil {
+			return SandboxRuntime{}, nil, err
+		}
 		lease, err = prepareSandboxRuntimeLease(root)
 		if err != nil {
 			return SandboxRuntime{}, nil, err
@@ -174,22 +218,109 @@ func combineSandboxCleanups(cleanups ...func()) func() {
 	}
 }
 
+// fallbackRuntimeAnchor is the per-user directory the deterministic fallback
+// runtime tree lives beneath.
+//
+// DETERMINISM IS NOT OWNERSHIP. The fallback moved from a fresh, private
+// os.MkdirTemp parent to a predictable name so that elevated setup and the
+// later command process agree on the runtime root. That agreement is required
+// and must stay. But a predictable path directly under the shared system temp
+// is not a private allocation: on an ordinary shared /tmp another local user
+// can create /tmp/zero with mode 0700 first, and every affected user then fails
+// before the sandbox command starts; a pre-existing redirected component makes
+// host-side preparation operate somewhere other than the derived tree.
+//
+// So the tree is rooted beneath an anchor that names THIS user, and the anchor
+// is created and validated by peermsg.EnsurePrivateDir before anything is
+// built under it: handle-relative descent, no-follow, refused if any component
+// is a link or the leaf is not owned by the current user, mode 0700. On Unix
+// the uid is in the name because /tmp is shared between users; on Windows
+// os.TempDir is already per-user, and the private descriptor plus the owner
+// check do the rest.
+func fallbackRuntimeAnchor() string {
+	tag := "user"
+	if runtime.GOOS != "windows" {
+		tag = strconv.Itoa(os.Getuid())
+	}
+	// PHYSICAL PARENT, OWNED LEAF. The validator walks every component no-follow
+	// and refuses links, which is right for the anchor Zero owns and wrong for
+	// the operator's ancestors above it: macOS puts TempDir under /var, a
+	// symlink to /private/var, and a redirected %LOCALAPPDATA% is ordinary on
+	// Windows. The first version handed the validator the unresolved path and
+	// refused every fallback on every Mac. Resolving the parent first means the
+	// only new component the validator sees is the anchor itself.
+	//
+	// On a resolve error the unresolved path is used, deliberately: the
+	// validator is still the fail-closed check, and refusing there names the
+	// real component rather than hiding it behind a resolver failure.
+	base := os.TempDir()
+	if physical, err := physicalTempDir(); err == nil && physical != "" {
+		base = physical
+	}
+	return filepath.Join(base, "zero-runtime-"+tag)
+}
+
+// isFallbackRuntimeRoot reports whether root was derived by
+// fallbackSandboxRuntimeRoot, so the caller knows the anchor must be validated
+// before the tree beneath it is prepared. Both candidates a command derives can
+// be fallback roots: sandboxRuntimeRootFor returns one itself when the cache
+// lands inside the workspace, and prepareSandboxRuntime asks for one when the
+// cache root cannot be leased.
+func isFallbackRuntimeRoot(root string) bool {
+	// pathWithinRoot(parent, child): is root beneath the anchor. The first
+	// draft had these reversed, which asked whether the anchor was beneath the
+	// root, answered false every time, and skipped the validation entirely
+	// while every test that depended on it reported success through a
+	// junction. Argument order is the whole function.
+	return pathWithinRoot(fallbackRuntimeAnchor(), root)
+}
+
+// ensureFallbackRuntimeAnchor validates the anchor beneath which a fallback
+// runtime tree is about to be built, and fails closed. Called only when the
+// root is a fallback root; the cache-derived root lives under the user cache,
+// which is already the user's own directory.
+func ensureFallbackRuntimeAnchor(root string) error {
+	if !isFallbackRuntimeRoot(root) {
+		return nil
+	}
+	anchor := fallbackRuntimeAnchor()
+	if err := peermsg.EnsurePrivateDir(anchor); err != nil {
+		return fmt.Errorf("sandbox runtime fallback anchor %s is not a private directory owned by this user: %w", anchor, err)
+	}
+	return nil
+}
+
+// fallbackSandboxRuntimeRoot returns the runtime root for a workspace whose
+// cache-derived root would land inside itself.
+//
+// DERIVED, not minted, and that is the whole of the fix. It used to call
+// os.MkdirTemp and remember the answer in a process-global map, which made the
+// result private to whichever process asked first. Elevated Windows setup
+// granted the sandbox principal write access to the directory IT created, then
+// every later __windows-command-runner process created a DIFFERENT one and
+// pointed TMP, GOCACHE, npm and the rest at it. Those directories are created by
+// the calling user and carry no ACE for the principal, so ordinary cache and
+// temp writes failed with a bare ACCESS_DENIED and nothing naming the sandbox.
+//
+// sandboxRuntimeRootFor already documents that both callers must agree exactly.
+// Hashing the workspace, the same way the cache-derived root does, is what makes
+// that true for this branch too: every process reaches the same path without
+// having to share any state.
+//
+// It creates nothing, so deterministicSandboxRuntimeRoot's promise about naming
+// a tree without materializing it now holds for the fallback as well.
 func fallbackSandboxRuntimeRoot(workspaceRoot string) (string, error) {
-	fallbackSandboxRuntimes.Lock()
-	defer fallbackSandboxRuntimes.Unlock()
-	if root := fallbackSandboxRuntimes.roots[workspaceRoot]; root != "" {
-		return root, nil
-	}
-	parent, err := os.MkdirTemp("", "zero-runtime-")
-	if err != nil {
-		return "", fmt.Errorf("create fallback sandbox runtime: %w", err)
-	}
-	root := filepath.Join(parent, "runtime")
+	digest := sha256.Sum256([]byte(workspaceRoot))
+	root := filepath.Join(fallbackRuntimeAnchor(), "v1", hex.EncodeToString(digest[:8]))
 	if pathWithinRoot(workspaceRoot, root) {
-		_ = os.RemoveAll(parent)
-		return "", fmt.Errorf("fallback sandbox runtime root %q must be outside workspace %q", root, workspaceRoot)
+		// Both candidates land inside the workspace, so there is nowhere left to
+		// put a runtime tree the workspace's own policy does not govern. Refused
+		// rather than pointed somewhere arbitrary: a runtime root inside the
+		// workspace makes the sandbox's own cache writes indistinguishable from
+		// the work it is meant to be confining.
+		return "", fmt.Errorf("sandbox runtime root %q would fall inside workspace %q; "+
+			"open the workspace somewhere other than the cache or temp directory", root, workspaceRoot)
 	}
-	fallbackSandboxRuntimes.roots[workspaceRoot] = root
 	return root, nil
 }
 
@@ -226,3 +357,59 @@ func permissionProfileWithRuntime(profile PermissionProfile, runtimeState Sandbo
 	profile.FileSystem.WriteRoots = append(profile.FileSystem.WriteRoots, WritableRoot{Root: runtimeState.Root})
 	return profile
 }
+
+// canonicalSandboxWorkspaceRoot normalizes a workspace root the way
+// Engine.resolveCommandDir already does — clean, absolutize, then resolve
+// symlinks — so every derivation keyed to a workspace agrees on the string.
+//
+// The runtime root is a hash of this, and the elevated Windows setup grants the
+// principal that tree while commands derive it again. Cleaning alone was not
+// enough for the two to agree, and it does not take a symlink for them to
+// differ: a path opened in different casing, or through an 8.3 short name (what
+// a Windows CI runner's TEMP looks like), resolves to a different spelling.
+// Setup then granted one tree and every command used another, so the grant that
+// makes npm/go/pip caches writable landed where nothing reads and surfaced as a
+// bare ACCESS_DENIED.
+//
+// Resolution failing is not an error: an unresolvable root still needs a stable
+// key, and falling back to the cleaned absolute path is what the command path
+// does too.
+func canonicalSandboxWorkspaceRoot(root string) string {
+	cleaned := filepath.Clean(strings.TrimSpace(root))
+	if cleaned == "" || cleaned == "." {
+		return ""
+	}
+	if !filepath.IsAbs(cleaned) {
+		if absolute, err := filepath.Abs(cleaned); err == nil {
+			cleaned = absolute
+		}
+	}
+	// EvalSymlinks fails outright when the LEAF does not exist, which is the
+	// normal case for a cache or runtime root that has not been created yet. A
+	// plain call therefore resolved an existing workspace while leaving a
+	// not-yet-created cache root unresolved, and the two were compared against
+	// each other — the containment check that decides whether the runtime tree
+	// must move out of the workspace then ran on /private/var/... versus
+	// /var/..., missed, and left the tree inside the workspace.
+	//
+	// Resolve the longest existing ancestor and re-append the rest, so a path
+	// normalizes the same way whether or not its final segments exist yet.
+	remainder := ""
+	current := cleaned
+	for {
+		if resolved, err := filepath.EvalSymlinks(current); err == nil {
+			if remainder == "" {
+				return resolved
+			}
+			return filepath.Join(resolved, remainder)
+		}
+		parent := filepath.Dir(current)
+		if parent == current {
+			// Nothing along the path resolved; the cleaned absolute form is the
+			// best stable key available.
+			return cleaned
+		}
+		remainder = filepath.Join(filepath.Base(current), remainder)
+		current = parent
+	}
+}

internal/sandbox/runtime_state_test.go

@@ -329,3 +329,79 @@ func TestEngineCommandPlanCarriesManagedRuntime(t *testing.T) {
 	}
 	cleanupLease.release()
 }
+
+// sandboxRuntimeRootFor compares the workspace root against the derived runtime
+// root to decide whether to fall back to a private temp tree. Both sides
+// therefore have to be the same spelling of the same path.
+//
+// Canonicalizing only the workspace root broke this on CI: the workspace
+// resolved (/var to /private/var on macOS, an 8.3 short name to its long form
+// on Windows) while the cache root kept its original spelling, so the
+// containment check compared two different strings, the fallback never fired,
+// and the runtime tree was placed inside the workspace it exists to stay out of.
+//
+// A symlink is the portable way to produce a spelling that only resolution
+// reconciles — Clean cannot see through one. Windows refuses to create symlinks
+// without privilege, so this skips there; the platforms that CI caught the bug
+// on are the ones that run it.
+func TestPrepareSandboxRuntimeNormalizesTheCacheRootBeforeComparingIt(t *testing.T) {
+	workspace := t.TempDir()
+	link := filepath.Join(t.TempDir(), "workspace-link")
+	if err := os.Symlink(workspace, link); err != nil {
+		t.Skipf("cannot create a symlink here: %v", err)
+	}
+	// The cache root reaches us spelled through the symlink; the workspace does
+	// not. Resolved, it is plainly inside the workspace and the fallback must
+	// fire. Unresolved, the two strings share no prefix and it does not.
+	original := sandboxUserCacheDir
+	sandboxUserCacheDir = func() (string, error) { return filepath.Join(link, ".cache"), nil }
+	t.Cleanup(func() { sandboxUserCacheDir = original })
+
+	runtimeState, release, err := prepareSandboxRuntime(workspace)
+	if err != nil {
+		t.Fatalf("prepareSandboxRuntime: %v", err)
+	}
+	if release != nil {
+		defer release()
+	}
+	// Resolve before comparing. Spelled through the link the runtime root shares
+	// no textual prefix with the workspace, so an unresolved comparison would
+	// call it "outside" while it sits physically inside — the test would pass
+	// against the very bug it exists for.
+	resolved := runtimeState.Root
+	if actual, err := filepath.EvalSymlinks(runtimeState.Root); err == nil {
+		resolved = actual
+	}
+	if pathWithinRoot(workspace, resolved) {
+		t.Fatalf("runtime root %q resolves to %q, inside workspace %q; the containment check did not see through the cache root's spelling", runtimeState.Root, resolved, workspace)
+	}
+}
+
+// A path whose final segments do not exist yet must still normalize the same
+// way as one that does. This is the shape macOS CI hit: t.TempDir() sits under
+// /var, a symlink to /private/var, and the cache root it derives has not been
+// created when it is first normalized. Resolving only the workspace left the
+// two sides of the containment check spelled differently.
+func TestCanonicalSandboxWorkspaceRootResolvesThroughAMissingLeaf(t *testing.T) {
+	real := t.TempDir()
+	link := filepath.Join(t.TempDir(), "link")
+	if err := os.Symlink(real, link); err != nil {
+		t.Skipf("cannot create a symlink here: %v", err)
+	}
+
+	existing := canonicalSandboxWorkspaceRoot(link)
+	if existing != canonicalSandboxWorkspaceRoot(real) {
+		t.Fatalf("an existing symlinked dir did not resolve: %q vs %q", existing, canonicalSandboxWorkspaceRoot(real))
+	}
+
+	// The leaf, and its parent, do not exist.
+	missing := filepath.Join(link, ".cache", "zero")
+	got := canonicalSandboxWorkspaceRoot(missing)
+	want := filepath.Join(existing, ".cache", "zero")
+	if got != want {
+		t.Errorf("missing leaf normalized to %q, want %q — the ancestor was not resolved", got, want)
+	}
+	if !pathWithinRoot(existing, got) {
+		t.Errorf("%q should be inside %q once both are canonical", got, existing)
+	}
+}

internal/sandbox/windows_acl.go

@@ -2,16 +2,77 @@ package sandbox
 
 import (
 	"errors"
+	"fmt"
 	"path/filepath"
 	"strings"
 )
 
+// isWindowsVolumeRoot reports whether a cleaned path is the top of a volume,
+// with nothing above it: `C:\`, a bare separator, or a UNC share root.
+//
+// Detected structurally rather than by pattern matching drive letters, because
+// filepath.Dir of a root is that same root and of anything else is strictly
+// shorter. That holds for drive-qualified paths, for the separator alone, and
+// for UNC roots, on either build host.
+func isWindowsVolumeRoot(path string) bool {
+	cleaned := filepath.Clean(strings.TrimSpace(path))
+	if cleaned == "" || cleaned == "." {
+		return false
+	}
+	return filepath.Dir(cleaned) == cleaned
+}
+
+// validateWindowsACLComponent rejects anything that is not a single path
+// component.
+//
+// This is load-bearing in two places, which is why it lives in the portable file
+// rather than beside either of them.
+//
+// At apply time NtCreateFile happily resolves a RELATIVE name containing
+// separators, and it resolves it the ordinary way, so an intermediate junction
+// inside that name is followed and the object lands outside the pinned parent.
+// A name with a separator reopens exactly the hole the parent handle exists to
+// close.
+//
+// At plan time the same shape escapes the write root: a name is joined onto the
+// root to place a deny ACE, so ".." or a separator puts that ACE on a directory
+// outside the workspace entirely.
+//
+// The separators are checked explicitly rather than via filepath.Base, because
+// these are Windows paths whatever the build host is, and on Linux
+// filepath.Base leaves a backslash-joined name untouched and would wave it
+// through. A colon is rejected too: it names an alternate data stream or a
+// drive, neither of which is a child.
+func validateWindowsACLComponent(name string) error {
+	switch {
+	case name == "":
+		return errors.New("windows ACL path component is empty")
+	case name == "." || name == "..":
+		return fmt.Errorf("windows ACL path component %q is a relative reference, not a child", name)
+	case strings.ContainsAny(name, `\/`):
+		return fmt.Errorf("windows ACL path component %q contains a separator, so it would resolve through intermediate directories instead of staying a child", name)
+	case strings.Contains(name, ":"):
+		return fmt.Errorf("windows ACL path component %q contains a colon, which names a stream or a drive rather than a child", name)
+	}
+	return nil
+}
+
 type WindowsACLAction string
 
 const (
 	WindowsACLAllowWrite WindowsACLAction = "allow-write"
 	WindowsACLDenyRead   WindowsACLAction = "deny-read"
 	WindowsACLDenyWrite  WindowsACLAction = "deny-write"
+	// WindowsACLDenyDelete denies removing or renaming the object it names,
+	// WITHOUT denying writes to it or inside it, and without inheriting.
+	//
+	// It exists for .git. The write-denied carveouts live on .git/config and
+	// .git/hooks as objects, so replacing the .git directory discards them: the
+	// recreated config and hooks inherit the workspace allow with no deny, which
+	// restores credential.helper and core.hooksPath. .git cannot simply join
+	// sandboxFullyProtectedMetadataNames, because DenyWrite's mask includes
+	// FILE_GENERIC_WRITE and git must write index, objects and refs.
+	WindowsACLDenyDelete WindowsACLAction = "deny-delete"
 )
 
 type WindowsACLEntry struct {
@@ -19,6 +80,11 @@ type WindowsACLEntry struct {
 	Path        string           `json:"path"`
 	Capability  string           `json:"capability"`
 	Materialize bool             `json:"materialize,omitempty"`
+	// MaterializeFile makes Materialize create an empty FILE instead of a
+	// directory. Only meaningful with Materialize. .git/config is the case that
+	// forces the distinction: created as a directory it does not merely carry
+	// the wrong ACL, it makes `git init` fail outright.
+	MaterializeFile bool `json:"materializeFile,omitempty"`
 }
 
 type WindowsACLPlan struct {
@@ -40,6 +106,15 @@ func BuildWindowsACLPlan(config WindowsSandboxCommandConfig) (WindowsACLPlan, er
 			Path:       capability.Root,
 			Capability: capability.SID,
 		})
+		// The grant above carries DELETE, so the same object guard the principal
+		// plan has must come with it: not DenyWrite (git writes index, objects and
+		// refs), not materialized (git creates .git, and an empty one breaks
+		// git init), and not inherited, so everything underneath stays writable.
+		entries = append(entries, WindowsACLEntry{
+			Action:     WindowsACLDenyDelete,
+			Path:       windowsRenameProtectedObject(capability.Root),
+			Capability: capability.SID,
+		})
 		for _, path := range capability.ProtectedWriteDenyPaths {
 			entries = append(entries, WindowsACLEntry{
 				Action:     WindowsACLDenyWrite,
@@ -48,6 +123,31 @@ func BuildWindowsACLPlan(config WindowsSandboxCommandConfig) (WindowsACLPlan, er
 			})
 		}
 	}
+	// Read roots, granted to the read-capability SID.
+	//
+	// A profile carrying DenyRead runs the command on a strict token rather than a
+	// WRITE_RESTRICTED one, and the strict token applies the restricted-SID check
+	// to reads. Read roots reached that check granted only to the principal's own
+	// account SID, which the write jail keeps out of the restricting set on
+	// purpose, so every read failed it — including opening the executable. The
+	// principal plan still grants the account SID; this is the other half of the
+	// same grant, so the allow-list and the restriction now come from one place.
+	readSID, err := windowsReadAllowCapabilitySID(config)
+	if err != nil {
+		return WindowsACLPlan{}, err
+	}
+	if readSID != "" {
+		for _, path := range config.PermissionProfile.FileSystem.ReadRoots {
+			if path = strings.TrimSpace(path); path == "" {
+				continue
+			}
+			entries = append(entries, WindowsACLEntry{
+				Action:     WindowsACLAllowRead,
+				Path:       path,
+				Capability: readSID,
+			})
+		}
+	}
 	writeSIDs := windowsWriteCapabilitySIDs(writeCapabilities)
 	for _, path := range config.PermissionProfile.FileSystem.DenyWrite {
 		path = strings.TrimSpace(path)
@@ -79,6 +179,39 @@ func BuildWindowsACLPlan(config WindowsSandboxCommandConfig) (WindowsACLPlan, er
 	return WindowsACLPlan{Entries: dedupeWindowsACLEntries(entries)}, nil
 }
 
+// windowsPlanVolumeRootGrant returns the first volume root the plan would have
+// to change permissions on, or empty when it needs none.
+//
+// Only elevated setup can write a volume-root DACL, so this is the one entry
+// that decides whether a plan is applicable by the unelevated tier at all. The
+// check is on the PLAN rather than on the profile because the plan is what gets
+// applied: a future entry that lands at a volume root for some other reason is
+// caught by the same test.
+func windowsPlanVolumeRootGrant(plan WindowsACLPlan) string {
+	for _, entry := range plan.Entries {
+		if isWindowsVolumeRoot(normalizeProfilePath(entry.Path)) {
+			return entry.Path
+		}
+	}
+	return ""
+}
+
+// windowsRenameProtectedObject names the object whose DELETE must be denied on
+// a write root, whatever trustee holds the grant.
+//
+// ONE DERIVATION, BECAUSE TWO PLANNERS CONSUME IT. The allow-write mask both
+// backends share includes DELETE, and it inherits from the write root onto .git.
+// The carveouts that actually protect git live on .git/config and .git/hooks as
+// OBJECTS, so renaming .git aside and recreating it discards them: the fresh
+// config and hooks inherit the workspace allow with no deny of their own, which
+// hands back credential.helper and core.hooksPath. The principal planner denied
+// DELETE here and the capability planner did not, so the default restricted-token
+// backend was missing the guard entirely. Deriving it in one place is what stops
+// a change to the shared mask from updating one consumer and not the other.
+func windowsRenameProtectedObject(root string) string {
+	return filepath.Join(root, sandboxRenameProtectedMetadataName)
+}
+
 type windowsWriteRootCapability struct {
 	Root                    string
 	SID                     string
@@ -140,17 +273,32 @@ func windowsWriteCapabilitySIDs(capabilities []windowsWriteRootCapability) []str
 }
 
 func windowsReadDenyCapabilitySIDs(config WindowsSandboxCommandConfig, writeSIDs []string) ([]string, error) {
-	if len(writeSIDs) > 0 {
-		return writeSIDs, nil
-	}
 	if len(config.PermissionProfile.FileSystem.DenyRead) == 0 {
 		return nil, nil
 	}
-	caps, err := LoadOrCreateWindowsCapabilitySIDs(config.SandboxHome)
+	// The read-capability SID must be denied here as well as granted above. It
+	// holds read on the read roots, and those start at the filesystem root, so a
+	// carveout sitting inside one of them would stay readable through that grant
+	// and the deny list would quietly stop meaning anything.
+	// Every SID the token can carry has to be denied, not just the newest one.
+	// With no write roots the token's only capability is ReadOnly, so dropping it
+	// here would leave the deny naming a SID the command never holds.
+	denySIDs := append([]string{}, writeSIDs...)
+	if len(denySIDs) == 0 {
+		caps, err := LoadOrCreateWindowsCapabilitySIDs(config.SandboxHome)
+		if err != nil {
+			return nil, err
+		}
+		denySIDs = append(denySIDs, caps.ReadOnly)
+	}
+	readSID, err := windowsReadAllowCapabilitySID(config)
 	if err != nil {
 		return nil, err
 	}
-	return []string{caps.ReadOnly}, nil
+	if readSID != "" {
+		denySIDs = append(denySIDs, readSID)
+	}
+	return denySIDs, nil
 }
 
 func planWindowsDenyReadPaths(paths []string) []string {
@@ -193,3 +341,19 @@ func dedupeWindowsACLEntries(entries []WindowsACLEntry) []WindowsACLEntry {
 	}
 	return out
 }
+
+// windowsReadAllowCapabilitySID returns the read-capability SID when this profile
+// will run on a strict token, and "" otherwise.
+//
+// Gated on DenyRead because that is exactly what selects the strict token in the
+// runner: with no DenyRead the token stays WRITE_RESTRICTED, reads skip the
+// restricted-SID check entirely, and granting read on roots as broad as the
+// filesystem root would add ACEs that buy nothing. Derived purely from the
+// profile, so the setup half and the command half reach the same answer without
+// consulting anything ambient.
+func windowsReadAllowCapabilitySID(config WindowsSandboxCommandConfig) (string, error) {
+	if len(config.PermissionProfile.FileSystem.DenyRead) == 0 {
+		return "", nil
+	}
+	return WindowsReadAllowSID(config.SandboxHome)
+}

internal/sandbox/windows_acl_apply_windows.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"sort"
 	"strings"
 
@@ -15,28 +16,121 @@ import (
 const windowsFileDeleteChild windows.ACCESS_MASK = 0x00000040
 
 type windowsACLPathGroup struct {
-	Path        string
-	Entries     []WindowsACLEntry
-	Materialize bool
+	Path            string
+	Entries         []WindowsACLEntry
+	Materialize     bool
+	MaterializeFile bool
 }
 
+// windowsACLChainStep is one directory component beneath the anchor, and
+// whether THIS run created it. The flag comes from the kernel rather than from
+// "it was missing when we looked", because something else can win the gap
+// between the probe and the create, and removing a directory the sandbox did not
+// make is how a rollback deletes a user's data.
+type windowsACLChainStep struct {
+	Name string
+	Made bool
+}
+
+// windowsACLMaterialization is exactly what materialization created, recorded in
+// the shape rollback needs to undo it without resolving a single pathname below
+// the anchor.
+//
+// The anchor is the deepest directory that already existed, and it is the ONLY
+// pathname rollback re-resolves. Everything under it is a list of single
+// components walked one handle at a time, because a name containing a separator
+// is resolved the ordinary way by the kernel and would follow an intermediate
+// junction straight out of the approved tree.
+type windowsACLMaterialization struct {
+	AnchorPath string
+	AnchorID   windowsFileIdentity
+	// Chain is every component between the anchor and the target, shallow to
+	// deep. All of them are needed to descend at rollback time; only the ones
+	// with Made set are removed.
+	Chain []windowsACLChainStep
+	// File is the leaf file component created inside the deepest Chain entry,
+	// for the .git/config carveout. Empty when the target is a directory.
+	File     string
+	FileMade bool
+}
+
+func (materialization windowsACLMaterialization) createdAnything() bool {
+	if materialization.FileMade {
+		return true
+	}
+	for _, step := range materialization.Chain {
+		if step.Made {
+			return true
+		}
+	}
+	return false
+}
+
+// windowsACLRestoreHook observes restore order. Nil in production; the ordering
+// it exposes is load-bearing and was previously pinned by nothing at all,
+// despite a comment claiming otherwise.
+var windowsACLRestoreHook func(path string)
+
 type windowsACLSnapshot struct {
-	Path         string
-	Descriptor   *windows.SECURITY_DESCRIPTOR
-	Materialized bool
+	Path       string
+	Descriptor *windows.SECURITY_DESCRIPTOR
+	Created    windowsACLMaterialization
+	// TargetID is the object the DACL was read from, so the restore can prove it
+	// is writing back to that same object rather than to whatever the pathname
+	// resolves to at rollback time.
+	//
+	// A no-follow re-open catches a REPARSE POINT swapped in since apply, and
+	// that is what the restore relied on. It cannot catch the other
+	// substitution: rename the target aside and put an ordinary directory of the
+	// same name in its place. Nothing about that decoy is a reparse point, so the
+	// re-open succeeds and the old DACL is written onto the attacker's object
+	// while the real one keeps the aborted setup's ACEs.
+	//
+	// Volume serial plus file index is the same identity the materialization
+	// unwind already anchors on. See reopenWindowsACLDirectoryAsIdentity.
+	TargetID windowsFileIdentity
 }
 
 func applyWindowsACLPlan(plan WindowsACLPlan) (func() error, error) {
 	groups := groupWindowsACLPlanByPath(plan)
 	snapshots := make([]windowsACLSnapshot, 0, len(groups))
+	abort := func(err error) (func() error, error) {
+		if rollbackErr := rollbackWindowsACLSnapshots(snapshots); rollbackErr != nil {
+			return nil, fmt.Errorf("%w; rollback failed: %v", err, rollbackErr)
+		}
+		return nil, err
+	}
+
+	var deferred []windowsACLPathGroup
 	for _, group := range groups {
 		snapshot, applied, err := applyWindowsACLPathGroup(group)
 		if err != nil {
-			rollbackErr := rollbackWindowsACLSnapshots(snapshots)
-			if rollbackErr != nil {
-				return nil, fmt.Errorf("%w; rollback failed: %v", err, rollbackErr)
-			}
-			return nil, err
+			return abort(err)
+		}
+		if applied {
+			snapshots = append(snapshots, snapshot)
+			continue
+		}
+		deferred = append(deferred, group)
+	}
+
+	// A group that applied to nothing was skipped because its target did not
+	// exist and the group does not materialize one. A LATER group can still
+	// create it, so the skip has to be retried rather than treated as final.
+	//
+	// The .git rename guard is exactly this shape and was silently absent
+	// because of it. .git must NOT be materialized, since an empty one breaks
+	// `git init`, so its deny-delete group carries no Materialize. But .git does
+	// get created, as the parent chain of the .git\config carveout, and that
+	// group sorts AFTER it. So on every workspace that did not already have a
+	// .git, the guard was skipped, the directory appeared moments later, and
+	// nothing went back for it: the principal could then rename .git aside and
+	// shed the config and hooks carveouts, which is the escape the guard exists
+	// to stop. Groups that are genuinely absent simply skip again here.
+	for _, group := range deferred {
+		snapshot, applied, err := applyWindowsACLPathGroup(group)
+		if err != nil {
+			return abort(err)
 		}
 		if applied {
 			snapshots = append(snapshots, snapshot)
@@ -61,6 +155,7 @@ func groupWindowsACLPlanByPath(plan WindowsACLPlan) []windowsACLPathGroup {
 		}
 		group.Entries = append(group.Entries, entry)
 		group.Materialize = group.Materialize || entry.Materialize
+		group.MaterializeFile = group.MaterializeFile || entry.MaterializeFile
 	}
 	out := make([]windowsACLPathGroup, 0, len(byPath))
 	for _, group := range byPath {
@@ -84,8 +179,40 @@ func applyWindowsACLPathGroup(group windowsACLPathGroup) (windowsACLSnapshot, bo
 	// elevated setup a lower-privileged local user could swap the target for a
 	// symlink/junction between operations and redirect the ACL change onto a
 	// system object it never validated (issue #728, a TOCTOU privilege boundary).
-	materialized := false
-	handle, isDir, err := openWindowsACLTarget(path)
+	// Reject a malformed group BEFORE touching the filesystem. This validation
+	// used to run after materialization, so a bad SID or an unknown action
+	// created a directory chain and only then failed, leaving the error path to
+	// unwind work that never needed doing. Nothing here depends on isDir: that
+	// argument only selects the inheritance flag, while the errors come from the
+	// action lookup and the SID parse.
+	if _, err := windowsExplicitAccessEntries(group.Entries, false); err != nil {
+		return windowsACLSnapshot{}, false, err
+	}
+
+	var created windowsACLMaterialization
+	var handle windows.Handle
+	// Every exit from here goes through one closure, because there are now two
+	// things to undo rather than one: the open handle, and whatever
+	// materialization created. Both are captured by reference and both start
+	// zero, so calling this before either is set is safe and does nothing.
+	//
+	// The unwind is handle-relative. It must never fall back to a pathname
+	// delete: the failure being cleaned up here can BE the path swap, and
+	// os.RemoveAll on a swapped ancestor is precisely the recursive elevated
+	// delete outside the workspace that this cleanup is supposed to prevent.
+	fail := func(err error) (windowsACLSnapshot, bool, error) {
+		if handle != 0 {
+			_ = windows.CloseHandle(handle)
+		}
+		if unwindErr := rollbackWindowsACLMaterialization(created); unwindErr != nil {
+			return windowsACLSnapshot{}, false, fmt.Errorf("%w; cleanup failed: %v", err, unwindErr)
+		}
+		return windowsACLSnapshot{}, false, err
+	}
+
+	var isDir bool
+	var err error
+	handle, isDir, err = openWindowsACLTarget(path)
 	if err != nil {
 		if !errors.Is(err, os.ErrNotExist) {
 			return windowsACLSnapshot{}, false, err
@@ -96,25 +223,19 @@ func applyWindowsACLPathGroup(group windowsACLPathGroup) (windowsACLSnapshot, bo
 			}
 			return windowsACLSnapshot{}, false, nil
 		}
-		if err := os.MkdirAll(path, 0o700); err != nil {
-			return windowsACLSnapshot{}, false, fmt.Errorf("materialize windows ACL target %s: %w", path, err)
+		// created is assigned even on failure: materialization reports what it
+		// managed to make before it stopped, and fail() unwinds exactly that.
+		created, err = materializeWindowsACLTarget(path, group.MaterializeFile)
+		if err != nil {
+			return fail(fmt.Errorf("materialize windows ACL target %s: %w", path, err))
 		}
-		materialized = true
 		handle, isDir, err = openWindowsACLTarget(path)
 		if err != nil {
-			_ = os.RemoveAll(path)
-			return windowsACLSnapshot{}, false, fmt.Errorf("open materialized windows ACL target %s: %w", path, err)
+			// This is the branch that fires when the post-create verify catches a
+			// swap, so it is the single most important cleanup in the file.
+			return fail(fmt.Errorf("open materialized windows ACL target %s: %w", path, err))
 		}
 	}
-	// From here the handle is open; every early return must close it first (and
-	// remove a freshly materialized target) so a failure leaks neither.
-	fail := func(err error) (windowsACLSnapshot, bool, error) {
-		_ = windows.CloseHandle(handle)
-		if materialized {
-			_ = os.RemoveAll(path)
-		}
-		return windowsACLSnapshot{}, false, err
-	}
 	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
 	if err != nil {
 		return fail(fmt.Errorf("read windows ACL for %s: %w", path, err))
@@ -138,8 +259,14 @@ func applyWindowsACLPathGroup(group windowsACLPathGroup) (windowsACLSnapshot, bo
 	// The handle has served its purpose (read+write bound to one object) and is
 	// closed now — rollback re-opens no-follow rather than holding a handle for
 	// the whole sandbox lifetime, since one caller discards the rollback closure.
+	// Captured BEFORE the handle closes, because that handle is the only thing
+	// that names the object rather than the path.
+	targetID, err := windowsIdentityOfHandle(handle)
+	if err != nil {
+		return fail(fmt.Errorf("read windows ACL target identity for %s: %w", path, err))
+	}
 	_ = windows.CloseHandle(handle)
-	return windowsACLSnapshot{Path: path, Descriptor: descriptor, Materialized: materialized}, true, nil
+	return windowsACLSnapshot{Path: path, Descriptor: descriptor, Created: created, TargetID: targetID}, true, nil
 }
 
 // openWindowsACLTarget opens path for reading and rewriting its DACL without
@@ -178,6 +305,12 @@ func openWindowsACLTarget(path string) (windows.Handle, bool, error) {
 		_ = windows.CloseHandle(handle)
 		return 0, false, fmt.Errorf("refusing to apply ACL to reparse-point target %s: possible path swap during elevated setup", path)
 	}
+	// Ancestors are resolved by CreateFile even with FILE_FLAG_OPEN_REPARSE_POINT,
+	// so the check above is not enough on its own.
+	if err := verifyWindowsACLTargetNotRedirected(handle, path); err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, false, err
+	}
 	isDir := info.FileAttributes&windows.FILE_ATTRIBUTE_DIRECTORY != 0
 	return handle, isDir, nil
 }
@@ -206,10 +339,19 @@ func windowsExplicitAccessEntries(entries []WindowsACLEntry, isDir bool) ([]wind
 		if err != nil {
 			return nil, err
 		}
+		entryInheritance := inheritance
+		// DenyDelete governs the object it names and nothing beneath it. Inherited
+		// onto .git's children it would deny DELETE on every file inside, so git
+		// could not remove a lock file, a ref, or anything else it rewrites, and
+		// the guard would read as a broken repository rather than as a blocked
+		// rename.
+		if entry.Action == WindowsACLDenyDelete {
+			entryInheritance = windows.NO_INHERITANCE
+		}
 		out = append(out, windows.EXPLICIT_ACCESS{
 			AccessPermissions: permissions,
 			AccessMode:        accessMode,
-			Inheritance:       inheritance,
+			Inheritance:       entryInheritance,
 			Trustee: windows.TRUSTEE{
 				TrusteeForm:  windows.TRUSTEE_IS_SID,
 				TrusteeType:  windows.TRUSTEE_IS_GROUP,
@@ -223,11 +365,59 @@ func windowsExplicitAccessEntries(entries []WindowsACLEntry, isDir bool) ([]wind
 func windowsACLAccess(action WindowsACLAction) (windows.ACCESS_MODE, windows.ACCESS_MASK, error) {
 	switch action {
 	case WindowsACLAllowWrite:
-		return windows.GRANT_ACCESS, windows.FILE_GENERIC_READ | windows.FILE_GENERIC_WRITE | windows.FILE_GENERIC_EXECUTE, nil
+		// DELETE is part of the grant, not an extra. FILE_GENERIC_WRITE covers
+		// creating and modifying but not removing or renaming, and a rename needs
+		// delete access on the source. Under the old same-user token that gap was
+		// invisible, because the caller already held inherited rights on its own
+		// tree; a sandbox principal is a separate account with no such
+		// inheritance, so without DELETE it can write a file it can never delete.
+		// Ordinary editing and most git operations rewrite files by replacing
+		// them, so the omission fails normal work rather than an edge case.
+		//
+		// FILE_DELETE_CHILD is deliberately NOT granted, for the same reason
+		// WRITE_DAC and WRITE_OWNER are not. On a parent it authorises deleting a
+		// child whatever the child's own DACL says, so granting it on a write root
+		// hands back the write-denied carve-outs underneath it: a principal could
+		// delete .git/config and recreate it, and the replacement inherits this
+		// grant with no deny of its own — restoring exactly the credential.helper
+		// and core.hooksPath control the carve-out exists to prevent.
+		//
+		// It was granted here originally to keep the mask symmetric with
+		// WindowsACLDenyWrite, which does treat FILE_DELETE_CHILD as part of
+		// write. Symmetry is the wrong goal: denying a capability is not a reason
+		// to grant it. DELETE alone covers removing and renaming files the
+		// principal owns inside its roots, which is what the grant is for.
+		return windows.GRANT_ACCESS, windows.FILE_GENERIC_READ | windows.FILE_GENERIC_WRITE | windows.FILE_GENERIC_EXECUTE | windows.DELETE, nil
+	case WindowsACLAllowRead:
+		// Read and traverse without write. A sandbox principal is a separate
+		// account with no inherent access to the caller's tree, so a read-only
+		// root has to be granted rather than assumed. Deliberately omits
+		// FILE_GENERIC_WRITE, DELETE and WRITE_DAC.
+		return windows.GRANT_ACCESS, windows.FILE_GENERIC_READ | windows.FILE_GENERIC_EXECUTE, nil
+	case windowsACLRevoke:
+		// REVOKE_ACCESS drops every ACE naming the trustee regardless of the mask,
+		// so the mask is ignored here. Used to retire a principal without having
+		// to remember which access each path was granted.
+		return windows.REVOKE_ACCESS, 0, nil
 	case WindowsACLDenyRead:
 		return windows.DENY_ACCESS, windows.FILE_GENERIC_READ | windows.FILE_GENERIC_EXECUTE, nil
 	case WindowsACLDenyWrite:
 		return windows.DENY_ACCESS, windows.FILE_GENERIC_WRITE | windows.DELETE | windowsFileDeleteChild | windows.WRITE_DAC | windows.WRITE_OWNER, nil
+	case WindowsACLDenyDelete:
+		// Deny removing or RENAMING the object itself, nothing more. Renaming a
+		// directory needs DELETE on that directory, so denying DELETE is what
+		// stops .git being moved aside and recreated without its carveouts.
+		//
+		// WRITE_DAC and WRITE_OWNER come along because a guard the principal can
+		// rewrite, or take ownership of and then rewrite, is not a guard.
+		//
+		// FILE_GENERIC_WRITE is deliberately absent: git writes index, objects and
+		// refs constantly, and denying it would break every commit rather than the
+		// rename. FILE_DELETE_CHILD is absent for the same reason one level down,
+		// since git deletes its own lock files and refs. Neither is needed here:
+		// this ACE does not inherit (see windowsExplicitAccessEntries), so it
+		// governs the .git directory object alone.
+		return windows.DENY_ACCESS, windows.DELETE | windows.WRITE_DAC | windows.WRITE_OWNER, nil
 	default:
 		return 0, 0, fmt.Errorf("unsupported windows ACL action %q", action)
 	}
@@ -235,11 +425,18 @@ func windowsACLAccess(action WindowsACLAction) (windows.ACCESS_MODE, windows.ACC
 
 func rollbackWindowsACLSnapshots(snapshots []windowsACLSnapshot) error {
 	var errs []error
+	// Reverse order is load-bearing twice over. Groups are sorted ascending by
+	// path key and an ancestor key is always a proper prefix of its descendants,
+	// so walking backwards unwinds descendant before ancestor: a materialized
+	// directory is therefore empty by the time its own removal is attempted.
+	// Restoring ACLs in the same order is independently right, because
+	// SetSecurityInfo propagates inheritable ACEs down, so the ancestor must go
+	// last. TestRollbackUnwindsDescendantsBeforeAncestors pins it.
 	for index := len(snapshots) - 1; index >= 0; index-- {
 		snapshot := snapshots[index]
-		if snapshot.Materialized {
-			if err := os.RemoveAll(snapshot.Path); err != nil {
-				errs = append(errs, fmt.Errorf("remove materialized windows ACL target %s: %w", snapshot.Path, err))
+		if snapshot.Created.createdAnything() {
+			if err := rollbackWindowsACLMaterialization(snapshot.Created); err != nil {
+				errs = append(errs, err)
 			}
 			continue
 		}
@@ -248,16 +445,36 @@ func rollbackWindowsACLSnapshots(snapshots []windowsACLSnapshot) error {
 			errs = append(errs, fmt.Errorf("read rollback windows DACL for %s: %w", snapshot.Path, err))
 			continue
 		}
-		// Re-open no-follow rather than restoring by pathname: the restore must
-		// land on the real object, not a reparse point swapped in since apply. The
-		// residual window is small because the target is ACL-restricted by now, but
-		// a handle keeps the restore honest. On a materialized-target rollback we
-		// remove it above, so only the restore-existing path opens here.
+		// Re-open no-follow AND prove it is the same object. The no-follow open
+		// alone only rules out a reparse point swapped in since apply; a target
+		// renamed aside and replaced by an ordinary directory of the same name
+		// passes it, and the old DACL would land on the decoy while the real
+		// object kept the aborted setup's ACEs. See windowsACLSnapshot.TargetID.
 		handle, _, err := openWindowsACLTarget(snapshot.Path)
 		if err != nil {
 			errs = append(errs, fmt.Errorf("re-open windows ACL target %s for rollback: %w", snapshot.Path, err))
 			continue
 		}
+		if !snapshot.TargetID.empty() {
+			got, identityErr := windowsIdentityOfHandle(handle)
+			if identityErr != nil {
+				_ = windows.CloseHandle(handle)
+				errs = append(errs, fmt.Errorf("read rollback identity for %s: %w", snapshot.Path, identityErr))
+				continue
+			}
+			if got != snapshot.TargetID {
+				// REFUSED, not forced through. Leaving the real object with the
+				// aborted setup's ACEs is the safe direction: the caller is failing
+				// anyway, and writing a stale DACL onto an object setup never
+				// touched is the one outcome rollback must not produce.
+				_ = windows.CloseHandle(handle)
+				errs = append(errs, fmt.Errorf("refusing to restore windows ACL for %s: it is no longer the object setup applied to", snapshot.Path))
+				continue
+			}
+		}
+		if windowsACLRestoreHook != nil {
+			windowsACLRestoreHook(snapshot.Path)
+		}
 		if err := windows.SetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION, nil, nil, dacl, nil); err != nil {
 			errs = append(errs, fmt.Errorf("rollback windows ACL for %s: %w", snapshot.Path, err))
 		}
@@ -265,3 +482,217 @@ func rollbackWindowsACLSnapshots(snapshots []windowsACLSnapshot) error {
 	}
 	return errors.Join(errs...)
 }
+
+// materializeWindowsACLTarget creates a missing ACL target with the shape the
+// owning tool expects. A directory target is created whole; a file target gets
+// its parent chain created and then an empty file, because creating it as a
+// directory would break the tool that owns it rather than just mis-ACL it.
+// The returned record is meaningful even when the error is non-nil: a chain that
+// got three levels deep and then failed still has three levels to unwind.
+func materializeWindowsACLTarget(path string, asFile bool) (windowsACLMaterialization, error) {
+	directory := path
+	leaf := ""
+	if asFile {
+		directory = filepath.Dir(path)
+		leaf = filepath.Base(path)
+	}
+	created, parent, err := makeWindowsACLDirChainNoFollow(directory)
+	if err != nil {
+		return created, err
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+	if !asFile {
+		return created, nil
+	}
+	// A racing creator winning is still fine: the target exists, which is all
+	// materialization needed. createWindowsACLChildFile reports that as
+	// created=false, so rollback will not delete a file the sandbox did not make.
+	created.File = leaf
+	madeFile, err := createWindowsACLChildFile(parent, leaf)
+	created.FileMade = madeFile
+	return created, err
+}
+
+// rollbackWindowsACLMaterialization removes exactly what materialization
+// created, deepest first, without resolving any pathname below the anchor.
+//
+// This is the other half of the pathname problem. The old cleanup called
+// os.RemoveAll on the target pathname, which re-resolves every ancestor at the
+// moment it runs, so an ancestor swapped to a junction after the object was
+// created sent a recursive elevated delete into an unrelated tree. It also only
+// ever removed the final component, leaving every intermediate directory the
+// chain had created behind.
+//
+// The anchor is the one pathname that has to be resolved again, and it is
+// checked by file identity rather than by name: replacing a directory with
+// another REAL directory of the same name needs no reparse point at all and
+// would otherwise pass every no-follow check there is.
+//
+// Residue is preferable to over-deletion throughout. When something cannot be
+// removed safely this reports it and leaves it, and never falls back to a
+// pathname delete.
+func rollbackWindowsACLMaterialization(materialization windowsACLMaterialization) error {
+	if !materialization.createdAnything() {
+		return nil
+	}
+	anchor, err := reopenWindowsACLDirectoryAsIdentity(materialization.AnchorPath, materialization.AnchorID)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			// The anchor is gone, so everything created beneath it is gone too.
+			// Nothing to undo, and no way to undo it if there were.
+			return nil
+		}
+		return fmt.Errorf("unwind windows ACL materialization under %s: %w", materialization.AnchorPath, err)
+	}
+
+	// One handle per level: handles[i] is the parent of Chain[i], which is what
+	// deleting Chain[i] relative to a pinned parent requires.
+	handles := []windows.Handle{anchor}
+	defer func() {
+		for _, handle := range handles {
+			if handle != 0 {
+				_ = windows.CloseHandle(handle)
+			}
+		}
+	}()
+
+	// Descend only as far as is actually required. Removing a directory needs its
+	// PARENT's handle, not its own, so the deepest component is opened only when
+	// a file leaf lives inside it. This is not just economy: the deepest
+	// component is usually the ACL target itself, so it may already carry the
+	// deny-read ACE this rollback is undoing, and opening it would be refused by
+	// the very ACL being unwound.
+	needed := len(materialization.Chain)
+	if !materialization.FileMade && needed > 0 {
+		needed--
+	}
+	depth := 0
+	for ; depth < needed; depth++ {
+		child, err := openWindowsACLChildDirectory(handles[depth], materialization.Chain[depth].Name)
+		if err != nil {
+			// Already removed by something else. Stop descending; whatever is
+			// below it is gone with it.
+			if isWindowsNotExist(err) {
+				break
+			}
+			return fmt.Errorf("unwind windows ACL materialization under %s: %w", materialization.AnchorPath, err)
+		}
+		handles = append(handles, child)
+	}
+
+	var errs []error
+	// The file leaf lives inside the deepest chain directory, so it goes first
+	// and only if the descent actually reached that far.
+	if materialization.FileMade && depth == needed {
+		if err := deleteWindowsACLChildFile(handles[len(handles)-1], materialization.File); err != nil {
+			errs = append(errs, fmt.Errorf("remove materialized windows ACL file %s: %w", materialization.File, err))
+		}
+	}
+	// Chain[i] is removed through handles[i], so the deepest one that can be
+	// removed is bounded by how far the descent actually got.
+	deepest := len(handles) - 1
+	if last := len(materialization.Chain) - 1; deepest > last {
+		deepest = last
+	}
+	for index := deepest; index >= 0; index-- {
+		// Close the child's own handle, if the descent opened one, before asking
+		// its parent to remove it. A directory with a live handle open still
+		// counts as present, so the parent's delete would come back as non-empty.
+		if index+1 < len(handles) && handles[index+1] != 0 {
+			_ = windows.CloseHandle(handles[index+1])
+			handles[index+1] = 0
+		}
+		if !materialization.Chain[index].Made {
+			continue
+		}
+		if err := deleteWindowsACLChildDirectory(handles[index], materialization.Chain[index].Name); err != nil {
+			errs = append(errs, fmt.Errorf("remove materialized windows ACL directory %s: %w", materialization.Chain[index].Name, err))
+		}
+	}
+	return errors.Join(errs...)
+}
+
+// windowsACLMaterializeSwapHook is a test seam and nothing else. It fires inside
+// makeWindowsACLDirChainNoFollow at the exact instant the race used to be
+// exploitable: the anchor is verified and pinned, and nothing has been created
+// yet. A race reproducible only by luck is not a regression test, so the instant
+// is made addressable rather than hoped for. Always nil in production.
+var windowsACLMaterializeSwapHook func(anchor string)
+
+// makeWindowsACLDirChainNoFollow is an os.MkdirAll that never resolves a
+// pathname below its anchor.
+//
+// It walks UP to the deepest ancestor that already exists and opens it
+// no-follow. Because GetFinalPathNameByHandle answers for the whole resolved
+// path, that single check clears every ancestor above it too. Then it walks back
+// DOWN, creating one component at a time relative to the HANDLE of the level
+// above, so the tree it descends is pinned to objects rather than named by
+// strings.
+//
+// That is the difference that matters. This used to verify a component by
+// pathname, close the handle, and then hand the same string to os.Mkdir: two
+// independent kernel resolutions with a gap between them. A workspace owner who
+// swapped the verified ancestor for a junction inside that gap got the component
+// created outside the approved tree, as Administrator, and verifying again
+// afterwards cannot un-create it. Junctions need no privilege, so this was
+// reachable by exactly the unprivileged user the sandbox exists to contain.
+//
+// It deliberately does NOT re-verify each created component by pathname. A child
+// created relative to a pinned parent is in the right place by construction, so
+// comparing pathnames afterwards would add nothing and would reject correct
+// creates whenever the tree was legitimately renamed mid-setup.
+//
+// Returns what it created, plus an open handle to the deepest directory which
+// the caller must close.
+func makeWindowsACLDirChainNoFollow(dir string) (windowsACLMaterialization, windows.Handle, error) {
+	cleaned := filepath.Clean(strings.TrimSpace(dir))
+	if cleaned == "" || cleaned == "." {
+		return windowsACLMaterialization{}, 0, fmt.Errorf("materialize windows ACL target: empty directory path %q", dir)
+	}
+
+	// Walk up to the deepest ancestor that exists, collecting the component
+	// NAMES that are missing. Names, not paths: everything below the anchor is
+	// addressed relative to a handle from here on.
+	var missing []string
+	current := cleaned
+	var anchor windows.Handle
+	var anchorID windowsFileIdentity
+	for {
+		handle, identity, err := openWindowsACLDirectoryNoFollowWithIdentity(current)
+		if err == nil {
+			anchor, anchorID = handle, identity
+			break
+		}
+		if !errors.Is(err, os.ErrNotExist) {
+			return windowsACLMaterialization{}, 0, err
+		}
+		parent := filepath.Dir(current)
+		if parent == current {
+			return windowsACLMaterialization{}, 0, fmt.Errorf("materialize windows ACL target %s: no existing ancestor to anchor on", dir)
+		}
+		missing = append(missing, filepath.Base(current))
+		current = parent
+	}
+
+	created := windowsACLMaterialization{AnchorPath: current, AnchorID: anchorID}
+
+	if hook := windowsACLMaterializeSwapHook; hook != nil {
+		hook(current)
+	}
+
+	// Walk back down, one component per handle. The anchor handle is released as
+	// soon as its child is open, so at most two levels are held at once.
+	parent := anchor
+	for index := len(missing) - 1; index >= 0; index-- {
+		name := missing[index]
+		child, madeNow, err := createWindowsACLChildDirectory(parent, name)
+		if err != nil {
+			_ = windows.CloseHandle(parent)
+			return created, 0, err
+		}
+		created.Chain = append(created.Chain, windowsACLChainStep{Name: name, Made: madeNow})
+		_ = windows.CloseHandle(parent)
+		parent = child
+	}
+	return created, parent, nil
+}

internal/sandbox/windows_acl_apply_windows_test.go

@@ -37,8 +37,15 @@ func TestApplyWindowsACLPathGroupHandleBasedRoundTrip(t *testing.T) {
 	if !applied {
 		t.Fatal("applied = false, want true for an existing directory target")
 	}
-	if snapshot.Path != dir || snapshot.Materialized {
-		t.Fatalf("snapshot = %#v, want Path=%q Materialized=false", snapshot, dir)
+	if snapshot.Path != dir {
+		t.Fatalf("snapshot.Path = %q, want %q", snapshot.Path, dir)
+	}
+	// The target already existed, so nothing was created and rollback must have
+	// nothing to remove. Asserting the chain rather than a bool matters: a
+	// rewiring that recorded the walked components instead of only the created
+	// ones would make rollback delete a directory the sandbox never made.
+	if snapshot.Created.createdAnything() {
+		t.Fatalf("snapshot recorded %#v as created for a target that already existed", snapshot.Created)
 	}
 	if snapshot.Descriptor == nil {
 		t.Fatal("snapshot has no captured descriptor to roll back to")
@@ -67,8 +74,16 @@ func TestApplyWindowsACLPathGroupMaterializes(t *testing.T) {
 	if err != nil {
 		t.Fatalf("applyWindowsACLPathGroup: %v", err)
 	}
-	if !applied || !snapshot.Materialized {
-		t.Fatalf("applied=%v materialized=%v, want both true", applied, snapshot.Materialized)
+	if !applied {
+		t.Fatal("applied = false, want true for a materialized target")
+	}
+	// Exactly one component was missing, so exactly one must be recorded as
+	// created, and it must be the leaf's own name rather than a path.
+	if len(snapshot.Created.Chain) != 1 || snapshot.Created.Chain[0] != (windowsACLChainStep{Name: "created", Made: true}) {
+		t.Fatalf("created chain = %#v, want one step {created true}", snapshot.Created.Chain)
+	}
+	if snapshot.Created.AnchorPath != filepath.Dir(target) {
+		t.Fatalf("anchor = %q, want the existing parent %q", snapshot.Created.AnchorPath, filepath.Dir(target))
 	}
 	if _, err := os.Stat(target); err != nil {
 		t.Fatalf("materialized target not created: %v", err)

internal/sandbox/windows_acl_git_guard_windows_test.go

@@ -0,0 +1,122 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// gitGuardDenyDeleteApplied reports whether the applied DACL on path carries a
+// DENY ace granting DELETE to sid. Read back from the object rather than from
+// the plan, because the plan is what was already wrong.
+func gitGuardDenyDeleteApplied(t *testing.T, path string, sid string) bool {
+	t.Helper()
+	handle, _, err := openWindowsACLTarget(path)
+	if err != nil {
+		t.Fatalf("open %s: %v", path, err)
+	}
+	defer func() { _ = windows.CloseHandle(handle) }()
+	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		t.Fatalf("read the DACL of %s: %v", path, err)
+	}
+	dacl, _, err := descriptor.DACL()
+	if err != nil || dacl == nil {
+		t.Fatalf("read the DACL of %s: %v", path, err)
+	}
+	wanted, err := windows.StringToSid(sid)
+	if err != nil {
+		t.Fatalf("parse the capability SID %q: %v", sid, err)
+	}
+	// GetAce hands back a generic header that is reinterpreted here. Sound only
+	// for the fixed-layout ACE types: an object ACE carries Flags and two GUIDs
+	// ahead of the trustee, so SidStart would land mid-structure. Allowed and
+	// denied ACEs share that fixed layout, and nothing under test builds an
+	// object ACE, so a type this does not recognise is skipped rather than
+	// decoded into a nonsense SID.
+	for index := uint32(0); index < uint32(dacl.AceCount); index++ {
+		var ace *windows.ACCESS_ALLOWED_ACE
+		if err := windows.GetAce(dacl, index, &ace); err != nil {
+			continue
+		}
+		if ace.Header.AceType != windows.ACCESS_DENIED_ACE_TYPE {
+			continue
+		}
+		aceSID := (*windows.SID)(unsafe.Pointer(&ace.SidStart))
+		if !aceSID.Equals(wanted) {
+			continue
+		}
+		if ace.Mask&windows.DELETE != 0 {
+			return true
+		}
+	}
+	return false
+}
+
+// THE GUARD BELONGS TO THE GRANT, NOT TO ONE PLANNER.
+//
+// The allow-write mask both backends share includes DELETE, and it inherits from
+// the write root onto .git. What protects git is attached to .git/config and
+// .git/hooks as OBJECTS, so renaming .git aside and recreating it discards those
+// carveouts: the fresh config and hooks inherit the workspace allow with no deny
+// of their own, which hands back credential.helper and core.hooksPath.
+//
+// The principal planner denied DELETE on .git; the capability planner did not,
+// and the capability backend is the default. So on the reachable path the
+// caller's own token and the capability restricting SID could both authorise the
+// rename.
+//
+// Read back from the applied object, not asserted on the plan, because the plan
+// is exactly what was wrong. DACL edits on a user-owned directory need no
+// Administrator rights, so this runs unelevated.
+func TestCapabilityPlanDeniesDeleteOnGit(t *testing.T) {
+	workspace := t.TempDir()
+	gitDir := filepath.Join(workspace, ".git")
+	if err := os.MkdirAll(gitDir, 0o700); err != nil {
+		t.Fatal(err)
+	}
+
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: workspace}},
+			},
+		},
+	}
+	plan, err := BuildWindowsACLPlan(config)
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	sid, err := windowsCapabilitySIDForWriteRoot(config, workspace)
+	if err != nil {
+		t.Fatalf("resolve the workspace capability SID: %v", err)
+	}
+	// SETUP: the plan must actually grant this root, or the guard below would be
+	// vacuously satisfied by a plan that grants nothing.
+	granted := false
+	for _, entry := range plan.Entries {
+		if entry.Action == WindowsACLAllowWrite && entry.Path == workspace {
+			granted = true
+		}
+	}
+	if !granted {
+		t.Fatalf("SETUP INVALID: the capability plan does not grant write on %s", workspace)
+	}
+
+	if _, err := applyWindowsACLPlan(plan); err != nil {
+		t.Skipf("cannot apply an ACL plan here: %v", err)
+	}
+
+	if !gitGuardDenyDeleteApplied(t, gitDir, sid) {
+		t.Fatalf(".git carries no deny-DELETE ace for the capability SID %s, so the sandboxed token can rename it and recreate it without the config and hooks carveouts", sid)
+	}
+}

internal/sandbox/windows_acl_junction_ancestor_windows_test.go

@@ -0,0 +1,94 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// makeJunction points link at target, skipping the test when the environment
+// refuses to create one. A junction needs no privilege, which is exactly why
+// this attack is reachable by an ordinary workspace owner.
+func makeJunction(t *testing.T, link, target string) {
+	t.Helper()
+	if out, err := exec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput(); err != nil {
+		t.Skipf("cannot create a junction here: %v: %s", err, out)
+	}
+	// Assert it actually redirects. A junction that silently did nothing would
+	// green this test while proving nothing.
+	probe := filepath.Join(link, "redirect-probe")
+	if err := os.WriteFile(probe, []byte("x"), 0o600); err != nil {
+		t.Fatalf("write through junction: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(target, "redirect-probe")); err != nil {
+		t.Fatalf("junction does not redirect, so this test would prove nothing: %v", err)
+	}
+	if err := os.Remove(probe); err != nil {
+		t.Fatalf("clean probe: %v", err)
+	}
+}
+
+// Elevated setup must not create anything through a reparse-point ancestor.
+//
+// FILE_FLAG_OPEN_REPARSE_POINT only stops the FINAL component being followed.
+// materializeWindowsACLTarget used os.MkdirAll/os.OpenFile on the pathname, both
+// of which resolve ancestors, so a workspace owner who turned .git into a
+// junction before setup ran got objects created at a location of their choosing
+// — as Administrator — and the no-follow check only rejected it afterwards, far
+// too late to un-create them.
+func TestMaterializeRefusesAncestorJunctionBeforeCreating(t *testing.T) {
+	for name, asFile := range map[string]bool{"file target": true, "directory target": false} {
+		t.Run(name, func(t *testing.T) {
+			external := t.TempDir()
+			workspace := t.TempDir()
+			gitDir := filepath.Join(workspace, ".git")
+			makeJunction(t, gitDir, external)
+
+			target := filepath.Join(gitDir, "hooks", "config")
+			_, err := materializeWindowsACLTarget(target, asFile)
+			if err == nil {
+				t.Fatalf("materialized %s through a junction ancestor instead of refusing", target)
+			}
+			if !strings.Contains(err.Error(), "reparse") {
+				t.Fatalf("refused for the wrong reason: %v", err)
+			}
+			// Nothing may survive on the other side of the junction. The old code
+			// left every intermediate directory MkdirAll had created.
+			leaked, lerr := os.ReadDir(external)
+			if lerr != nil {
+				t.Fatalf("read external dir: %v", lerr)
+			}
+			if len(leaked) != 0 {
+				names := make([]string, 0, len(leaked))
+				for _, entry := range leaked {
+					names = append(names, entry.Name())
+				}
+				t.Fatalf("created %v outside the workspace through the junction", names)
+			}
+		})
+	}
+}
+
+// The ordinary path still works: no reparse point anywhere, target gets made.
+func TestMaterializeStillCreatesOrdinaryTargets(t *testing.T) {
+	root := t.TempDir()
+	for name, asFile := range map[string]bool{"file target": true, "directory target": false} {
+		t.Run(name, func(t *testing.T) {
+			target := filepath.Join(root, name, "nested", "deeper", "target")
+			if _, err := materializeWindowsACLTarget(target, asFile); err != nil {
+				t.Fatalf("materializeWindowsACLTarget: %v", err)
+			}
+			info, err := os.Stat(target)
+			if err != nil {
+				t.Fatalf("target was not created: %v", err)
+			}
+			if info.IsDir() == asFile {
+				t.Fatalf("target isDir=%v, wanted file=%v", info.IsDir(), asFile)
+			}
+		})
+	}
+}

internal/sandbox/windows_acl_materialize_swap_windows_test.go

@@ -0,0 +1,453 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// swapAncestorAside moves approved out of the way and leaves a junction wearing
+// its name, pointing at elsewhere. This is the whole attack in three lines, and
+// it needs no privilege: junctions are creatable by any user, which is exactly
+// why an unprivileged workspace owner can aim elevated setup wherever they like.
+//
+// Returns the path the real directory now lives at.
+func swapAncestorAside(t *testing.T, approved, elsewhere string) string {
+	t.Helper()
+	moved := approved + "-moved"
+	if err := os.Rename(approved, moved); err != nil {
+		t.Skipf("cannot rename a directory with an open handle on this filesystem: %v", err)
+	}
+	makeJunction(t, approved, elsewhere)
+	return moved
+}
+
+// requireNothingEscaped fails when anything at all was created on the far side
+// of the junction.
+func requireNothingEscaped(t *testing.T, elsewhere string) {
+	t.Helper()
+	leaked, err := os.ReadDir(elsewhere)
+	if err != nil {
+		t.Fatalf("read the decoy directory: %v", err)
+	}
+	if len(leaked) == 0 {
+		return
+	}
+	names := make([]string, 0, len(leaked))
+	for _, entry := range leaked {
+		names = append(names, entry.Name())
+	}
+	t.Fatalf("ESCAPED: created %v outside the approved tree, as Administrator", names)
+}
+
+// THE MATERIALIZATION RACE, ON THE PRODUCTION CALL PATH.
+//
+// The existing junction test plants its junction before the walk even starts, so
+// the very first check sees it and refuses. That proves the easy half. The half
+// the reviewer filed is the swap that happens AFTER a component has been
+// verified and BEFORE it is used, and no test reached it: a race nobody can
+// trigger on demand is not a regression test, so makeWindowsACLDirChainNoFollow
+// carries a seam that fires at exactly that instant.
+//
+// The control arm matters as much as the fixed one. It performs the identical
+// swap and then does what this code used to do, creating by pathname, and
+// asserts that the object DOES escape. Without it, the fixed arm passing proves
+// only that some code ran, not that the hole it closes was ever open.
+func TestMaterializeSurvivesAnAncestorSwappedMidWalk(t *testing.T) {
+	t.Run("control: creating by pathname escapes", func(t *testing.T) {
+		root := t.TempDir()
+		approved := filepath.Join(root, "ws")
+		elsewhere := filepath.Join(root, "OUTSIDE")
+		for _, dir := range []string{approved, elsewhere} {
+			if err := os.Mkdir(dir, 0o700); err != nil {
+				t.Fatalf("seed %s: %v", dir, err)
+			}
+		}
+		// Verified, exactly as the walk verifies its anchor.
+		if err := verifyWindowsACLPathComponentNotRedirected(approved); err != nil {
+			t.Fatalf("anchor did not verify before the swap: %v", err)
+		}
+		moved := swapAncestorAside(t, approved, elsewhere)
+
+		// The old create: a pathname, re-resolved by the kernel right now.
+		if err := os.MkdirAll(filepath.Join(approved, "a", "b"), 0o700); err != nil {
+			t.Fatalf("pathname create: %v", err)
+		}
+		if _, err := os.Stat(filepath.Join(elsewhere, "a", "b")); err != nil {
+			t.Fatalf("the control arm did not reproduce the escape, so the fixed arm below proves nothing: %v", err)
+		}
+		if _, err := os.Stat(filepath.Join(moved, "a", "b")); err == nil {
+			t.Error("the control arm created inside the verified directory, which is not the behaviour being contrasted")
+		}
+	})
+
+	t.Run("fixed: creating through the pinned handle stays put", func(t *testing.T) {
+		root := t.TempDir()
+		approved := filepath.Join(root, "ws")
+		elsewhere := filepath.Join(root, "OUTSIDE")
+		for _, dir := range []string{approved, elsewhere} {
+			if err := os.Mkdir(dir, 0o700); err != nil {
+				t.Fatalf("seed %s: %v", dir, err)
+			}
+		}
+
+		var moved string
+		swapped := false
+		windowsACLMaterializeSwapHook = func(anchor string) {
+			if swapped {
+				return
+			}
+			swapped = true
+			if anchor != approved {
+				t.Errorf("anchored on %q, want the deepest existing ancestor %q", anchor, approved)
+			}
+			moved = swapAncestorAside(t, approved, elsewhere)
+		}
+		t.Cleanup(func() { windowsACLMaterializeSwapHook = nil })
+
+		created, err := materializeWindowsACLTarget(filepath.Join(approved, "a", "b"), false)
+		if err != nil {
+			t.Fatalf("materializeWindowsACLTarget: %v", err)
+		}
+		if !swapped {
+			t.Fatal("the seam never fired, so the swap never happened and this test proved nothing")
+		}
+		requireNothingEscaped(t, elsewhere)
+		if _, err := os.Stat(filepath.Join(moved, "a", "b")); err != nil {
+			t.Errorf("the target did not land in the directory that was verified: %v", err)
+		}
+		// And the record must describe what to unwind, in components rather than
+		// paths, shallowest first.
+		if len(created.Chain) != 2 || created.Chain[0].Name != "a" || created.Chain[1].Name != "b" {
+			t.Fatalf("created chain = %#v, want [a b] shallow to deep", created.Chain)
+		}
+		for _, step := range created.Chain {
+			if !step.Made {
+				t.Errorf("component %q was not recorded as created, so rollback would leave it behind", step.Name)
+			}
+		}
+	})
+}
+
+// The FILE target has the same race, and it is the one that matters most:
+// .git/config is materialized as a file on every stock setup, and it is the file
+// whose credential.helper is worth stealing.
+func TestMaterializeFileSurvivesAnAncestorSwappedMidWalk(t *testing.T) {
+	root := t.TempDir()
+	approved := filepath.Join(root, "ws")
+	elsewhere := filepath.Join(root, "OUTSIDE")
+	for _, dir := range []string{approved, elsewhere} {
+		if err := os.Mkdir(dir, 0o700); err != nil {
+			t.Fatalf("seed %s: %v", dir, err)
+		}
+	}
+
+	var moved string
+	swapped := false
+	windowsACLMaterializeSwapHook = func(string) {
+		if swapped {
+			return
+		}
+		swapped = true
+		moved = swapAncestorAside(t, approved, elsewhere)
+	}
+	t.Cleanup(func() { windowsACLMaterializeSwapHook = nil })
+
+	created, err := materializeWindowsACLTarget(filepath.Join(approved, ".git", "config"), true)
+	if err != nil {
+		t.Fatalf("materializeWindowsACLTarget: %v", err)
+	}
+	if !swapped {
+		t.Fatal("the seam never fired, so this test proved nothing")
+	}
+	requireNothingEscaped(t, elsewhere)
+
+	landed := filepath.Join(moved, ".git", "config")
+	info, err := os.Stat(landed)
+	if err != nil {
+		t.Fatalf("the file did not land in the directory that was verified: %v", err)
+	}
+	if info.IsDir() {
+		t.Error("materialized .git/config as a directory, which breaks git init")
+	}
+	if !created.FileMade || created.File != "config" {
+		t.Errorf("file record = %q made=%v, want config/true", created.File, created.FileMade)
+	}
+}
+
+// THE ROLLBACK RACE. The ancestor is swapped AFTER the target was created, which
+// is the window the teardown path lives in: minutes or hours, not microseconds.
+//
+// The bystander is the point. If the unwind resolves by pathname it walks into
+// the decoy and deletes what it finds there, recursively and elevated. Its
+// survival is the only thing that proves the unwind did not.
+func TestRollbackDoesNotFollowAnAncestorSwappedAfterCreation(t *testing.T) {
+	root := t.TempDir()
+	approved := filepath.Join(root, "ws")
+	elsewhere := filepath.Join(root, "OUTSIDE")
+	for _, dir := range []string{approved, elsewhere} {
+		if err := os.Mkdir(dir, 0o700); err != nil {
+			t.Fatalf("seed %s: %v", dir, err)
+		}
+	}
+
+	created, err := materializeWindowsACLTarget(filepath.Join(approved, "a", "b"), false)
+	if err != nil {
+		t.Fatalf("materializeWindowsACLTarget: %v", err)
+	}
+
+	// A bystander tree under the decoy, shaped exactly like what we created, so a
+	// pathname unwind would find something to destroy at every level.
+	bystander := filepath.Join(elsewhere, "a", "b")
+	if err := os.MkdirAll(bystander, 0o700); err != nil {
+		t.Fatalf("seed bystander: %v", err)
+	}
+	witness := filepath.Join(bystander, "irreplaceable.txt")
+	if err := os.WriteFile(witness, []byte("not yours to delete"), 0o600); err != nil {
+		t.Fatalf("seed witness: %v", err)
+	}
+
+	moved := swapAncestorAside(t, approved, elsewhere)
+
+	// The anchor pathname now names the decoy, and the decoy is a junction, so
+	// the unwind must refuse rather than proceed. Either way it must not delete.
+	err = rollbackWindowsACLMaterialization(created)
+
+	if _, statErr := os.Stat(witness); statErr != nil {
+		t.Fatalf("DESTRUCTIVE: rollback followed the junction and deleted a tree outside the approved directory: %v", statErr)
+	}
+	if _, statErr := os.Stat(bystander); statErr != nil {
+		t.Fatalf("DESTRUCTIVE: rollback removed the bystander directory outside the approved directory: %v", statErr)
+	}
+	if err == nil {
+		t.Error("rollback reported success while unwinding through a swapped ancestor; it must say it could not")
+	}
+	// Residue inside the real tree is the accepted price: leaving it is strictly
+	// better than a recursive delete through a path someone else controls.
+	if _, statErr := os.Stat(filepath.Join(moved, "a", "b")); statErr != nil {
+		t.Logf("note: the real tree was also unwound (%v); leaving it would be acceptable too", statErr)
+	}
+}
+
+// A real directory wearing the anchor's name is a swap with NO reparse point
+// anywhere, so every no-follow check in this package passes it. Only the file
+// identity notices.
+func TestRollbackRefusesAnAnchorReplacedByARealDirectory(t *testing.T) {
+	root := t.TempDir()
+	approved := filepath.Join(root, "ws")
+	if err := os.Mkdir(approved, 0o700); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	created, err := materializeWindowsACLTarget(filepath.Join(approved, "a"), false)
+	if err != nil {
+		t.Fatalf("materializeWindowsACLTarget: %v", err)
+	}
+
+	if err := os.Rename(approved, approved+"-moved"); err != nil {
+		t.Skipf("cannot rename here: %v", err)
+	}
+	// An ordinary directory. Nothing is a link; nothing is a reparse point.
+	if err := os.Mkdir(approved, 0o700); err != nil {
+		t.Fatalf("plant the replacement: %v", err)
+	}
+	decoy := filepath.Join(approved, "a")
+	if err := os.Mkdir(decoy, 0o700); err != nil {
+		t.Fatalf("plant the decoy child: %v", err)
+	}
+
+	err = rollbackWindowsACLMaterialization(created)
+	if err == nil {
+		t.Error("rollback accepted a different directory wearing the anchor's name")
+	} else if !strings.Contains(err.Error(), "no longer the directory") {
+		t.Errorf("refused for the wrong reason: %v", err)
+	}
+	if _, statErr := os.Stat(decoy); statErr != nil {
+		t.Errorf("rollback deleted a directory it never created: %v", statErr)
+	}
+}
+
+// Rollback removes ONLY what this run created. A pre-existing ancestor is walked
+// through and left alone.
+func TestRollbackLeavesDirectoriesItDidNotCreate(t *testing.T) {
+	root := t.TempDir()
+	existing := filepath.Join(root, "ws", "already-here")
+	if err := os.MkdirAll(existing, 0o700); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	target := filepath.Join(existing, "made", "deeper")
+
+	created, err := materializeWindowsACLTarget(target, false)
+	if err != nil {
+		t.Fatalf("materializeWindowsACLTarget: %v", err)
+	}
+	if created.AnchorPath != existing {
+		t.Fatalf("anchor = %q, want the deepest pre-existing directory %q", created.AnchorPath, existing)
+	}
+	if err := rollbackWindowsACLMaterialization(created); err != nil {
+		t.Fatalf("rollbackWindowsACLMaterialization: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(existing, "made")); !errors.Is(err, os.ErrNotExist) {
+		t.Errorf("created directory survived rollback: stat err = %v", err)
+	}
+	if _, err := os.Stat(existing); err != nil {
+		t.Errorf("rollback removed a directory that already existed: %v", err)
+	}
+}
+
+// The whole apply path, through the closure callers actually hold, rather than
+// through rollbackWindowsACLSnapshots directly. Every other rollback test in
+// this package calls the unwind by hand, which cannot catch applyWindowsACLPlan
+// failing to carry the created record into the snapshots it hands over.
+func TestApplyWindowsACLPlanClosureRemovesWhatItMaterialized(t *testing.T) {
+	root := t.TempDir()
+	directoryTarget := filepath.Join(root, "ws", "hooks")
+	fileTarget := filepath.Join(root, "ws", "config")
+
+	plan := WindowsACLPlan{Entries: []WindowsACLEntry{
+		{Action: WindowsACLDenyWrite, Path: directoryTarget, Capability: "S-1-1-0", Materialize: true},
+		{Action: WindowsACLDenyWrite, Path: fileTarget, Capability: "S-1-1-0", Materialize: true, MaterializeFile: true},
+	}}
+
+	rollback, err := applyWindowsACLPlan(plan)
+	if err != nil {
+		t.Fatalf("applyWindowsACLPlan: %v", err)
+	}
+	for _, path := range []string{directoryTarget, fileTarget} {
+		if _, err := os.Stat(path); err != nil {
+			t.Fatalf("%s was not materialized: %v", path, err)
+		}
+	}
+	if err := rollback(); err != nil {
+		t.Fatalf("rollback closure: %v", err)
+	}
+	for _, path := range []string{directoryTarget, fileTarget} {
+		if _, err := os.Stat(path); !errors.Is(err, os.ErrNotExist) {
+			t.Errorf("%s survived the rollback closure: stat err = %v", path, err)
+		}
+	}
+	// The shared prefix both targets needed must go too, and it is created by
+	// whichever group runs first rather than being owned by both.
+	if _, err := os.Stat(filepath.Join(root, "ws")); !errors.Is(err, os.ErrNotExist) {
+		t.Errorf("the shared parent survived: stat err = %v", err)
+	}
+}
+
+// A rollback that cannot remove something must SAY so. This is the regression
+// guard for the trap a naive handle-relative port walks straight into:
+// FILE_DELETE_ON_CLOSE reports success on a non-empty directory and leaves it
+// there, which turns a loud failure into a silent lie. The directory being
+// populated is not adversarial; .git/hooks fills up the moment git runs.
+func TestRollbackReportsWhatItCouldNotRemove(t *testing.T) {
+	root := t.TempDir()
+	target := filepath.Join(root, "ws", "hooks")
+	created, err := materializeWindowsACLTarget(target, false)
+	if err != nil {
+		t.Fatalf("materializeWindowsACLTarget: %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(target, "pre-commit"), []byte("#!/bin/sh\n"), 0o600); err != nil {
+		t.Fatalf("populate: %v", err)
+	}
+
+	err = rollbackWindowsACLMaterialization(created)
+	if err == nil {
+		t.Fatal("rollback reported success on a directory it could not empty, so callers cannot tell teardown failed")
+	}
+	if !strings.Contains(strings.ToLower(err.Error()), "hooks") {
+		t.Errorf("the error does not name what was left behind: %v", err)
+	}
+	// Left in place deliberately. Removing it would mean recursing, and recursion
+	// through a path the workspace owner controls is the thing being avoided.
+	if _, statErr := os.Stat(target); statErr != nil {
+		t.Errorf("rollback recursed into a populated directory instead of reporting it: %v", statErr)
+	}
+}
+
+// The primitives take a single component and the walk relies on that. A joined
+// name is resolved the ordinary way by the kernel, so an intermediate junction
+// inside it is followed and the object lands outside the anchor: the pinned
+// parent buys nothing if the name itself walks.
+func TestChildOperationsRefuseNamesThatAreNotSingleComponents(t *testing.T) {
+	root := t.TempDir()
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	for _, name := range []string{`sub\child`, "sub/child", "..", ".", "", `C:\absolute`, "stream:name"} {
+		t.Run("create dir "+name, func(t *testing.T) {
+			handle, _, err := createWindowsACLChildDirectory(parent, name)
+			if err == nil {
+				_ = windows.CloseHandle(handle)
+				t.Fatalf("accepted %q, which the kernel would resolve through intermediate directories", name)
+			}
+		})
+		t.Run("delete dir "+name, func(t *testing.T) {
+			if err := deleteWindowsACLChildDirectory(parent, name); err == nil {
+				t.Fatalf("accepted %q for deletion", name)
+			}
+		})
+		t.Run("create file "+name, func(t *testing.T) {
+			if _, err := createWindowsACLChildFile(parent, name); err == nil {
+				t.Fatalf("accepted %q for file creation", name)
+			}
+		})
+	}
+}
+
+// A junction sitting where a chain component should be must be refused when it
+// is OPENED, not merely when it is created. FILE_OPEN_REPARSE_POINT hands back a
+// handle to the junction itself, and using that as the next parent puts every
+// deeper create on the far side of it.
+func TestChildOperationsRefuseAnExistingJunction(t *testing.T) {
+	root := t.TempDir()
+	elsewhere := t.TempDir()
+	makeJunction(t, filepath.Join(root, "hop"), elsewhere)
+
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	handle, _, err := createWindowsACLChildDirectory(parent, "hop")
+	if err == nil {
+		_ = windows.CloseHandle(handle)
+		t.Error("createWindowsACLChildDirectory returned a junction as the next parent in the walk")
+	}
+	opened, err := openWindowsACLChildDirectory(parent, "hop")
+	if err == nil {
+		_ = windows.CloseHandle(opened)
+		t.Error("openWindowsACLChildDirectory returned a junction to descend through")
+	}
+}
+
+// Rollback descends; it must never create. If a component was removed by
+// something else in the meantime, re-making it and then deleting it would remove
+// a directory the sandbox never made.
+func TestRollbackDescentNeverCreatesAMissingComponent(t *testing.T) {
+	root := t.TempDir()
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	handle, err := openWindowsACLChildDirectory(parent, "never-existed")
+	if err == nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatal("the descent open created a directory that did not exist")
+	}
+	if !isWindowsNotExist(err) {
+		t.Errorf("a missing component reported %v, which rollback cannot distinguish from a real failure", err)
+	}
+	if _, statErr := os.Stat(filepath.Join(root, "never-existed")); statErr == nil {
+		t.Error("a directory appeared on disk from an open that should never create")
+	}
+}

internal/sandbox/windows_acl_relative_windows.go

@@ -0,0 +1,470 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// Handle-relative directory operations.
+//
+// WHY THESE EXIST. Every pathname-based call re-resolves the whole path inside
+// the kernel at the moment it runs. So verifying a component and then creating
+// through it are two separate resolutions of the same string, and a workspace
+// owner can swap an ancestor for a junction in the gap between them: setup
+// verifies, the attacker swaps, setup creates, and the object lands outside the
+// approved tree. Checking again afterwards is too late, because the thing has
+// already been created somewhere it should not be.
+//
+// A HANDLE pins the object rather than the name. Once a directory is open, that
+// handle keeps referring to the same directory however the path is later
+// rearranged, so creating a child relative to it cannot be redirected. os.Mkdir,
+// os.OpenFile and os.RemoveAll are pathname-based by construction with no
+// relative form on Windows, which is why this drops to NtCreateFile with
+// OBJECT_ATTRIBUTES.RootDirectory.
+
+// IO_STATUS_BLOCK.Information values for a create/open, named because "2 means
+// it was created" is not something a reader should have to look up.
+const (
+	windowsFileOpened  uintptr = 1
+	windowsFileCreated uintptr = 2
+)
+
+// windowsACLDirectoryShare is the share mode every open here uses. A sandbox
+// tree is live, so refusing to share would fail on any directory something else
+// happens to have open: a denial of service on ourselves rather than a security
+// property.
+const windowsACLDirectoryShare = windows.FILE_SHARE_READ | windows.FILE_SHARE_WRITE | windows.FILE_SHARE_DELETE
+
+// windowsFileIdentity is the kernel's own answer to "is this the same object",
+// independent of what it is currently called.
+//
+// It exists because rollback cannot hold the anchor handle open for the whole
+// sandbox lifetime (see rollbackWindowsACLSnapshots), so it has to re-open the
+// anchor by pathname, and a pathname can be made to name a different object.
+// Crucially that substitution needs NO reparse point: rename the real directory
+// aside and create an ordinary directory wearing its name, and every no-follow
+// check still passes because nothing anywhere is a link. Comparing the volume
+// and file index catches it, because those identify the object the kernel
+// actually opened.
+type windowsFileIdentity struct {
+	Volume    uint32
+	IndexHigh uint32
+	IndexLow  uint32
+}
+
+func (identity windowsFileIdentity) empty() bool {
+	return identity == windowsFileIdentity{}
+}
+
+// windowsIdentityOfHandle reads the identity of an already-open object.
+func windowsIdentityOfHandle(handle windows.Handle) (windowsFileIdentity, error) {
+	var info windows.ByHandleFileInformation
+	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
+		return windowsFileIdentity{}, fmt.Errorf("read windows file identity: %w", err)
+	}
+	return windowsFileIdentity{
+		Volume:    info.VolumeSerialNumber,
+		IndexHigh: info.FileIndexHigh,
+		IndexLow:  info.FileIndexLow,
+	}, nil
+}
+
+// openWindowsACLDirectoryNoFollow opens an existing directory by pathname,
+// refusing to traverse or land on a reparse point.
+//
+// This is the ANCHOR for a handle-relative walk: the one pathname resolution
+// that has to happen, with everything below it relative to the handle it
+// returns. FILE_FLAG_OPEN_REPARSE_POINT stops the final component being
+// followed, and verifyWindowsACLTargetNotRedirected then confirms no ancestor
+// redirected either, because GetFinalPathNameByHandle answers for the whole
+// resolved path.
+func openWindowsACLDirectoryNoFollow(path string) (windows.Handle, error) {
+	utf16Path, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return 0, fmt.Errorf("encode windows ACL directory %s: %w", path, err)
+	}
+	handle, err := windows.CreateFile(
+		utf16Path,
+		windows.FILE_LIST_DIRECTORY|windows.FILE_READ_ATTRIBUTES|windows.SYNCHRONIZE,
+		windowsACLDirectoryShare,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OPEN_REPARSE_POINT,
+		0,
+	)
+	if err != nil {
+		// Errno.Is maps the not-found codes to os.ErrNotExist, so a caller
+		// walking up to find the deepest existing ancestor keeps working.
+		return 0, fmt.Errorf("open windows ACL directory %s: %w", path, err)
+	}
+	if err := verifyWindowsACLHandleIsCleanDirectory(handle, path); err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, err
+	}
+	return handle, nil
+}
+
+// openWindowsACLDirectoryNoFollowWithIdentity is the anchor open plus the
+// identity a later rollback needs in order to prove it re-opened the same
+// object.
+func openWindowsACLDirectoryNoFollowWithIdentity(path string) (windows.Handle, windowsFileIdentity, error) {
+	handle, err := openWindowsACLDirectoryNoFollow(path)
+	if err != nil {
+		return 0, windowsFileIdentity{}, err
+	}
+	identity, err := windowsIdentityOfHandle(handle)
+	if err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, windowsFileIdentity{}, fmt.Errorf("identify windows ACL directory %s: %w", path, err)
+	}
+	return handle, identity, nil
+}
+
+// reopenWindowsACLDirectoryAsIdentity re-opens an anchor by pathname and refuses
+// it unless the kernel says it is the object that was opened before.
+//
+// Used only by rollback. See windowsFileIdentity for why the pathname alone is
+// not enough, and rollbackWindowsACLSnapshots for why a handle cannot simply be
+// held instead.
+func reopenWindowsACLDirectoryAsIdentity(path string, want windowsFileIdentity) (windows.Handle, error) {
+	handle, err := openWindowsACLDirectoryNoFollow(path)
+	if err != nil {
+		return 0, err
+	}
+	if want.empty() {
+		return handle, nil
+	}
+	got, err := windowsIdentityOfHandle(handle)
+	if err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, fmt.Errorf("identify windows ACL directory %s: %w", path, err)
+	}
+	if got != want {
+		_ = windows.CloseHandle(handle)
+		return 0, fmt.Errorf("refusing to unwind under %s: it is no longer the directory setup created into, so something replaced it since (possible path swap during elevated setup)", path)
+	}
+	return handle, nil
+}
+
+// createWindowsACLChildDirectory creates one directory directly beneath parent,
+// or opens it when it already exists, and reports which happened.
+//
+// name must be a single component; see validateWindowsACLComponent for why that
+// is enforced rather than assumed. The kernel resolves it relative to the parent
+// HANDLE, so nothing above it is consulted and nothing above it can be swapped
+// underneath us.
+//
+// created is true only when this call made the directory, which the rollback
+// needs: removing one that already existed would delete a user's data over a
+// failure that had nothing to do with it.
+func createWindowsACLChildDirectory(parent windows.Handle, name string) (handle windows.Handle, created bool, err error) {
+	if err := validateWindowsACLComponent(name); err != nil {
+		return 0, false, err
+	}
+	objectName, err := windows.NewNTUnicodeString(name)
+	if err != nil {
+		return 0, false, fmt.Errorf("encode windows ACL directory component %s: %w", name, err)
+	}
+	attributes := windows.OBJECT_ATTRIBUTES{
+		RootDirectory: parent,
+		ObjectName:    objectName,
+		Attributes:    windows.OBJ_CASE_INSENSITIVE,
+	}
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+
+	var status windows.IO_STATUS_BLOCK
+	if err := windows.NtCreateFile(
+		&handle,
+		windows.FILE_LIST_DIRECTORY|windows.FILE_READ_ATTRIBUTES|windows.SYNCHRONIZE|windows.DELETE,
+		&attributes,
+		&status,
+		nil,
+		windows.FILE_ATTRIBUTE_DIRECTORY,
+		windowsACLDirectoryShare,
+		windows.FILE_OPEN_IF,
+		windows.FILE_DIRECTORY_FILE|windows.FILE_OPEN_REPARSE_POINT|windows.FILE_SYNCHRONOUS_IO_NONALERT,
+		0,
+		0,
+	); err != nil {
+		return 0, false, fmt.Errorf("create windows ACL directory component %s: %w", name, err)
+	}
+	// FILE_OPEN_IF means an EXISTING child is opened rather than created, and
+	// FILE_OPEN_REPARSE_POINT means a junction is opened AS the junction. Without
+	// this check that junction becomes the parent of the next level and every
+	// create beneath it lands wherever it points, which is the mid-walk swap this
+	// whole file exists to stop.
+	if err := rejectWindowsACLReparseHandle(handle, name); err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, false, err
+	}
+	return handle, status.Information == windowsFileCreated, nil
+}
+
+// openWindowsACLChildDirectory opens an EXISTING directory beneath parent and
+// never creates one.
+//
+// Rollback walks back down the chain it created, and it must not conjure a
+// component that has since been removed: FILE_OPEN_IF would recreate it, and
+// then the unwind would delete a directory setup never made. FILE_OPEN is the
+// whole difference from createWindowsACLChildDirectory.
+func openWindowsACLChildDirectory(parent windows.Handle, name string) (handle windows.Handle, err error) {
+	if err := validateWindowsACLComponent(name); err != nil {
+		return 0, err
+	}
+	objectName, err := windows.NewNTUnicodeString(name)
+	if err != nil {
+		return 0, fmt.Errorf("encode windows ACL directory component %s: %w", name, err)
+	}
+	attributes := windows.OBJECT_ATTRIBUTES{
+		RootDirectory: parent,
+		ObjectName:    objectName,
+		Attributes:    windows.OBJ_CASE_INSENSITIVE,
+	}
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+
+	// Ask for the least that still allows passing through and checking the
+	// reparse attribute. This runs during rollback, on directories whose ACEs
+	// have already been applied, so every extra right is another way for the
+	// unwind to be refused by the very ACL it is unwinding. Notably absent:
+	// SYNCHRONIZE, and with it FILE_SYNCHRONOUS_IO_NONALERT, since nothing is
+	// read or written through this handle.
+	var status windows.IO_STATUS_BLOCK
+	if err := windows.NtCreateFile(
+		&handle,
+		windows.FILE_TRAVERSE|windows.FILE_READ_ATTRIBUTES,
+		&attributes,
+		&status,
+		nil,
+		windows.FILE_ATTRIBUTE_DIRECTORY,
+		windowsACLDirectoryShare,
+		windows.FILE_OPEN,
+		windows.FILE_DIRECTORY_FILE|windows.FILE_OPEN_REPARSE_POINT,
+		0,
+		0,
+	); err != nil {
+		return 0, fmt.Errorf("open windows ACL directory component %s: %w", name, err)
+	}
+	if err := rejectWindowsACLReparseHandle(handle, name); err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, err
+	}
+	return handle, nil
+}
+
+// createWindowsACLChildFile creates an empty file directly beneath parent, or
+// opens it when it already exists, and reports which happened.
+//
+// The counterpart to createWindowsACLChildDirectory for the one materialized
+// target that must be a FILE: .git/config, where creating a directory instead
+// would break `git init` outright rather than merely mis-ACL it.
+//
+// FILE_OPEN_IF rather than FILE_CREATE deliberately. The pathname version this
+// replaces used O_CREATE|O_EXCL and then tolerated os.ErrExist, so a racing
+// creator winning was fine. FILE_CREATE's collision status is
+// STATUS_OBJECT_NAME_COLLISION, which errors.Is(err, os.ErrExist) does NOT
+// match, so porting it literally would have turned that tolerated race into a
+// hard failure. FILE_OPEN_IF keeps the old behaviour and reports the truth in
+// created, which rollback needs so it never deletes a file it did not make.
+func createWindowsACLChildFile(parent windows.Handle, name string) (created bool, err error) {
+	if err := validateWindowsACLComponent(name); err != nil {
+		return false, err
+	}
+	objectName, err := windows.NewNTUnicodeString(name)
+	if err != nil {
+		return false, fmt.Errorf("encode windows ACL file component %s: %w", name, err)
+	}
+	attributes := windows.OBJECT_ATTRIBUTES{
+		RootDirectory: parent,
+		ObjectName:    objectName,
+		Attributes:    windows.OBJ_CASE_INSENSITIVE,
+	}
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+
+	var handle windows.Handle
+	var status windows.IO_STATUS_BLOCK
+	if err := windows.NtCreateFile(
+		&handle,
+		windows.FILE_READ_ATTRIBUTES|windows.SYNCHRONIZE,
+		&attributes,
+		&status,
+		nil,
+		windows.FILE_ATTRIBUTE_NORMAL,
+		windowsACLDirectoryShare,
+		windows.FILE_OPEN_IF,
+		windows.FILE_NON_DIRECTORY_FILE|windows.FILE_OPEN_REPARSE_POINT|windows.FILE_SYNCHRONOUS_IO_NONALERT,
+		0,
+		0,
+	); err != nil {
+		return false, fmt.Errorf("create windows ACL file component %s: %w", name, err)
+	}
+	if err := rejectWindowsACLReparseHandle(handle, name); err != nil {
+		_ = windows.CloseHandle(handle)
+		return false, err
+	}
+	createdNow := status.Information == windowsFileCreated
+	if err := windows.CloseHandle(handle); err != nil {
+		return createdNow, fmt.Errorf("close windows ACL file component %s: %w", name, err)
+	}
+	return createdNow, nil
+}
+
+// deleteWindowsACLChildDirectory removes one directory directly beneath parent.
+//
+// The counterpart to the create above, and the reason rollback cannot use
+// os.RemoveAll: that takes a pathname, so an ancestor swapped to a junction
+// AFTER the object was created sends the recursive delete somewhere else and
+// takes unrelated trees with it. Resolving relative to the parent handle makes
+// that impossible.
+//
+// A missing child is not an error: rollback runs on failure paths where the
+// object may never have been created.
+func deleteWindowsACLChildDirectory(parent windows.Handle, name string) error {
+	return deleteWindowsACLChild(parent, name, true)
+}
+
+// deleteWindowsACLChildFile removes one file directly beneath parent, for the
+// materialized .git/config carveout. Rollback picks between this and the
+// directory form from the shape recorded at materialization time rather than by
+// stat-ing the pathname, because a stat is another pathname resolution and this
+// whole file exists to avoid those.
+func deleteWindowsACLChildFile(parent windows.Handle, name string) error {
+	return deleteWindowsACLChild(parent, name, false)
+}
+
+// deleteWindowsACLChild opens one child relative to parent and deletes it by
+// SETTING ITS DISPOSITION, not with FILE_DELETE_ON_CLOSE.
+//
+// That distinction is the whole point, and it was measured rather than assumed.
+// FILE_DELETE_ON_CLOSE defers the removal to cleanup, where a non-empty
+// directory makes it fail with nothing to report it to: NtCreateFile returns
+// success, CloseHandle returns success, and the directory is still there. A
+// rollback built on it would report success while leaving materialized state on
+// disk, which is strictly worse than the os.RemoveAll it replaces, because
+// os.RemoveAll at least removed it.
+//
+// NtSetInformationFile answers synchronously and to the caller, so a non-empty
+// directory comes back as STATUS_DIRECTORY_NOT_EMPTY. Leaving residue is
+// acceptable, since the alternative is a recursive delete through a pathname an
+// attacker may control; lying about having removed it is not.
+//
+// FILE_DIRECTORY_FILE / FILE_NON_DIRECTORY_FILE also make the open refuse an
+// object of the wrong shape rather than deleting it.
+func deleteWindowsACLChild(parent windows.Handle, name string, directory bool) error {
+	if err := validateWindowsACLComponent(name); err != nil {
+		return err
+	}
+	objectName, err := windows.NewNTUnicodeString(name)
+	if err != nil {
+		return fmt.Errorf("encode windows ACL component %s: %w", name, err)
+	}
+	attributes := windows.OBJECT_ATTRIBUTES{
+		RootDirectory: parent,
+		ObjectName:    objectName,
+		Attributes:    windows.OBJ_CASE_INSENSITIVE,
+	}
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+
+	shapeOption := uint32(windows.FILE_NON_DIRECTORY_FILE)
+	shapeAttribute := uint32(windows.FILE_ATTRIBUTE_NORMAL)
+	if directory {
+		shapeOption = windows.FILE_DIRECTORY_FILE
+		shapeAttribute = windows.FILE_ATTRIBUTE_DIRECTORY
+	}
+
+	// DELETE alone. Asking for SYNCHRONIZE as well would make this fail on any
+	// object already carrying a deny-read ACE, because FILE_GENERIC_READ and
+	// FILE_GENERIC_EXECUTE both include SYNCHRONIZE, and rollback exists
+	// precisely to undo objects that have just been ACL'd. Nothing is read or
+	// written through this handle, so synchronous IO is not needed either.
+	var handle windows.Handle
+	var status windows.IO_STATUS_BLOCK
+	if err := windows.NtCreateFile(
+		&handle,
+		windows.DELETE,
+		&attributes,
+		&status,
+		nil,
+		shapeAttribute,
+		windowsACLDirectoryShare,
+		windows.FILE_OPEN,
+		shapeOption|windows.FILE_OPEN_REPARSE_POINT,
+		0,
+		0,
+	); err != nil {
+		if isWindowsNotExist(err) {
+			return nil
+		}
+		return fmt.Errorf("open windows ACL component %s for delete: %w", name, err)
+	}
+	defer func() { _ = windows.CloseHandle(handle) }()
+
+	// One BOOLEAN: FILE_DISPOSITION_INFORMATION.DeleteFile = TRUE.
+	disposition := byte(1)
+	var setStatus windows.IO_STATUS_BLOCK
+	if err := windows.NtSetInformationFile(
+		handle,
+		&setStatus,
+		&disposition,
+		uint32(unsafe.Sizeof(disposition)),
+		windows.FileDispositionInformation,
+	); err != nil {
+		return fmt.Errorf("delete windows ACL component %s: %w", name, err)
+	}
+	return nil
+}
+
+// rejectWindowsACLReparseHandle refuses a handle that landed on a reparse point.
+//
+// Deliberately NOT verifyWindowsACLTargetNotRedirected: that one compares the
+// handle's resolved path against an expected pathname, which is exactly the
+// pathname dependency the handle-relative walk removes. A child opened relative
+// to a pinned parent is in the right place by construction even when the
+// pathname no longer leads there, so comparing paths would reject correct, safe
+// creates whenever the tree was legitimately renamed. The attribute is the only
+// thing worth checking here.
+func rejectWindowsACLReparseHandle(handle windows.Handle, name string) error {
+	var info windows.ByHandleFileInformation
+	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
+		return fmt.Errorf("inspect windows ACL component %s: %w", name, err)
+	}
+	if info.FileAttributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+		return fmt.Errorf("refusing to work through reparse-point component %s: possible path swap during elevated setup", name)
+	}
+	return nil
+}
+
+// verifyWindowsACLHandleIsCleanDirectory rejects a handle that landed on a
+// reparse point, or on an object other than the path asked for.
+func verifyWindowsACLHandleIsCleanDirectory(handle windows.Handle, path string) error {
+	var info windows.ByHandleFileInformation
+	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
+		return fmt.Errorf("inspect windows ACL directory %s: %w", path, err)
+	}
+	if info.FileAttributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+		return fmt.Errorf("refusing to materialize under reparse-point path component %s: possible path swap during elevated setup", path)
+	}
+	return verifyWindowsACLTargetNotRedirected(handle, path)
+}
+
+// isWindowsNotExist reports a missing-object error from either side of the API.
+// NtCreateFile returns NTSTATUS values, which do not map to os.ErrNotExist the
+// way the Win32 error codes do.
+func isWindowsNotExist(err error) bool {
+	if err == nil {
+		return false
+	}
+	if os.IsNotExist(err) {
+		return true
+	}
+	var status windows.NTStatus
+	if errors.As(err, &status) {
+		return status == windows.STATUS_OBJECT_NAME_NOT_FOUND || status == windows.STATUS_OBJECT_PATH_NOT_FOUND
+	}
+	return false
+}

internal/sandbox/windows_acl_relative_windows_test.go

@@ -0,0 +1,230 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// THE SWAP THAT PATHNAMES CANNOT SURVIVE.
+//
+// This is the race behind the materialization P1. A pathname-based create
+// resolves the whole path again at the moment it runs, so an ancestor replaced
+// between the verification and the create sends the new directory somewhere else
+// entirely. Verifying afterwards is too late: the object already exists in the
+// wrong place.
+//
+// A handle pins the OBJECT, not the name. This test performs the swap for real,
+// in the window that used to be exploitable, and asserts the child still lands
+// in the directory that was verified.
+//
+// Worth noting for anyone extending this: the fix is what makes the race
+// testable at all. Against the old code the swap had to be threaded into the
+// middle of a function; here it is three ordinary lines between an open and a
+// create, because the handle is held across them.
+func TestChildCreationFollowsTheHandleNotThePath(t *testing.T) {
+	root := t.TempDir()
+	approved := filepath.Join(root, "approved")
+	elsewhere := filepath.Join(root, "elsewhere")
+	for _, dir := range []string{approved, elsewhere} {
+		if err := os.Mkdir(dir, 0o700); err != nil {
+			t.Fatalf("seed %s: %v", dir, err)
+		}
+	}
+
+	// Verified once, exactly as setup does before it materializes anything.
+	parent, err := openWindowsACLDirectoryNoFollow(approved)
+	if err != nil {
+		t.Fatalf("open approved directory: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	// THE SWAP, in the window that used to be exploitable: move the verified
+	// directory aside and leave a junction to somewhere else wearing its name.
+	moved := filepath.Join(root, "approved-moved")
+	if err := os.Rename(approved, moved); err != nil {
+		t.Skipf("cannot rename a directory with an open handle on this filesystem: %v", err)
+	}
+	makeJunction(t, approved, elsewhere)
+
+	// The create resolves against the handle, so it must ignore the junction now
+	// sitting at the original pathname.
+	child, created, err := createWindowsACLChildDirectory(parent, "materialized")
+	if err != nil {
+		t.Fatalf("create child relative to the pinned handle: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(child) }()
+	if !created {
+		t.Error("created = false for a directory that did not exist")
+	}
+
+	if _, err := os.Stat(filepath.Join(moved, "materialized")); err != nil {
+		t.Errorf("the child did not land in the verified directory: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(elsewhere, "materialized")); err == nil {
+		t.Fatal("ESCAPED: the child was created through the junction, outside the approved tree")
+	}
+}
+
+// Materialization runs on trees that may already be half-built, so creating an
+// existing directory has to be a no-op rather than a failure. created must still
+// report the truth, because rollback deletes only what this call made.
+func TestCreatingAnExistingChildOpensItInstead(t *testing.T) {
+	root := t.TempDir()
+	if err := os.Mkdir(filepath.Join(root, "already"), 0o700); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	handle, created, err := createWindowsACLChildDirectory(parent, "already")
+	if err != nil {
+		t.Fatalf("open existing child: %v", err)
+	}
+	_ = windows.CloseHandle(handle)
+	if created {
+		t.Error("created = true for a directory that already existed; rollback would delete a user's data")
+	}
+}
+
+// The rollback counterpart. os.RemoveAll on a pathname whose ancestor has since
+// become a junction recurses outside the workspace and deletes unrelated trees,
+// which is the second P1. Resolving relative to the parent handle cannot.
+func TestDeleteFollowsTheHandleNotThePath(t *testing.T) {
+	root := t.TempDir()
+	approved := filepath.Join(root, "approved")
+	elsewhere := filepath.Join(root, "elsewhere")
+	for _, dir := range []string{approved, elsewhere} {
+		if err := os.Mkdir(dir, 0o700); err != nil {
+			t.Fatalf("seed %s: %v", dir, err)
+		}
+	}
+	// A bystander under the decoy: if the delete ever resolves by pathname it is
+	// reachable, and its survival is what proves the delete did not.
+	if err := os.Mkdir(filepath.Join(elsewhere, "victim"), 0o700); err != nil {
+		t.Fatalf("seed victim: %v", err)
+	}
+	if err := os.Mkdir(filepath.Join(approved, "victim"), 0o700); err != nil {
+		t.Fatalf("seed target: %v", err)
+	}
+
+	parent, err := openWindowsACLDirectoryNoFollow(approved)
+	if err != nil {
+		t.Fatalf("open approved directory: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	moved := filepath.Join(root, "approved-moved")
+	if err := os.Rename(approved, moved); err != nil {
+		t.Skipf("cannot rename a directory with an open handle on this filesystem: %v", err)
+	}
+	makeJunction(t, approved, elsewhere)
+
+	if err := deleteWindowsACLChildDirectory(parent, "victim"); err != nil {
+		t.Fatalf("delete relative to the pinned handle: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(elsewhere, "victim")); err != nil {
+		t.Fatal("DESTRUCTIVE: rollback followed the junction and deleted a directory outside the approved tree")
+	}
+	if _, err := os.Stat(filepath.Join(moved, "victim")); err == nil {
+		t.Error("the directory inside the approved tree was not removed")
+	}
+}
+
+// A delete that cannot happen must SAY so.
+//
+// This is the gap that let a silent bug ship in the first version of this file.
+// It deleted with FILE_DELETE_ON_CLOSE, which defers the removal to cleanup,
+// where a non-empty directory makes it fail with nowhere to report it: the open
+// returned success, the close returned success, and the directory was still
+// there. The only test covering deletion used an EMPTY directory, so it passed
+// throughout. Rollback built on that would have reported success while leaving
+// materialized state on disk, which is worse than the os.RemoveAll it replaced,
+// because os.RemoveAll actually removed it.
+func TestDeletingANonEmptyDirectoryIsReported(t *testing.T) {
+	root := t.TempDir()
+	populated := filepath.Join(root, "populated")
+	if err := os.Mkdir(populated, 0o700); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(populated, "occupant"), []byte("x"), 0o600); err != nil {
+		t.Fatalf("seed occupant: %v", err)
+	}
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	err = deleteWindowsACLChildDirectory(parent, "populated")
+	if _, statErr := os.Stat(populated); statErr != nil {
+		t.Fatalf("the directory was removed with its contents, which this delete must never do: %v", statErr)
+	}
+	if err == nil {
+		t.Fatal("reported success while leaving the directory in place")
+	}
+}
+
+// The directory form must refuse a file rather than delete it, and the file form
+// must handle the one materialized target that is a file.
+func TestDeleteDistinguishesFilesFromDirectories(t *testing.T) {
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, "config"), []byte("x"), 0o600); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	if err := deleteWindowsACLChildDirectory(parent, "config"); err == nil {
+		t.Error("the directory delete accepted a file")
+	}
+	if err := deleteWindowsACLChildFile(parent, "config"); err != nil {
+		t.Fatalf("deleteWindowsACLChildFile: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(root, "config")); err == nil {
+		t.Error("the file survived its delete")
+	}
+}
+
+// Rollback runs on failure paths where the object may never have been created,
+// so a missing child is success rather than an error to report.
+func TestDeletingAMissingChildIsNotAnError(t *testing.T) {
+	root := t.TempDir()
+	parent, err := openWindowsACLDirectoryNoFollow(root)
+	if err != nil {
+		t.Fatalf("open root: %v", err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	if err := deleteWindowsACLChildDirectory(parent, "never-existed"); err != nil {
+		t.Fatalf("deleting a missing child reported an error: %v", err)
+	}
+}
+
+// The anchor open is the one pathname resolution in the walk, so it has to
+// refuse a junction itself rather than leaving it to a later check.
+func TestOpeningAJunctionAnchorIsRefused(t *testing.T) {
+	root := t.TempDir()
+	real := filepath.Join(root, "real")
+	if err := os.Mkdir(real, 0o700); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	link := filepath.Join(root, "link")
+	makeJunction(t, link, real)
+
+	handle, err := openWindowsACLDirectoryNoFollow(link)
+	if err == nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatal("opened a junction as the materialization anchor; every create beneath it would land outside the approved tree")
+	}
+}

internal/sandbox/windows_acl_reparse_windows.go

@@ -0,0 +1,161 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+
+	"golang.org/x/sys/windows"
+)
+
+// GetFinalPathNameByHandle flags. x/sys/windows does not export these.
+const (
+	windowsFileNameNormalized uint32 = 0x0
+	windowsVolumeNameDOS      uint32 = 0x0
+)
+
+// verifyWindowsACLTargetNotRedirected fails when an opened handle resolved
+// somewhere other than the requested path, which means a component along the way
+// is a reparse point.
+//
+// openWindowsACLTarget's FILE_FLAG_OPEN_REPARSE_POINT check covers the FINAL
+// component only; CreateFile still resolves ANCESTORS. A user who controls the
+// workspace can turn an ancestor — .git, say — into a junction before elevated
+// setup runs, and setup would then apply its DACL change to an object outside
+// the approved tree while the final-component check still passed. Junctions need
+// no privilege to create, unlike symlinks, so this is reachable by exactly the
+// unprivileged user the sandbox exists to contain.
+//
+// GetFinalPathNameByHandle answers where the handle actually landed, covering
+// every component in one call rather than walking the path and re-checking each
+// component (which would also race between the checks).
+//
+// The comparison basis must NEVER follow a reparse point, and getting that wrong
+// is what made this guard mostly ornamental.
+//
+// It used to compare against canonicalSandboxWorkspaceRoot(path), which runs
+// filepath.EvalSymlinks — the same resolution the kernel just performed. So for a
+// directory SYMLINK the two sides agreed precisely BECAUSE the redirect happened:
+// GetFinalPathNameByHandle followed the link to produce one side, EvalSymlinks
+// followed the same link to produce the other, and elevated setup went on to
+// rewrite the DACL of an object outside the workspace.
+//
+// Junctions were rejected, but by accident rather than by design. Go reports a
+// junction as ModeIrregular rather than ModeSymlink, so EvalSymlinks refuses it,
+// canonicalSandboxWorkspaceRoot falls back to the lexical path, and the mismatch
+// surfaces. That is a property of the standard library's mode bits, not of this
+// check: a Go release that resolved mount points would silently disarm the one
+// case this was known to catch.
+//
+// So the expected side is now normalized WITHOUT resolving anything. Casing and
+// 8.3 short names still have to compare equal, since a legitimate target can be
+// spelled either way and still be the same object — EqualFold covers the first
+// and GetLongPathName the second, and neither follows a link.
+//
+// This does mean a target deliberately spelled through a symlink is now refused.
+// That is the intended direction: the whole question here is whether the object
+// the handle landed on is the object that was named, and a caller that wants a
+// link's target should name the target.
+func verifyWindowsACLTargetNotRedirected(handle windows.Handle, path string) error {
+	buffer := make([]uint16, windows.MAX_LONG_PATH)
+	n, err := windows.GetFinalPathNameByHandle(handle, &buffer[0], uint32(len(buffer)), windowsFileNameNormalized|windowsVolumeNameDOS)
+	if err != nil {
+		return fmt.Errorf("resolve windows ACL target %s: %w", path, err)
+	}
+	if int(n) < len(buffer) {
+		buffer = buffer[:n]
+	}
+	actual := trimWindowsExtendedPathPrefix(windows.UTF16ToString(buffer))
+	expected := trimWindowsExtendedPathPrefix(windowsACLComparablePath(path))
+	if !strings.EqualFold(filepath.Clean(actual), filepath.Clean(expected)) {
+		return fmt.Errorf("refusing to apply ACL to %s: it resolves to %s, so a parent directory is a reparse point (possible path swap during elevated setup)", path, actual)
+	}
+	return nil
+}
+
+// windowsACLComparablePath normalizes a path for comparison against where a
+// handle actually landed, WITHOUT resolving any reparse point.
+//
+// The two things that must still compare equal are casing and 8.3 short names,
+// because either can spell the same object. GetLongPathName expands the short
+// form by reading directory entries; it does not follow a symlink or a junction
+// to its target, which is the entire reason it is used here instead of
+// EvalSymlinks. Casing is left to the caller's EqualFold.
+//
+// Every failure degrades to the lexically cleaned path rather than to a resolved
+// one. A path this cannot expand is compared as written, which can only produce
+// a spurious MISMATCH — a refusal — and never a spurious match. That is the
+// direction a redirect check has to fail in.
+func windowsACLComparablePath(path string) string {
+	cleaned := filepath.Clean(strings.TrimSpace(path))
+	if cleaned == "" || cleaned == "." {
+		return cleaned
+	}
+	if absolute, err := filepath.Abs(cleaned); err == nil {
+		cleaned = absolute
+	}
+	wide, err := windows.UTF16PtrFromString(cleaned)
+	if err != nil {
+		return cleaned
+	}
+	buffer := make([]uint16, windows.MAX_LONG_PATH)
+	n, err := windows.GetLongPathName(wide, &buffer[0], uint32(len(buffer)))
+	if err != nil || n == 0 || int(n) >= len(buffer) {
+		return cleaned
+	}
+	return filepath.Clean(windows.UTF16ToString(buffer[:n]))
+}
+
+// trimWindowsExtendedPathPrefix strips the \?\ form GetFinalPathNameByHandle
+// returns so it can be compared with an ordinary path.
+func trimWindowsExtendedPathPrefix(path string) string {
+	// Built from filepath.Separator rather than written as literals so the
+	// backslashes cannot be miscounted by whatever writes this file.
+	sep := string(filepath.Separator)
+	devicePrefix := sep + sep + "?" + sep
+	uncPrefix := devicePrefix + "UNC" + sep
+	if strings.HasPrefix(path, uncPrefix) {
+		return sep + sep + strings.TrimPrefix(path, uncPrefix)
+	}
+	return strings.TrimPrefix(path, devicePrefix)
+}
+
+// verifyWindowsACLPathComponentNotRedirected opens one path component no-follow
+// and refuses it if it is a reparse point or resolves anywhere other than its own
+// pathname. Because GetFinalPathNameByHandle answers for the WHOLE resolved path,
+// verifying a single existing component also clears every ancestor above it.
+//
+// A missing component surfaces as os.ErrNotExist so the caller can walk further
+// up. Only FILE_READ_ATTRIBUTES is requested: this inspects, it never writes, and
+// asking for more would fail on ancestors the setup process has no rights on.
+func verifyWindowsACLPathComponentNotRedirected(path string) error {
+	utf16Path, err := windows.UTF16PtrFromString(path)
+	if err != nil {
+		return fmt.Errorf("encode windows ACL path component %s: %w", path, err)
+	}
+	handle, err := windows.CreateFile(
+		utf16Path,
+		windows.FILE_READ_ATTRIBUTES,
+		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
+		nil,
+		windows.OPEN_EXISTING,
+		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OPEN_REPARSE_POINT,
+		0,
+	)
+	if err != nil {
+		// syscall.Errno.Is maps ERROR_FILE_NOT_FOUND/ERROR_PATH_NOT_FOUND to
+		// os.ErrNotExist, so the caller's errors.Is check keeps working.
+		return fmt.Errorf("open windows ACL path component %s: %w", path, err)
+	}
+	defer func() { _ = windows.CloseHandle(handle) }()
+	var info windows.ByHandleFileInformation
+	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
+		return fmt.Errorf("inspect windows ACL path component %s: %w", path, err)
+	}
+	if info.FileAttributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0 {
+		return fmt.Errorf("refusing to materialize under reparse-point path component %s: possible path swap during elevated setup", path)
+	}
+	return verifyWindowsACLTargetNotRedirected(handle, path)
+}

internal/sandbox/windows_acl_reparse_windows_test.go

@@ -0,0 +1,79 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// An unprivileged user who controls the workspace can turn an ancestor of a
+// configured ACL target into a junction before elevated setup runs. CreateFile
+// resolves ancestors even with FILE_FLAG_OPEN_REPARSE_POINT, so the
+// final-component check passes and setup would rewrite the DACL of an object
+// outside the approved tree.
+//
+// Junctions, unlike symlinks, need no privilege — which is what makes this
+// reachable by exactly the user the sandbox is containing.
+func TestOpenWindowsACLTargetRefusesAJunctionAncestor(t *testing.T) {
+	base := t.TempDir()
+	workspace := filepath.Join(base, "workspace")
+	outside := filepath.Join(base, "OUTSIDE")
+	for _, dir := range []string{workspace, outside} {
+		if err := os.MkdirAll(dir, 0o700); err != nil {
+			t.Fatalf("mkdir %s: %v", dir, err)
+		}
+	}
+	// The object the attacker wants setup to touch.
+	victim := filepath.Join(outside, "hooks")
+	if err := os.MkdirAll(victim, 0o700); err != nil {
+		t.Fatalf("mkdir victim: %v", err)
+	}
+
+	// .git is the ancestor, and it is a junction to OUTSIDE.
+	gitDir := filepath.Join(workspace, ".git")
+	if out, err := exec.Command("cmd", "/c", "mklink", "/J", gitDir, outside).CombinedOutput(); err != nil {
+		t.Skipf("cannot create a junction here: %v %s", err, out)
+	}
+
+	// This is the path setup would be configured with.
+	target := filepath.Join(gitDir, "hooks")
+	if _, err := os.Stat(target); err != nil {
+		t.Fatalf("precondition: the junction should make %s reachable: %v", target, err)
+	}
+
+	handle, _, err := openWindowsACLTarget(target)
+	if err == nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatal("opened an ACL target through a junction ancestor; setup would have rewritten a DACL outside the workspace")
+	}
+	if !strings.Contains(err.Error(), "reparse point") {
+		t.Errorf("error = %v, want it to name the reparse point", err)
+	}
+	t.Logf("refused as expected: %v", err)
+}
+
+// The guard must not reject ordinary targets, including ones spelled
+// non-canonically — a differently-cased path is the same object, not a redirect.
+func TestOpenWindowsACLTargetAcceptsAnOrdinaryTarget(t *testing.T) {
+	dir := t.TempDir()
+	nested := filepath.Join(dir, "Nested")
+	if err := os.MkdirAll(nested, 0o700); err != nil {
+		t.Fatalf("mkdir: %v", err)
+	}
+	for _, spelling := range []string{nested, strings.ToLower(nested)} {
+		handle, isDir, err := openWindowsACLTarget(spelling)
+		if err != nil {
+			t.Fatalf("openWindowsACLTarget(%q): %v", spelling, err)
+		}
+		if !isDir {
+			t.Errorf("%q reported as not a directory", spelling)
+		}
+		_ = windows.CloseHandle(handle)
+	}
+}

internal/sandbox/windows_acl_restore_identity_windows_test.go

@@ -0,0 +1,148 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// THE RESTORE MUST LAND ON THE OBJECT THE DACL CAME FROM.
+//
+// Creation and the materialization unwind were both bound to handles, and the
+// restore branch was left re-resolving the target by pathname. Its comment
+// justified that with a no-follow re-open, which rules out a REPARSE POINT
+// swapped in since apply and nothing else. The other substitution passes it
+// untouched: rename the target aside and put an ordinary directory of the same
+// name in its place. Nothing in that decoy is a link, so the no-follow open
+// succeeds, the pre-apply DACL is written onto the attacker's object, and the
+// real one keeps the ACEs from the setup that just aborted.
+func TestRollbackRefusesToRestoreOntoAReplacedTarget(t *testing.T) {
+	root := t.TempDir()
+	approved := filepath.Join(root, "ws")
+	target := filepath.Join(approved, "target")
+	if err := os.MkdirAll(target, 0o700); err != nil {
+		t.Fatalf("seed the approved tree: %v", err)
+	}
+
+	handle, _, err := openWindowsACLTarget(target)
+	if err != nil {
+		t.Fatalf("open the target: %v", err)
+	}
+	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatalf("read the target DACL: %v", err)
+	}
+	identity, err := windowsIdentityOfHandle(handle)
+	if err != nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatalf("read the target identity: %v", err)
+	}
+	_ = windows.CloseHandle(handle)
+
+	// The swap, exactly as it would happen between apply and rollback. Ordinary
+	// directories throughout: no links, no reparse points.
+	if err := os.Rename(target, target+"-moved"); err != nil {
+		t.Skipf("cannot rename here: %v", err)
+	}
+	if err := os.Mkdir(target, 0o700); err != nil {
+		t.Fatalf("plant the replacement: %v", err)
+	}
+
+	snapshot := windowsACLSnapshot{Path: target, Descriptor: descriptor, TargetID: identity}
+	err = rollbackWindowsACLSnapshots([]windowsACLSnapshot{snapshot})
+	if err == nil {
+		t.Fatal("rollback wrote the pre-apply DACL onto a different object wearing the target's name")
+	}
+	if !strings.Contains(err.Error(), "no longer the object") {
+		t.Errorf("refused for the wrong reason: %v", err)
+	}
+}
+
+// And an untouched target still restores, or the guard above would be satisfied
+// by a rollback that refuses everything.
+func TestRollbackRestoresAnUnchangedTarget(t *testing.T) {
+	target := filepath.Join(t.TempDir(), "target")
+	if err := os.Mkdir(target, 0o700); err != nil {
+		t.Fatalf("seed the target: %v", err)
+	}
+
+	handle, _, err := openWindowsACLTarget(target)
+	if err != nil {
+		t.Fatalf("open the target: %v", err)
+	}
+	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatalf("read the target DACL: %v", err)
+	}
+	identity, err := windowsIdentityOfHandle(handle)
+	if err != nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatalf("read the target identity: %v", err)
+	}
+	_ = windows.CloseHandle(handle)
+
+	snapshot := windowsACLSnapshot{Path: target, Descriptor: descriptor, TargetID: identity}
+	if err := rollbackWindowsACLSnapshots([]windowsACLSnapshot{snapshot}); err != nil {
+		t.Fatalf("an unchanged target was refused: %v", err)
+	}
+}
+
+// THE COMMENT AT rollbackWindowsACLSnapshots CLAIMED A TEST THAT DID NOT EXIST.
+//
+// It said "TestRollbackUnwindsDescendantsBeforeAncestors pins it" about the
+// reverse iteration order, and grepping the repo found only that sentence. The
+// ordering is load-bearing twice over: a materialized directory must be empty
+// before its own removal is attempted, and SetSecurityInfo propagates
+// inheritable ACEs downward so the ancestor has to be restored last. Neither
+// was pinned by anything.
+func TestRollbackUnwindsDescendantsBeforeAncestors(t *testing.T) {
+	root := t.TempDir()
+	ancestor := filepath.Join(root, "ws")
+	descendant := filepath.Join(ancestor, "child")
+	if err := os.MkdirAll(descendant, 0o700); err != nil {
+		t.Fatalf("seed the tree: %v", err)
+	}
+
+	snapshotFor := func(path string) windowsACLSnapshot {
+		t.Helper()
+		handle, _, err := openWindowsACLTarget(path)
+		if err != nil {
+			t.Fatalf("open %s: %v", path, err)
+		}
+		defer windows.CloseHandle(handle)
+		descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+		if err != nil {
+			t.Fatalf("read the DACL for %s: %v", path, err)
+		}
+		identity, err := windowsIdentityOfHandle(handle)
+		if err != nil {
+			t.Fatalf("read the identity for %s: %v", path, err)
+		}
+		return windowsACLSnapshot{Path: path, Descriptor: descriptor, TargetID: identity}
+	}
+
+	// Ascending by path key, the order applyWindowsACLPlan produces.
+	snapshots := []windowsACLSnapshot{snapshotFor(ancestor), snapshotFor(descendant)}
+
+	var order []string
+	restore := windowsACLRestoreHook
+	windowsACLRestoreHook = func(path string) { order = append(order, path) }
+	t.Cleanup(func() { windowsACLRestoreHook = restore })
+
+	if err := rollbackWindowsACLSnapshots(snapshots); err != nil {
+		t.Fatalf("rollbackWindowsACLSnapshots: %v", err)
+	}
+	if len(order) != 2 {
+		t.Fatalf("restored %d targets, want 2: %v", len(order), order)
+	}
+	if order[0] != descendant || order[1] != ancestor {
+		t.Errorf("restore order was %v; the descendant must be restored before its ancestor, because SetSecurityInfo propagates inheritable ACEs downward", order)
+	}
+}

internal/sandbox/windows_acl_symlink_ancestor_windows_test.go

@@ -0,0 +1,113 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// THE COMPARISON BASIS MUST NOT FOLLOW WHAT IT IS CHECKING FOR.
+//
+// verifyWindowsACLTargetNotRedirected asks GetFinalPathNameByHandle where the
+// handle landed and compares that against the requested path. It used to build
+// the second side with filepath.EvalSymlinks, which is the same resolution the
+// kernel had just performed, so for a directory symlink the two sides agreed
+// BECAUSE the redirect happened, and elevated setup rewrote the DACL of an
+// object outside the workspace.
+//
+// Junctions were caught, but only because Go reports one as ModeIrregular and
+// EvalSymlinks declines it, leaving the lexical path to disagree. That is a
+// standard-library detail, not a property of the guard.
+
+// The property, testable with no privilege at all: the normalizer must not
+// resolve a reparse point.
+//
+// HONEST LIMIT, because it would be easy to read more into this than it proves.
+// A junction is used because creating one needs no privilege, and EvalSymlinks
+// does not resolve a junction either — so this test passes against BOTH the old
+// basis and the new one, and reverting the fix does not fail it. It is an
+// invariant test, not a regression for this change: it pins that normalization
+// leaves a reparse point alone, which is what would break first if a future Go
+// started resolving mount points and silently disarmed the junction case.
+//
+// The case that actually separates the two bases is the directory symlink below,
+// and that one skips without privilege. On this machine it skipped.
+func TestACLComparablePathDoesNotResolveAReparsePoint(t *testing.T) {
+	base := t.TempDir()
+	outside := filepath.Join(base, "outside")
+	if err := os.MkdirAll(filepath.Join(outside, "hooks"), 0o700); err != nil {
+		t.Fatalf("mkdir outside: %v", err)
+	}
+	link := filepath.Join(base, "link")
+	if out, err := exec.Command("cmd", "/c", "mklink", "/J", link, outside).CombinedOutput(); err != nil {
+		t.Skipf("cannot create a junction here: %v (%s)", err, out)
+	}
+
+	through := filepath.Join(link, "hooks")
+	comparable := windowsACLComparablePath(through)
+
+	// Compared against the normalizer's OWN answer for each path, never against
+	// the raw input. GetLongPathName legitimately rewrites the string: on a CI
+	// runner the temp directory is an 8.3 short name, so expanding RUNNER~1 to
+	// runneradmin makes the result differ from what was passed in while naming
+	// exactly the same object. An earlier version of this test asserted equality
+	// with the input and failed on Windows CI for that reason, which was the test
+	// being wrong rather than the code.
+	target := windowsACLComparablePath(filepath.Join(outside, "hooks"))
+
+	// The whole property: normalizing a path THROUGH the reparse point must not
+	// produce the target's normalization. If it did, the guard would be comparing
+	// a redirected handle against a redirected expectation and matching itself.
+	if strings.EqualFold(filepath.Clean(comparable), filepath.Clean(target)) {
+		t.Errorf("windowsACLComparablePath resolved through the reparse point:\n  through %s\n  target  %s", comparable, target)
+	}
+	// And it still has to name the link component, or it resolved to something
+	// else entirely rather than leaving the path alone.
+	if !strings.Contains(strings.ToLower(comparable), "link") {
+		t.Errorf("windowsACLComparablePath lost the link component: %s", comparable)
+	}
+
+	// The old basis is checked alongside for contrast: where it resolves, it
+	// could not have been the comparison basis.
+	if resolved := canonicalSandboxWorkspaceRoot(through); strings.EqualFold(filepath.Clean(resolved), filepath.Clean(filepath.Join(outside, "hooks"))) {
+		t.Logf("confirmed: canonicalSandboxWorkspaceRoot resolves %s to %s, which is why it cannot be the basis", through, resolved)
+	}
+}
+
+// The end-to-end symlink case the finding names. Directory symlinks need either
+// Developer Mode or SeCreateSymbolicLinkPrivilege, so this skips where it cannot
+// build the fixture rather than passing vacuously.
+func TestOpenWindowsACLTargetRefusesASymlinkAncestor(t *testing.T) {
+	base := t.TempDir()
+	outside := filepath.Join(base, "outside")
+	if err := os.MkdirAll(filepath.Join(outside, "hooks"), 0o700); err != nil {
+		t.Fatalf("mkdir outside: %v", err)
+	}
+	workspace := filepath.Join(base, "workspace")
+	if err := os.MkdirAll(workspace, 0o700); err != nil {
+		t.Fatalf("mkdir workspace: %v", err)
+	}
+
+	// A DIRECTORY symlink, not a junction: os.Symlink maps to CreateSymbolicLinkW
+	// and the directory flag is inferred from the existing target.
+	link := filepath.Join(workspace, ".git")
+	if err := os.Symlink(outside, link); err != nil {
+		t.Skipf("cannot create a directory symlink here (needs Developer Mode or SeCreateSymbolicLinkPrivilege): %v", err)
+	}
+
+	target := filepath.Join(link, "hooks")
+	handle, _, err := openWindowsACLTarget(target)
+	if err == nil {
+		_ = windows.CloseHandle(handle)
+		t.Fatal("elevated setup opened a target through a symlinked ancestor; its DACL change would land outside the workspace")
+	}
+	if !strings.Contains(err.Error(), "reparse point") {
+		t.Errorf("refusal does not name the cause: %v", err)
+	}
+}

internal/sandbox/windows_acl_test.go

@@ -73,10 +73,19 @@ func TestBuildWindowsACLPlanUsesReadOnlySIDWithoutWriteRoots(t *testing.T) {
 	if err != nil {
 		t.Fatalf("BuildWindowsACLPlan: %v", err)
 	}
-	if len(plan.Entries) != 1 {
-		t.Fatalf("ACL entries = %#v, want one deny-read entry", plan.Entries)
+	// Two deny entries, one per SID the token can carry here. ReadOnly is the
+	// only capability a profile with no write roots gets, and ReadAllow is the
+	// read grant a DenyRead profile's strict token carries: denying just one of
+	// them leaves the carveout readable through the other.
+	if len(plan.Entries) != 2 {
+		t.Fatalf("ACL entries = %#v, want a deny-read entry per capability SID", plan.Entries)
 	}
 	assertWindowsACLEntry(t, plan, WindowsACLDenyRead, `C:\workspace\secret-read`, caps.ReadOnly, true)
+	readSID, err := WindowsReadAllowSID(home)
+	if err != nil {
+		t.Fatalf("WindowsReadAllowSID: %v", err)
+	}
+	assertWindowsACLEntry(t, plan, WindowsACLDenyRead, `C:\workspace\secret-read`, readSID, true)
 }
 
 func TestBuildWindowsACLPlanRejectsUnrestrictedProfiles(t *testing.T) {

internal/sandbox/windows_candidate_creation_windows_test.go

@@ -0,0 +1,87 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// ELEVATED SETUP MUST NOT CREATE THROUGH A JUNCTION THE CALLER PLANTED.
+//
+// ensureWindowsSandboxRuntimeCandidates runs as Administrator, and os.MkdirAll
+// follows links. Both candidate ancestries are prepared by the invoking user, so
+// a junction at a not-yet-created component below the cache directory — or at the
+// fallback anchor — made elevated setup build the runtime tree beneath a
+// redirected, Administrator-writable target. The ACL applier's no-follow check
+// runs later and spots the redirection, but the privileged create has already
+// happened by then, outside its rollback boundary.
+//
+// A junction needs no privilege on Windows, so this reproduces the caller's half
+// of the race on an ordinary unelevated box.
+func TestRuntimeCandidateCreationRefusesAJunctionedComponent(t *testing.T) {
+	cacheRoot := t.TempDir()
+	target := t.TempDir()
+
+	previous := sandboxUserCacheDir
+	t.Cleanup(func() { sandboxUserCacheDir = previous })
+	sandboxUserCacheDir = func() (string, error) { return cacheRoot, nil }
+
+	workspace := t.TempDir()
+	candidate, ok := deterministicSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(cacheRoot))
+	if !ok {
+		t.Fatalf("SETUP INVALID: no deterministic candidate for %s under %s", workspace, cacheRoot)
+	}
+	// The component the caller controls: the first one Zero owns, below the
+	// operator's cache directory.
+	owned := filepath.Join(canonicalSandboxWorkspaceRoot(cacheRoot), "zero")
+	if !strings.HasPrefix(strings.ToLower(candidate), strings.ToLower(owned)) {
+		t.Fatalf("SETUP INVALID: candidate %s does not sit under the owned component %s", candidate, owned)
+	}
+	if out, err := exec.Command("cmd", "/c", "mklink", "/J", owned, target).CombinedOutput(); err != nil {
+		t.Fatalf("SETUP INVALID: mklink /J: %v\n%s", err, out)
+	}
+
+	err := ensureRuntimeCandidateDir(candidate)
+	if err == nil {
+		t.Fatal("elevated setup created the runtime tree through a junction the caller planted")
+	}
+
+	// The whole point: nothing may appear beneath the redirected target.
+	entries, readErr := os.ReadDir(target)
+	if readErr != nil {
+		t.Fatal(readErr)
+	}
+	if len(entries) != 0 {
+		names := make([]string, 0, len(entries))
+		for _, entry := range entries {
+			names = append(names, entry.Name())
+		}
+		t.Fatalf("setup created %v beneath the redirected target", names)
+	}
+}
+
+// And an ordinary cache directory still gets its candidate, or the refusal above
+// would be satisfied by a creator that refuses everything.
+func TestRuntimeCandidateCreationStillCreatesAnOrdinaryTree(t *testing.T) {
+	cacheRoot := t.TempDir()
+	previous := sandboxUserCacheDir
+	t.Cleanup(func() { sandboxUserCacheDir = previous })
+	sandboxUserCacheDir = func() (string, error) { return cacheRoot, nil }
+
+	workspace := t.TempDir()
+	candidate, ok := deterministicSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(cacheRoot))
+	if !ok {
+		t.Fatalf("SETUP INVALID: no deterministic candidate for %s", workspace)
+	}
+	if err := ensureRuntimeCandidateDir(candidate); err != nil {
+		t.Fatalf("an ordinary cache tree was refused: %v", err)
+	}
+	info, err := os.Stat(candidate)
+	if err != nil || !info.IsDir() {
+		t.Fatalf("the candidate was not created: err=%v", err)
+	}
+}

internal/sandbox/windows_command_runner_windows.go

@@ -3,8 +3,11 @@
 package sandbox
 
 import (
+	"errors"
 	"fmt"
 	"io"
+	"os"
+	"strings"
 )
 
 func runWindowsSandboxCommand(config WindowsSandboxCommandConfig, stderr io.Writer) int {
@@ -14,6 +17,15 @@ func runWindowsSandboxCommand(config WindowsSandboxCommandConfig, stderr io.Writ
 			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
 			return 1
 		}
+		// The marker attests to a PLAN. This attests to the OBJECT that plan was
+		// applied to, which cleanup can reclaim and an ordinary run then recreates
+		// without the capability ACE. Checked before the token is minted, so the
+		// operator is told to rerun setup instead of watching every sandboxed write
+		// fail with a bare access-denied.
+		if err := verifyWindowsRuntimeRootCapability(config); err != nil {
+			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
+			return 1
+		}
 	case WindowsSandboxLevelUnelevated:
 		if err := ensureWindowsUnelevatedSetup(config); err != nil {
 			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
@@ -59,7 +71,7 @@ func runWindowsSandboxCommand(config WindowsSandboxCommandConfig, stderr io.Writ
 	// granting only the user, Administrators, and SYSTEM (msys2-runtime
 	// sigproc.cc sigproc_init -> sec_user_nih -> __sec_user), and a
 	// WRITE_RESTRICTED write check must ALSO match one of the token's
-	// restricted SIDs (logon SID, Everyone, capability SIDs). None of the
+	// restricted SIDs (logon SID and capability SIDs). None of the
 	// granted SIDs can be added to the restricted list without collapsing the
 	// write jail (each has write access nearly everywhere), so MSYS startup
 	// dies with "couldn't create signal pipe" or "CreateFileMapping <SID>.1",
@@ -75,6 +87,124 @@ func runWindowsSandboxCommand(config WindowsSandboxCommandConfig, stderr io.Writ
 	// reads under that flag (#612). Profiles with DenyRead keep the fully
 	// restricted token, trading spawn capability for read-deny enforcement.
 	writeRestricted := len(config.PermissionProfile.FileSystem.DenyRead) == 0
+
+	// That strict token then needs the read capability in its SID list, because
+	// the restricted-SID check covers reads once WRITE_RESTRICTED is gone. See
+	// windowsRestrictedTokenSIDsForProfile.
+	tokenSIDs, err = windowsRestrictedTokenSIDsForProfile(tokenSIDs, config.SandboxHome, writeRestricted)
+	if err != nil {
+		fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
+		return 1
+	}
+
+	// A provisioned sandbox principal replaces the restricted token entirely: it
+	// is a separate account, so reads outside its granted roots are denied by the
+	// filesystem rather than left open the way a same-user restricted token has
+	// to leave them (#662). Absent, unprovisioned or opted-out, ok is false and
+	// the restricted-token backend below runs exactly as before.
+	principalToken, ok, err := windowsSandboxPrincipalToken(config)
+	if err != nil {
+		// The one path here that does not fall back, because a provisioned but
+		// unusable principal means the sandbox is broken rather than absent. Say
+		// how to get out of it, since the whole backend is opt-in.
+		fmt.Fprintf(stderr, "%s: sandbox principal is provisioned but unusable: %v. Re-run `zero sandbox setup` from an elevated terminal, or unset %s to fall back to the restricted-token sandbox.\n",
+			WindowsSandboxCommandRunnerName, err, windowsSandboxIdentityEnv)
+		return 1
+	}
+	if ok {
+		defer principalToken.Close()
+		// The principal gets its own identity AND the write jail, not one or the
+		// other. Its ACEs confine reads; without the restricted token it would
+		// still hold every write its ambient memberships grant, so a profile
+		// permitting writes only to the workspace could still write anywhere
+		// BATCH or BUILTIN\Users may — C:\Users\Public\Documents, for one.
+		//
+		// The principal's own SID is deliberately NOT a restricting SID, and that
+		// is the whole of the write jail on this path.
+		//
+		// WRITE_RESTRICTED allows a write only when BOTH the normal token and the
+		// restricting-SID list allow it. The account SID is already enabled in the
+		// normal token, so listing it as a restricting SID makes the second check
+		// a formality for anything granted to that account: every path carrying a
+		// direct principal ACE passes both halves and is writable wherever it
+		// sits. The principal's own profile directory, which Windows creates on
+		// first logon with exactly such an ACE, is outside every configured write
+		// root and was writable for that reason.
+		//
+		// Same defect as the World SID in the restricted list (#865), with the
+		// account SID in place of Everyone: a SID already carried by the normal
+		// token cannot also serve as the restriction on it.
+		//
+		// Confining to the capability SIDs alone restores the intersection the
+		// jail is supposed to be. A configured write root carries BOTH a principal
+		// ACE, from the principal plan, satisfying the normal token, AND a
+		// capability ACE, from BuildWindowsACLPlan which setup applies on every
+		// path, satisfying the restriction. So the principal keeps the tree it
+		// owns, while anything holding only a principal ACE now fails the
+		// restricted check.
+		principalUser, err := principalToken.GetTokenUser()
+		if err != nil {
+			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": read sandbox principal SID: "+err.Error())
+			return 1
+		}
+		jailSIDs := windowsPrincipalJailSIDs(tokenSIDs, principalUser.User.Sid.String())
+		// A strict token restricts READS too, and the account SID that carries the
+		// read roots was just removed above. Without the read capability the jail
+		// stops being a write jail and becomes a total one: no read root, no
+		// executable, nothing. BuildWindowsACLPlan grants this same SID on every
+		// read root and denies it on every DenyRead path, so the allow-list and the
+		// restriction stay one decision.
+		if !writeRestricted {
+			readSID, err := WindowsReadAllowSID(config.SandboxHome)
+			if err != nil {
+				fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": resolve sandbox read capability: "+err.Error())
+				return 1
+			}
+			jailSIDs = append(jailSIDs, readSID)
+		}
+		jailedToken, err := restrictWindowsTokenForCapabilitySIDs(principalToken, jailSIDs, writeRestricted)
+		if err != nil {
+			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
+			return 1
+		}
+		defer jailedToken.Close()
+		// CreateProcessAsUser below is about to be handed a token for a DIFFERENT
+		// account, which forfeits the own-token exemption the ordinary restricted
+		// path relies on. Enable what that needs, and refuse with a specific reason
+		// when this process cannot, rather than letting an unelevated run surface a
+		// bare "Access is denied" that reads as the command being rejected.
+		//
+		// Refusing rather than falling back: an operator who set the opt-in to
+		// confine reads must not be handed the same-user restricted token while
+		// believing otherwise.
+		if err := enableWindowsPrincipalLaunchPrivileges(); err != nil {
+			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
+			return 1
+		}
+		// Stop the child describing the CALLER. Everything identifying the account
+		// survived from the invoking user's environment, so a principal command
+		// resolved its per-user state through paths inside a profile it cannot
+		// open. Applied here, on the principal path only, because this is the first
+		// point that knows which account the command is about to run as.
+		// The role has to be the one the token above was minted for, or the child
+		// is told it is the other account and resolves its per-user state under a
+		// profile it cannot open.
+		config.Env = windowsPrincipalIdentityEnvironment(
+			config.Env,
+			windowsSandboxUserName(
+				windowsSandboxPrincipalKey(config),
+				windowsSandboxRoleForNetwork(config.PermissionProfile.Network.Mode),
+			),
+			config.PermissionProfile.Runtime,
+		)
+		exitCode, err := runWindowsCommandAsUser(jailedToken, config)
+		if err != nil {
+			fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
+			return 1
+		}
+		return exitCode
+	}
+
 	token, err := createWindowsRestrictedTokenForCapabilitySIDs(tokenSIDs, writeRestricted)
 	if err != nil {
 		fmt.Fprintln(stderr, WindowsSandboxCommandRunnerName+": "+err.Error())
@@ -111,9 +241,111 @@ func ensureWindowsUnelevatedSetup(config WindowsSandboxCommandConfig) error {
 	if marker.contains(applied) {
 		return nil
 	}
+	// THE TIER BOUNDARY, STATED BEFORE ANY MUTATION IS ATTEMPTED.
+	//
+	// One plan is consumed by two tiers with different authority. A profile that
+	// carries DenyRead runs on a strict token, and the strict token applies the
+	// restricted-SID check to READS, so the read capability has to be granted
+	// wherever the command reads from -- including the volume root that
+	// permissionProfileReadRoots seeds. Elevated setup can write that DACL. An
+	// ordinary user cannot, and the common opener asks for WRITE_DAC on every
+	// entry, so this tier fails on that one root every time.
+	//
+	// Dropping the root ACE instead is not an option: the strict token would then
+	// fail its own read check for the executable and every ambient read. So the
+	// tier is refused explicitly, naming the root and the reason, rather than
+	// discovered as an ACCESS_DENIED after the fact. The real smoke test misses
+	// this because it substitutes a user-owned temporary directory for the
+	// production read root.
+	if root := windowsPlanVolumeRootGrant(plan); root != "" {
+		return fmt.Errorf("unelevated sandbox setup cannot grant the read capability at the volume root %s, which this profile needs because it configures denyRead and therefore runs on a fully restricted token: "+
+			"changing that directory's permissions requires Administrator rights. "+
+			"Run `zero sandbox setup` from an elevated (Administrator) terminal, "+
+			"or remove denyRead from the sandbox configuration so the command can run on a write-restricted token instead", root)
+	}
 	if _, err := applyWindowsACLPlan(plan); err != nil {
+		// Refusing to run is right: without these ACEs the write jail does not
+		// exist, so continuing would run the command believing it is sandboxed
+		// when it is not. What was wrong was the diagnosis. Every failure got the
+		// same "the workspace may be on a filesystem you do not own" guess, and
+		// the suggested remedy was elevated setup, which does not help at all
+		// when the real problem is one root in the plan that nobody can ACL.
+		//
+		// Being precise matters because this failure repeats: the success marker
+		// is only recorded on success, so the same plan fails identically on
+		// every later command until the offending root leaves it. A reader who
+		// cannot tell which root is at fault has no way out of that.
+		// Every remedy named below is one the reader can actually carry out.
+		// Both messages used to offer `--sandbox forbid`, which is not an option
+		// at all: SandboxPreferenceForbid is an internal engine state with no flag
+		// behind it, so acting on it produced an unknown option and left the reader
+		// stuck on the failure they had just been told how to clear. Advice that
+		// does not work costs more than none, since finding that out takes time.
+		if denied := windowsACLPlanDeniedPath(err); denied != "" {
+			return fmt.Errorf("apply unelevated workspace ACLs: %w; %s cannot have its permissions changed by this user, "+
+				"so the sandbox cannot enforce a write boundary there and will not run the command. "+
+				"That path is one of this workspace's sandbox roots, usually a system directory that arrived via TEMP or TMP. "+
+				"Check those, or turn the sandbox off in your user config with "+
+				`"sandbox": {"enabled": false}. `+
+				"Running `zero sandbox setup` elevated will NOT fix this", err, denied)
+		}
 		return fmt.Errorf("apply unelevated workspace ACLs: %w — the workspace may be on a filesystem the current user does not own; "+
-			"run `zero sandbox setup` from an elevated (Administrator) terminal, or re-run with `--sandbox forbid` to skip OS sandboxing", err)
+			"run `zero sandbox setup` from an elevated (Administrator) terminal, "+
+			`or turn the sandbox off in your user config with "sandbox": {"enabled": false}`, err)
 	}
 	return recordWindowsUnelevatedAppliedPlan(config.SandboxHome, applied)
 }
+
+// windowsACLPlanDeniedPath pulls the target path out of an apply failure that
+// was an access denial, and returns "" for anything else.
+//
+// applyWindowsACLPathGroup already wraps the path into its error, so this reads
+// the message rather than threading a typed error through four layers for one
+// diagnostic. The string it matches is produced in the same package by
+// openWindowsACLTarget, and a test pins the pairing so the two cannot drift
+// apart silently.
+func windowsACLPlanDeniedPath(err error) string {
+	if err == nil || !errors.Is(err, os.ErrPermission) {
+		return ""
+	}
+	const marker = "open windows ACL target "
+	message := err.Error()
+	start := strings.Index(message, marker)
+	if start < 0 {
+		return ""
+	}
+	// Colon-SPACE, not colon. The wrapper is "...target %s: %w", and on Windows
+	// the path itself starts with a drive colon, so splitting on the first colon
+	// returns "C". A drive colon is always followed by a separator, never a
+	// space, which makes ": " the only unambiguous boundary here.
+	rest := message[start+len(marker):]
+	end := strings.Index(rest, ": ")
+	if end <= 0 {
+		return ""
+	}
+	return strings.TrimSpace(rest[:end])
+}
+
+// windowsPrincipalJailSIDs returns the restricting SIDs for a principal command.
+//
+// A named function with its own test rather than an inline slice literal,
+// because the defect it encodes was invisible: the production composition added
+// the account's own SID while the only jail test granted a GROUP, so the
+// configuration that shipped was never the configuration under test.
+//
+// accountSID is taken and DELIBERATELY EXCLUDED rather than simply not passed.
+// Naming it here makes the exclusion the function's stated contract instead of an
+// omission a later edit could undo without noticing, and it also removes the SID
+// should it ever arrive through capabilitySIDs, which is the same route the World
+// SID took in #865.
+func windowsPrincipalJailSIDs(capabilitySIDs []string, accountSID string) []string {
+	account := strings.TrimSpace(accountSID)
+	jail := make([]string, 0, len(capabilitySIDs))
+	for _, sid := range capabilitySIDs {
+		if account != "" && strings.EqualFold(strings.TrimSpace(sid), account) {
+			continue
+		}
+		jail = append(jail, sid)
+	}
+	return jail
+}

internal/sandbox/windows_dualrole_rollback_windows_test.go

@@ -0,0 +1,161 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// The outer rollback must only delete principals this run created.
+//
+// Dual-role setup provisions offline then online. If the offline role adopts an
+// account that already existed and anything afterwards fails, the outer rollback
+// used to delete it anyway, turning a partial re-setup failure into the loss of
+// a principal that was working before the run started. Two roles make that the
+// common case rather than an unlucky one: the first role usually succeeds, so
+// there is nearly always something for a later failure to destroy.
+func TestDualRoleSetupRollbackSparesAdoptedPrincipals(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		createdByRole map[windowsSandboxRole]bool
+		wantRemoved   []windowsSandboxRole
+	}{
+		"offline adopted, online created": {
+			createdByRole: map[windowsSandboxRole]bool{
+				windowsSandboxRoleOffline: false,
+				windowsSandboxRoleOnline:  true,
+			},
+			// Only the one this run made. Deleting the adopted offline principal
+			// is the data loss this guards against.
+			wantRemoved: []windowsSandboxRole{windowsSandboxRoleOnline},
+		},
+		"both adopted": {
+			createdByRole: map[windowsSandboxRole]bool{
+				windowsSandboxRoleOffline: false,
+				windowsSandboxRoleOnline:  false,
+			},
+			wantRemoved: nil,
+		},
+		"both created": {
+			createdByRole: map[windowsSandboxRole]bool{
+				windowsSandboxRoleOffline: true,
+				windowsSandboxRoleOnline:  true,
+			},
+			wantRemoved: []windowsSandboxRole{windowsSandboxRoleOffline, windowsSandboxRoleOnline},
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			var removed []windowsSandboxRole
+			restoreDualRoleSeams(t, testCase.createdByRole, &removed)
+
+			// Fail on the SECOND role, not the first. ACL application happens per
+			// role inside the loop, so failing the first aborts before the second
+			// is provisioned at all and the rollback never has more than one
+			// principal to consider. The case worth testing is the one review
+			// described: the first role succeeds, the second fails, and the
+			// question is whether the first gets destroyed on the way out.
+			// Count GRANT plans, not ACL calls. applyWindowsPrincipalACLs revokes
+			// this trustee's existing ACEs before applying the new plan, so there
+			// are two calls per role now. The contract under test is "the second
+			// ROLE fails"; counting raw calls would fail the first role's grant
+			// instead and prove something else entirely.
+			grants := 0
+			applyWindowsACLPlanFn = func(plan WindowsACLPlan) (func() error, error) {
+				if len(plan.Entries) > 0 && plan.Entries[0].Action == windowsACLRevoke {
+					return func() error { return nil }, nil
+				}
+				grants++
+				if grants < 2 {
+					return func() error { return nil }, nil
+				}
+				return nil, errors.New("ACL apply refused")
+			}
+
+			if _, err := setupWindowsSandboxPrincipal(windowsSandboxTestConfig()); err == nil {
+				t.Fatal("setup reported success despite an injected ACL failure")
+			}
+			assertRolesEqual(t, removed, testCase.wantRemoved)
+		})
+	}
+}
+
+// The same contract on the success path: the rollback handed back to the caller
+// for a LATER setup step to invoke must be just as reluctant.
+func TestDualRoleSetupReturnedRollbackSparesAdoptedPrincipals(t *testing.T) {
+	var removed []windowsSandboxRole
+	restoreDualRoleSeams(t, map[windowsSandboxRole]bool{
+		windowsSandboxRoleOffline: false,
+		windowsSandboxRoleOnline:  true,
+	}, &removed)
+
+	rollback, err := setupWindowsSandboxPrincipal(windowsSandboxTestConfig())
+	if err != nil {
+		t.Fatalf("setup: %v", err)
+	}
+	if len(removed) != 0 {
+		t.Fatalf("setup removed principals before anything failed: %v", removed)
+	}
+	if err := rollback(); err != nil {
+		t.Fatalf("rollback: %v", err)
+	}
+	assertRolesEqual(t, removed, []windowsSandboxRole{windowsSandboxRoleOnline})
+}
+
+func restoreDualRoleSeams(t *testing.T, createdByRole map[windowsSandboxRole]bool, removed *[]windowsSandboxRole) {
+	t.Helper()
+	prevProvision := provisionWindowsSandboxPrincipalForSetupFn
+	prevApply := applyWindowsACLPlanFn
+	prevRemove := removeWindowsSandboxPrincipalForSetupFn
+	t.Cleanup(func() {
+		provisionWindowsSandboxPrincipalForSetupFn = prevProvision
+		applyWindowsACLPlanFn = prevApply
+		removeWindowsSandboxPrincipalForSetupFn = prevRemove
+	})
+
+	provisionWindowsSandboxPrincipalForSetupFn = func(_ WindowsSandboxCommandConfig, role windowsSandboxRole) (windowsSandboxIdentity, bool, error) {
+		sid, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+		if err != nil {
+			return windowsSandboxIdentity{}, false, err
+		}
+		return windowsSandboxIdentity{Username: "zero-sbx-test", SID: sid}, createdByRole[role], nil
+	}
+	applyWindowsACLPlanFn = func(WindowsACLPlan) (func() error, error) {
+		return func() error { return nil }, nil
+	}
+	removeWindowsSandboxPrincipalForSetupFn = func(_ WindowsSandboxCommandConfig, role windowsSandboxRole) error {
+		*removed = append(*removed, role)
+		return nil
+	}
+}
+
+func windowsSandboxTestConfig() WindowsSandboxCommandConfig {
+	return WindowsSandboxCommandConfig{
+		SandboxHome:    `C:\sandboxhome`,
+		CommandCWD:     `C:\ws`,
+		WorkspaceRoots: []string{`C:\ws`},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: `C:\ws`}},
+			},
+		},
+	}
+}
+
+func assertRolesEqual(t *testing.T, got []windowsSandboxRole, want []windowsSandboxRole) {
+	t.Helper()
+	if len(got) != len(want) {
+		t.Fatalf("removed %v, want %v", got, want)
+	}
+	seen := map[windowsSandboxRole]bool{}
+	for _, role := range got {
+		seen[role] = true
+	}
+	for _, role := range want {
+		if !seen[role] {
+			t.Fatalf("removed %v, want %v", got, want)
+		}
+	}
+}

internal/sandbox/windows_git_carveout_windows_test.go

@@ -0,0 +1,66 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+// The git control-plane carveouts are two different shapes: .git/hooks is a
+// directory, .git/config is a FILE. Materialization has to respect that.
+//
+// On a fresh workspace neither exists yet, which is precisely the case
+// Materialize was added for — so this is the common path, not a corner. Creating
+// .git/config as a directory does not just mis-ACL it: it makes the workspace
+// permanently unusable, because git refuses to initialise over a directory
+// where its config file belongs.
+func TestPrincipalACLPlanMaterializesGitConfigAsFile(t *testing.T) {
+	workspace := t.TempDir()
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		// Guests, deliberately: the deny-write ACE must land on the sandbox
+		// principal, not on whoever runs the test. With Everyone (S-1-1-0) the
+		// ACE denies the test process too and `git init` fails with "Permission
+		// denied" for a reason that has nothing to do with the shape bug.
+		PrincipalSID: "S-1-5-32-546",
+		WriteRoots: []WritableRoot{{
+			Root:             workspace,
+			ReadOnlySubpaths: gitMetadataWriteCarveouts(workspace),
+		}},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+
+	rollback, err := applyWindowsACLPlan(plan)
+	if err != nil {
+		t.Fatalf("applyWindowsACLPlan: %v", err)
+	}
+	t.Cleanup(func() {
+		if rollback != nil {
+			_ = rollback()
+		}
+	})
+
+	configPath := filepath.Join(workspace, ".git", "config")
+	if info, err := os.Stat(configPath); err == nil && info.IsDir() {
+		t.Errorf(".git/config was materialized as a directory; git requires a file")
+	}
+	hooksPath := filepath.Join(workspace, ".git", "hooks")
+	if info, err := os.Stat(hooksPath); err == nil && !info.IsDir() {
+		t.Errorf(".git/hooks was materialized as a file; git requires a directory")
+	}
+
+	// The failure users would actually hit.
+	if _, err := exec.LookPath("git"); err != nil {
+		t.Skip("git not on PATH; shape assertions above still ran")
+	}
+	cmd := exec.Command("git", "init")
+	cmd.Dir = workspace
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Errorf("git init failed on a workspace after sandbox setup: %v\n%s", err, out)
+	}
+}

internal/sandbox/windows_git_rename_guard_apply_windows_test.go

@@ -0,0 +1,145 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// windowsPathDeniesDelete reports whether the object's real DACL carries a deny
+// ACE covering DELETE for the given SID.
+//
+// The plan-shape tests assert what the planner emits. This reads what actually
+// landed on disk, which is the gap that let the guard go missing: the plan was
+// right the whole time and the apply silently dropped it.
+func windowsPathDeniesDelete(t *testing.T, path, sid string) bool {
+	t.Helper()
+	handle, _, err := openWindowsACLTarget(path)
+	if err != nil {
+		t.Fatalf("open %s: %v", path, err)
+	}
+	defer func() { _ = windows.CloseHandle(handle) }()
+
+	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		t.Fatalf("read DACL for %s: %v", path, err)
+	}
+	// SDDL rather than walking the ACE buffer by hand: this x/sys does not export
+	// an ACE enumerator, and unsafe pointer arithmetic in a test that exists to
+	// catch a security regression is its own hazard.
+	//
+	// An ACE renders as (type;flags;rights;guid;inherit_guid;sid), so the deny
+	// entries are the ones whose type field is D. Rights come back as SDDL
+	// abbreviations when they fit and as a hex mask when they do not, so both are
+	// accepted; SD is the abbreviation for DELETE.
+	for _, ace := range strings.Split(descriptor.String(), "(") {
+		fields := strings.Split(strings.TrimSuffix(strings.TrimSpace(ace), ")"), ";")
+		if len(fields) != 6 || fields[0] != "D" || !strings.EqualFold(fields[5], sid) {
+			continue
+		}
+		rights := fields[2]
+		if strings.Contains(rights, "SD") {
+			return true
+		}
+		if mask, err := strconv.ParseUint(strings.TrimPrefix(strings.ToLower(rights), "0x"), 16, 32); err == nil {
+			if uint32(mask)&uint32(windows.DELETE) != 0 {
+				return true
+			}
+		}
+	}
+	return false
+}
+
+// THE GUARD MUST REACH DISK ON A WORKSPACE THAT HAD NO .git.
+//
+// This is the case the whole plan is built around and it was the one case where
+// the guard was absent. .git deliberately carries no Materialize, because an
+// empty .git breaks `git init`. Groups are applied in ascending path order, so
+// the .git group ran while .git did not yet exist, was skipped as a
+// non-materializing missing target, and nothing revisited it once the
+// .git\config carveout created .git as its parent chain moments later.
+//
+// Four tests already covered the deny-delete mask and the plan emitting it.
+// None of them applied the plan and looked at the object, which is exactly why
+// a green suite said nothing.
+func TestGitRenameGuardReachesDiskOnAWorkspaceWithoutGit(t *testing.T) {
+	const principal = testPrincipalSID // Unaliased, so it renders literally in SDDL.
+	workspace := t.TempDir()
+	gitDir := filepath.Join(workspace, ".git")
+	if _, err := os.Stat(gitDir); err == nil {
+		t.Fatal("the workspace already has .git, so this proves nothing")
+	}
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: principal,
+		WriteRoots: []WritableRoot{{
+			Root:                   workspace,
+			ProtectedMetadataNames: sandboxFullyProtectedMetadataNames,
+			ReadOnlySubpaths:       gitMetadataWriteCarveouts(workspace),
+		}},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+
+	rollback, err := applyWindowsACLPlan(plan)
+	if err != nil {
+		t.Fatalf("applyWindowsACLPlan: %v", err)
+	}
+	t.Cleanup(func() {
+		if rollback != nil {
+			_ = rollback()
+		}
+	})
+
+	// The carveouts create .git on the way to .git\config.
+	if _, err := os.Stat(gitDir); err != nil {
+		t.Fatalf(".git was never created by the carveout materialization: %v", err)
+	}
+	if !windowsPathDeniesDelete(t, gitDir, principal) {
+		t.Fatal("no deny-delete ACE on .git after applying the plan: the principal can rename it aside and recreate it without the config and hooks carveouts")
+	}
+}
+
+// The same guard on a workspace that already had .git must keep working. This
+// case was already correct, and it is kept so a fix aimed at the fresh
+// workspace cannot quietly trade one for the other.
+func TestGitRenameGuardStillReachesDiskWhenGitAlreadyExists(t *testing.T) {
+	const principal = testPrincipalSID
+	workspace := t.TempDir()
+	gitDir := filepath.Join(workspace, ".git")
+	if err := os.Mkdir(gitDir, 0o700); err != nil {
+		t.Fatalf("seed .git: %v", err)
+	}
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: principal,
+		WriteRoots: []WritableRoot{{
+			Root:                   workspace,
+			ProtectedMetadataNames: sandboxFullyProtectedMetadataNames,
+			ReadOnlySubpaths:       gitMetadataWriteCarveouts(workspace),
+		}},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+	rollback, err := applyWindowsACLPlan(plan)
+	if err != nil {
+		t.Fatalf("applyWindowsACLPlan: %v", err)
+	}
+	t.Cleanup(func() {
+		if rollback != nil {
+			_ = rollback()
+		}
+	})
+
+	if !windowsPathDeniesDelete(t, gitDir, principal) {
+		t.Fatal("no deny-delete ACE on a .git that already existed")
+	}
+}

internal/sandbox/windows_git_rename_guard_test.go

@@ -0,0 +1,98 @@
+package sandbox
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+// A sandbox principal must not be able to REPLACE .git.
+//
+// The write-denied carveouts are attached to .git/config and .git/hooks as
+// objects. Rename .git aside, recreate it, and those objects are gone: the fresh
+// config and hooks inherit the workspace allow with no deny of their own, which
+// hands back credential.helper and core.hooksPath, and with them arbitrary code
+// execution on the next git command.
+//
+// .git cannot join sandboxFullyProtectedMetadataNames to fix this, because that
+// list emits DenyWrite, whose mask includes FILE_GENERIC_WRITE. Git has to write
+// index, objects and refs. So the directory needs DELETE denied on itself while
+// staying writable underneath.
+func TestThePrincipalCannotRenameTheGitDirectory(t *testing.T) {
+	root := filepath.FromSlash("/ws/repo")
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: "S-1-5-21-1-2-3-1001",
+		WriteRoots: []WritableRoot{{
+			Root:                   root,
+			ProtectedMetadataNames: sandboxFullyProtectedMetadataNames,
+			ReadOnlySubpaths:       gitMetadataWriteCarveouts(root),
+		}},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+
+	// The plan normalizes every write root before it names an ACE, so the
+	// expected path has to be normalized too or this compares two spellings of
+	// the same directory. Hardcoding a drive letter instead would pass on
+	// Windows and fail everywhere else, since the builder is portable code.
+	gitDir := filepath.Join(normalizeProfilePath(root), ".git")
+	var denyDelete *WindowsACLEntry
+	for index := range plan.Entries {
+		if plan.Entries[index].Action == WindowsACLDenyDelete && plan.Entries[index].Path == gitDir {
+			denyDelete = &plan.Entries[index]
+			break
+		}
+	}
+	if denyDelete == nil {
+		t.Fatalf("no deny-delete entry for %s, so the principal can rename .git and recreate it without the carveouts:\n%#v", gitDir, plan.Entries)
+	}
+	if denyDelete.Capability != "S-1-5-21-1-2-3-1001" {
+		t.Errorf("deny-delete names %q, want the principal SID", denyDelete.Capability)
+	}
+}
+
+// The guard must not become a write ban. Git writes constantly inside .git, so a
+// deny that reached the children would break every commit rather than just the
+// rename. It also must not be materialized into existence: .git is git's to
+// create, and an empty .git directory made by setup breaks `git init`.
+func TestTheGitRenameGuardDoesNotBlockGitsOwnWrites(t *testing.T) {
+	root := filepath.FromSlash("/ws/repo")
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: "S-1-5-21-1-2-3-1001",
+		WriteRoots: []WritableRoot{{
+			Root:                   root,
+			ProtectedMetadataNames: sandboxFullyProtectedMetadataNames,
+			ReadOnlySubpaths:       gitMetadataWriteCarveouts(root),
+		}},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+
+	gitDir := filepath.Join(normalizeProfilePath(root), ".git")
+	for _, entry := range plan.Entries {
+		if entry.Path != gitDir {
+			continue
+		}
+		if entry.Action == WindowsACLDenyWrite {
+			t.Errorf("deny-write on %s would stop git writing index/objects/refs", gitDir)
+		}
+		if entry.Action == WindowsACLDenyDelete && entry.Materialize {
+			t.Errorf("the rename guard materializes %s; git must create it, an empty .git breaks git init", gitDir)
+		}
+	}
+
+	// The existing carveouts must survive unchanged.
+	for _, want := range []string{filepath.Join(gitDir, "config"), filepath.Join(gitDir, "hooks")} {
+		found := false
+		for _, entry := range plan.Entries {
+			if entry.Action == WindowsACLDenyWrite && entry.Path == want {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("the write-deny carveout for %s disappeared", want)
+		}
+	}
+}

internal/sandbox/windows_git_rename_guard_windows_test.go

@@ -0,0 +1,81 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// The mask is the whole point of the action, so it is asserted bit by bit.
+//
+// Renaming a directory needs DELETE on the directory itself, so denying DELETE
+// is what stops .git being replaced. Everything else in the mask is there to
+// stop the principal removing the guard: WRITE_DAC would let it rewrite the
+// DACL, WRITE_OWNER would let it take ownership and then rewrite the DACL.
+//
+// What must NOT be in it matters just as much. FILE_GENERIC_WRITE would stop git
+// writing index, objects and refs. FILE_DELETE_CHILD would stop git deleting its
+// own lock files and refs. Either one turns a rename guard into a broken repo.
+func TestDenyDeleteMaskStopsRenameWithoutStoppingGit(t *testing.T) {
+	mode, mask, err := windowsACLAccess(WindowsACLDenyDelete)
+	if err != nil {
+		t.Fatalf("windowsACLAccess(deny-delete): %v", err)
+	}
+	if mode != windows.DENY_ACCESS {
+		t.Fatalf("access mode = %v, want DENY_ACCESS", mode)
+	}
+
+	for _, required := range []struct {
+		name string
+		bit  windows.ACCESS_MASK
+	}{
+		{"DELETE", windows.DELETE},
+		{"WRITE_DAC", windows.WRITE_DAC},
+		{"WRITE_OWNER", windows.WRITE_OWNER},
+	} {
+		if mask&required.bit == 0 {
+			t.Errorf("mask %#x is missing %s, so the guard can be removed or bypassed", mask, required.name)
+		}
+	}
+	for _, forbidden := range []struct {
+		name   string
+		bit    windows.ACCESS_MASK
+		breaks string
+	}{
+		{"FILE_GENERIC_WRITE", windows.FILE_GENERIC_WRITE, "git writing index/objects/refs"},
+		{"FILE_DELETE_CHILD", windowsFileDeleteChild, "git deleting its own lock files and refs"},
+	} {
+		if mask&forbidden.bit != 0 {
+			t.Errorf("mask %#x includes %s, which breaks %s", mask, forbidden.name, forbidden.breaks)
+		}
+	}
+}
+
+// Inheritance is the other half. An inherited deny would reach every file inside
+// .git and stop git deleting anything at all, so this ACE has to apply to the
+// directory object alone while the other actions keep inheriting as before.
+func TestDenyDeleteDoesNotInheritWhileOtherActionsStillDo(t *testing.T) {
+	entries := []WindowsACLEntry{
+		{Action: WindowsACLDenyDelete, Path: `C:\work\repo\.git`, Capability: "S-1-5-32-9999"},
+		{Action: WindowsACLDenyWrite, Path: `C:\work\repo\.zero`, Capability: "S-1-5-32-9999"},
+		{Action: WindowsACLAllowWrite, Path: `C:\work\repo`, Capability: "S-1-5-32-9999"},
+	}
+
+	access, err := windowsExplicitAccessEntries(entries, true)
+	if err != nil {
+		t.Fatalf("windowsExplicitAccessEntries: %v", err)
+	}
+	if len(access) != len(entries) {
+		t.Fatalf("got %d access entries, want %d", len(access), len(entries))
+	}
+	if access[0].Inheritance != windows.NO_INHERITANCE {
+		t.Errorf("deny-delete inheritance = %#x, want NO_INHERITANCE; an inherited deny would stop git deleting inside .git", access[0].Inheritance)
+	}
+	for index, entry := range entries[1:] {
+		if got := access[index+1].Inheritance; got != windows.SUB_CONTAINERS_AND_OBJECTS_INHERIT {
+			t.Errorf("%s inheritance = %#x, want the directory default to be unchanged", entry.Action, got)
+		}
+	}
+}

internal/sandbox/windows_group_adoption_windows_test.go

@@ -0,0 +1,86 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+// A NAME IS NOT PROOF OF PROVENANCE, for the USERS group specifically.
+//
+// ensureWindowsLocalGroup covers both managed groups, and the sibling test in
+// windows_offline_group_ownership_windows_test.go pins the offline one. This
+// pins ZeroSandboxUsers, which is the group anandh8x reported: it holds every
+// sandbox principal, so adopting a same-named group created by an installer or
+// by policy would hand the principal whatever that group already grants.
+//
+// Driven through the seams so it needs no Administrator and leaves no real local
+// group on the machine running the suite.
+func TestAdoptingAForeignUsersGroupIsRefused(t *testing.T) {
+	for _, status := range []uintptr{nerrGroupExists, errorAliasExists} {
+		prevAdd, prevOwned := addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn
+		t.Cleanup(func() { addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn = prevAdd, prevOwned })
+
+		addWindowsLocalGroupFn = func(string, string) (uintptr, error) { return status, nil }
+		windowsLocalGroupOwnedByZeroFn = func(string, string) (bool, error) { return false, nil }
+
+		err := ensureWindowsSandboxGroup()
+		if err == nil {
+			t.Fatalf("status %d adopted a group Zero did not create, so the principal inherits whatever it already grants", status)
+		}
+		if !strings.Contains(err.Error(), windowsSandboxGroupName) {
+			t.Errorf("the refusal must name the group in the way, got %q", err)
+		}
+	}
+}
+
+// Our own group is still adopted, or re-running setup would fail on the group it
+// created a moment ago and provisioning would never converge.
+func TestAdoptingOurOwnUsersGroupSucceeds(t *testing.T) {
+	prevAdd, prevOwned := addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn
+	t.Cleanup(func() { addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn = prevAdd, prevOwned })
+
+	addWindowsLocalGroupFn = func(string, string) (uintptr, error) { return nerrGroupExists, nil }
+	windowsLocalGroupOwnedByZeroFn = func(name, comment string) (bool, error) {
+		if name != windowsSandboxGroupName || comment != windowsSandboxGroupComment {
+			t.Errorf("ownership probed for %q/%q, want the users group", name, comment)
+		}
+		return true, nil
+	}
+	if err := ensureWindowsSandboxGroup(); err != nil {
+		t.Errorf("refused a group carrying Zero's own comment: %v", err)
+	}
+}
+
+// A freshly created group is ours by construction, so no ownership probe runs.
+func TestCreatingTheUsersGroupDoesNotProbeOwnership(t *testing.T) {
+	prevAdd, prevOwned := addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn
+	t.Cleanup(func() { addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn = prevAdd, prevOwned })
+
+	probed := false
+	addWindowsLocalGroupFn = func(string, string) (uintptr, error) { return nerrSuccess, nil }
+	windowsLocalGroupOwnedByZeroFn = func(string, string) (bool, error) { probed = true; return false, nil }
+
+	if err := ensureWindowsSandboxGroup(); err != nil {
+		t.Fatalf("a successful create was rejected: %v", err)
+	}
+	if probed {
+		t.Error("ownership was probed for a group we had just created ourselves")
+	}
+}
+
+// A failed ownership probe is not read as "ours" and not as "not ours" either.
+func TestAnUnreadableUsersGroupIsNotGuessedEitherWay(t *testing.T) {
+	prevAdd, prevOwned := addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn
+	t.Cleanup(func() { addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn = prevAdd, prevOwned })
+
+	sentinel := errors.New("NetLocalGroupGetInfo: status 5")
+	addWindowsLocalGroupFn = func(string, string) (uintptr, error) { return nerrGroupExists, nil }
+	windowsLocalGroupOwnedByZeroFn = func(string, string) (bool, error) { return false, sentinel }
+
+	if err := ensureWindowsSandboxGroup(); !errors.Is(err, sentinel) {
+		t.Fatalf("a failed ownership probe was swallowed, got %v", err)
+	}
+}

internal/sandbox/windows_identity_acl.go

@@ -0,0 +1,231 @@
+package sandbox
+
+// ACLs for a sandbox principal.
+//
+// The capability-SID model this sits beside starts from "the caller can already
+// read everything" and narrows writes, because the sandboxed child runs as the
+// caller. A principal inverts that: a separate local account has no access to
+// the caller's profile at all, so the interesting direction is what to GRANT.
+//
+// That inversion is the point. Credential stores under the user's profile are
+// unreachable because the principal is a different account, not because a deny
+// rule enumerated them, which is what makes this able to close #662 and #675 on
+// Windows where a deny-read ACE against the caller's own SID never could. Deny
+// rules stay useful only for objects that are readable by everyone.
+//
+// Grants are explicit and narrow: the workspace and any extra write roots get
+// read+write, declared read-only roots get read, and the protected metadata
+// carve-outs the profile already defines stay denied so .git internals and
+// .zero/.agents cannot be rewritten from inside the sandbox.
+
+import (
+	"errors"
+	"fmt"
+	"path/filepath"
+)
+
+// WindowsACLAllowRead grants read and execute without write. It exists for the
+// principal model, where a read root must be granted rather than assumed.
+const WindowsACLAllowRead WindowsACLAction = "allow-read"
+
+// windowsPrincipalACLInput is everything needed to describe a principal's
+// access. It is deliberately a plain struct rather than the full command config
+// so the plan can be built and tested without a live sandbox.
+type windowsPrincipalACLInput struct {
+	// PrincipalSID is the string SID of the sandbox account every ACE names.
+	PrincipalSID string
+	// WriteRoots receive read+write+execute. The workspace lives here.
+	WriteRoots []WritableRoot
+	// ReadRoots receive read+execute only.
+	ReadRoots []string
+	// DenyRead covers objects a principal could otherwise reach because they are
+	// world-readable; per-user secrets need no entry.
+	DenyRead []string
+	// DenyWrite carries the policy's own deny-write paths. The capability plan
+	// has always emitted these; the principal plan denied write only on
+	// protected metadata and read-only subpaths inside write roots, so a policy
+	// deny sitting anywhere else was simply not enforced once the runner used a
+	// principal token, and a shell child could write where the restricted-token
+	// backend would have blocked it.
+	DenyWrite []string
+}
+
+// buildWindowsPrincipalACLPlan turns a principal's access into ACL entries.
+//
+// Ordering matters at apply time: deny entries are emitted before allows so a
+// carve-out inside a granted root survives, which mirrors how Windows evaluates
+// an explicit DACL (deny ACEs first).
+func buildWindowsPrincipalACLPlan(input windowsPrincipalACLInput) (WindowsACLPlan, error) {
+	if input.PrincipalSID == "" {
+		return WindowsACLPlan{}, errors.New("windows principal ACL plan requires a principal SID")
+	}
+	if len(input.WriteRoots) == 0 && len(input.ReadRoots) == 0 {
+		return WindowsACLPlan{}, errors.New("windows principal ACL plan requires at least one root")
+	}
+
+	entries := make([]WindowsACLEntry, 0, len(input.WriteRoots)*2+len(input.ReadRoots)+len(input.DenyRead))
+
+	// Deny first. A deny ACE inside a write root (protected metadata, git
+	// internals) has to win over the grant that follows it.
+	// Materialized, matching the capability plan. applyWindowsACLPlan skips a
+	// target that does not exist, so without this a deny-read path created after
+	// setup ran never got an ACE at all and the principal could read it. The
+	// deny has to be in place before the object is.
+	for _, path := range normalizeProfilePaths(input.DenyRead) {
+		entries = append(entries, WindowsACLEntry{
+			Action:      WindowsACLDenyRead,
+			Path:        path,
+			Capability:  input.PrincipalSID,
+			Materialize: true,
+		})
+	}
+	for _, path := range normalizeProfilePaths(input.DenyWrite) {
+		entries = append(entries, WindowsACLEntry{
+			Action:      WindowsACLDenyWrite,
+			Path:        path,
+			Capability:  input.PrincipalSID,
+			Materialize: true,
+		})
+	}
+	for _, root := range input.WriteRoots {
+		// Normalized the same way as read and deny paths: a write root may arrive
+		// with "~" or as a relative path, and an ACE has to name the same absolute,
+		// symlink-resolved object the deny entries do or the two disagree.
+		cleaned := normalizeProfilePath(root.Root)
+		if cleaned == "" {
+			return WindowsACLPlan{}, fmt.Errorf("windows principal ACL plan: unusable write root %q", root.Root)
+		}
+		// Materialized, like the metadata and policy denies below and above.
+		//
+		// These are the git control-plane carveouts (.git/config, .git/hooks). On a
+		// workspace where git has not run yet they do not exist at setup time, and
+		// applyWindowsACLPlan skips a target that is absent, so the ACEs were never
+		// written. Once git created those paths the principal still held inherited
+		// write access to the workspace and could install a hook or rewrite
+		// credential.helper. The capability plan gets away without this because its
+		// child runs as the caller; a separate principal account does not.
+		// Which carveouts are files rather than directories comes from the same
+		// spec list the profile built ReadOnlySubpaths from, so a new carveout
+		// cannot be added without its shape coming along.
+		for _, subpath := range normalizeProfilePaths(root.ReadOnlySubpaths) {
+			entries = append(entries, WindowsACLEntry{
+				Action:          WindowsACLDenyWrite,
+				Path:            subpath,
+				Capability:      input.PrincipalSID,
+				Materialize:     true,
+				MaterializeFile: gitMetadataCarveoutIsFile(subpath),
+			})
+		}
+		for _, name := range root.ProtectedMetadataNames {
+			// These are documented as NAMES, and the join below is what makes that
+			// documentation load-bearing rather than descriptive: ".." or a
+			// separator would place this deny ACE, and the directory it
+			// materializes, outside the write root entirely. Today every caller
+			// passes a package constant, so this is unreachable; it is here so that
+			// stays true when a future caller sources these from config.
+			if err := validateWindowsACLComponent(name); err != nil {
+				return WindowsACLPlan{}, fmt.Errorf("windows principal ACL plan: protected metadata name: %w", err)
+			}
+			entries = append(entries, WindowsACLEntry{
+				Action:      WindowsACLDenyWrite,
+				Path:        filepath.Join(cleaned, name),
+				Capability:  input.PrincipalSID,
+				Materialize: true,
+			})
+		}
+		// .git gets DELETE denied on the directory itself, because the carveouts
+		// that protect it are attached to .git/config and .git/hooks as OBJECTS.
+		// Rename .git aside and recreate it and those objects are gone, so the
+		// fresh config and hooks inherit the workspace allow with no deny of their
+		// own, handing back credential.helper and core.hooksPath.
+		//
+		// Not DenyWrite (git writes index, objects and refs), not materialized
+		// (git creates .git, and an empty one breaks git init), and not inherited,
+		// so everything underneath stays writable.
+		entries = append(entries, WindowsACLEntry{
+			Action:     WindowsACLDenyDelete,
+			Path:       windowsRenameProtectedObject(cleaned),
+			Capability: input.PrincipalSID,
+		})
+	}
+
+	// Then the grants the principal cannot work without.
+	for _, root := range input.WriteRoots {
+		cleaned := normalizeProfilePath(root.Root)
+		if cleaned == "" {
+			continue
+		}
+		entries = append(entries, WindowsACLEntry{
+			Action:     WindowsACLAllowWrite,
+			Path:       cleaned,
+			Capability: input.PrincipalSID,
+		})
+	}
+	for _, path := range normalizeProfilePaths(input.ReadRoots) {
+		// NEVER grant at a volume root.
+		//
+		// permissionProfileReadRoots seeds its list with profileRootPath(), which
+		// is the separator alone, because the workspace-write posture is
+		// read-all/write-jail. That is harmless for the capability backend, whose
+		// child runs as the CALLER and therefore reads what the caller could read
+		// anyway. It is not harmless here: a principal is a separate local
+		// account, so this loop turns that synthetic entry into a real,
+		// persistent, inheritable allow-read ACE for that account at the root of
+		// the current drive, reaching every directory that does not block
+		// inheritance.
+		//
+		// Dropping it does not take away the reads the principal needs to run
+		// commands. NetUserAdd with USER_PRIV_USER puts the account in the
+		// built-in Users group (see usrPrivUser in windows_identity_windows.go),
+		// and the machine's own ACLs already grant Users read on the system and
+		// program directories. What the grant added on top was read access to the
+		// places Users are deliberately kept out of, which is the opposite of what
+		// a sandbox is for.
+		//
+		// Note this inherits rather than asserts: nothing here checks that those
+		// default ACLs are actually in place, so a hardened image that strips
+		// Users read would need an explicit bounded read set instead.
+		if isWindowsVolumeRoot(path) {
+			continue
+		}
+		entries = append(entries, WindowsACLEntry{
+			Action:     WindowsACLAllowRead,
+			Path:       path,
+			Capability: input.PrincipalSID,
+		})
+	}
+
+	return WindowsACLPlan{Entries: dedupeWindowsACLEntries(entries)}, nil
+}
+
+// windowsPrincipalRevokePlan returns the entries whose ACEs should be removed
+// when a principal is retired. Revocation is by TRUSTEE rather than by path:
+// every ACE naming this principal is dropped, so cleanup does not depend on
+// remembering which paths were granted, and a grant added by an older version
+// is still removed.
+//
+// This is the removal path the capability-SID model never had, where a synthetic
+// SID left ACEs behind on the user's tree with nothing to match them against.
+func windowsPrincipalRevokePlan(principalSID string, paths []string) (WindowsACLPlan, error) {
+	if principalSID == "" {
+		return WindowsACLPlan{}, errors.New("windows principal revoke plan requires a principal SID")
+	}
+	cleaned := normalizeProfilePaths(paths)
+	entries := make([]WindowsACLEntry, 0, len(cleaned))
+	for _, path := range cleaned {
+		entries = append(entries, WindowsACLEntry{
+			Action:     windowsACLRevoke,
+			Path:       path,
+			Capability: principalSID,
+		})
+	}
+	return WindowsACLPlan{Entries: entries}, nil
+}
+
+// windowsACLRevoke removes every ACE naming the trustee on a path, whatever
+// access it granted or denied.
+const windowsACLRevoke WindowsACLAction = "revoke"
+
+// windowsACLPlanPaths lives in windows_identity_runtime_windows.go, beside its
+// only callers. It was here, in the portable file, which made it dead code on
+// every non-Windows build and failed the static analysis gate.

internal/sandbox/windows_identity_acl_test.go

@@ -0,0 +1,275 @@
+package sandbox
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+const testPrincipalSID = "S-1-5-21-1111111111-2222222222-3333333333-1005"
+
+func testPrincipalInput() windowsPrincipalACLInput {
+	return windowsPrincipalACLInput{
+		PrincipalSID: testPrincipalSID,
+		WriteRoots: []WritableRoot{{
+			Root:                   filepath.FromSlash("/ws/project"),
+			ReadOnlySubpaths:       []string{filepath.FromSlash("/ws/project/.git/config")},
+			ProtectedMetadataNames: []string{".zero", ".agents"},
+		}},
+		ReadRoots: []string{filepath.FromSlash("/usr/lib")},
+		DenyRead:  []string{filepath.FromSlash("/shared/secrets")},
+	}
+}
+
+// Windows evaluates an explicit DACL deny-before-allow, so a carve-out inside a
+// granted root only survives if its deny ACE is written first. If the grant on
+// the workspace landed before the deny on .zero, the protected metadata would be
+// writable from inside the sandbox.
+func TestPrincipalACLPlanEmitsDeniesBeforeAllows(t *testing.T) {
+	plan, err := buildWindowsPrincipalACLPlan(testPrincipalInput())
+	if err != nil {
+		t.Fatalf("build: %v", err)
+	}
+	lastDeny, firstAllow := -1, -1
+	for index, entry := range plan.Entries {
+		switch entry.Action {
+		case WindowsACLDenyRead, WindowsACLDenyWrite:
+			lastDeny = index
+		case WindowsACLAllowWrite, WindowsACLAllowRead:
+			if firstAllow == -1 {
+				firstAllow = index
+			}
+		}
+	}
+	if firstAllow == -1 || lastDeny == -1 {
+		t.Fatalf("plan is missing a deny or an allow: %+v", plan.Entries)
+	}
+	if lastDeny > firstAllow {
+		t.Fatalf("deny at %d comes after allow at %d; carve-outs would be overridden", lastDeny, firstAllow)
+	}
+}
+
+// Every ACE must name the sandbox principal. An entry with any other trustee
+// would change access for a real user.
+func TestPrincipalACLPlanNamesOnlyThePrincipal(t *testing.T) {
+	plan, err := buildWindowsPrincipalACLPlan(testPrincipalInput())
+	if err != nil {
+		t.Fatalf("build: %v", err)
+	}
+	for _, entry := range plan.Entries {
+		if entry.Capability != testPrincipalSID {
+			t.Fatalf("entry %+v names %q, want the principal SID", entry, entry.Capability)
+		}
+	}
+}
+
+// A principal is a separate account with no inherent access, so a write root
+// must be granted read+write and a read root granted read. Without the grant the
+// sandbox cannot open its own workspace.
+func TestPrincipalACLPlanGrantsRoots(t *testing.T) {
+	plan, err := buildWindowsPrincipalACLPlan(testPrincipalInput())
+	if err != nil {
+		t.Fatalf("build: %v", err)
+	}
+	var grantedWrite, grantedRead bool
+	for _, entry := range plan.Entries {
+		if entry.Action == WindowsACLAllowWrite && entry.Path == normalizeProfilePath(filepath.FromSlash("/ws/project")) {
+			grantedWrite = true
+		}
+		if entry.Action == WindowsACLAllowRead && entry.Path == normalizeProfilePath(filepath.FromSlash("/usr/lib")) {
+			grantedRead = true
+		}
+	}
+	if !grantedWrite {
+		t.Fatal("write root was not granted; the sandbox could not write its workspace")
+	}
+	if !grantedRead {
+		t.Fatal("read root was not granted; the sandbox could not read it")
+	}
+}
+
+// Protected metadata is denied write and marked Materialize so the ACE is
+// created even when the directory does not exist yet, closing the window where
+// a sandboxed command creates .zero before the deny lands.
+func TestPrincipalACLPlanProtectsMetadata(t *testing.T) {
+	plan, err := buildWindowsPrincipalACLPlan(testPrincipalInput())
+	if err != nil {
+		t.Fatalf("build: %v", err)
+	}
+	found := map[string]WindowsACLEntry{}
+	for _, entry := range plan.Entries {
+		found[entry.Path] = entry
+	}
+	for _, name := range []string{".zero", ".agents"} {
+		path := filepath.Join(normalizeProfilePath(filepath.FromSlash("/ws/project")), name)
+		entry, ok := found[path]
+		if !ok {
+			t.Fatalf("no entry protecting %s", path)
+		}
+		if entry.Action != WindowsACLDenyWrite {
+			t.Fatalf("%s has action %q, want deny-write", path, entry.Action)
+		}
+		if !entry.Materialize {
+			t.Fatalf("%s must be materialized so the deny exists before the directory does", path)
+		}
+	}
+}
+
+// A missing principal SID must be a hard error: an empty trustee would either
+// fail at apply time or, worse, be interpreted as some other account.
+func TestPrincipalACLPlanRequiresSID(t *testing.T) {
+	input := testPrincipalInput()
+	input.PrincipalSID = ""
+	if _, err := buildWindowsPrincipalACLPlan(input); err == nil {
+		t.Fatal("an empty principal SID must be rejected")
+	}
+}
+
+// A plan with no roots at all is a caller mistake rather than a valid empty
+// grant, since the resulting sandbox could not run anything.
+func TestPrincipalACLPlanRequiresRoots(t *testing.T) {
+	input := windowsPrincipalACLInput{PrincipalSID: testPrincipalSID}
+	if _, err := buildWindowsPrincipalACLPlan(input); err == nil {
+		t.Fatal("a plan with no roots must be rejected")
+	}
+}
+
+// Revocation is keyed to the trustee, so retiring a principal removes every ACE
+// naming it without having to remember what was granted. This is the cleanup
+// path the capability-SID model lacks.
+func TestPrincipalRevokePlanTargetsTrustee(t *testing.T) {
+	paths := []string{filepath.FromSlash("/ws/project"), filepath.FromSlash("/usr/lib")}
+	plan, err := windowsPrincipalRevokePlan(testPrincipalSID, paths)
+	if err != nil {
+		t.Fatalf("revoke plan: %v", err)
+	}
+	if len(plan.Entries) != len(paths) {
+		t.Fatalf("got %d entries, want %d", len(plan.Entries), len(paths))
+	}
+	for _, entry := range plan.Entries {
+		if entry.Action != windowsACLRevoke {
+			t.Fatalf("entry %+v is not a revoke", entry)
+		}
+		if entry.Capability != testPrincipalSID {
+			t.Fatalf("revoke names %q, want the principal", entry.Capability)
+		}
+	}
+}
+
+func TestPrincipalRevokePlanRequiresSID(t *testing.T) {
+	if _, err := windowsPrincipalRevokePlan("", []string{"/ws"}); err == nil {
+		t.Fatal("revoking without a principal SID must be rejected")
+	}
+}
+
+// The action strings end up in a serialized plan consumed by the elevated
+// helper, so they must stay stable and distinct from the existing actions.
+func TestPrincipalACLActionsAreDistinct(t *testing.T) {
+	actions := []WindowsACLAction{
+		WindowsACLAllowWrite, WindowsACLAllowRead,
+		WindowsACLDenyRead, WindowsACLDenyWrite, windowsACLRevoke,
+	}
+	seen := map[WindowsACLAction]bool{}
+	for _, action := range actions {
+		if strings.TrimSpace(string(action)) == "" {
+			t.Fatal("an action string is empty")
+		}
+		if seen[action] {
+			t.Fatalf("duplicate action %q", action)
+		}
+		seen[action] = true
+	}
+}
+
+// ProtectedMetadataNames is joined onto the write root to place a deny ACE and to
+// materialize the directory it names. A value that is not a single component
+// therefore puts both OUTSIDE the workspace: ".." walks up out of it, and a
+// separator reaches through whatever sits in between. Every caller passes a
+// package constant today, so this is the guard that keeps it true if one ever
+// sources these from config.
+func TestPrincipalACLPlanRefusesProtectedNamesThatEscapeTheWriteRoot(t *testing.T) {
+	root := filepath.FromSlash("/ws/project")
+	for _, name := range []string{"..", ".", "", `..\..\Windows\System32`, "nested/child", `nested\child`, "C:", "stream:name"} {
+		t.Run(name, func(t *testing.T) {
+			input := testPrincipalInput()
+			input.WriteRoots = []WritableRoot{{
+				Root:                   root,
+				ProtectedMetadataNames: []string{name},
+			}}
+			plan, err := buildWindowsPrincipalACLPlan(input)
+			if err == nil {
+				t.Fatalf("accepted protected metadata name %q, which would place a deny ACE outside %s:\n%#v", name, root, plan.Entries)
+			}
+			if len(plan.Entries) != 0 {
+				t.Errorf("returned %d entries alongside the error, so a caller ignoring err would still apply them", len(plan.Entries))
+			}
+		})
+	}
+}
+
+// A PRINCIPAL MUST NEVER BE GRANTED READ AT A VOLUME ROOT.
+//
+// permissionProfileReadRoots seeds its list with the bare separator, because
+// the workspace-write posture is read-all with a write jail. That costs nothing
+// for the capability backend, whose child runs as the caller and could read
+// those paths anyway. For a principal it is a real, persistent, inheritable
+// allow-read ACE for a separate local account at the root of the drive.
+//
+// Built from the production profile rather than a synthetic fixture, because
+// the whole point is that the shipped configuration produced it.
+func TestPrincipalACLPlanNeverGrantsReadAtAVolumeRoot(t *testing.T) {
+	workspace := filepath.FromSlash("/ws/project")
+	profile := DefaultPermissionProfile(workspace)
+
+	// If the profile ever stops carrying a volume root, this test proves nothing
+	// and should be retired rather than left passing vacuously.
+	seeded := false
+	for _, root := range profile.FileSystem.ReadRoots {
+		if isWindowsVolumeRoot(normalizeProfilePath(root)) {
+			seeded = true
+			break
+		}
+	}
+	if !seeded {
+		t.Skipf("the production profile no longer contains a volume read root: %v", profile.FileSystem.ReadRoots)
+	}
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: testPrincipalSID,
+		ReadRoots:    profile.FileSystem.ReadRoots,
+		WriteRoots:   profile.FileSystem.WriteRoots,
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+	for _, entry := range plan.Entries {
+		if entry.Action == WindowsACLAllowRead && isWindowsVolumeRoot(entry.Path) {
+			t.Errorf("plan grants the principal read at the volume root %q, which inherits into every directory on the drive", entry.Path)
+		}
+	}
+	// And the workspace itself must still be reachable, or this traded a real
+	// grant for a broken sandbox.
+	wantWorkspace := normalizeProfilePath(workspace)
+	reachable := false
+	for _, entry := range plan.Entries {
+		if entry.Path == wantWorkspace && (entry.Action == WindowsACLAllowRead || entry.Action == WindowsACLAllowWrite) {
+			reachable = true
+			break
+		}
+	}
+	if !reachable {
+		t.Errorf("no grant for the workspace root %q, so the principal could not read its own workspace", wantWorkspace)
+	}
+}
+
+// The ordinary names must keep working, or the guard above is just a break.
+func TestPrincipalACLPlanStillAcceptsTheRealProtectedNames(t *testing.T) {
+	input := testPrincipalInput()
+	input.WriteRoots = []WritableRoot{{
+		Root:                   filepath.FromSlash("/ws/project"),
+		ProtectedMetadataNames: sandboxFullyProtectedMetadataNames,
+	}}
+	if _, err := buildWindowsPrincipalACLPlan(input); err != nil {
+		t.Fatalf("the shipped protected names were rejected: %v", err)
+	}
+}

internal/sandbox/windows_identity_dpapi_windows.go

@@ -0,0 +1,76 @@
+//go:build windows
+
+package sandbox
+
+// DPAPI wrapping for the stored sandbox principal password.
+//
+// The file ACL is the primary control and remains the thing that keeps the
+// sandbox principal itself from reading its own credential. This adds the layer
+// the ACL cannot: an ACL is only meaningful while the filesystem is being asked
+// to enforce it, so a backup, a mounted disk image, or a copy taken by anyone
+// who can bypass the DACL yields the password in the clear. CryptProtectData
+// binds the ciphertext to the invoking user's logon secret, so an offline copy
+// is inert without that user's credentials.
+//
+// The principal name is passed as optional entropy, which makes a blob usable
+// only for the account it was minted for; moving one secret file over another
+// then fails to decrypt instead of silently authenticating the wrong principal.
+
+import (
+	"errors"
+	"fmt"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// protectWindowsSecret encrypts a password to the current user.
+//
+// CRYPTPROTECT_UI_FORBIDDEN matters here: this runs inside a CLI and, on the
+// setup path, potentially without an interactive desktop, so DPAPI must fail
+// rather than try to prompt.
+func protectWindowsSecret(plaintext string, entropy string) ([]byte, error) {
+	if plaintext == "" {
+		return nil, errors.New("windows sandbox secret: refusing to protect an empty password")
+	}
+	in := windows.DataBlob{
+		Size: uint32(len(plaintext)),
+		Data: &[]byte(plaintext)[0],
+	}
+	entropyBytes := []byte(entropy)
+	var entropyBlob *windows.DataBlob
+	if len(entropyBytes) > 0 {
+		entropyBlob = &windows.DataBlob{Size: uint32(len(entropyBytes)), Data: &entropyBytes[0]}
+	}
+	var out windows.DataBlob
+	if err := windows.CryptProtectData(&in, nil, entropyBlob, 0, nil, windows.CRYPTPROTECT_UI_FORBIDDEN, &out); err != nil {
+		return nil, fmt.Errorf("protect sandbox secret: %w", err)
+	}
+	// DPAPI allocates the output with LocalAlloc; copy it out and hand it back.
+	defer windows.LocalFree(windows.Handle(unsafe.Pointer(out.Data)))
+	return append([]byte(nil), unsafe.Slice(out.Data, out.Size)...), nil
+}
+
+// unprotectWindowsSecret reverses protectWindowsSecret. It fails for any user
+// other than the one that wrote the blob, and for a blob minted with a different
+// principal name as entropy.
+func unprotectWindowsSecret(ciphertext []byte, entropy string) (string, error) {
+	if len(ciphertext) == 0 {
+		return "", errWindowsSandboxIdentityUnavailable
+	}
+	in := windows.DataBlob{
+		Size: uint32(len(ciphertext)),
+		Data: &ciphertext[0],
+	}
+	entropyBytes := []byte(entropy)
+	var entropyBlob *windows.DataBlob
+	if len(entropyBytes) > 0 {
+		entropyBlob = &windows.DataBlob{Size: uint32(len(entropyBytes)), Data: &entropyBytes[0]}
+	}
+	var out windows.DataBlob
+	if err := windows.CryptUnprotectData(&in, nil, entropyBlob, 0, nil, windows.CRYPTPROTECT_UI_FORBIDDEN, &out); err != nil {
+		return "", fmt.Errorf("unprotect sandbox secret: %w", err)
+	}
+	defer windows.LocalFree(windows.Handle(unsafe.Pointer(out.Data)))
+	return string(unsafe.Slice(out.Data, out.Size)), nil
+}

internal/sandbox/windows_identity_logon_windows.go

@@ -0,0 +1,270 @@
+//go:build windows
+
+package sandbox
+
+// Minting a token for a sandbox principal.
+//
+// A provisioned account is inert until something can log on as it. Windows
+// gates that behind account rights held in the local security policy, so setup
+// grants the principal exactly one: the right to be logged on as a batch job,
+// which is what a non-interactive service-style logon needs. It is deliberately
+// NOT granted interactive, network or remote-interactive logon, and those three
+// are explicitly DENIED, so the account cannot be used to sign in at the
+// console, over SMB, or through RDP even if its password leaked. The password
+// exists only so LogonUser can mint a token; nobody is meant to type it.
+//
+// Rights are granted at setup (elevated) because LsaAddAccountRights requires
+// administrator privileges. The per-command path only calls LogonUser, which
+// needs no special privilege once the batch right is in place.
+
+import (
+	"errors"
+	"fmt"
+	"runtime"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+const (
+	// Logon type/provider for a non-interactive token. Batch is the closest
+	// match for "run this command as a service-like principal": it produces a
+	// full token without a desktop or network-credential footprint.
+	logon32LogonBatch      = 4
+	logon32ProviderDefault = 0
+
+	// LSA policy access rights needed to add account rights.
+	policyCreateAccount = 0x00000010
+	policyLookupNames   = 0x00000800
+
+	// Account rights. The sandbox principal gets the batch right and is denied
+	// every interactive path.
+	seBatchLogonRight              = "SeBatchLogonRight"
+	seDenyInteractiveLogonRight    = "SeDenyInteractiveLogonRight"
+	seDenyNetworkLogonRight        = "SeDenyNetworkLogonRight"
+	seDenyRemoteInteractiveRight   = "SeDenyRemoteInteractiveLogonRight"
+	seDenyServiceLogonRightName    = "SeDenyServiceLogonRight"
+	windowsIdentityLogonRightsNote = "granted by `zero sandbox setup`"
+)
+
+var (
+	procLogonUserW          = windows.NewLazySystemDLL("advapi32.dll").NewProc("LogonUserW")
+	procLsaOpenPolicy       = windows.NewLazySystemDLL("advapi32.dll").NewProc("LsaOpenPolicy")
+	procLsaClose            = windows.NewLazySystemDLL("advapi32.dll").NewProc("LsaClose")
+	procLsaAddAccountRights = windows.NewLazySystemDLL("advapi32.dll").NewProc("LsaAddAccountRights")
+	// Retiring a principal has to drop its rights as well as its account, or the
+	// LSA policy database keeps an entry keyed to a SID that no longer resolves.
+	procLsaRemoveAccountRights = windows.NewLazySystemDLL("advapi32.dll").NewProc("LsaRemoveAccountRights")
+	procLsaNtStatusToWinErr    = windows.NewLazySystemDLL("advapi32.dll").NewProc("LsaNtStatusToWinError")
+)
+
+// lsaUnicodeString mirrors LSA_UNICODE_STRING. Length and MaximumLength are
+// BYTE counts, not rune counts, which is the usual source of bugs here.
+type lsaUnicodeString struct {
+	Length        uint16
+	MaximumLength uint16
+	Buffer        *uint16
+}
+
+// lsaObjectAttributes mirrors LSA_OBJECT_ATTRIBUTES. Every field except Length
+// is unused for LsaOpenPolicy, but the struct must still be the right size.
+type lsaObjectAttributes struct {
+	Length                   uint32
+	RootDirectory            windows.Handle
+	ObjectName               *lsaUnicodeString
+	Attributes               uint32
+	SecurityDescriptor       unsafe.Pointer
+	SecurityQualityOfService unsafe.Pointer
+}
+
+// newLSAString builds an LSA_UNICODE_STRING over a UTF-16 buffer the caller
+// keeps alive. The returned value borrows that buffer, so the buffer must
+// outlive every use of the string.
+func newLSAString(buffer []uint16) lsaUnicodeString {
+	if len(buffer) == 0 {
+		return lsaUnicodeString{}
+	}
+	// The buffer is NUL-terminated; the LSA length counts bytes WITHOUT the
+	// terminator, while MaximumLength counts bytes WITH it.
+	runes := len(buffer) - 1
+	return lsaUnicodeString{
+		Length:        uint16(runes * 2),
+		MaximumLength: uint16(len(buffer) * 2),
+		Buffer:        &buffer[0],
+	}
+}
+
+// lsaStatusError converts an NTSTATUS from an Lsa* call into a Go error, going
+// through LsaNtStatusToWinError so the message is the familiar Win32 one rather
+// than a raw NTSTATUS.
+func lsaStatusError(call string, status uintptr) error {
+	if status == 0 {
+		return nil
+	}
+	winErr, _, _ := procLsaNtStatusToWinErr.Call(status)
+	return fmt.Errorf("%s: %w", call, windows.Errno(winErr))
+}
+
+// grantWindowsSandboxLogonRights gives the principal the batch logon right and
+// denies every interactive logon path. Idempotent: LsaAddAccountRights silently
+// succeeds when the account already holds a right, so setup can re-run.
+//
+// Requires an elevated caller.
+func grantWindowsSandboxLogonRights(sid *windows.SID) error {
+	if sid == nil {
+		return fmt.Errorf("grant sandbox logon rights: nil SID")
+	}
+	var attributes lsaObjectAttributes
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+	var policy windows.Handle
+	status, _, _ := procLsaOpenPolicy.Call(
+		0, // local system
+		uintptr(unsafe.Pointer(&attributes)),
+		uintptr(policyCreateAccount|policyLookupNames),
+		uintptr(unsafe.Pointer(&policy)),
+	)
+	// LsaOpenPolicy borrows the attributes struct by address, so it has to stay
+	// reachable until the call has returned.
+	runtime.KeepAlive(attributes)
+	if err := lsaStatusError("LsaOpenPolicy", status); err != nil {
+		return err
+	}
+	defer procLsaClose.Call(uintptr(policy))
+
+	rights := []string{
+		seBatchLogonRight,
+		seDenyInteractiveLogonRight,
+		seDenyNetworkLogonRight,
+		seDenyRemoteInteractiveRight,
+		seDenyServiceLogonRightName,
+	}
+	// Each right is added on its own call so one unsupported name on an odd SKU
+	// cannot silently drop the others.
+	for _, right := range rights {
+		buffer, err := windows.UTF16FromString(right)
+		if err != nil {
+			return err
+		}
+		entry := newLSAString(buffer)
+		status, _, _ := procLsaAddAccountRights.Call(
+			uintptr(policy),
+			uintptr(unsafe.Pointer(sid)),
+			uintptr(unsafe.Pointer(&entry)),
+			1,
+		)
+		// Both the descriptor and the buffer it points at are borrowed by the
+		// call. Kept alive before the error check, not after, so the failure path
+		// does not return with them already collectable.
+		runtime.KeepAlive(entry)
+		runtimeKeepAliveUint16(buffer)
+		if err := lsaStatusError("LsaAddAccountRights("+right+")", status); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// revokeWindowsSandboxLogonRights drops every account right held by a principal
+// and removes its entry from the LSA policy database.
+//
+// This is the logon-rights counterpart to revoking ACEs by trustee, and it has
+// the same reason to exist: deleting the account on its own leaves the rights
+// behind, keyed to a SID that no longer resolves, which is the orphaned residue
+// this model is supposed to avoid. It must therefore run BEFORE the account is
+// deleted, while the SID is still resolvable.
+//
+// Removing all rights rather than naming them is deliberate. The principal is
+// being retired, so anything keyed to it should go, including rights a previous
+// version of setup granted and this one no longer knows about.
+//
+// Requires an elevated caller. A principal that holds no rights is not an error:
+// LsaRemoveAccountRights reports ERROR_FILE_NOT_FOUND for an account with no LSA
+// entry, which is the state teardown is trying to reach anyway.
+func revokeWindowsSandboxLogonRights(sid *windows.SID) error {
+	if sid == nil {
+		return fmt.Errorf("revoke sandbox logon rights: nil SID")
+	}
+	var attributes lsaObjectAttributes
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+	var policy windows.Handle
+	status, _, _ := procLsaOpenPolicy.Call(
+		0, // local system
+		uintptr(unsafe.Pointer(&attributes)),
+		uintptr(policyCreateAccount|policyLookupNames),
+		uintptr(unsafe.Pointer(&policy)),
+	)
+	runtime.KeepAlive(attributes)
+	if err := lsaStatusError("LsaOpenPolicy", status); err != nil {
+		return err
+	}
+	defer procLsaClose.Call(uintptr(policy))
+
+	status, _, _ = procLsaRemoveAccountRights.Call(
+		uintptr(policy),
+		uintptr(unsafe.Pointer(sid)),
+		1, // AllRights: drop everything and delete the LSA account object
+		0, // UserRights ignored when AllRights is set
+		0, // CountOfRights likewise
+	)
+	runtime.KeepAlive(sid)
+	if err := lsaStatusError("LsaRemoveAccountRights", status); err != nil {
+		if errors.Is(err, windows.ERROR_FILE_NOT_FOUND) {
+			return nil
+		}
+		return err
+	}
+	return nil
+}
+
+// logonWindowsSandboxPrincipal mints a primary token for the sandbox account.
+// The caller owns the returned token and must Close it.
+//
+// This needs no elevation: the batch logon right granted at setup is what makes
+// it work, which is why the per-command path can run unelevated once setup has
+// been done once.
+func logonWindowsSandboxPrincipal(username string, password string) (windows.Token, error) {
+	user, err := windows.UTF16PtrFromString(username)
+	if err != nil {
+		return 0, err
+	}
+	// "." is the local machine, so the lookup never leaves this host even if the
+	// machine is domain-joined and a same-named domain account exists.
+	domain, err := windows.UTF16PtrFromString(".")
+	if err != nil {
+		return 0, err
+	}
+	secret, err := windows.UTF16PtrFromString(password)
+	if err != nil {
+		return 0, err
+	}
+	var token windows.Token
+	result, _, callErr := procLogonUserW.Call(
+		uintptr(unsafe.Pointer(user)),
+		uintptr(unsafe.Pointer(domain)),
+		uintptr(unsafe.Pointer(secret)),
+		logon32LogonBatch,
+		logon32ProviderDefault,
+		uintptr(unsafe.Pointer(&token)),
+	)
+	// The three strings are borrowed for the duration of the call.
+	runtime.KeepAlive(user)
+	runtime.KeepAlive(domain)
+	runtime.KeepAlive(secret)
+	if result == 0 {
+		if callErr != nil && callErr != windows.ERROR_SUCCESS {
+			return 0, fmt.Errorf("LogonUser(%s): %w", username, callErr)
+		}
+		return 0, fmt.Errorf("LogonUser(%s) failed", username)
+	}
+	return token, nil
+}
+
+// runtimeKeepAliveUint16 keeps a UTF-16 buffer reachable across a syscall that
+// borrows it. Declared rather than inlined so the intent is explicit at each
+// call site; the compiler must not free the slice while LSA holds the pointer.
+func runtimeKeepAliveUint16(buffer []uint16) {
+	if len(buffer) == 0 {
+		return
+	}
+	_ = buffer[0]
+}

internal/sandbox/windows_identity_logon_windows_test.go

@@ -0,0 +1,41 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// revokeWindowsSandboxLogonRights treats "this account holds no rights" as
+// success, because that is the state teardown is trying to reach anyway. That
+// relies on the NTSTATUS for it surviving the trip through LsaNtStatusToWinError
+// as an error errors.Is can still match, which is exactly the kind of Windows
+// errno assumption that quietly turns out to be false. Assert it rather than
+// trust it.
+//
+// Needs no privilege: LsaNtStatusToWinError is a pure status translation, so
+// this runs everywhere rather than joining the gated set.
+func TestLsaStatusErrorMapsObjectNameNotFound(t *testing.T) {
+	// STATUS_OBJECT_NAME_NOT_FOUND, what LsaRemoveAccountRights reports for an
+	// account that has no LSA entry.
+	const statusObjectNameNotFound = 0xC0000034
+
+	err := lsaStatusError("LsaRemoveAccountRights", statusObjectNameNotFound)
+	if err == nil {
+		t.Fatal("a nonzero NTSTATUS produced no error")
+	}
+	if !errors.Is(err, windows.ERROR_FILE_NOT_FOUND) {
+		t.Fatalf("error = %v, want one errors.Is matches against ERROR_FILE_NOT_FOUND; "+
+			"without that, revoking a principal that simply holds no rights fails teardown", err)
+	}
+
+	// The tolerance must be specific. If any failure matched it, revoke would
+	// swallow a real one and teardown would report success having done nothing.
+	const statusAccessDenied = 0xC0000022
+	if other := lsaStatusError("LsaRemoveAccountRights", statusAccessDenied); errors.Is(other, windows.ERROR_FILE_NOT_FOUND) {
+		t.Fatalf("access denied matched the not-found tolerance: %v", other)
+	}
+}

internal/sandbox/windows_identity_policy_windows_test.go

@@ -0,0 +1,409 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// Policy deny-write has to reach the principal plan.
+//
+// The capability plan has always emitted these. The principal plan denied write
+// only on protected metadata and read-only subpaths inside write roots, so once
+// the runner used a principal token a policy deny sitting anywhere else was not
+// enforced at the OS layer at all, and a shell child could write where the
+// restricted-token backend would have stopped it.
+func TestPrincipalACLPlanCarriesPolicyDenyWrite(t *testing.T) {
+	root := filepath.Join(t.TempDir(), "workspace")
+	denied := filepath.Join(root, "protected", "keep-out")
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: "S-1-5-21-0-0-0-1001",
+		WriteRoots:   []WritableRoot{{Root: root}},
+		DenyWrite:    []string{denied},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+
+	entry, ok := findPrincipalACLEntry(plan, WindowsACLDenyWrite, denied)
+	if !ok {
+		t.Fatalf("no deny-write ACE for the policy path; plan = %+v", plan.Entries)
+	}
+	// Materialized for the same reason the capability plan does it: the applier
+	// skips targets that do not exist, so a deny on a path created after setup
+	// would never be written.
+	if !entry.Materialize {
+		t.Error("policy deny-write ACE is not materialized, so it is skipped when the path does not exist yet")
+	}
+}
+
+// Deny-read has to be materialized too, which it was not.
+func TestPrincipalACLPlanMaterializesDenyRead(t *testing.T) {
+	root := filepath.Join(t.TempDir(), "workspace")
+	secret := filepath.Join(t.TempDir(), "elsewhere", "creds")
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: "S-1-5-21-0-0-0-1001",
+		WriteRoots:   []WritableRoot{{Root: root}},
+		DenyRead:     []string{secret},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+	entry, ok := findPrincipalACLEntry(plan, WindowsACLDenyRead, secret)
+	if !ok {
+		t.Fatalf("no deny-read ACE emitted; plan = %+v", plan.Entries)
+	}
+	if !entry.Materialize {
+		t.Fatal("deny-read ACE is not materialized, so a path created after setup never gets one")
+	}
+}
+
+// Deny entries must still precede the grants they carve out of, which is what
+// makes them win under Windows DACL evaluation. Adding deny-write to the plan is
+// only safe if it did not disturb that ordering.
+func TestPrincipalACLPlanKeepsDeniesBeforeGrants(t *testing.T) {
+	root := filepath.Join(t.TempDir(), "workspace")
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: "S-1-5-21-0-0-0-1001",
+		WriteRoots:   []WritableRoot{{Root: root}},
+		DenyWrite:    []string{filepath.Join(root, "nope")},
+		DenyRead:     []string{filepath.Join(root, "secret")},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+	firstGrant := -1
+	for i, entry := range plan.Entries {
+		switch entry.Action {
+		case WindowsACLAllowWrite, WindowsACLAllowRead:
+			if firstGrant == -1 {
+				firstGrant = i
+			}
+		case WindowsACLDenyRead, WindowsACLDenyWrite:
+			if firstGrant != -1 {
+				t.Fatalf("deny entry at %d follows a grant at %d; the grant would win", i, firstGrant)
+			}
+		}
+	}
+}
+
+func findPrincipalACLEntry(plan WindowsACLPlan, action WindowsACLAction, path string) (WindowsACLEntry, bool) {
+	want := normalizeProfilePath(path)
+	for _, entry := range plan.Entries {
+		if entry.Action == action && entry.Path == want {
+			return entry, true
+		}
+	}
+	return WindowsACLEntry{}, false
+}
+
+// An adopted account must not have its password rotated during provisioning.
+//
+// Rotating there left every later step running against an account whose password
+// had already been replaced with nothing on disk holding it. Any failure in
+// between stranded a working principal: the rollback correctly declined to
+// delete an account it had not created, so what remained was a live account
+// authenticated by a password no longer stored anywhere, and the command path
+// read the absent secret as "not provisioned" and quietly fell back to the
+// weaker backend.
+func TestProvisionWindowsSandboxIdentityDefersPasswordRotation(t *testing.T) {
+	stubWindowsProvisioning(t, true, nil, nil, nil)
+
+	rotated := false
+	previous := resetWindowsSandboxUserPasswordFn
+	t.Cleanup(func() { resetWindowsSandboxUserPasswordFn = previous })
+	resetWindowsSandboxUserPasswordFn = func(string, string) error {
+		rotated = true
+		return nil
+	}
+
+	if _, _, _, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline); err != nil {
+		t.Fatalf("provisioning an adopted account: %v", err)
+	}
+	if rotated {
+		t.Fatal("provisioning rotated the password; the window this closes lasts until the secret is committed")
+	}
+}
+
+// The ownership comment carries the full workspace key, so two workspaces whose
+// digests collide in the 11 characters the account name can hold are refused
+// rather than silently sharing one account, one secret and one ACL identity.
+func TestWindowsSandboxUserCommentDistinguishesWorkspaces(t *testing.T) {
+	first := windowsSandboxUserCommentFor("aaaaaaaaaaaabbbbbbbb")
+	second := windowsSandboxUserCommentFor("aaaaaaaaaaaacccccccc")
+	if first == second {
+		t.Fatal("two workspaces produced the same ownership comment, so a name collision would be adopted")
+	}
+	if !strings.HasPrefix(first, windowsSandboxUserComment) {
+		t.Fatalf("comment %q lost the marker prefix that identifies it as ours", first)
+	}
+	// The names DO collide, which is the whole reason the comment has to carry
+	// the key. If this stops being true the test is no longer covering anything.
+	if windowsSandboxUserName("aaaaaaaaaaaabbbbbbbb", windowsSandboxRoleOffline) != windowsSandboxUserName("aaaaaaaaaaaacccccccc", windowsSandboxRoleOffline) {
+		t.Skip("account names no longer collide for these keys; revisit what this test is for")
+	}
+}
+
+// Rollback must not strip an adopted principal's logon rights.
+//
+// revokeWindowsSandboxLogonRights passes AllRights, which drops every right the
+// account holds and deletes its LSA object. On an account this run created that
+// is a rollback; on one it adopted it destroys the SeBatchLogonRight and
+// deny-logon rights an earlier setup established, which is the working
+// principal this path exists to preserve.
+func TestSetupRollbackRevokesRightsOnlyForCreatedPrincipals(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		existed     bool
+		wantRevoked bool
+	}{
+		"adopted principal": {existed: true, wantRevoked: false},
+		"created principal": {existed: false, wantRevoked: true},
+	} {
+		t.Run(name, func(t *testing.T) {
+			stubWindowsProvisioning(t, testCase.existed, nil, nil, nil)
+
+			revoked := false
+			prevGrant, prevRevoke := grantWindowsSandboxLogonRightsFn, revokeWindowsSandboxLogonRightsFn
+			t.Cleanup(func() {
+				grantWindowsSandboxLogonRightsFn, revokeWindowsSandboxLogonRightsFn = prevGrant, prevRevoke
+			})
+			// Fail the grant so the undo path runs with rights already attempted,
+			// which is the state that used to revoke unconditionally.
+			grantWindowsSandboxLogonRightsFn = func(*windows.SID) error {
+				return errors.New("LSA grant refused by policy")
+			}
+			revokeWindowsSandboxLogonRightsFn = func(*windows.SID) error {
+				revoked = true
+				return nil
+			}
+
+			config := WindowsSandboxCommandConfig{
+				SandboxHome:    t.TempDir(),
+				WorkspaceRoots: []string{`C:\ws`},
+			}
+			if _, _, err := provisionWindowsSandboxPrincipalForSetup(config, windowsSandboxRoleOffline); err == nil {
+				t.Fatal("provisioning reported success despite an injected grant failure")
+			}
+			if revoked != testCase.wantRevoked {
+				if testCase.wantRevoked {
+					t.Fatal("rights were not revoked for an account this run created, leaving LSA entries keyed to a SID about to be deleted")
+				}
+				t.Fatal("rights were revoked for an adopted account; AllRights drops its pre-existing rights and deletes the LSA object")
+			}
+		})
+	}
+}
+
+// A secret the current user cannot read is unavailability, not breakage.
+//
+// The secret's DACL names whoever ran setup. An operator who elevated with a
+// separate administrative account, via runas or an over-the-shoulder UAC
+// prompt, leaves a secret their ordinary account cannot open. Treating that as a
+// hard error made every sandboxed command fail on a machine that was merely set
+// up by a different admin; it belongs in the same fail-soft path as a missing
+// secret, so the warning fires and the restricted token takes over.
+func TestReadWindowsSandboxSecretTreatsPermissionDeniedAsUnavailable(t *testing.T) {
+	previous := readWindowsSandboxSecretFile
+	t.Cleanup(func() { readWindowsSandboxSecretFile = previous })
+	readWindowsSandboxSecretFile = func(string) ([]byte, error) {
+		return nil, &os.PathError{Op: "open", Path: "secret", Err: windows.ERROR_ACCESS_DENIED}
+	}
+
+	if _, err := readWindowsSandboxSecret(`C:\anything.secret`); err != errWindowsSandboxIdentityUnavailable {
+		t.Fatalf("permission-denied read returned %v, want errWindowsSandboxIdentityUnavailable so the command falls back", err)
+	}
+}
+
+// A workspace must not bind to another workspace's principal.
+//
+// The account name carries only 11 characters of the workspace digest, so two
+// workspaces can derive the same name. Provisioning refuses that case by
+// checking the full key in the account comment, but the command path resolved
+// the name straight to a SID. The workspace that lost the race would have failed
+// setup and then quietly run as the other one's principal, using its secret and
+// its ACL identity.
+func TestLookupWindowsSandboxIdentityRejectsForeignWorkspace(t *testing.T) {
+	previous := windowsSandboxUserIsManagedFn
+	t.Cleanup(func() { windowsSandboxUserIsManagedFn = previous })
+
+	prevSID := resolveWindowsSandboxSIDFn
+	t.Cleanup(func() { resolveWindowsSandboxSIDFn = prevSID })
+	// The account resolves; whether it BELONGS to this workspace is the question.
+	resolveWindowsSandboxSIDFn = func(string) (*windows.SID, error) {
+		return windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	}
+
+	var askedKey string
+	windowsSandboxUserIsManagedFn = func(_ string, workspaceKey string) (bool, error) {
+		askedKey = workspaceKey
+		return false, nil
+	}
+	_, err := lookupWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline)
+	if err == nil {
+		t.Fatal("lookup accepted an account belonging to another workspace")
+	}
+	if errors.Is(err, errWindowsSandboxIdentityUnavailable) {
+		t.Fatal("a foreign account must not read as unprovisioned; that would silently fall back instead of reporting the conflict")
+	}
+	if askedKey != "" && askedKey != "workspacekey" {
+		t.Fatalf("ownership was checked against %q, want the caller's workspace key", askedKey)
+	}
+}
+
+// An account that does not exist must stay the unavailable sentinel rather than
+// becoming a collision error, since that is the ordinary not-set-up state.
+func TestLookupWindowsSandboxIdentityAbsentAccountIsUnavailable(t *testing.T) {
+	previous := windowsSandboxUserIsManagedFn
+	t.Cleanup(func() { windowsSandboxUserIsManagedFn = previous })
+	windowsSandboxUserIsManagedFn = func(string, string) (bool, error) {
+		t.Fatal("ownership must not be consulted for an account that does not resolve")
+		return false, nil
+	}
+	if _, err := lookupWindowsSandboxIdentity("nosuchworkspacekey", windowsSandboxRoleOffline); !errors.Is(err, errWindowsSandboxIdentityUnavailable) {
+		t.Fatalf("absent account returned %v, want errWindowsSandboxIdentityUnavailable", err)
+	}
+}
+
+// The git control-plane carveouts must be materialized.
+//
+// gitMetadataWriteCarveouts supplies .git/config and .git/hooks as
+// ReadOnlySubpaths of the workspace. On a workspace where git has not run yet
+// they do not exist when setup applies the plan, and applyWindowsACLPlan skips
+// an absent target, so the deny ACEs were never written. Once git created those
+// paths the principal still held inherited write access and could install a
+// hook or rewrite credential.helper.
+func TestPrincipalACLPlanMaterializesReadOnlySubpaths(t *testing.T) {
+	root := filepath.Join(t.TempDir(), "workspace")
+	carveout := filepath.Join(root, ".git", "config")
+
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: "S-1-5-21-0-0-0-1001",
+		WriteRoots:   []WritableRoot{{Root: root, ReadOnlySubpaths: []string{carveout}}},
+	})
+	if err != nil {
+		t.Fatalf("buildWindowsPrincipalACLPlan: %v", err)
+	}
+	entry, ok := findPrincipalACLEntry(plan, WindowsACLDenyWrite, carveout)
+	if !ok {
+		t.Fatalf("no deny-write ACE for the git carveout; plan = %+v", plan.Entries)
+	}
+	if !entry.Materialize {
+		t.Fatal("git carveout deny-write is not materialized, so it is skipped on a workspace where .git does not exist yet")
+	}
+}
+
+// Setup must grant the principal the same runtime root that commands write to.
+//
+// permissionProfileWithRuntime appends this root to WriteRoots on every command
+// and redirects HOME, GOCACHE and npm_config_cache into it, but it lives under
+// the user cache rather than the workspace, so the profile setup sees never
+// contains it. A principal is a separate account with no rights there, so
+// without a grant every npm install or go build fails on a cache write.
+//
+// The assertion that matters is that the two derivations agree. If they drift,
+// setup writes the ACE on one directory while commands use another, and the
+// symptom is a bare ACCESS_DENIED with nothing pointing at the sandbox.
+func TestSetupGrantsTheRuntimeRootCommandsActuallyUse(t *testing.T) {
+	workspace := filepath.Join(t.TempDir(), "ws")
+	if err := os.MkdirAll(workspace, 0o700); err != nil {
+		t.Fatal(err)
+	}
+
+	granted, err := setupWindowsSandboxRuntimeRoot(WindowsSandboxCommandConfig{
+		WorkspaceRoots: []string{workspace},
+	})
+	if err != nil {
+		t.Fatalf("setupWindowsSandboxRuntimeRoot: %v", err)
+	}
+	if len(granted) == 0 {
+		t.Fatal("no runtime root resolved for a configured workspace")
+	}
+	// EVERY granted candidate has to exist, not just the one this process would
+	// pick: applyWindowsACLPlan fails the whole run on a target that is absent.
+	for _, root := range granted {
+		if info, err := os.Stat(root); err != nil || !info.IsDir() {
+			t.Fatalf("runtime root %q was not created; applyWindowsACLPlan skips absent targets so the grant would no-op (stat err %v)", root, err)
+		}
+	}
+
+	// What a command would actually use.
+	runtimeState, release, err := prepareSandboxRuntime(workspace)
+	if err != nil {
+		t.Fatalf("prepareSandboxRuntime: %v", err)
+	}
+	if release != nil {
+		defer release()
+	}
+	// The command's choice has to be one setup provisioned. Setup grants the whole
+	// candidate set precisely because the choice is made per process, so the
+	// contract is membership rather than equality.
+	if !grantedRuntimeRootsCover(granted, runtimeState.Root) {
+		t.Fatalf("setup granted %q but commands write to %q", granted, runtimeState.Root)
+	}
+}
+
+// grantedRuntimeRootsCover reports whether the root a command selected is one of
+// the roots setup granted an ACE on.
+func grantedRuntimeRootsCover(granted []string, selected string) bool {
+	for _, root := range granted {
+		if filepath.Clean(root) == filepath.Clean(selected) {
+			return true
+		}
+	}
+	return false
+}
+
+// No workspace root means nothing to grant, which is not an error.
+func TestSetupRuntimeRootWithoutWorkspaceIsNotAnError(t *testing.T) {
+	granted, err := setupWindowsSandboxRuntimeRoot(WindowsSandboxCommandConfig{})
+	if err != nil {
+		t.Fatalf("no workspace root should not error: %v", err)
+	}
+	if len(granted) != 0 {
+		t.Fatalf("granted %q with no workspace configured", granted)
+	}
+}
+
+// An account in a privileged group must not be adopted, even when name and
+// ownership comment say it is ours.
+//
+// Adoption resets the password and hands the account to the sandbox. If that
+// account is also in Administrators, the sandbox gains the rights the sandbox
+// exists to withhold: rewriting the ACLs confining it, reading the secret locked
+// to the invoking user, and stopping Zero.
+func TestProvisionWindowsSandboxIdentityRefusesPrivilegedAccount(t *testing.T) {
+	stubWindowsProvisioning(t, true, nil, nil, nil)
+
+	previous := windowsSandboxUserIsPrivilegedFn
+	t.Cleanup(func() { windowsSandboxUserIsPrivilegedFn = previous })
+	windowsSandboxUserIsPrivilegedFn = func(string) (bool, error) { return true, nil }
+
+	_, _, created, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline)
+	if !errors.Is(err, errWindowsSandboxPrivilegedAccount) {
+		t.Fatalf("provisioning adopted a privileged account, err = %v", err)
+	}
+	if created {
+		t.Fatal("created must stay false for an account this run refused to adopt")
+	}
+}
+
+// The ordinary adopted account is unaffected.
+func TestProvisionWindowsSandboxIdentityAdoptsUnprivilegedAccount(t *testing.T) {
+	stubWindowsProvisioning(t, true, nil, nil, nil)
+
+	previous := windowsSandboxUserIsPrivilegedFn
+	t.Cleanup(func() { windowsSandboxUserIsPrivilegedFn = previous })
+	windowsSandboxUserIsPrivilegedFn = func(string) (bool, error) { return false, nil }
+
+	if _, _, _, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline); err != nil {
+		t.Fatalf("an unprivileged managed account must still be adopted: %v", err)
+	}
+}

internal/sandbox/windows_identity_privilege_recheck_windows_test.go

@@ -0,0 +1,92 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// Group membership is not frozen at setup. An account provisioned clean can be
+// added to Administrators afterwards — by an operator, or by an attacker who
+// already has that access and wants the sandbox to hand it back. Provisioning's
+// refusal cannot see that; only the path that mints the token can.
+func TestLookupPrincipalForCommandRefusesAnAccountThatBecamePrivileged(t *testing.T) {
+	sid, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	if err != nil {
+		t.Fatalf("CreateWellKnownSid: %v", err)
+	}
+	restoreLookupSeams(t, sid)
+
+	privilegedCalls := 0
+	windowsSandboxUserIsPrivilegedFn = func(string) (bool, error) {
+		privilegedCalls++
+		return true, nil
+	}
+
+	_, err = lookupWindowsSandboxPrincipalForCommand("workspace-key", windowsSandboxRoleOnline)
+	if !errors.Is(err, errWindowsSandboxPrivilegedAccount) {
+		t.Fatalf("err = %v, want errWindowsSandboxPrivilegedAccount", err)
+	}
+	if privilegedCalls != 1 {
+		t.Errorf("privileged check ran %d times, want exactly 1", privilegedCalls)
+	}
+	// It must be a hard refusal, not the unavailable sentinel — that one is the
+	// quiet "not provisioned" fallback and would silently drop the sandbox back
+	// to the restricted token instead of telling the operator.
+	if errors.Is(err, errWindowsSandboxIdentityUnavailable) {
+		t.Error("privileged refusal must not read as the not-provisioned fallback")
+	}
+}
+
+func TestLookupPrincipalForCommandAcceptsAnUnprivilegedAccount(t *testing.T) {
+	sid, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	if err != nil {
+		t.Fatalf("CreateWellKnownSid: %v", err)
+	}
+	restoreLookupSeams(t, sid)
+	windowsSandboxUserIsPrivilegedFn = func(string) (bool, error) { return false, nil }
+
+	identity, err := lookupWindowsSandboxPrincipalForCommand("workspace-key", windowsSandboxRoleOnline)
+	if err != nil {
+		t.Fatalf("lookupWindowsSandboxPrincipalForCommand: %v", err)
+	}
+	if identity.Username == "" {
+		t.Error("expected the resolved principal")
+	}
+}
+
+// Teardown must stay able to clean up an account that has become privileged.
+// If the refusal lived inside lookupWindowsSandboxIdentity, the account this
+// guard exists to catch would become undeletable by Zero.
+func TestLookupIdentityItselfDoesNotConsultPrivilege(t *testing.T) {
+	sid, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	if err != nil {
+		t.Fatalf("CreateWellKnownSid: %v", err)
+	}
+	restoreLookupSeams(t, sid)
+	windowsSandboxUserIsPrivilegedFn = func(string) (bool, error) {
+		t.Error("teardown's lookup must not be gated on privilege")
+		return true, nil
+	}
+
+	if _, err := lookupWindowsSandboxIdentity("workspace-key", windowsSandboxRoleOnline); err != nil {
+		t.Fatalf("lookupWindowsSandboxIdentity: %v", err)
+	}
+}
+
+func restoreLookupSeams(t *testing.T, sid *windows.SID) {
+	t.Helper()
+	prevResolve := resolveWindowsSandboxSIDFn
+	prevManaged := windowsSandboxUserIsManagedFn
+	prevPrivileged := windowsSandboxUserIsPrivilegedFn
+	t.Cleanup(func() {
+		resolveWindowsSandboxSIDFn = prevResolve
+		windowsSandboxUserIsManagedFn = prevManaged
+		windowsSandboxUserIsPrivilegedFn = prevPrivileged
+	})
+	resolveWindowsSandboxSIDFn = func(string) (*windows.SID, error) { return sid, nil }
+	windowsSandboxUserIsManagedFn = func(string, string) (bool, error) { return true, nil }
+}

internal/sandbox/windows_identity_rollback_windows_test.go

@@ -0,0 +1,187 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// stubWindowsProvisioning replaces the four provisioning syscalls so the
+// function can run on an ordinary machine. Every one of them needs an elevated
+// caller and a real local account, so without this the test would stop at the
+// first call and never reach the behaviour it is named for.
+func stubWindowsProvisioning(t *testing.T, existed bool, groupErr error, offlineErr error, sidErr error) {
+	t.Helper()
+	prevGroup, prevUser := ensureWindowsSandboxGroupFn, ensureWindowsSandboxUserFn
+	prevOfflineGroup := ensureWindowsSandboxOfflineGroupFn
+	prevAdd, prevSID := addWindowsSandboxUserToGroupFn, resolveWindowsSandboxSIDFn
+	prevManaged := windowsSandboxUserIsManagedFn
+	prevOffline := addWindowsSandboxUserToOfflineGroupFn
+	prevReset := resetWindowsSandboxUserPasswordFn
+	t.Cleanup(func() {
+		ensureWindowsSandboxGroupFn, ensureWindowsSandboxUserFn = prevGroup, prevUser
+		ensureWindowsSandboxOfflineGroupFn = prevOfflineGroup
+		addWindowsSandboxUserToGroupFn, resolveWindowsSandboxSIDFn = prevAdd, prevSID
+		windowsSandboxUserIsManagedFn = prevManaged
+		addWindowsSandboxUserToOfflineGroupFn = prevOffline
+		resetWindowsSandboxUserPasswordFn = prevReset
+	})
+
+	ensureWindowsSandboxGroupFn = func() error { return nil }
+	ensureWindowsSandboxOfflineGroupFn = func() error { return nil }
+	// Adopted accounts are ours in these tests; the ownership check is a real
+	// syscall and would otherwise refuse before the code under test runs.
+	windowsSandboxUserIsManagedFn = func(string, string) (bool, error) { return true, nil }
+	// Stubbed even though provisioning no longer rotates: rotation moved to the
+	// caller, so nothing under test reaches this today. Left in so that if it
+	// ever moves back, these tests fail rather than resetting the password of a
+	// real managed account that happens to exist on the machine running them.
+	resetWindowsSandboxUserPasswordFn = func(string, string) error {
+		t.Fatal("provisioning must not rotate an account password; rotation belongs to the caller, immediately before the secret is written")
+		return nil
+	}
+	ensureWindowsSandboxUserFn = func(string, string, string) (bool, error) { return existed, nil }
+	addWindowsSandboxUserToGroupFn = func(string) error { return groupErr }
+	addWindowsSandboxUserToOfflineGroupFn = func(string) error { return offlineErr }
+	resolveWindowsSandboxSIDFn = func(username string) (*windows.SID, error) {
+		if sidErr != nil {
+			return nil, sidErr
+		}
+		return windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	}
+}
+
+// A failure after the account has been created must still hand back the name.
+//
+// The caller's rollback deletes by identity.Username, so returning a zero
+// identity alongside created=true asked it to delete "" and quietly left the
+// account this run had just made. Group attachment is the case that matters
+// most: it is the enforcement boundary and it can fail under local policy.
+func TestProvisionWindowsSandboxIdentityReturnsNameForRollback(t *testing.T) {
+	groupFailure := errors.New("group attachment refused by policy")
+	offlineFailure := errors.New("offline group attachment refused by policy")
+	sidFailure := errors.New("sid lookup failed")
+
+	for name, testCase := range map[string]struct {
+		groupErr   error
+		offlineErr error
+		sidErr     error
+	}{
+		"group attachment fails":         {groupErr: groupFailure},
+		"offline group attachment fails": {offlineErr: offlineFailure},
+		"sid resolution fails":           {sidErr: sidFailure},
+	} {
+		t.Run(name, func(t *testing.T) {
+			stubWindowsProvisioning(t, false, testCase.groupErr, testCase.offlineErr, testCase.sidErr)
+
+			identity, _, created, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline)
+			if err == nil {
+				t.Fatal("provisioning reported success despite an injected failure")
+			}
+			if !created {
+				t.Fatal("created = false, so the rollback would skip an account this run made")
+			}
+			// The whole point: without a name there is nothing to delete.
+			if identity.Username == "" {
+				t.Fatal("identity carries no username, so the rollback deletes \"\" and strands the account")
+			}
+			if want := windowsSandboxUserName("workspacekey", windowsSandboxRoleOffline); identity.Username != want {
+				t.Fatalf("username = %q, want %q", identity.Username, want)
+			}
+		})
+	}
+}
+
+// An account that already existed must not be deleted because a later step
+// failed. created=false is what stops the rollback turning a partial failure
+// into the loss of a working principal from an earlier setup.
+func TestProvisionWindowsSandboxIdentityDoesNotClaimPreexistingAccount(t *testing.T) {
+	stubWindowsProvisioning(t, true, errors.New("group attachment refused"), nil, nil)
+
+	_, _, created, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline)
+	if err == nil {
+		t.Fatal("provisioning reported success despite an injected failure")
+	}
+	// created is the whole assertion. It is what stops the rollback deleting an
+	// account it did not make, and it must stay false however provisioning
+	// fails. The identity is deliberately not checked: an adopted account exits
+	// early at the ownership check, which is a real syscall and not stubbed
+	// here, so asserting on the name would be testing the stub rather than the
+	// contract.
+	if created {
+		t.Fatal("created = true for an account this run did not create; rollback would delete a working principal")
+	}
+}
+
+// The write grant has to include delete.
+//
+// FILE_GENERIC_WRITE covers creating and modifying but not removing or
+// renaming, and a rename needs delete on the source. The old same-user token hid
+// this because the caller already held inherited rights on its own tree; a
+// principal is a separate account with none, so without these it can write files
+// it can never remove, which fails ordinary editing and most git operations
+// rather than an edge case.
+func TestWindowsACLAllowWriteGrantsDelete(t *testing.T) {
+	mode, mask, err := windowsACLAccess(WindowsACLAllowWrite)
+	if err != nil {
+		t.Fatalf("windowsACLAccess: %v", err)
+	}
+	if mode != windows.GRANT_ACCESS {
+		t.Fatalf("mode = %v, want GRANT_ACCESS", mode)
+	}
+	// Atomic bits only. FILE_GENERIC_READ and FILE_GENERIC_WRITE both carry
+	// READ_CONTROL and SYNCHRONIZE, so testing a composite constant with & is
+	// satisfied by any grant at all and proves nothing.
+	for label, bit := range map[string]windows.ACCESS_MASK{
+		"DELETE":          windows.DELETE,
+		"FILE_WRITE_DATA": windows.FILE_WRITE_DATA,
+	} {
+		if mask&bit == 0 {
+			t.Errorf("write grant is missing %s", label)
+		}
+	}
+	// Granting these would let the principal rewrite the very restrictions
+	// placed on it. They are in the deny mask for that reason and must not
+	// appear here.
+	//
+	// FILE_DELETE_CHILD belongs in this set and was originally in the one
+	// above, on the reasoning that the grant should mirror the deny mask. On a
+	// parent it authorises deleting a child whatever the child's own DACL says,
+	// so on a write root it hands back the write-denied carve-outs underneath:
+	// delete .git/config, recreate it, and the replacement inherits the grant
+	// with no deny of its own. Mirroring the deny mask is the wrong instinct —
+	// denying a capability is not a reason to grant it.
+	for label, bit := range map[string]windows.ACCESS_MASK{
+		"WRITE_DAC":         windows.WRITE_DAC,
+		"WRITE_OWNER":       windows.WRITE_OWNER,
+		"FILE_DELETE_CHILD": windowsFileDeleteChild,
+	} {
+		if mask&bit != 0 {
+			t.Errorf("write grant unexpectedly includes %s", label)
+		}
+	}
+}
+
+// The read grant must stay read-only. Widening the write mask above is only
+// safe if this one did not move with it.
+func TestWindowsACLAllowReadGrantsNoDelete(t *testing.T) {
+	_, mask, err := windowsACLAccess(WindowsACLAllowRead)
+	if err != nil {
+		t.Fatalf("windowsACLAccess: %v", err)
+	}
+	// Atomic bits, for the same reason as above: the read and write composites
+	// overlap on the standard rights, so a composite check here would report a
+	// failure that is not real.
+	for label, bit := range map[string]windows.ACCESS_MASK{
+		"DELETE":            windows.DELETE,
+		"FILE_DELETE_CHILD": windowsFileDeleteChild,
+		"FILE_WRITE_DATA":   windows.FILE_WRITE_DATA,
+	} {
+		if mask&bit != 0 {
+			t.Errorf("read grant unexpectedly includes %s", label)
+		}
+	}
+}

internal/sandbox/windows_identity_runtime_windows_test.go

@@ -0,0 +1,292 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"strings"
+	"sync"
+	"testing"
+)
+
+// An opted-in command that ends up on the restricted token anyway must say so.
+// Both cases below are correct fallbacks, not errors — but silence leaves the
+// operator believing an account boundary is isolating them when it is not, which
+// is the same failure the setup-protocol opt-in check exists to prevent, reached
+// from the other side. The deny case matters most: deny is the DEFAULT network
+// mode, so a fully provisioned, fully agreeing setup still never uses the
+// principal for an ordinary command.
+func TestWindowsSandboxPrincipalFallbackIsAnnounced(t *testing.T) {
+	testCases := []struct {
+		name   string
+		mode   NetworkMode
+		reason string
+	}{
+		// The network-deny case is deliberately absent. It used to warn here, but
+		// this runner is re-exec'd per command, so the sync.Once guarding the notice
+		// is once per COMMAND — and deny is the default mode, so the warning landed
+		// on nearly every tool call. That fact belongs to `zero doctor` now, which is
+		// read once. The deny-mode BEHAVIOUR is still pinned, by
+		// TestPrincipalBackendDefersToRestrictedTokenWhenNetworkDenied below.
+		{name: "no principal provisioned on this machine", mode: NetworkAllow, reason: "no sandbox principal is provisioned"},
+	}
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			var warned []string
+			originalWarn := warnWindowsSandboxPrincipalNotUsed
+			warnWindowsSandboxPrincipalNotUsed = func(reason string) { warned = append(warned, reason) }
+			windowsSandboxPrincipalNotUsedWarnOnce = sync.Once{}
+			t.Cleanup(func() {
+				warnWindowsSandboxPrincipalNotUsed = originalWarn
+				windowsSandboxPrincipalNotUsedWarnOnce = sync.Once{}
+			})
+
+			config := WindowsSandboxCommandConfig{
+				SandboxHome:    t.TempDir(),
+				CommandCWD:     `C:\workspace`,
+				WorkspaceRoots: []string{`C:\workspace`},
+				PermissionProfile: PermissionProfile{
+					FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: `C:\workspace`}}},
+					Network:    NetworkPolicy{Mode: testCase.mode},
+				},
+				Env:          map[string]string{windowsSandboxIdentityEnv: "1"},
+				SandboxLevel: WindowsSandboxLevelRestrictedToken,
+				Command:      []string{"cmd.exe", "/c", "echo"},
+			}
+			// Assert the precondition rather than assume it: this must be the quiet
+			// fallback path, not a token this host actually minted and not an error.
+			token, ok, err := windowsSandboxPrincipalToken(config)
+			if ok {
+				token.Close()
+				t.Fatalf("host unexpectedly provisioned a principal; this test cannot measure the fallback")
+			}
+			if err != nil {
+				t.Fatalf("windowsSandboxPrincipalToken error = %v, want the quiet fallback", err)
+			}
+			if len(warned) != 1 {
+				t.Fatalf("opted-in fallback warnings = %v, want exactly one naming %q", warned, testCase.reason)
+			}
+			if !strings.Contains(warned[0], testCase.reason) {
+				t.Fatalf("warning = %q, want it to name %q", warned[0], testCase.reason)
+			}
+		})
+	}
+}
+
+// The opt-out must stay silent, or the warning becomes noise every user learns
+// to ignore.
+func TestWindowsSandboxPrincipalFallbackIsSilentWhenOptedOut(t *testing.T) {
+	var warned []string
+	originalWarn := warnWindowsSandboxPrincipalNotUsed
+	warnWindowsSandboxPrincipalNotUsed = func(reason string) { warned = append(warned, reason) }
+	windowsSandboxPrincipalNotUsedWarnOnce = sync.Once{}
+	t.Cleanup(func() {
+		warnWindowsSandboxPrincipalNotUsed = originalWarn
+		windowsSandboxPrincipalNotUsedWarnOnce = sync.Once{}
+	})
+
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     `C:\workspace`,
+		WorkspaceRoots: []string{`C:\workspace`},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: `C:\workspace`}}},
+			Network:    NetworkPolicy{Mode: NetworkDeny},
+		},
+		Env:          map[string]string{windowsSandboxIdentityEnv: "0"},
+		SandboxLevel: WindowsSandboxLevelRestrictedToken,
+		Command:      []string{"cmd.exe", "/c", "echo"},
+	}
+	if _, ok, err := windowsSandboxPrincipalToken(config); ok || err != nil {
+		t.Fatalf("windowsSandboxPrincipalToken ok=%v err=%v, want the quiet opted-out fallback", ok, err)
+	}
+	if len(warned) != 0 {
+		t.Fatalf("opted-out command warned %v, want silence", warned)
+	}
+}
+
+// Setup must stay inert unless the principal backend is explicitly opted into.
+// This is the property that makes the branch safe to merge while the privileged
+// paths are still being validated: without the opt-in, `zero sandbox setup`
+// creates no local account and the capability-SID backend is the whole of setup.
+func TestWindowsSandboxIdentityGating(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		env  map[string]string
+		want bool
+	}{
+		// An explicit map entry is authoritative; these cases never reach the
+		// process environment.
+		"empty":          {env: map[string]string{windowsSandboxIdentityEnv: ""}, want: false},
+		"zero":           {env: map[string]string{windowsSandboxIdentityEnv: "0"}, want: false},
+		"true not one":   {env: map[string]string{windowsSandboxIdentityEnv: "true"}, want: false},
+		"one":            {env: map[string]string{windowsSandboxIdentityEnv: "1"}, want: true},
+		"one with space": {env: map[string]string{windowsSandboxIdentityEnv: " 1 "}, want: true},
+	} {
+		t.Run(name, func(t *testing.T) {
+			// Pin the process variable too. Every case here supplies an explicit
+			// map entry so none of them should consult it, and pinning proves
+			// that rather than assuming it: without this a developer who exports
+			// the opt-in would see different results from CI.
+			t.Setenv(windowsSandboxIdentityEnv, "1")
+			if got := windowsSandboxIdentityEnabled(testCase.env); got != testCase.want {
+				t.Fatalf("enabled = %v, want %v for %q", got, testCase.want, testCase.env[windowsSandboxIdentityEnv])
+			}
+		})
+	}
+}
+
+// With no map entry the process environment decides. That fallback is what the
+// elevated setup path actually runs on — it passes no Env — so it needs its own
+// coverage rather than riding on a case that also has a map entry.
+func TestWindowsSandboxIdentityGatingFallsBackToTheProcessEnvironment(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		value string
+		set   bool
+		want  bool
+	}{
+		"unset":          {set: false, want: false},
+		"empty":          {value: "", set: true, want: false},
+		"zero":           {value: "0", set: true, want: false},
+		"one":            {value: "1", set: true, want: true},
+		"one with space": {value: " 1 ", set: true, want: true},
+	} {
+		t.Run(name, func(t *testing.T) {
+			// t.Setenv registers the restore even when the variable is then
+			// cleared, which is the only way to test a genuinely absent variable
+			// without leaking that state into the rest of the package.
+			t.Setenv(windowsSandboxIdentityEnv, testCase.value)
+			if !testCase.set {
+				if err := os.Unsetenv(windowsSandboxIdentityEnv); err != nil {
+					t.Fatalf("unset %s: %v", windowsSandboxIdentityEnv, err)
+				}
+			}
+			if got := windowsSandboxIdentityEnabled(nil); got != testCase.want {
+				t.Fatalf("enabled = %v, want %v (set=%v value=%q)", got, testCase.want, testCase.set, testCase.value)
+			}
+		})
+	}
+}
+
+// The command environment wins over the process environment, so a run can opt in
+// or out without depending on how the parent shell was launched.
+func TestWindowsSandboxIdentityEnvOverridesProcess(t *testing.T) {
+	t.Setenv(windowsSandboxIdentityEnv, "1")
+	if windowsSandboxIdentityEnabled(map[string]string{windowsSandboxIdentityEnv: "0"}) {
+		t.Fatal("command env set to 0 must override a process env of 1")
+	}
+	if !windowsSandboxIdentityEnabled(map[string]string{}) {
+		t.Fatal("with no command-env entry the process env should apply")
+	}
+}
+
+// One workspace maps to one principal, and different workspaces must not share
+// an account, or two projects would run under the same identity and could reach
+// each other's granted roots.
+func TestWindowsSandboxWorkspaceKeyIsStableAndDistinct(t *testing.T) {
+	first := windowsSandboxWorkspaceKey([]string{`C:\ws\alpha`})
+	again := windowsSandboxWorkspaceKey([]string{`C:\ws\alpha`})
+	other := windowsSandboxWorkspaceKey([]string{`C:\ws\beta`})
+	if first != again {
+		t.Fatalf("key is not stable: %q vs %q", first, again)
+	}
+	if first == other {
+		t.Fatal("two different workspaces produced the same principal key")
+	}
+	if first == "" {
+		t.Fatal("empty key")
+	}
+	// An empty root list still has to yield a usable key rather than a blank one.
+	if windowsSandboxWorkspaceKey(nil) == "" {
+		t.Fatal("no workspace roots produced an empty key")
+	}
+}
+
+// Network denial is enforced by WFP filters keyed to the offline-marker SID,
+// which only the restricted token carries. A principal token would leave those
+// filters matching nothing, so the principal backend must stand down whenever
+// the network is denied rather than silently trading network enforcement for
+// read confinement.
+// The backend now runs under both network modes, so the opt-in is the only thing
+// eligibility turns on. The mode selects which principal is used, not whether one
+// is used at all.
+func TestPrincipalBackendEligibilityTurnsOnlyOnTheOptIn(t *testing.T) {
+	eligible := func(mode NetworkMode, optIn string) bool {
+		config := WindowsSandboxCommandConfig{
+			SandboxHome:    t.TempDir(),
+			WorkspaceRoots: []string{t.TempDir()},
+			Env:            map[string]string{windowsSandboxIdentityEnv: optIn},
+		}
+		config.PermissionProfile.Network.Mode = mode
+		return windowsSandboxPrincipalEligible(config)
+	}
+
+	for _, mode := range []NetworkMode{NetworkDeny, NetworkAllow} {
+		if !eligible(mode, "1") {
+			t.Fatalf("principal backend refused with network %q; the mode should choose a principal, not disable the backend", mode)
+		}
+		if eligible(mode, "0") {
+			t.Fatalf("principal backend eligible without the opt-in for network %q", mode)
+		}
+	}
+}
+
+// The network mode is enforced by WHICH principal runs, because the offline one
+// is a member of the group the block filters match. Picking the wrong role is
+// therefore a silent loss of network enforcement, so the mapping is asserted
+// directly rather than inferred from a token that only exists on a provisioned
+// machine.
+func TestPrincipalRoleFollowsNetworkMode(t *testing.T) {
+	if got := windowsSandboxRoleForNetwork(NetworkDeny); got != windowsSandboxRoleOffline {
+		t.Fatalf("deny selected %q, want the offline principal; the online one is not in the blocked group and would have the network", got)
+	}
+	if got := windowsSandboxRoleForNetwork(NetworkAllow); got != windowsSandboxRoleOnline {
+		t.Fatalf("allow selected %q, want the online principal", got)
+	}
+	// Fail closed. An unrecognised mode must lose the network rather than keep it,
+	// so anything that is not an explicit allow maps to the offline principal.
+	for _, mode := range []NetworkMode{NetworkMode(""), NetworkMode("bogus"), NetworkMode("ALLOW")} {
+		if got := windowsSandboxRoleForNetwork(mode); got != windowsSandboxRoleOffline {
+			t.Fatalf("unrecognised mode %q selected %q, want the offline principal so an unknown mode fails closed", mode, got)
+		}
+	}
+}
+
+// The account key has to separate USERS, not only workspaces.
+//
+// The name is machine-wide while the DPAPI secret and the ACL ledger both live
+// under the invoking user's sandbox home. Keyed on the workspace alone, two
+// Windows users sharing one path derive the same account with private
+// bookkeeping: the second finds no ledger of its own and either retires the
+// first user's working principal or adopts it and rotates the password while
+// storing only its own secret. The first user's marker still validates and its
+// next command cannot log on.
+//
+// Asserting the principal key DIFFERS from the workspace key is what pins that.
+// It is the cheapest observable difference; a test that only checked the key was
+// stable would pass against the broken version.
+func TestPrincipalKeyIsScopedToTheUserNotOnlyTheWorkspace(t *testing.T) {
+	roots := []string{`C:\ws\shared`}
+	workspace := windowsSandboxWorkspaceKey(roots)
+	config := WindowsSandboxCommandConfig{WorkspaceRoots: roots}
+	principal := windowsSandboxPrincipalKey(config)
+
+	if principal == workspace {
+		t.Fatal("the principal key equals the workspace key, so two users sharing this workspace would derive one account with separate secrets and ledgers")
+	}
+	if principal != windowsSandboxPrincipalKey(config) {
+		t.Error("the principal key is not stable for one user, so a second setup would not find its own account")
+	}
+	if other := windowsSandboxPrincipalKey(WindowsSandboxCommandConfig{WorkspaceRoots: []string{`C:\ws\other`}}); other == principal {
+		t.Error("two workspaces derived the same principal key, so they would share one account")
+	}
+}
+
+// The setup lock must stay keyed to the WORKSPACE alone. Two users setting up
+// one shared workspace now provision separate accounts, but they still write
+// DACLs on the same paths, so they have to keep serializing against each other.
+func TestSetupLockStaysWorkspaceScoped(t *testing.T) {
+	roots := []string{`C:\ws\shared`}
+	if windowsSandboxWorkspaceKey(roots) == windowsSandboxPrincipalKey(WindowsSandboxCommandConfig{WorkspaceRoots: roots}) {
+		t.Fatal("the lock key and the principal key are the same, so making the account per-user also made the lock per-user")
+	}
+}

internal/sandbox/windows_identity_secret_handle_windows.go

@@ -0,0 +1,119 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"fmt"
+	"path/filepath"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// createWindowsSecretFileNoFollow creates (or replaces) the secret file as a
+// CHILD of a pinned parent handle and returns a handle to it.
+//
+// The pathname version this replaces resolved the path four separate times in an
+// elevated process: MkdirAll, an O_TRUNC open, SetNamedSecurityInfo by name, then
+// WriteFile by name. The sandbox home belongs to the invoking user, who is the
+// party this sandbox exists to contain, so each of those resolutions was a place
+// to swap a component. Worst case an attacker-chosen file gets truncated and then
+// has its DACL rewritten by an Administrator; the milder junction-only case still
+// plants the deterministic secret somewhere the caller then owns.
+//
+// One handle, one resolution. The parent is opened no-follow, the leaf is created
+// relative to that handle so its name can never be resolved again, and both the
+// DACL and the bytes are applied to the handle rather than to a path.
+//
+// FILE_OVERWRITE_IF, not FILE_CREATE: an existing secret must be replaced, since
+// a stale password makes LogonUser fail in a way that reads as a sandbox bug.
+// FILE_OPEN_REPARSE_POINT means a leaf that IS a reparse point comes back as
+// itself rather than being followed, and the attribute check below then refuses
+// it instead of writing through it.
+func createWindowsSecretFileNoFollow(path string) (windows.Handle, error) {
+	parentPath := filepath.Dir(path)
+	name := filepath.Base(path)
+	if err := validateWindowsACLComponent(name); err != nil {
+		return 0, fmt.Errorf("secret file name: %w", err)
+	}
+	parent, err := openWindowsACLDirectoryNoFollow(parentPath)
+	if err != nil {
+		return 0, fmt.Errorf("open secret directory %s: %w", parentPath, err)
+	}
+	defer func() { _ = windows.CloseHandle(parent) }()
+
+	objectName, err := windows.NewNTUnicodeString(name)
+	if err != nil {
+		return 0, fmt.Errorf("encode secret file name %s: %w", name, err)
+	}
+	attributes := windows.OBJECT_ATTRIBUTES{
+		RootDirectory: parent,
+		ObjectName:    objectName,
+		Attributes:    windows.OBJ_CASE_INSENSITIVE,
+	}
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+
+	var handle windows.Handle
+	var status windows.IO_STATUS_BLOCK
+	if err := windows.NtCreateFile(
+		&handle,
+		// WRITE_DAC to lock it down and FILE_WRITE_DATA to fill it, both through
+		// this one handle. READ_CONTROL because SetSecurityInfo reads the existing
+		// descriptor before replacing it, and FILE_READ_ATTRIBUTES because the
+		// reparse-point check below asks the handle what it landed on.
+		windows.FILE_WRITE_DATA|windows.FILE_READ_ATTRIBUTES|windows.WRITE_DAC|windows.READ_CONTROL|windows.SYNCHRONIZE,
+		&attributes,
+		&status,
+		nil,
+		windows.FILE_ATTRIBUTE_NORMAL,
+		windows.FILE_SHARE_READ,
+		windows.FILE_OVERWRITE_IF,
+		windows.FILE_NON_DIRECTORY_FILE|windows.FILE_OPEN_REPARSE_POINT|windows.FILE_SYNCHRONOUS_IO_NONALERT,
+		0,
+		0,
+	); err != nil {
+		return 0, fmt.Errorf("create secret file %s: %w", path, err)
+	}
+	if err := rejectWindowsACLReparseHandle(handle, name); err != nil {
+		_ = windows.CloseHandle(handle)
+		return 0, err
+	}
+	return handle, nil
+}
+
+// lockWindowsSecretHandleToOwner applies the owner-and-SYSTEM-only DACL to an
+// open handle instead of to a pathname, so the object being locked is provably
+// the object that was just created rather than whatever the name resolves to now.
+func lockWindowsSecretHandleToOwner(handle windows.Handle, owner *windows.SID) error {
+	acl, err := windowsSecretOwnerACL(owner)
+	if err != nil {
+		return err
+	}
+	if err := windows.SetSecurityInfo(
+		handle,
+		windows.SE_FILE_OBJECT,
+		windows.DACL_SECURITY_INFORMATION|windows.PROTECTED_DACL_SECURITY_INFORMATION,
+		nil,
+		nil,
+		acl,
+		nil,
+	); err != nil {
+		return fmt.Errorf("lock secret to owner: %w", err)
+	}
+	return nil
+}
+
+// writeWindowsSecretHandle writes the whole payload through the handle.
+func writeWindowsSecretHandle(handle windows.Handle, payload []byte) error {
+	for written := 0; written < len(payload); {
+		var n uint32
+		if err := windows.WriteFile(handle, payload[written:], &n, nil); err != nil {
+			return fmt.Errorf("write secret: %w", err)
+		}
+		if n == 0 {
+			return fmt.Errorf("write secret: wrote no bytes with %d remaining", len(payload)-written)
+		}
+		written += int(n)
+	}
+	return nil
+}

internal/sandbox/windows_identity_secret_junction_windows_test.go

@@ -0,0 +1,71 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// THE SECRET MUST NOT BE WRITTEN THROUGH A REPARSE POINT.
+//
+// The sandbox home belongs to the invoking user, who is the party this sandbox
+// exists to contain, so they can put a junction where the secret directory is
+// expected before elevated setup runs. The pathname version resolved the path
+// four times over (MkdirAll, an O_TRUNC open, SetNamedSecurityInfo by name, then
+// WriteFile by name) and every one of them followed it, so an Administrator
+// process created the principal's password in a directory of the caller's
+// choosing and then granted them control of it.
+//
+// A junction rather than a symlink on purpose: it needs no privilege, so it is
+// reachable by exactly the unprivileged user this guards against. os.Symlink
+// would need SeCreateSymbolicLinkPrivilege and would skip on an ordinary
+// developer or CI account, which is where this most needs to run.
+func TestWriteWindowsSandboxSecretRefusesAJunctionedDirectory(t *testing.T) {
+	base := t.TempDir()
+	outside := filepath.Join(base, "outside")
+	if err := os.MkdirAll(outside, 0o700); err != nil {
+		t.Fatalf("MkdirAll: %v", err)
+	}
+	// The swap: the directory the secret is addressed into is a junction that
+	// leads out of the sandbox home. The pathname never changes.
+	cfg := filepath.Join(base, "cfg")
+	makeJunction(t, cfg, outside)
+
+	path := filepath.Join(cfg, "zero-sbx-test.secret")
+	err := writeWindowsSandboxSecret(path, "Zs1!EXAMPLEPASSWORDVALUE")
+	if err == nil {
+		t.Fatal("wrote the principal secret through a junction, so an elevated setup would have placed it in a directory the caller controls")
+	}
+	if !strings.Contains(err.Error(), "reparse") {
+		t.Fatalf("refused for the wrong reason: %v", err)
+	}
+	// Refusing is only half of it. Nothing may survive on the other side, or the
+	// caller still ends up holding a file the sandbox created for them.
+	entries, readErr := os.ReadDir(outside)
+	if readErr != nil {
+		t.Fatalf("ReadDir: %v", readErr)
+	}
+	if len(entries) != 0 {
+		t.Fatalf("junction target holds %d entries after the refusal, want none", len(entries))
+	}
+}
+
+// The ordinary path must still work, or the refusal above proves only that the
+// function fails.
+func TestWriteWindowsSandboxSecretStillWritesAnOrdinaryDirectory(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "cfg", "zero-sbx-test.secret")
+	const password = "Zs1!EXAMPLEPASSWORDVALUE"
+	if err := writeWindowsSandboxSecret(path, password); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	got, err := readWindowsSandboxSecret(path)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if got != password {
+		t.Fatalf("read %q, want the stored password", got)
+	}
+}

internal/sandbox/windows_identity_secret_windows.go

@@ -0,0 +1,257 @@
+//go:build windows
+
+package sandbox
+
+// Storing a sandbox principal's password.
+//
+// The elevated setup path provisions the account, but the per-command path runs
+// UNELEVATED and needs the password to call LogonUser. So the secret has to
+// cross that boundary on disk, and the only thing standing between it and the
+// sandboxed child is the file's ACL.
+//
+// The file is locked to the invoking user: an explicit, INHERITANCE-PROTECTED
+// DACL granting that user and SYSTEM, and nobody else. The sandbox principal is
+// deliberately absent from it, which is the property that matters, because a
+// principal that could read this file could mint its own token and the whole
+// identity boundary would be decorative. Administrators are not added either;
+// an admin can already take ownership, so naming them buys nothing and widens
+// the visible grant.
+//
+// Ordering is load-bearing: the ACL is applied to an EMPTY file before the
+// password is written, so the bytes never exist under the directory's inherited
+// permissions even briefly.
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"golang.org/x/sys/windows"
+)
+
+// windowsSandboxSecretDirName holds per-principal secrets under the Zero config
+// directory. Kept in its own directory so the whole set can be removed when the
+// sandbox is torn down.
+const windowsSandboxSecretDirName = "windows-sandbox"
+
+// windowsSandboxSecretPath returns where a principal's password lives. The
+// account name is already sanitised to [a-z0-9-] by windowsSandboxUserName, so
+// it cannot escape the directory.
+func windowsSandboxSecretPath(configDir string, username string) (string, error) {
+	if strings.TrimSpace(configDir) == "" {
+		return "", errors.New("windows sandbox secret: empty config directory")
+	}
+	if strings.TrimSpace(username) == "" {
+		return "", errors.New("windows sandbox secret: empty principal name")
+	}
+	// Defence in depth against a caller passing something windowsSandboxUserName
+	// did not produce: refuse anything with a separator or a parent reference.
+	if strings.ContainsAny(username, `\/:`) || strings.Contains(username, "..") {
+		return "", fmt.Errorf("windows sandbox secret: unsafe principal name %q", username)
+	}
+	return filepath.Join(configDir, windowsSandboxSecretDirName, username+".secret"), nil
+}
+
+// currentTokenUserSID returns the SID of the user this process runs as. Under
+// UAC the elevated token keeps the same user SID as the desktop session, so
+// setup and the later unelevated command path agree on the owner, which is what
+// makes an owner-scoped ACL usable across the elevation boundary.
+func currentTokenUserSID() (*windows.SID, error) {
+	var token windows.Token
+	if err := windows.OpenProcessToken(windows.CurrentProcess(), windows.TOKEN_QUERY, &token); err != nil {
+		return nil, fmt.Errorf("open process token: %w", err)
+	}
+	defer token.Close()
+	user, err := token.GetTokenUser()
+	if err != nil {
+		return nil, fmt.Errorf("get token user: %w", err)
+	}
+	// The SID points into a buffer owned by the Tokenuser, so copy it out before
+	// that buffer goes away.
+	copied, err := user.User.Sid.Copy()
+	if err != nil {
+		return nil, fmt.Errorf("copy token user SID: %w", err)
+	}
+	return copied, nil
+}
+
+// windowsSecretOwnerACL builds the explicit, inheritance-protected DACL granting
+// only owner and SYSTEM. PROTECTED is what drops any ACE inherited from the
+// config directory; without it a permissive parent would still grant access to
+// whoever it names.
+//
+// Returns the ACL rather than applying it, because the caller applies it to an
+// open HANDLE. Applying by pathname meant the object being locked was whatever
+// the name resolved to at that instant, which is not necessarily the object that
+// was just created.
+func windowsSecretOwnerACL(owner *windows.SID) (*windows.ACL, error) {
+	if owner == nil {
+		return nil, errors.New("windows sandbox secret: nil owner SID")
+	}
+	system, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	if err != nil {
+		return nil, fmt.Errorf("resolve SYSTEM SID: %w", err)
+	}
+	entries := []windows.EXPLICIT_ACCESS{
+		{
+			AccessPermissions: windows.GENERIC_ALL,
+			AccessMode:        windows.GRANT_ACCESS,
+			Inheritance:       windows.NO_INHERITANCE,
+			Trustee: windows.TRUSTEE{
+				TrusteeForm:  windows.TRUSTEE_IS_SID,
+				TrusteeType:  windows.TRUSTEE_IS_USER,
+				TrusteeValue: windows.TrusteeValueFromSID(owner),
+			},
+		},
+		{
+			AccessPermissions: windows.GENERIC_ALL,
+			AccessMode:        windows.GRANT_ACCESS,
+			Inheritance:       windows.NO_INHERITANCE,
+			Trustee: windows.TRUSTEE{
+				TrusteeForm:  windows.TRUSTEE_IS_SID,
+				TrusteeType:  windows.TRUSTEE_IS_USER,
+				TrusteeValue: windows.TrusteeValueFromSID(system),
+			},
+		},
+	}
+	acl, err := windows.ACLFromEntries(entries, nil)
+	if err != nil {
+		return nil, fmt.Errorf("build secret ACL: %w", err)
+	}
+	return acl, nil
+}
+
+// writeWindowsSandboxSecret stores a principal's password readable only by the
+// invoking user.
+//
+// The file is created empty, locked down, and only then written, so the secret
+// is never on disk under the directory's inherited ACL. An existing file is
+// replaced rather than appended, since a stale password would make LogonUser
+// fail in a way that looks like a sandbox bug.
+func writeWindowsSandboxSecret(path string, password string) error {
+	owner, err := currentTokenUserSID()
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return fmt.Errorf("create secret directory: %w", err)
+	}
+	// Sealed BEFORE the file exists, so a failure here leaves nothing on disk at
+	// all rather than an empty file for someone else to win a race on.
+	//
+	// Encrypted to the invoking user on top of the ACL, so a copy taken outside
+	// the filesystem's enforcement (backup, disk image) is inert. The principal
+	// name is the entropy, which keeps one principal's blob from authenticating
+	// another.
+	sealed, err := protectWindowsSecret(password, windowsSandboxSecretEntropy(path))
+	if err != nil {
+		return err
+	}
+	// ONE resolution of the path, not four. The leaf is created relative to a
+	// pinned no-follow parent handle, and the DACL and the bytes are both applied
+	// to that handle. Resolving the name again between those steps is what let a
+	// junction or symlink swap point an elevated write at a file of the caller's
+	// choosing.
+	handle, err := createWindowsSecretFileNoFollow(path)
+	if err != nil {
+		return err
+	}
+	defer func() { _ = windows.CloseHandle(handle) }()
+	if err := lockWindowsSecretHandleToOwner(handle, owner); err != nil {
+		// Do not leave an unprotected file behind. Removal is by name, which is
+		// safe in a way the write was not: worst case a swapped name means the
+		// cleanup misses and leaves a locked-down file, never that it deletes
+		// something it did not create.
+		_ = os.Remove(path)
+		return err
+	}
+	if err := writeWindowsSecretHandle(handle, sealed); err != nil {
+		_ = os.Remove(path)
+		return err
+	}
+	return nil
+}
+
+// windowsSandboxSecretEntropy derives the DPAPI entropy from the secret's own
+// filename, which is the principal name. Deriving it rather than threading the
+// name through keeps read and write agreeing by construction.
+func windowsSandboxSecretEntropy(path string) string {
+	return strings.TrimSuffix(filepath.Base(path), ".secret")
+}
+
+// Seamed so the permission-denied mapping in readWindowsSandboxSecret is
+// testable. Producing a real ERROR_ACCESS_DENIED needs DACL surgery on Windows,
+// since a 0000 file is still readable and reading a directory reports
+// "Incorrect function", so a test built that way would exercise the platform
+// rather than the mapping.
+var readWindowsSandboxSecretFile = os.ReadFile
+
+// readWindowsSandboxSecret loads a principal's password. A missing file means
+// setup has not run for this workspace, which the caller turns into a fallback
+// rather than a hard failure.
+func readWindowsSandboxSecret(path string) (string, error) {
+	data, err := readWindowsSandboxSecretFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", errWindowsSandboxIdentityUnavailable
+		}
+		// Permission denied is unavailability, not breakage. The secret's DACL
+		// names whoever ran setup, so an operator who elevated with a separate
+		// administrative account, through runas or an over-the-shoulder UAC
+		// prompt, ends up with a secret their ordinary account cannot open. That
+		// is the documented fail-soft case: fall back to the restricted token and
+		// let the warning say so. Treating it as a hard error instead made every
+		// sandboxed command fail on a machine that was merely set up by a
+		// different admin, which is a common way to run an elevated setup.
+		if os.IsPermission(err) {
+			return "", errWindowsSandboxIdentityUnavailable
+		}
+		return "", fmt.Errorf("read sandbox secret: %w", err)
+	}
+	if len(data) == 0 {
+		return "", errWindowsSandboxIdentityUnavailable
+	}
+	secret, err := unprotectWindowsSecret(data, windowsSandboxSecretEntropy(path))
+	if err != nil {
+		// A blob written by another user, for another principal, or by an older
+		// build that stored the password in the clear. Report it as unavailable so
+		// the caller falls back to the restricted token; the next elevated setup
+		// rewrites the secret in the current format.
+		return "", errWindowsSandboxIdentityUnavailable
+	}
+	if strings.TrimSpace(secret) == "" {
+		return "", errWindowsSandboxIdentityUnavailable
+	}
+	return secret, nil
+}
+
+// removeWindowsSandboxSecret deletes a stored password. Called before the
+// account itself is removed so a secret never outlives the principal it
+// authenticates.
+// errWindowsSandboxSecretNotOurs reports a secret this account cannot delete
+// because another administrator's setup owns its DACL.
+//
+// Distinguishable so teardown can carry on. The alternative is what it used to
+// do: abort before removing the account, its logon rights, its ACEs and its
+// ledger, leaving a fully provisioned principal on the machine because one file
+// could not be unlinked.
+var errWindowsSandboxSecretNotOurs = errors.New("the sandbox secret belongs to a different administrator's setup")
+
+func removeWindowsSandboxSecret(path string) error {
+	err := os.Remove(path)
+	if err == nil || os.IsNotExist(err) {
+		return nil
+	}
+	// The read path already treats permission-denied as unavailability rather
+	// than breakage, for the reason documented on readWindowsSandboxSecret: an
+	// operator who elevated with a separate administrative account gets a secret
+	// their ordinary account cannot open. Removal has to agree. It did not, so on
+	// exactly those machines teardown stopped at the first file and stranded
+	// everything after it.
+	if os.IsPermission(err) {
+		return fmt.Errorf("%w: %s", errWindowsSandboxSecretNotOurs, path)
+	}
+	return fmt.Errorf("remove sandbox secret: %w", err)
+}

internal/sandbox/windows_identity_secret_windows_test.go

@@ -0,0 +1,300 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+func TestWindowsSandboxSecretRoundTrip(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "cfg", "zero-sbx-test.secret")
+	const password = "Zs1!EXAMPLEPASSWORDVALUE"
+
+	if err := writeWindowsSandboxSecret(path, password); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	got, err := readWindowsSandboxSecret(path)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if got != password {
+		t.Fatalf("read %q, want the stored password", got)
+	}
+}
+
+// THE security property: the stored password must be readable only by the user
+// who owns it. If any other trustee appears in the DACL, and in particular the
+// sandbox principal, that account could mint its own token and the identity
+// boundary would be worthless.
+func TestWindowsSandboxSecretIsLockedToOwner(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "locked.secret")
+	if err := writeWindowsSandboxSecret(path, "Zs1!SECRET"); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	descriptor, err := windows.GetNamedSecurityInfo(
+		path,
+		windows.SE_FILE_OBJECT,
+		windows.DACL_SECURITY_INFORMATION,
+	)
+	if err != nil {
+		t.Fatalf("read back security info: %v", err)
+	}
+	dacl, _, err := descriptor.DACL()
+	if err != nil {
+		t.Fatalf("read DACL: %v", err)
+	}
+	if dacl == nil {
+		t.Fatal("secret has a nil DACL, which grants everyone access")
+	}
+
+	owner, err := currentTokenUserSID()
+	if err != nil {
+		t.Fatalf("owner SID: %v", err)
+	}
+	system, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+	if err != nil {
+		t.Fatalf("SYSTEM SID: %v", err)
+	}
+
+	entries, err := windowsSecretACEList(dacl)
+	if err != nil {
+		t.Fatalf("enumerate ACEs: %v", err)
+	}
+	if len(entries) == 0 {
+		t.Fatal("secret DACL has no ACEs")
+	}
+	for _, sid := range entries {
+		if sid.Equals(owner) || sid.Equals(system) {
+			continue
+		}
+		t.Fatalf("secret DACL grants an unexpected trustee %s; only the owner and SYSTEM may appear", sid)
+	}
+}
+
+// The DACL must be inheritance-protected, otherwise a permissive ACE on the
+// config directory would still reach the secret.
+func TestWindowsSandboxSecretDaclIsProtected(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "protected.secret")
+	if err := writeWindowsSandboxSecret(path, "Zs1!SECRET"); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	descriptor, err := windows.GetNamedSecurityInfo(path, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		t.Fatalf("read back security info: %v", err)
+	}
+	control, _, err := descriptor.Control()
+	if err != nil {
+		t.Fatalf("read control bits: %v", err)
+	}
+	if control&windows.SE_DACL_PROTECTED == 0 {
+		t.Fatal("secret DACL is not protected, so inherited ACEs still apply")
+	}
+}
+
+// Rewriting must replace the previous secret rather than append to it, or
+// LogonUser would be handed two concatenated passwords.
+func TestWindowsSandboxSecretOverwrites(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "rewrite.secret")
+	if err := writeWindowsSandboxSecret(path, "Zs1!FIRST"); err != nil {
+		t.Fatalf("first write: %v", err)
+	}
+	if err := writeWindowsSandboxSecret(path, "Zs1!SECOND"); err != nil {
+		t.Fatalf("second write: %v", err)
+	}
+	got, err := readWindowsSandboxSecret(path)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	if got != "Zs1!SECOND" {
+		t.Fatalf("read %q, want only the newest password", got)
+	}
+}
+
+// A workspace whose setup has not run must report the actionable sentinel so the
+// command path falls back to the restricted-token backend instead of failing.
+func TestWindowsSandboxSecretMissingIsSentinel(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "absent.secret")
+	if _, err := readWindowsSandboxSecret(path); err != errWindowsSandboxIdentityUnavailable {
+		t.Fatalf("missing secret returned %v, want errWindowsSandboxIdentityUnavailable", err)
+	}
+}
+
+// An empty file is a half-written secret, not a valid empty password.
+//
+// Both seeds matter and only one of them tests what the name says. A
+// zero-length file is the truncated-write case, and it is the only one that
+// reaches the length check. Whitespace is several bytes, so it travels on to
+// DPAPI and fails to unprotect instead, which is the path
+// TestWindowsSandboxSecretRejectsLegacyPlaintext already covers. Seeding only
+// the whitespace, as this did, left the branch in the test's own name
+// unexercised.
+func TestWindowsSandboxSecretEmptyIsSentinel(t *testing.T) {
+	for name, seed := range map[string][]byte{
+		"truncated write": {},
+		"whitespace only": []byte("   \r\n"),
+	} {
+		t.Run(name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "empty.secret")
+			if err := os.WriteFile(path, seed, 0o600); err != nil {
+				t.Fatalf("seed: %v", err)
+			}
+			if _, err := readWindowsSandboxSecret(path); err != errWindowsSandboxIdentityUnavailable {
+				t.Fatalf("secret of %d bytes returned %v, want the unavailable sentinel", len(seed), err)
+			}
+		})
+	}
+}
+
+// The principal name lands in a filename, so anything that could escape the
+// directory has to be refused even though windowsSandboxUserName already
+// sanitises its output.
+func TestWindowsSandboxSecretPathRejectsTraversal(t *testing.T) {
+	for _, name := range []string{`..\evil`, "sub/dir", `C:\abs`, "..", ""} {
+		if _, err := windowsSandboxSecretPath(`C:\cfg`, name); err == nil {
+			t.Fatalf("principal name %q was accepted", name)
+		}
+	}
+	if _, err := windowsSandboxSecretPath("", "zero-sbx-a"); err == nil {
+		t.Fatal("empty config directory was accepted")
+	}
+	path, err := windowsSandboxSecretPath(`C:\cfg`, "zero-sbx-abc")
+	if err != nil {
+		t.Fatalf("valid name rejected: %v", err)
+	}
+	if !strings.HasSuffix(path, `zero-sbx-abc.secret`) {
+		t.Fatalf("unexpected secret path %q", path)
+	}
+}
+
+// Removal must be idempotent so teardown converges the same way provisioning
+// does, and must actually delete the secret.
+func TestWindowsSandboxSecretRemoveIsIdempotent(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "gone.secret")
+	if err := writeWindowsSandboxSecret(path, "Zs1!SECRET"); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	if err := removeWindowsSandboxSecret(path); err != nil {
+		t.Fatalf("first remove: %v", err)
+	}
+	if _, err := os.Stat(path); !os.IsNotExist(err) {
+		t.Fatalf("secret still present after removal (stat err %v)", err)
+	}
+	if err := removeWindowsSandboxSecret(path); err != nil {
+		t.Fatalf("removing an absent secret must succeed, got %v", err)
+	}
+}
+
+// windowsSecretACEList returns the trustee SID of every ACE in a DACL so a test
+// can assert exactly who is named.
+func windowsSecretACEList(dacl *windows.ACL) ([]*windows.SID, error) {
+	var out []*windows.SID
+	for index := uint32(0); index < uint32(dacl.AceCount); index++ {
+		var ace *windows.ACCESS_ALLOWED_ACE
+		if err := windows.GetAce(dacl, index, &ace); err != nil {
+			return nil, err
+		}
+		// GetAce hands back a generic ACE_HEADER and we reinterpret it. That is
+		// only sound for the fixed-layout types: an object ACE carries Flags and
+		// two GUIDs ahead of the trustee, so SidStart would land mid-structure
+		// and Copy would read whatever bytes happen to be there. The caller's
+		// "unexpected trustee" assertion would then print a nonsense SID instead
+		// of naming the ACE that does not belong, which is the opposite of what
+		// a failing test should do. Nothing under test builds anything but
+		// allowed ACEs today, so this exists to keep the failure legible if that
+		// ever changes.
+		if ace.Header.AceType != windows.ACCESS_ALLOWED_ACE_TYPE {
+			return nil, fmt.Errorf("ACE %d has type %d, want ACCESS_ALLOWED_ACE_TYPE (%d); refusing to decode its trustee", index, ace.Header.AceType, windows.ACCESS_ALLOWED_ACE_TYPE)
+		}
+		sid := (*windows.SID)(unsafe.Pointer(&ace.SidStart))
+		copied, err := sid.Copy()
+		if err != nil {
+			return nil, err
+		}
+		out = append(out, copied)
+	}
+	return out, nil
+}
+
+// DPAPI round-trip through the real store, which needs no privilege and so is
+// genuine coverage rather than a gated stub.
+func TestWindowsSandboxSecretRoundTripsThroughDPAPI(t *testing.T) {
+	path, err := windowsSandboxSecretPath(t.TempDir(), "zero-sbx-roundtrip")
+	if err != nil {
+		t.Fatalf("secret path: %v", err)
+	}
+	const password = "S0me-Sandbox-P@ssw0rd-value"
+	if err := writeWindowsSandboxSecret(path, password); err != nil {
+		t.Fatalf("write secret: %v", err)
+	}
+	got, err := readWindowsSandboxSecret(path)
+	if err != nil {
+		t.Fatalf("read secret: %v", err)
+	}
+	if got != password {
+		t.Fatalf("round-trip returned %q, want %q", got, password)
+	}
+	// The point of the exercise: the password must not be recoverable by reading
+	// the file, or the encryption layer is decorative.
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read raw secret file: %v", err)
+	}
+	if bytes.Contains(raw, []byte(password)) {
+		t.Fatal("the password appears verbatim in the stored file; it was not encrypted")
+	}
+}
+
+// Entropy is the principal name, so a blob moved onto another principal's secret
+// path must fail to decrypt rather than authenticate the wrong account.
+func TestWindowsSandboxSecretDoesNotTransferBetweenPrincipals(t *testing.T) {
+	home := t.TempDir()
+	minePath, err := windowsSandboxSecretPath(home, "zero-sbx-mine")
+	if err != nil {
+		t.Fatalf("secret path: %v", err)
+	}
+	theirsPath, err := windowsSandboxSecretPath(home, "zero-sbx-theirs")
+	if err != nil {
+		t.Fatalf("secret path: %v", err)
+	}
+	if err := writeWindowsSandboxSecret(minePath, "a-password-for-mine"); err != nil {
+		t.Fatalf("write secret: %v", err)
+	}
+	blob, err := os.ReadFile(minePath)
+	if err != nil {
+		t.Fatalf("read blob: %v", err)
+	}
+	if err := os.WriteFile(theirsPath, blob, 0o600); err != nil {
+		t.Fatalf("plant blob: %v", err)
+	}
+	if _, err := readWindowsSandboxSecret(theirsPath); !errors.Is(err, errWindowsSandboxIdentityUnavailable) {
+		t.Fatalf("a blob planted at another principal's path decrypted; got err = %v", err)
+	}
+}
+
+// An older plaintext secret must degrade to a fallback rather than being handed
+// to LogonUser as if it were a password.
+func TestWindowsSandboxSecretRejectsLegacyPlaintext(t *testing.T) {
+	path, err := windowsSandboxSecretPath(t.TempDir(), "zero-sbx-legacy")
+	if err != nil {
+		t.Fatalf("secret path: %v", err)
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		t.Fatalf("mkdir: %v", err)
+	}
+	if err := os.WriteFile(path, []byte("plaintext-password"), 0o600); err != nil {
+		t.Fatalf("write legacy secret: %v", err)
+	}
+	if _, err := readWindowsSandboxSecret(path); !errors.Is(err, errWindowsSandboxIdentityUnavailable) {
+		t.Fatalf("legacy plaintext secret was accepted; got err = %v", err)
+	}
+}

internal/sandbox/windows_identity_windows_test.go

@@ -0,0 +1,498 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"strings"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// A local Windows account name is capped at 20 characters, so the derived name
+// must truncate rather than produce a name NetUserAdd rejects.
+func TestWindowsSandboxUserNameRespectsLengthLimit(t *testing.T) {
+	name := windowsSandboxUserName(strings.Repeat("a", 64), windowsSandboxRoleOffline)
+	if len(name) > windowsSandboxUserNameMax {
+		t.Fatalf("name %q is %d chars, want at most %d", name, len(name), windowsSandboxUserNameMax)
+	}
+	if !strings.HasPrefix(name, windowsSandboxUserPrefix) {
+		t.Fatalf("name %q lost the managed prefix", name)
+	}
+}
+
+// The same workspace must map to the same principal, otherwise re-running setup
+// would accumulate a new local account every time.
+func TestWindowsSandboxUserNameIsStable(t *testing.T) {
+	first := windowsSandboxUserName("abc123", windowsSandboxRoleOffline)
+	second := windowsSandboxUserName("abc123", windowsSandboxRoleOffline)
+	if first != second {
+		t.Fatalf("name is not stable: %q vs %q", first, second)
+	}
+	if other := windowsSandboxUserName("def456", windowsSandboxRoleOffline); other == first {
+		t.Fatalf("different workspaces produced the same principal %q", first)
+	}
+}
+
+// The key is sanitised to characters a local account name accepts, so a hash or
+// path fragment cannot smuggle a separator or a space into the name.
+func TestWindowsSandboxUserNameRejectsUnsafeCharacters(t *testing.T) {
+	name := windowsSandboxUserName(`C:\Users\me\proj ect`, windowsSandboxRoleOffline)
+	for _, r := range strings.TrimPrefix(name, windowsSandboxUserPrefix) {
+		isLower := r >= 'a' && r <= 'z'
+		isDigit := r >= '0' && r <= '9'
+		if !isLower && !isDigit {
+			t.Fatalf("name %q contains unsafe rune %q", name, r)
+		}
+	}
+	if name == windowsSandboxUserPrefix {
+		t.Fatal("sanitising removed every character, leaving a bare prefix")
+	}
+}
+
+// An empty or fully-sanitised-away key must still yield a usable name rather
+// than the bare prefix.
+func TestWindowsSandboxUserNameHandlesEmptyKey(t *testing.T) {
+	for _, key := range []string{"", "///", "   "} {
+		if got := windowsSandboxUserName(key, windowsSandboxRoleOffline); got == windowsSandboxUserPrefix {
+			t.Fatalf("key %q produced a bare prefix", key)
+		}
+	}
+}
+
+// The password must be fresh per call and carry the character classes a default
+// Windows complexity policy demands, or NetUserAdd fails with ERROR_PASSWORD_RESTRICTION.
+func TestNewWindowsSandboxPasswordIsRandomAndComplex(t *testing.T) {
+	first, err := newWindowsSandboxPassword()
+	if err != nil {
+		t.Fatalf("generate: %v", err)
+	}
+	second, err := newWindowsSandboxPassword()
+	if err != nil {
+		t.Fatalf("generate: %v", err)
+	}
+	if first == second {
+		t.Fatal("two generated passwords are identical, so they are not random")
+	}
+	if len(first) < 12 {
+		t.Fatalf("password is only %d chars", len(first))
+	}
+	var hasUpper, hasLower, hasDigit bool
+	for _, r := range first {
+		switch {
+		case r >= 'A' && r <= 'Z':
+			hasUpper = true
+		case r >= 'a' && r <= 'z':
+			hasLower = true
+		case r >= '0' && r <= '9':
+			hasDigit = true
+		}
+	}
+	if !hasUpper || !hasLower || !hasDigit {
+		t.Fatalf("password %q lacks a required character class", first)
+	}
+}
+
+// "Already exists" is the normal result of re-running setup and must not surface
+// as an error, while a genuine failure must.
+func TestNetAPIStatusTreatsExistingAsSuccess(t *testing.T) {
+	if err := netAPIStatus("NetUserAdd", nerrSuccess); err != nil {
+		t.Fatalf("success status returned %v", err)
+	}
+	if err := netAPIStatus("NetUserAdd", nerrUserExists, nerrUserExists); err != nil {
+		t.Fatalf("existing user must be success, got %v", err)
+	}
+	if err := netAPIStatus("NetLocalGroupAdd", nerrGroupExists, nerrGroupExists, errorAliasExists); err != nil {
+		t.Fatalf("existing group must be success, got %v", err)
+	}
+	if err := netAPIStatus("NetUserAdd", 2245); err == nil {
+		t.Fatal("an unexpected status must surface as an error")
+	}
+}
+
+// Access-denied is the status an unelevated run gets, and it must say so rather
+// than reporting a bare number the user cannot act on.
+func TestNetAPIStatusExplainsAccessDenied(t *testing.T) {
+	err := netAPIStatus("NetUserAdd", errorAccessDenied32)
+	if err == nil {
+		t.Fatal("access denied must be an error")
+	}
+	if !strings.Contains(err.Error(), "elevated") {
+		t.Fatalf("error %q should point at elevation", err)
+	}
+}
+
+// The Win32 structs are passed to netapi32 as raw buffers, so their layout must
+// match what the API expects. A wrong size means silent memory corruption.
+func TestWindowsIdentityStructLayouts(t *testing.T) {
+	ptr := unsafe.Sizeof(uintptr(0))
+	if got, want := unsafe.Sizeof(localGroupMembersInfo3{}), ptr; got != want {
+		t.Fatalf("LOCALGROUP_MEMBERS_INFO_3 size = %d, want %d", got, want)
+	}
+	if got, want := unsafe.Sizeof(localGroupInfo1{}), 2*ptr; got != want {
+		t.Fatalf("LOCALGROUP_INFO_1 size = %d, want %d", got, want)
+	}
+	// USER_INFO_1 is four pointers plus three DWORDs, with the compiler padding
+	// each DWORD pair up to pointer alignment on amd64.
+	if got := unsafe.Sizeof(userInfo1{}); got < 4*ptr {
+		t.Fatalf("USER_INFO_1 size = %d, smaller than its four pointer fields", got)
+	}
+	if unsafe.Offsetof(userInfo1{}.Password) != ptr {
+		t.Fatal("USER_INFO_1.Password must directly follow Name")
+	}
+}
+
+// LSA_UNICODE_STRING counts BYTES, not runes, and excludes the NUL terminator
+// from Length while including it in MaximumLength. Getting either wrong makes
+// LsaAddAccountRights read past the buffer or silently match no right, so pin it.
+func TestNewLSAStringUsesByteLengths(t *testing.T) {
+	buffer, err := windows.UTF16FromString("SeBatchLogonRight")
+	if err != nil {
+		t.Fatalf("encode: %v", err)
+	}
+	entry := newLSAString(buffer)
+	const runes uint16 = uint16(len("SeBatchLogonRight"))
+	if entry.Length != runes*2 {
+		t.Fatalf("Length = %d, want %d (bytes, excluding NUL)", entry.Length, runes*2)
+	}
+	if entry.MaximumLength != (runes+1)*2 {
+		t.Fatalf("MaximumLength = %d, want %d (bytes, including NUL)", entry.MaximumLength, (runes+1)*2)
+	}
+	if entry.Buffer == nil {
+		t.Fatal("Buffer must point at the encoded string")
+	}
+}
+
+// An empty buffer must not produce a struct pointing at nothing with a nonzero
+// length, which would hand LSA a wild pointer.
+func TestNewLSAStringHandlesEmptyBuffer(t *testing.T) {
+	entry := newLSAString(nil)
+	if entry.Buffer != nil || entry.Length != 0 || entry.MaximumLength != 0 {
+		t.Fatalf("empty buffer produced %+v, want a zero value", entry)
+	}
+}
+
+// The LSA structs are passed to advapi32 as raw buffers, so their sizes must
+// match the Win32 definitions.
+func TestLSAStructLayouts(t *testing.T) {
+	ptr := unsafe.Sizeof(uintptr(0))
+	// LSA_UNICODE_STRING: two uint16 then a pointer, padded to pointer alignment.
+	if got, want := unsafe.Sizeof(lsaUnicodeString{}), 2*ptr; got != want {
+		t.Fatalf("LSA_UNICODE_STRING size = %d, want %d", got, want)
+	}
+	if unsafe.Offsetof(lsaUnicodeString{}.Buffer) != ptr {
+		t.Fatal("LSA_UNICODE_STRING.Buffer must sit at the second pointer slot")
+	}
+	var attributes lsaObjectAttributes
+	if unsafe.Sizeof(attributes) < 6*ptr-ptr {
+		t.Fatalf("LSA_OBJECT_ATTRIBUTES size = %d, smaller than its fields", unsafe.Sizeof(attributes))
+	}
+	if unsafe.Offsetof(attributes.ObjectName) == 0 {
+		t.Fatal("LSA_OBJECT_ATTRIBUTES.ObjectName must not alias Length")
+	}
+}
+
+// Provisioning creates real local accounts, so it only runs when explicitly
+// opted into on an elevated machine. Everything above covers the logic that can
+// be exercised without touching the account database.
+func TestProvisionWindowsSandboxIdentityRoundTrip(t *testing.T) {
+	if os.Getenv("ZERO_WINDOWS_IDENTITY_PROVISION_TEST") != "1" {
+		// Spelled out per shell because `set VAR=1` is cmd syntax and silently
+		// sets a shell variable rather than an environment variable in
+		// PowerShell, which makes this skip look like the elevation check failing.
+		t.Skip("provisioning test not enabled: PowerShell `$env:ZERO_WINDOWS_IDENTITY_PROVISION_TEST = \"1\"`, cmd `set ZERO_WINDOWS_IDENTITY_PROVISION_TEST=1`, bash `export ZERO_WINDOWS_IDENTITY_PROVISION_TEST=1` (also needs an elevated terminal)")
+	}
+	if !windowsProcessIsElevated() {
+		t.Skip("provisioning requires an elevated process")
+	}
+	// Starting clean keeps a failure here from being explained by residue from a
+	// previous run. Provisioning no longer resets an adopted account's password,
+	// so a leftover account would otherwise be adopted with a password this test
+	// never learns.
+	_ = removeWindowsSandboxIdentity(windowsSandboxUserName("ziptest01", windowsSandboxRoleOffline), "ziptest01")
+
+	identity, password, _, err := provisionWindowsSandboxIdentity("ziptest01", windowsSandboxRoleOffline)
+	if err != nil {
+		t.Fatalf("provision: %v", err)
+	}
+	// Registered immediately after provisioning so every failure path below is
+	// covered. This test grants a real batch-logon right to a real local account;
+	// leaving either behind on a developer machine is not acceptable residue, and
+	// rights are revoked before the account so nothing is left keyed to a SID that
+	// no longer resolves.
+	t.Cleanup(func() {
+		if err := revokeWindowsSandboxLogonRights(identity.SID); err != nil {
+			t.Errorf("cleanup: revoke logon rights: %v", err)
+		}
+		if err := removeWindowsSandboxIdentity(identity.Username, "ziptest01"); err != nil && !errors.Is(err, errWindowsSandboxForeignAccountRetained) {
+			t.Errorf("cleanup: remove principal: %v", err)
+		}
+	})
+	if identity.SID == nil {
+		t.Fatal("provisioned identity has no SID")
+	}
+	if password == "" {
+		t.Fatal("provisioning returned an empty password")
+	}
+	// Re-running must converge on the same principal rather than failing or
+	// creating a second account.
+	again, secondPassword, _, err := provisionWindowsSandboxIdentity("ziptest01", windowsSandboxRoleOffline)
+	if err != nil {
+		t.Fatalf("second provision: %v", err)
+	}
+	if again.Username != identity.Username || !again.SID.Equals(identity.SID) {
+		t.Fatalf("provisioning is not idempotent: %s then %s", identity, again)
+	}
+	if secondPassword == "" {
+		t.Fatal("second provision returned an empty password")
+	}
+	// Deliberately NOT logging on with secondPassword. Provisioning does not
+	// rotate an adopted account any more, so that value is a fresh random string
+	// the account does not hold; rotation happens in
+	// provisionWindowsSandboxPrincipalForSetup, immediately before the secret is
+	// written, to keep the window where no stored password authenticates as small
+	// as possible.
+	//
+	// The guarantee worth asserting is therefore the one the setup path makes:
+	// after it returns, the stored secret logs the principal on. That covers
+	// rotation, the secret write and the logon right in one assertion, and it is
+	// the thing a broken re-setup would actually break.
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:    t.TempDir(),
+		WorkspaceRoots: []string{`C:\ziptest01`},
+	}
+	setupIdentity, _, err := provisionWindowsSandboxPrincipalForSetup(config, windowsSandboxRoleOffline)
+	if err != nil {
+		t.Fatalf("setup provision: %v", err)
+	}
+	secretPath, err := windowsSandboxSecretPath(config.SandboxHome, setupIdentity.Username)
+	if err != nil {
+		t.Fatalf("secret path: %v", err)
+	}
+	storedPassword, err := readWindowsSandboxSecret(secretPath)
+	if err != nil {
+		t.Fatalf("read stored secret: %v", err)
+	}
+	token, err := logonWindowsSandboxPrincipal(setupIdentity.Username, storedPassword)
+	if err != nil {
+		t.Fatalf("logon with the secret the setup path stored: %v", err)
+	}
+	_ = token.Close()
+	t.Cleanup(func() {
+		_ = revokeWindowsSandboxLogonRights(setupIdentity.SID)
+		_ = removeWindowsSandboxIdentity(setupIdentity.Username, "ziptest01")
+	})
+	// Lookup must find what provisioning created.
+	found, err := lookupWindowsSandboxIdentity("ziptest01", windowsSandboxRoleOffline)
+	if err != nil {
+		t.Fatalf("lookup after provision: %v", err)
+	}
+	if !found.SID.Equals(identity.SID) {
+		t.Fatalf("lookup returned %s, want %s", found, identity)
+	}
+}
+
+// The other half of the privileged chain: granting logon rights and actually
+// minting a token. Provisioning proves the account exists; this proves it is
+// USABLE, which is the part the runner depends on.
+//
+// The batch logon doubles as the assertion that LsaAddAccountRights worked. A
+// LOGON32_LOGON_BATCH logon fails with ERROR_LOGON_TYPE_NOT_GRANTED (1385)
+// unless SeBatchLogonRight is actually held, so a token coming back is proof the
+// grant landed rather than merely that the call returned success.
+//
+// Creates a real local account and removes it again, so it is gated the same way
+// as the provisioning round-trip.
+func TestGrantLogonRightsAndMintPrincipalToken(t *testing.T) {
+	if os.Getenv("ZERO_WINDOWS_IDENTITY_PROVISION_TEST") != "1" {
+		t.Skip("provisioning test not enabled: PowerShell `$env:ZERO_WINDOWS_IDENTITY_PROVISION_TEST = \"1\"`, cmd `set ZERO_WINDOWS_IDENTITY_PROVISION_TEST=1`, bash `export ZERO_WINDOWS_IDENTITY_PROVISION_TEST=1` (also needs an elevated terminal)")
+	}
+	if !windowsProcessIsElevated() {
+		t.Skip("granting logon rights requires an elevated process")
+	}
+
+	const key = "ziplogon01"
+	// A leftover account from an interrupted run would keep its old password,
+	// which the freshly generated one will not match, so start from a clean slate.
+	_ = removeWindowsSandboxIdentity(windowsSandboxUserName(key, windowsSandboxRoleOffline), key)
+
+	identity, password, _, err := provisionWindowsSandboxIdentity(key, windowsSandboxRoleOffline)
+	if err != nil {
+		t.Fatalf("provision: %v", err)
+	}
+	t.Cleanup(func() {
+		// Rights first, then the account: the reverse order strands an LSA entry
+		// keyed to a SID that no longer resolves.
+		if err := revokeWindowsSandboxLogonRights(identity.SID); err != nil {
+			t.Errorf("cleanup: revoke logon rights: %v", err)
+		}
+		if err := removeWindowsSandboxIdentity(identity.Username, key); err != nil && !errors.Is(err, errWindowsSandboxForeignAccountRetained) {
+			t.Errorf("cleanup: remove principal: %v", err)
+		}
+	})
+
+	// Exercises LsaAddAccountRights, including the LSA_UNICODE_STRING byte-length
+	// handling that nothing else has run.
+	if err := grantWindowsSandboxLogonRights(identity.SID); err != nil {
+		t.Fatalf("grant logon rights: %v", err)
+	}
+	// Idempotent: setup re-runs must not fail on rights the account already holds.
+	if err := grantWindowsSandboxLogonRights(identity.SID); err != nil {
+		t.Fatalf("granting logon rights twice must succeed: %v", err)
+	}
+
+	token, err := logonWindowsSandboxPrincipal(identity.Username, password)
+	if err != nil {
+		t.Fatalf("logon as principal: %v", err)
+	}
+	defer token.Close()
+
+	// The token must BE the principal. If this came back as the caller, the whole
+	// identity boundary would be an illusion and reads would still run as the user.
+	user, err := token.GetTokenUser()
+	if err != nil {
+		t.Fatalf("token user: %v", err)
+	}
+	if !user.User.Sid.Equals(identity.SID) {
+		t.Fatalf("token belongs to %s, want the principal %s", user.User.Sid, identity.SID)
+	}
+	t.Logf("minted a token for %s", identity)
+}
+
+// A workspace with no provisioned principal must report the actionable
+// "run setup" error rather than a raw lookup failure, so the command path can
+// fall back instead of surfacing a Win32 code.
+func TestLookupWindowsSandboxIdentityUnprovisioned(t *testing.T) {
+	_, err := lookupWindowsSandboxIdentity("nosuchworkspacekey9z", windowsSandboxRoleOffline)
+	if err == nil {
+		t.Skip("a principal for this key unexpectedly exists on this machine")
+	}
+	if err != errWindowsSandboxIdentityUnavailable {
+		t.Fatalf("error = %v, want errWindowsSandboxIdentityUnavailable", err)
+	}
+}
+
+// A name that resolves to something other than a user account is a conflict,
+// not an absent principal, and must not be reported as "setup has not run": the
+// command path treats that sentinel as permission to fall back silently, so
+// collapsing the two would hide a squatted account behind a quiet downgrade to
+// the restricted token.
+//
+// Every machine already has well-known non-user names to test against, so this
+// needs no privilege and no provisioning.
+func TestLookupWindowsSandboxIdentityRejectsNonUserAccount(t *testing.T) {
+	// Groups that exist on any Windows install. Whichever resolves first is
+	// enough; localized machines may not carry the English name.
+	for _, group := range []string{"Administrators", "Users", "Guests"} {
+		sid, _, accountType, err := windows.LookupSID("", group)
+		if err != nil || sid == nil {
+			continue
+		}
+		if accountType == windows.SidTypeUser {
+			continue
+		}
+		resolveErr := func() error {
+			_, err := resolveWindowsSandboxSID(group)
+			return err
+		}()
+		if resolveErr == nil {
+			t.Fatalf("resolveWindowsSandboxSID(%q) accepted a non-user account (type %d)", group, accountType)
+		}
+		// The classification is the part that matters: the sentinel is what
+		// licenses the command path to fall back silently, so this refusal must
+		// survive it rather than be folded into it.
+		if classified := classifyWindowsSandboxLookupError(resolveErr); errors.Is(classified, errWindowsSandboxIdentityUnavailable) {
+			t.Fatalf("non-user account %q classified as unprovisioned, which would silently downgrade to the restricted token: %v", group, classified)
+		}
+		return
+	}
+	t.Skip("no well-known non-user account resolved on this machine")
+}
+
+// The account name is derived from a workspace hash, not discovered, so it can
+// be occupied by a local account that has nothing to do with Zero. Adopting one
+// means resetting a stranger's password, so provisioning has to prove ownership
+// first and refuse otherwise.
+//
+// Driven against real accounts every Windows install carries, which are
+// definitively not ours. Unprivileged: it only has to establish that they are
+// not classified as managed, so nothing is ever created or modified.
+func TestWindowsSandboxUserIsManagedRefusesForeignAccounts(t *testing.T) {
+	checked := 0
+	for _, name := range []string{"Administrator", "Guest", "DefaultAccount"} {
+		managed, err := windowsSandboxUserIsManaged(name, "workspacekey")
+		if err != nil {
+			// Localized or disabled installs may not carry every one of these.
+			continue
+		}
+		checked++
+		if managed {
+			t.Fatalf("%q classified as a Zero sandbox principal; provisioning would reset its password", name)
+		}
+	}
+	if checked == 0 {
+		t.Skip("no well-known local account could be queried on this machine")
+	}
+	// An absent account must answer false rather than error, since provisioning
+	// asks this question about names that usually do not exist yet.
+	managed, err := windowsSandboxUserIsManaged("zero-sbx-nosuchacct", "workspacekey")
+	if err != nil {
+		t.Fatalf("querying a missing account: %v", err)
+	}
+	if managed {
+		t.Fatal("a missing account was classified as managed")
+	}
+}
+
+// The refusal has to be a typed, recognisable collision rather than a generic
+// failure, so setup can say what is wrong instead of reporting a Win32 status.
+func TestWindowsSandboxNameCollisionIsTyped(t *testing.T) {
+	wrapped := fmt.Errorf("%w: %q", errWindowsSandboxNameCollision, "zero-sbx-dexample")
+	if !errors.Is(wrapped, errWindowsSandboxNameCollision) {
+		t.Fatal("collision error does not match its sentinel")
+	}
+	if !strings.Contains(wrapped.Error(), "not created by Zero") {
+		t.Fatalf("collision message = %q, want it to say the account is not ours", wrapped.Error())
+	}
+}
+
+// The two roles must never collide onto one account. If they did, the online
+// principal would be whatever the offline one is, which means either an approved
+// network command silently has no network, or an ordinary command silently has
+// one. The second is the dangerous direction.
+func TestWindowsSandboxUserNameSeparatesRoles(t *testing.T) {
+	const key = "abc123"
+	offline := windowsSandboxUserName(key, windowsSandboxRoleOffline)
+	online := windowsSandboxUserName(key, windowsSandboxRoleOnline)
+	if offline == online {
+		t.Fatalf("both roles produced %q", offline)
+	}
+	// Still stable per role, or setup would create a new account every run.
+	if again := windowsSandboxUserName(key, windowsSandboxRoleOnline); again != online {
+		t.Fatalf("online name is not stable: %q then %q", online, again)
+	}
+
+	// The role tag has to survive truncation. A very long key is exactly when the
+	// name is cut to the 20-character limit, and losing the tag there is what
+	// would collide the roles on precisely the workspaces most likely to hit it.
+	longKey := strings.Repeat("f", 200)
+	longOffline := windowsSandboxUserName(longKey, windowsSandboxRoleOffline)
+	longOnline := windowsSandboxUserName(longKey, windowsSandboxRoleOnline)
+	if longOffline == longOnline {
+		t.Fatalf("truncation collided the roles onto %q", longOffline)
+	}
+	for _, name := range []string{longOffline, longOnline} {
+		if len(name) > windowsSandboxUserNameMax {
+			t.Fatalf("name %q is %d chars, want at most %d", name, len(name), windowsSandboxUserNameMax)
+		}
+	}
+
+	// Different workspaces still get different accounts within a role.
+	if other := windowsSandboxUserName("def456", windowsSandboxRoleOffline); other == offline {
+		t.Fatalf("two workspaces produced the same offline account %q", other)
+	}
+}

internal/sandbox/windows_legacy_comment_upgrade_windows_test.go

@@ -0,0 +1,90 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// THE UPGRADE HAS TO BE OBSERVABLE, or it is not really there.
+//
+// windowsSandboxUserIsManaged accepts the old bare account comment and promises
+// the workspace key gets stamped on so the bare form can eventually be retired.
+// The probe for the legacy comment was seamed and the upgrade itself was not, so
+// nothing in the suite could tell whether the call ran: deleting it left
+// everything green. This repository has already had a fix silently reverted by a
+// later change, which is the failure this pins.
+func TestAdoptingALegacyAccountStampsTheWorkspaceKey(t *testing.T) {
+	stub := func(t *testing.T, legacy bool) *string {
+		t.Helper()
+		var upgraded string
+
+		prevGroup := ensureWindowsSandboxGroupFn
+		prevOffline := ensureWindowsSandboxOfflineGroupFn
+		prevEnsure := ensureWindowsSandboxUserFn
+		prevManaged := windowsSandboxUserIsManagedFn
+		prevLegacy := windowsSandboxUserHasLegacyCommentFn
+		prevPrivileged := windowsSandboxUserIsPrivilegedFn
+		prevUpgrade := upgradeWindowsSandboxUserCommentFn
+		prevReset := resetWindowsSandboxUserPasswordFn
+		prevGroupAdd := addWindowsSandboxUserToGroupFn
+		prevOfflineAdd := addWindowsSandboxUserToOfflineGroupFn
+		prevSID := resolveWindowsSandboxSIDFn
+		t.Cleanup(func() {
+			ensureWindowsSandboxGroupFn = prevGroup
+			ensureWindowsSandboxOfflineGroupFn = prevOffline
+			ensureWindowsSandboxUserFn = prevEnsure
+			windowsSandboxUserIsManagedFn = prevManaged
+			windowsSandboxUserHasLegacyCommentFn = prevLegacy
+			windowsSandboxUserIsPrivilegedFn = prevPrivileged
+			upgradeWindowsSandboxUserCommentFn = prevUpgrade
+			resetWindowsSandboxUserPasswordFn = prevReset
+			addWindowsSandboxUserToGroupFn = prevGroupAdd
+			addWindowsSandboxUserToOfflineGroupFn = prevOfflineAdd
+			resolveWindowsSandboxSIDFn = prevSID
+		})
+
+		ensureWindowsSandboxGroupFn = func() error { return nil }
+		ensureWindowsSandboxOfflineGroupFn = func() error { return nil }
+		// existed = true: the adoption path, which is the only one that upgrades.
+		ensureWindowsSandboxUserFn = func(string, string, string) (bool, error) { return true, nil }
+		windowsSandboxUserIsManagedFn = func(string, string) (bool, error) { return true, nil }
+		windowsSandboxUserHasLegacyCommentFn = func(string) (bool, error) { return legacy, nil }
+		windowsSandboxUserIsPrivilegedFn = func(string) (bool, error) { return false, nil }
+		upgradeWindowsSandboxUserCommentFn = func(_ string, workspaceKey string) error {
+			upgraded = workspaceKey
+			return nil
+		}
+		resetWindowsSandboxUserPasswordFn = func(string, string) error { return nil }
+		addWindowsSandboxUserToGroupFn = func(string) error { return nil }
+		addWindowsSandboxUserToOfflineGroupFn = func(string) error { return nil }
+		resolveWindowsSandboxSIDFn = func(string) (*windows.SID, error) {
+			return windows.StringToSid("S-1-5-32-546")
+		}
+		return &upgraded
+	}
+
+	t.Run("a legacy comment is upgraded", func(t *testing.T) {
+		upgraded := stub(t, true)
+		if _, _, _, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline); err != nil {
+			t.Fatalf("provisionWindowsSandboxIdentity: %v", err)
+		}
+		if *upgraded != "workspacekey" {
+			t.Errorf("the workspace key was never stamped onto the adopted legacy account (got %q); windowsSandboxUserIsManaged goes on accepting the bare comment forever", *upgraded)
+		}
+	})
+
+	// And an account that already carries the key is left alone, or the
+	// assertion above would be satisfied by an unconditional rewrite.
+	t.Run("a current comment is left alone", func(t *testing.T) {
+		upgraded := stub(t, false)
+		if _, _, _, err := provisionWindowsSandboxIdentity("workspacekey", windowsSandboxRoleOffline); err != nil {
+			t.Fatalf("provisionWindowsSandboxIdentity: %v", err)
+		}
+		if *upgraded != "" {
+			t.Errorf("an account that already carried the workspace key was rewritten anyway: %q", *upgraded)
+		}
+	})
+}

internal/sandbox/windows_legacy_principal_windows_test.go

@@ -0,0 +1,108 @@
+package sandbox
+
+import (
+	"errors"
+	"sort"
+	"strings"
+	"testing"
+)
+
+// The predecessor of this branch provisioned ONE account per workspace, named
+// "zero-sbx-<key>" with no role tag. Splitting the roles changed both names, so
+// on an upgraded machine neither role resolves to the account that is actually
+// installed. Every other path on this branch then reports the principal as never
+// provisioned, and the untagged account keeps its secret, its logon rights, its
+// ACEs and its ledger with nothing left that looks for them.
+//
+// windowsSandboxRoleLegacy exists to reproduce that name exactly so retirement
+// can still find it. If its tag ever stops being empty, this stops matching what
+// the predecessor wrote and the residue becomes unreachable again.
+func TestLegacyRoleReproducesTheUntaggedPredecessorName(t *testing.T) {
+	// Short enough that the 20-character account-name limit does not truncate,
+	// so the comparison is against the spelling itself rather than the cap.
+	const key = "abc123"
+
+	legacy := windowsSandboxUserName(key, windowsSandboxRoleLegacy)
+	if want := windowsSandboxUserPrefix + key; legacy != want {
+		t.Fatalf("legacy account name = %q, want the pre-split spelling %q", legacy, want)
+	}
+	// And the cap still applies to it exactly as it did before the split, since
+	// the predecessor truncated the same way.
+	long := windowsSandboxUserName("averylongworkspacekeyindeed", windowsSandboxRoleLegacy)
+	if len(long) > windowsSandboxUserNameMax {
+		t.Errorf("legacy name %q is %d characters, over the %d-character limit", long, len(long), windowsSandboxUserNameMax)
+	}
+	if tag := windowsSandboxRoleLegacy.roleTag(); tag != "" {
+		t.Errorf("legacy role tag = %q, want empty; a tag here would stop matching the installed account", tag)
+	}
+
+	// It must not collide with either live role, or retiring the predecessor
+	// would delete an account this branch is still using.
+	offline := windowsSandboxUserName(key, windowsSandboxRoleOffline)
+	online := windowsSandboxUserName(key, windowsSandboxRoleOnline)
+	for _, live := range []string{offline, online} {
+		if strings.EqualFold(legacy, live) {
+			t.Errorf("legacy name %q collides with a live role name %q", legacy, live)
+		}
+	}
+	if strings.EqualFold(offline, online) {
+		t.Fatalf("the two live roles derived the same account name %q", offline)
+	}
+}
+
+// THE LEGACY ROLE IS RETIRED, NEVER PROVISIONED. It derives the untagged name
+// on purpose, so provisioning it would create the very account this branch
+// replaced, on every setup, forever. Adding it to the retirement list and the
+// provisioning list are one character apart in the source and the difference is
+// invisible at a glance, so it is asserted rather than left to review.
+func TestSetupNeverProvisionsTheLegacyPrincipal(t *testing.T) {
+	previous := provisionWindowsSandboxPrincipalForSetupFn
+	t.Cleanup(func() { provisionWindowsSandboxPrincipalForSetupFn = previous })
+
+	var provisioned []windowsSandboxRole
+	provisionWindowsSandboxPrincipalForSetupFn = func(_ WindowsSandboxCommandConfig, role windowsSandboxRole) (windowsSandboxIdentity, bool, error) {
+		provisioned = append(provisioned, role)
+		return windowsSandboxIdentity{}, false, errStopProvisioningForTest
+	}
+
+	// The error stops setup at the first role; what matters is which roles it
+	// was willing to ask for, and legacy must never be among them.
+	_, _ = setupWindowsSandboxPrincipal(WindowsSandboxCommandConfig{SandboxHome: t.TempDir()})
+
+	for _, role := range provisioned {
+		if role == windowsSandboxRoleLegacy {
+			t.Fatalf("setup tried to provision the legacy role, which would recreate the pre-split account: %v", provisioned)
+		}
+	}
+}
+
+var errStopProvisioningForTest = errors.New("stop provisioning for test")
+
+// Opting out or re-running setup has to retire the predecessor as well as both
+// live roles. Retiring only the two this branch knows about is what leaves a
+// fully provisioned account on a machine whose marker says it has none.
+func TestSetupRetirementCoversTheLegacyPrincipal(t *testing.T) {
+	previous := removeWindowsSandboxPrincipalForSetupFn
+	t.Cleanup(func() { removeWindowsSandboxPrincipalForSetupFn = previous })
+
+	var retired []string
+	removeWindowsSandboxPrincipalForSetupFn = func(_ WindowsSandboxCommandConfig, role windowsSandboxRole) error {
+		retired = append(retired, string(role))
+		return nil
+	}
+
+	if err := removeWindowsSandboxPrincipalsForSetup(WindowsSandboxCommandConfig{SandboxHome: t.TempDir()}); err != nil {
+		t.Fatalf("removeWindowsSandboxPrincipalsForSetup: %v", err)
+	}
+
+	sort.Strings(retired)
+	want := []string{"legacy", "offline", "online"}
+	if len(retired) != len(want) {
+		t.Fatalf("retired %v, want all of %v; a role left out keeps its account and everything it owns", retired, want)
+	}
+	for index := range want {
+		if retired[index] != want[index] {
+			t.Fatalf("retired %v, want %v", retired, want)
+		}
+	}
+}

internal/sandbox/windows_network.go

@@ -62,15 +62,190 @@ func BuildWindowsNetworkInfraPlan(config WindowsSandboxCommandConfig) (WindowsNe
 	if err != nil {
 		return WindowsNetworkPlan{}, err
 	}
+	// The offline-marker SID stays first: the setup marker records
+	// IdentitySIDs[0] as the offline filter identity.
+	identitySIDs := []string{offlineSID}
+	// A sandbox principal cannot carry the offline-marker SID, because LogonUser
+	// builds a token from an account's real group memberships and the marker is a
+	// synthetic capability SID. So the same filters additionally match a real
+	// local group that network-denied principals belong to.
+	//
+	// Gated on THIS home's opt-in, not merely on the group existing.
+	//
+	// The group is machine-global, so keying off its existence meant the first
+	// workspace to opt in changed the computed plan for every OTHER sandbox home
+	// on the machine. The plan is hashed into the setup marker and compared on
+	// every command, so those homes started failing every command with "setup is
+	// out of date" until each was re-run from an elevated terminal, having never
+	// opted into anything. Existing markers have to keep validating.
+	//
+	// Reading the opt-in instead keeps an opted-out home computing exactly the
+	// plan it computed before any of this existed. An opted-in home records the
+	// group in its own marker, and setup and the command path both derive the
+	// flag from the same environment, so they agree. Opting in AFTER setup does
+	// invalidate that home's marker, which is correct: it has no principals yet
+	// and setup is genuinely required.
+	if windowsSandboxIdentityEnabled(config.Env) && resolveWindowsSandboxOfflineGroupSIDHook != nil {
+		groupSID, err := resolveWindowsSandboxOfflineGroupSIDHook()
+		if err != nil {
+			return WindowsNetworkPlan{}, err
+		}
+		if trimmed := strings.TrimSpace(groupSID); trimmed != "" {
+			identitySIDs = append(identitySIDs, trimmed)
+		}
+	}
 	return WindowsNetworkPlan{
 		Mode:         NetworkDeny,
 		ProviderKey:  windowsWFPProviderKey,
 		SubLayerKey:  windowsWFPSubLayerKey,
-		IdentitySIDs: []string{offlineSID},
+		IdentitySIDs: identitySIDs,
 		Filters:      windowsDenyWFPFilterSpecs(),
 	}, nil
 }
 
+// WindowsNetworkPlanForApply returns the plan to INSTALL, which is not always
+// the plan a home fingerprints.
+//
+// The filters are machine-global and every setup installs them by deleting and
+// recreating the fixed set. So an opted-OUT setup for workspace B was replacing
+// filters that workspace A's opted-in setup had installed, dropping the offline
+// group SID as it went. A's offline principal still passed the runtime
+// membership check and A's marker still validated, but no filter matched its
+// token any more, so a NetworkDeny command in A quietly gained egress. Nothing
+// on either side reported anything: B did exactly what it was asked to.
+//
+// The gate in BuildWindowsNetworkInfraPlan stays as it is, and this is
+// deliberately a SEPARATE function rather than a change to it. That gate exists
+// because the plan is hashed into each home's setup marker: keying it on the
+// group's existence made the first workspace to opt in invalidate every other
+// home's marker on the machine, and those homes then failed every command with
+// "setup is out of date" having opted into nothing. That has to keep working.
+//
+// The two questions are simply different. What a home RECORDS is about that
+// home's own configuration; what setup INSTALLS is about the machine, where the
+// group either exists or does not. Answering both from one plan is what forced a
+// choice between stale markers and a silent hole.
+//
+// The group not existing is the ordinary opted-out machine, and it adds nothing.
+//
+// A resolution FAILURE is a different answer and is fatal, because the filters
+// are replaced wholesale. The earlier reasoning here was that refusing to
+// install would trade a partial denial for no denial at all, and that is wrong:
+// the alternative to installing is leaving the filters that are already there
+// alone. Returning a marker-only plan does not decline to add coverage, it
+// actively REMOVES coverage another workspace is relying on. An opted-out home
+// skips assertWindowsNetworkPlanCoversOfflineGroup entirely (it is gated on
+// provisioned), so this is the only thing between a transient lookup error and
+// workspace A silently regaining egress under NetworkDeny.
+//
+// "Absent" and "could not be read" have to be different answers, which is the
+// same distinction the marker/apply split above is built on.
+func WindowsNetworkPlanForApply(plan WindowsNetworkPlan, resolveOfflineGroupSID func() (string, error)) (WindowsNetworkPlan, error) {
+	if resolveOfflineGroupSID == nil {
+		return plan, nil
+	}
+	groupSID, err := resolveOfflineGroupSID()
+	if err != nil {
+		return WindowsNetworkPlan{}, fmt.Errorf("resolve the sandbox offline group before replacing the machine network filters: %w", err)
+	}
+	trimmed := strings.TrimSpace(groupSID)
+	if trimmed == "" {
+		return plan, nil
+	}
+	for _, existing := range plan.IdentitySIDs {
+		if strings.EqualFold(strings.TrimSpace(existing), trimmed) {
+			return plan, nil
+		}
+	}
+	// Copied rather than appended in place: the caller's plan is what gets
+	// fingerprinted, and growing its backing array would risk changing the
+	// recorded plan as a side effect of installing one.
+	augmented := plan
+	augmented.IdentitySIDs = append(append([]string{}, plan.IdentitySIDs...), trimmed)
+	return augmented, nil
+}
+
+// WindowsNetworkPlanCoversPrincipals reports whether a plan's block filters name
+// the offline group, which is the only thing that makes them apply to a sandbox
+// principal.
+//
+// This exists to be asserted, not consulted. The plan must be built AFTER
+// provisioning, because provisioning is what creates the group it resolves; a
+// plan built first names only the offline marker and leaves every offline
+// principal with an open network while setup reports success. That is a control
+// that enforces nothing while claiming to work, and the ordering which prevents
+// it is invisible at the call site, so a later refactor can undo it silently.
+func WindowsNetworkPlanCoversPrincipals(plan WindowsNetworkPlan, offlineGroupSID string) bool {
+	offlineGroupSID = strings.TrimSpace(offlineGroupSID)
+	if offlineGroupSID == "" {
+		// No group provisioned on this host, so there is no principal for the
+		// filters to miss and nothing to assert.
+		return true
+	}
+	for _, sid := range plan.IdentitySIDs {
+		if strings.EqualFold(strings.TrimSpace(sid), offlineGroupSID) {
+			return true
+		}
+	}
+	return false
+}
+
+// assertWindowsNetworkPlanCoversOfflineGroup is the post-provisioning check that
+// setup runs before it installs filters and reports success.
+//
+// It fails closed on every answer that is not a definite yes, because the thing
+// it guards is invisible when it goes wrong: filters that do not name the
+// offline group leave every offline principal with an open network on a machine
+// whose setup marker says it is protected. Nobody sees an error, and the sandbox
+// looks correctly installed.
+//
+// Two answers other than "the plan omits the group" also mean the check did not
+// happen and must not be treated as a pass:
+//
+//   - The lookup failed. Whether the group is covered is then unknown, and the
+//     Win32 reason is worth surfacing: an operator seeing "access is denied"
+//     knows to re-run elevated.
+//   - The lookup succeeded and found nothing. ("", nil) means the group does not
+//     exist, which is the ordinary state BEFORE provisioning and an impossible
+//     one after it, so here it means the group vanished or was never created.
+//     WindowsNetworkPlanCoversPrincipals answers true for an empty SID, correctly
+//     for the pre-provisioning callers that ask it, so the emptiness has to be
+//     rejected here rather than delegated to it.
+//
+// provisioned says whether principals were actually provisioned on this run.
+// When they were not there is no offline group, no principal carrying it, and
+// nothing for the filters to miss, so there is nothing to assert. Running the
+// check anyway turns the ordinary opt-out setup into a hard failure, because a
+// machine with no group resolves to ("", nil) and the empty-SID rejection below
+// is correct only after provisioning.
+func assertWindowsNetworkPlanCoversOfflineGroup(plan WindowsNetworkPlan, resolve func() (string, error), provisioned bool) error {
+	if !provisioned {
+		return nil
+	}
+	if resolve == nil {
+		return errors.New("sandbox offline group resolver is not wired up, so filter coverage cannot be verified")
+	}
+	groupSID, err := resolve()
+	if err != nil {
+		return fmt.Errorf("resolve the sandbox offline group after provisioning, so filter coverage cannot be verified: %w", err)
+	}
+	if strings.TrimSpace(groupSID) == "" {
+		return errors.New("the sandbox offline group does not exist after provisioning, so the block filters would not apply to any sandbox principal")
+	}
+	if !WindowsNetworkPlanCoversPrincipals(plan, groupSID) {
+		return errors.New("network block filters do not name the sandbox offline group, so they would not apply to any sandbox principal; the network plan must be built after principals are provisioned")
+	}
+	return nil
+}
+
+// resolveWindowsSandboxOfflineGroupSIDHook resolves the local group that
+// network-denied principals belong to. It is wired up on Windows only, so this
+// file stays free of Win32 calls and the plan on other platforms is unchanged.
+//
+// Returning ("", nil) means the group does not exist yet, which is the state
+// before principals have ever been provisioned.
+var resolveWindowsSandboxOfflineGroupSIDHook func() (string, error)
+
 // WindowsNetworkInfraHash fingerprints the provisioned (mode-independent) network
 // infrastructure so the setup marker validates against the same setup for BOTH
 // command modes. It never folds in the per-command network mode.

internal/sandbox/windows_network_coverage_assert_test.go

@@ -0,0 +1,129 @@
+package sandbox
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+// The post-provisioning coverage assertion is the last thing standing between a
+// mis-ordered plan and a machine that reports a successful setup while every
+// offline principal has an open network. It has to fail closed on every way of
+// not knowing the answer, not just on a definite negative.
+func TestAssertWindowsNetworkPlanCoversOfflineGroupFailsClosed(t *testing.T) {
+	const groupSID = "S-1-5-32-9999"
+	covering := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-marker", groupSID}}
+	notCovering := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-marker"}}
+	lookupFailed := errors.New("look up zero-sandbox-offline: access is denied")
+
+	tests := []struct {
+		name     string
+		plan     WindowsNetworkPlan
+		resolve  func() (string, error)
+		wantErr  bool
+		wantText string
+	}{
+		{
+			name:    "plan names the group",
+			plan:    covering,
+			resolve: func() (string, error) { return groupSID, nil },
+		},
+		{
+			// The finding anandh8x raised: a failed lookup skipped the assertion
+			// entirely and setup carried on to write a success marker.
+			name:     "lookup fails",
+			plan:     covering,
+			resolve:  func() (string, error) { return "", lookupFailed },
+			wantErr:  true,
+			wantText: "access is denied",
+		},
+		{
+			// Empty-and-no-error means "the group does not exist", which is a
+			// legitimate answer BEFORE provisioning and an impossible one after it.
+			// WindowsNetworkPlanCoversPrincipals answers true for an empty SID, so
+			// without an explicit check the assertion passes vacuously.
+			name:     "group missing after provisioning",
+			plan:     covering,
+			resolve:  func() (string, error) { return "", nil },
+			wantErr:  true,
+			wantText: "does not exist",
+		},
+		{
+			name:     "whitespace-only SID",
+			plan:     covering,
+			resolve:  func() (string, error) { return "   ", nil },
+			wantErr:  true,
+			wantText: "does not exist",
+		},
+		{
+			name:     "plan omits the group",
+			plan:     notCovering,
+			resolve:  func() (string, error) { return groupSID, nil },
+			wantErr:  true,
+			wantText: "do not name the sandbox offline group",
+		},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			err := assertWindowsNetworkPlanCoversOfflineGroup(test.plan, test.resolve, true)
+			if test.wantErr && err == nil {
+				t.Fatalf("assertWindowsNetworkPlanCoversOfflineGroup() = nil, want an error so setup rolls back")
+			}
+			if !test.wantErr {
+				if err != nil {
+					t.Fatalf("assertWindowsNetworkPlanCoversOfflineGroup() = %v, want nil", err)
+				}
+				return
+			}
+			if !strings.Contains(err.Error(), test.wantText) {
+				t.Fatalf("error = %q, want it to mention %q so the operator can tell the cases apart", err, test.wantText)
+			}
+		})
+	}
+
+	// The lookup failure has to stay unwrapped-comparable: setup logs it and an
+	// operator needs the underlying Win32 reason, not a flattened string.
+	err := assertWindowsNetworkPlanCoversOfflineGroup(covering, func() (string, error) { return "", lookupFailed }, true)
+	if !errors.Is(err, lookupFailed) {
+		t.Fatalf("errors.Is(err, lookupFailed) = false; the cause was dropped: %v", err)
+	}
+}
+
+// The assert only means something AFTER principals were provisioned. The offline
+// group is created inside provisionWindowsSandboxIdentity, which runs only under
+// the ZERO_WINDOWS_SANDBOX_IDENTITY opt-in, so on a default machine there is no
+// group, no principal, and nothing for the filters to miss.
+//
+// Rejecting an empty SID is right once provisioning ran (that is the vacuous
+// pass anandh8x found) and wrong before it: it turned the ordinary opt-out setup
+// into a hard failure, which is the regression jatmn caught on #812.
+func TestAssertWindowsNetworkPlanCoversOfflineGroupSkipsWhenNothingWasProvisioned(t *testing.T) {
+	plan := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-marker"}}
+
+	// The default machine: no group exists, so the resolver reports ("", nil).
+	if err := assertWindowsNetworkPlanCoversOfflineGroup(plan, func() (string, error) { return "", nil }, false); err != nil {
+		t.Fatalf("opt-out setup was refused: %v", err)
+	}
+	// Same state WITH provisioning claimed: now the missing group is a real fault.
+	if err := assertWindowsNetworkPlanCoversOfflineGroup(plan, func() (string, error) { return "", nil }, true); err == nil {
+		t.Fatal("a missing group after provisioning must still fail closed")
+	}
+	// Not provisioned must not become a licence to skip a lookup failure either:
+	// the resolver is never consulted, so a broken lookup cannot matter here.
+	if err := assertWindowsNetworkPlanCoversOfflineGroup(plan, func() (string, error) {
+		t.Fatal("the resolver must not be consulted when nothing was provisioned")
+		return "", nil
+	}, false); err != nil {
+		t.Fatalf("opt-out setup was refused: %v", err)
+	}
+}
+
+// A nil resolver is a wiring mistake, and guessing that coverage is fine is the
+// one thing it must not do.
+func TestAssertWindowsNetworkPlanCoversOfflineGroupRejectsNilResolver(t *testing.T) {
+	plan := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-32-9999"}}
+	if err := assertWindowsNetworkPlanCoversOfflineGroup(plan, nil, true); err == nil {
+		t.Fatal("assertWindowsNetworkPlanCoversOfflineGroup(nil resolver) = nil, want an error")
+	}
+}

internal/sandbox/windows_network_mixed_optin_test.go

@@ -0,0 +1,163 @@
+package sandbox
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+// THE BLOCK FILTERS ARE MACHINE-GLOBAL, SO ONE WORKSPACE'S SETUP DECIDES ANOTHER
+// WORKSPACE'S NETWORK.
+//
+// Every setup installs them by deleting and recreating one fixed set. The plan
+// each home builds names the offline group only when THAT home opted in, which
+// is right for the marker it fingerprints and wrong for the filters it installs:
+// an ordinary opted-out setup for a second workspace replaced the filters
+// without the group SID, and the first workspace's offline principal — still in
+// the group, still passing the runtime membership check, still holding a valid
+// marker — was no longer matched by any filter. A NetworkDeny command there
+// gained egress, silently, because the second setup did exactly what it was
+// asked to do.
+//
+// The plan a home installs and the plan it records are therefore different
+// questions, and these pin both.
+
+const testOfflineGroupSID = "S-1-5-32-9990"
+
+func offlineGroupResolver(sid string) func() (string, error) {
+	return func() (string, error) { return sid, nil }
+}
+
+// The regression itself: an opted-OUT home must still install filters covering
+// the managed offline group, because an opted-IN home's principals depend on it.
+func TestOptedOutSetupKeepsOfflineGroupCoverageForOtherWorkspaces(t *testing.T) {
+	optedOut, err := BuildWindowsNetworkInfraPlan(WindowsSandboxCommandConfig{
+		SandboxHome: t.TempDir(),
+		// No opt-in in the environment: this is the ordinary workspace.
+		Env: map[string]string{},
+	})
+	if err != nil {
+		t.Fatalf("BuildWindowsNetworkInfraPlan: %v", err)
+	}
+
+	// What this home RECORDS must not change, or every opted-out marker on the
+	// machine goes stale the moment another workspace opts in.
+	if WindowsNetworkPlanCoversPrincipals(optedOut, testOfflineGroupSID) {
+		t.Fatal("the recorded plan named the offline group; that is what invalidated other homes' markers")
+	}
+
+	// What this home INSTALLS must cover the group, since the group exists.
+	applied, err := WindowsNetworkPlanForApply(optedOut, offlineGroupResolver(testOfflineGroupSID))
+	if err != nil {
+		t.Fatalf("WindowsNetworkPlanForApply: %v", err)
+	}
+	if !WindowsNetworkPlanCoversPrincipals(applied, testOfflineGroupSID) {
+		t.Errorf("an opted-out setup would install filters that do not match the offline group, so another workspace's NetworkDeny commands gain egress: %v", applied.IdentitySIDs)
+	}
+	// The offline-marker SID has to survive alongside it: the restricted-token
+	// tier is matched by that one, and it is the whole denial on machines with no
+	// principals at all.
+	if len(applied.IdentitySIDs) == 0 || strings.TrimSpace(applied.IdentitySIDs[0]) == "" {
+		t.Fatalf("the offline-marker SID was lost from the applied plan: %v", applied.IdentitySIDs)
+	}
+	if applied.IdentitySIDs[0] != optedOut.IdentitySIDs[0] {
+		t.Errorf("the marker SID must stay first, since the setup marker records IdentitySIDs[0]: %v", applied.IdentitySIDs)
+	}
+}
+
+// Augmenting must not mutate the plan the caller fingerprints. Appending in
+// place would let installing a plan change the plan that was recorded.
+func TestPlanForApplyDoesNotMutateTheRecordedPlan(t *testing.T) {
+	recorded, err := BuildWindowsNetworkInfraPlan(WindowsSandboxCommandConfig{
+		SandboxHome: t.TempDir(),
+		Env:         map[string]string{},
+	})
+	if err != nil {
+		t.Fatalf("BuildWindowsNetworkInfraPlan: %v", err)
+	}
+	before := append([]string{}, recorded.IdentitySIDs...)
+
+	if _, err := WindowsNetworkPlanForApply(recorded, offlineGroupResolver(testOfflineGroupSID)); err != nil {
+		t.Fatalf("WindowsNetworkPlanForApply: %v", err)
+	}
+
+	if len(recorded.IdentitySIDs) != len(before) {
+		t.Fatalf("the recorded plan grew from %v to %v", before, recorded.IdentitySIDs)
+	}
+	for index := range before {
+		if recorded.IdentitySIDs[index] != before[index] {
+			t.Errorf("the recorded plan changed at %d: %v vs %v", index, before, recorded.IdentitySIDs)
+		}
+	}
+}
+
+// An opted-IN home already names the group, and must not name it twice: a
+// duplicate identity would change the installed filter set for no reason.
+func TestPlanForApplyDoesNotDuplicateAnAlreadyCoveredGroup(t *testing.T) {
+	plan := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-marker", testOfflineGroupSID}}
+	applied, err := WindowsNetworkPlanForApply(plan, offlineGroupResolver(strings.ToLower(testOfflineGroupSID)))
+	if err != nil {
+		t.Fatalf("WindowsNetworkPlanForApply: %v", err)
+	}
+	if len(applied.IdentitySIDs) != 2 {
+		t.Errorf("the group was added again despite already being covered: %v", applied.IdentitySIDs)
+	}
+}
+
+// On an ordinary machine with no principals the group does not exist, and the
+// plan must be left exactly as it was.
+func TestPlanForApplyLeavesAMachineWithoutTheGroupAlone(t *testing.T) {
+	plan := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-marker"}}
+
+	for _, testCase := range []struct {
+		name    string
+		resolve func() (string, error)
+	}{
+		{name: "no resolver", resolve: nil},
+		{name: "group absent", resolve: func() (string, error) { return "", nil }},
+		{name: "blank SID", resolve: func() (string, error) { return "   ", nil }},
+	} {
+		t.Run(testCase.name, func(t *testing.T) {
+			applied, err := WindowsNetworkPlanForApply(plan, testCase.resolve)
+			if err != nil {
+				t.Fatalf("a machine with no offline group is not a failure: %v", err)
+			}
+			if len(applied.IdentitySIDs) != 1 || applied.IdentitySIDs[0] != "S-1-5-21-marker" {
+				t.Errorf("plan changed on a machine with no offline group: %v", applied.IdentitySIDs)
+			}
+		})
+	}
+}
+
+// A LOOKUP FAILURE IS NOT AN ABSENT GROUP. The filters are machine-global and
+// are replaced wholesale, so returning a marker-only plan here does not decline
+// to add coverage, it removes coverage another workspace is relying on.
+//
+// This is reachable only from an opted-OUT home, because
+// assertWindowsNetworkPlanCoversOfflineGroup is gated on provisioned and never
+// runs for one. Workspace A opts in, the group SID lands in the global filters;
+// workspace B is opted out and its lookup fails transiently; B installs a plan
+// without the group and A's NetworkDeny commands silently regain egress while
+// A's marker and its direct membership check both still pass.
+func TestPlanForApplyRefusesToReplaceFiltersWhenTheGroupCannotBeRead(t *testing.T) {
+	// Opted-out home B: the coverage assert is skipped for it, so nothing else
+	// stands between the failure and the wholesale replace.
+	if err := assertWindowsNetworkPlanCoversOfflineGroup(
+		WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-B-marker"}},
+		func() (string, error) { return "", errors.New("group lookup failed") },
+		false,
+	); err != nil {
+		t.Fatalf("the coverage assert is meant to be skipped for an opted-out home: %v", err)
+	}
+
+	plan := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-5-21-B-marker"}}
+	applied, err := WindowsNetworkPlanForApply(plan, func() (string, error) {
+		return "", errors.New("group lookup failed")
+	})
+	if err == nil {
+		t.Fatalf("a failed group lookup installed %v, stripping the offline group from the machine's filters", applied.IdentitySIDs)
+	}
+	if !strings.Contains(err.Error(), "group lookup failed") {
+		t.Errorf("the underlying failure must be reported so the operator can act on it: %v", err)
+	}
+}

internal/sandbox/windows_network_test.go

@@ -118,3 +118,154 @@ func assertWindowsWFPCommonFilter(t *testing.T, specs map[string]WindowsWFPFilte
 
 // Coverage for the network infra plan + hash and the per-mode token-SID
 // composition lives in windows_online_offline_test.go.
+
+// The block filters have to name the offline group as well as the offline-marker
+// SID, or a sandbox principal is not covered by them at all: LogonUser builds a
+// token from real group memberships and cannot carry the synthetic marker. This
+// is the single property that makes network denial work for the principal
+// backend, so assert it on the plan rather than trusting the wiring.
+func TestNetworkInfraPlanIncludesOfflineGroupIdentity(t *testing.T) {
+	previous := resolveWindowsSandboxOfflineGroupSIDHook
+	t.Cleanup(func() { resolveWindowsSandboxOfflineGroupSIDHook = previous })
+
+	config := WindowsSandboxCommandConfig{SandboxHome: t.TempDir(), Env: map[string]string{windowsSandboxIdentityEnv: "1"}}
+
+	// Before principals have ever been provisioned the group does not exist, and
+	// the plan must be exactly what it was before this feature. The plan is hashed
+	// into the setup marker and re-derived on every command, so an identity set
+	// that appeared out of nowhere would fail every command with "setup is out of
+	// date".
+	resolveWindowsSandboxOfflineGroupSIDHook = func() (string, error) { return "", nil }
+	base, err := BuildWindowsNetworkInfraPlan(config)
+	if err != nil {
+		t.Fatalf("build plan without the group: %v", err)
+	}
+	if len(base.IdentitySIDs) != 1 {
+		t.Fatalf("identity SIDs = %v, want only the offline marker when the group is absent", base.IdentitySIDs)
+	}
+
+	const groupSID = "S-1-5-32-9999"
+	resolveWindowsSandboxOfflineGroupSIDHook = func() (string, error) { return groupSID, nil }
+	withGroup, err := BuildWindowsNetworkInfraPlan(config)
+	if err != nil {
+		t.Fatalf("build plan with the group: %v", err)
+	}
+	if len(withGroup.IdentitySIDs) != 2 || withGroup.IdentitySIDs[1] != groupSID {
+		t.Fatalf("identity SIDs = %v, want the offline marker plus %s", withGroup.IdentitySIDs, groupSID)
+	}
+	// The marker records IdentitySIDs[0] as the offline filter identity, so the
+	// marker SID has to stay first.
+	if withGroup.IdentitySIDs[0] != base.IdentitySIDs[0] {
+		t.Fatalf("offline marker moved from position 0: %v", withGroup.IdentitySIDs)
+	}
+	// Adding the group must change the fingerprint, or setup and the command path
+	// could disagree about the installed filters without anything noticing.
+	baseHash, err := WindowsNetworkInfraHash(base)
+	if err != nil {
+		t.Fatalf("hash base: %v", err)
+	}
+	groupHash, err := WindowsNetworkInfraHash(withGroup)
+	if err != nil {
+		t.Fatalf("hash with group: %v", err)
+	}
+	if baseHash == groupHash {
+		t.Fatal("the infra hash ignored the offline group identity")
+	}
+}
+
+// A lookup failure must not be swallowed into "no group", because that silently
+// produces a plan whose filters do not cover sandbox principals.
+func TestNetworkInfraPlanPropagatesOfflineGroupLookupFailure(t *testing.T) {
+	previous := resolveWindowsSandboxOfflineGroupSIDHook
+	t.Cleanup(func() { resolveWindowsSandboxOfflineGroupSIDHook = previous })
+
+	resolveWindowsSandboxOfflineGroupSIDHook = func() (string, error) {
+		return "", errors.New("boom")
+	}
+	if _, err := BuildWindowsNetworkInfraPlan(WindowsSandboxCommandConfig{SandboxHome: t.TempDir(), Env: map[string]string{windowsSandboxIdentityEnv: "1"}}); err == nil {
+		t.Fatal("a failed offline-group lookup produced a plan; the filters would not cover any principal")
+	}
+}
+
+// The ordering that makes the block filters apply to sandbox principals is
+// invisible at the call site: the plan must be built AFTER provisioning,
+// because provisioning creates the group the filters name. A plan built first
+// names only the offline marker, and a machine would then report a successful
+// setup while every offline principal had an open network.
+//
+// Asserted on the predicate setup checks before installing anything, so moving
+// the plan build back ahead of provisioning fails loudly instead of silently
+// producing a control that enforces nothing.
+func TestNetworkPlanCoverageDetectsPrincipalsLeftUncovered(t *testing.T) {
+	const groupSID = "S-1-5-32-4242"
+
+	// What a plan built BEFORE provisioning looks like: marker only.
+	tooEarly := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-15-3-1111"}}
+	if WindowsNetworkPlanCoversPrincipals(tooEarly, groupSID) {
+		t.Fatal("a plan naming only the offline marker was reported as covering principals; the ordering guard would not fire")
+	}
+
+	// And after: marker plus the group.
+	correct := WindowsNetworkPlan{IdentitySIDs: []string{"S-1-15-3-1111", groupSID}}
+	if !WindowsNetworkPlanCoversPrincipals(correct, groupSID) {
+		t.Fatal("a plan naming the offline group was reported as not covering principals; setup would refuse a correct plan")
+	}
+	// SID comparison is case-insensitive, so a differently-cased resolve does not
+	// read as a missing group and block a valid setup.
+	mixed := WindowsNetworkPlan{IdentitySIDs: []string{strings.ToLower(groupSID)}}
+	if !WindowsNetworkPlanCoversPrincipals(mixed, groupSID) {
+		t.Fatal("case difference read as a missing group")
+	}
+
+	// A host with no group provisioned has no principal to miss, so there is
+	// nothing to assert and setup must not refuse.
+	if !WindowsNetworkPlanCoversPrincipals(tooEarly, "") {
+		t.Fatal("an unprovisioned host was treated as a coverage failure")
+	}
+}
+
+// AN OPTED-OUT HOME MUST KEEP ITS EXISTING MARKER VALID.
+//
+// The offline group is machine-global. Keyed on its existence, the first
+// workspace to opt in changed the computed plan for every OTHER sandbox home on
+// the machine, so their stored NetworkInfraHash stopped matching and every one
+// of their commands failed with "setup is out of date" until each was re-run
+// from an elevated terminal. They had opted into nothing.
+//
+// The hashes are compared rather than the SID lists because the hash is what the
+// marker actually stores and what validation compares.
+func TestOfflineGroupDoesNotChangeAnOptedOutHomesInfraHash(t *testing.T) {
+	previous := resolveWindowsSandboxOfflineGroupSIDHook
+	t.Cleanup(func() { resolveWindowsSandboxOfflineGroupSIDHook = previous })
+
+	optedOut := WindowsSandboxCommandConfig{SandboxHome: t.TempDir()}
+
+	// No group on the machine yet: the state before anyone opted in.
+	resolveWindowsSandboxOfflineGroupSIDHook = nil
+	before, err := BuildWindowsNetworkInfraPlan(optedOut)
+	if err != nil {
+		t.Fatalf("build plan before any opt-in: %v", err)
+	}
+	beforeHash, err := WindowsNetworkInfraHash(before)
+	if err != nil {
+		t.Fatalf("hash before: %v", err)
+	}
+
+	// Another workspace has now opted in and created the machine-global group.
+	resolveWindowsSandboxOfflineGroupSIDHook = func() (string, error) {
+		return "S-1-5-21-1111111111-1111111111-1111111111-4002", nil
+	}
+	after, err := BuildWindowsNetworkInfraPlan(optedOut)
+	if err != nil {
+		t.Fatalf("build plan after another home opted in: %v", err)
+	}
+	afterHash, err := WindowsNetworkInfraHash(after)
+	if err != nil {
+		t.Fatalf("hash after: %v", err)
+	}
+
+	if beforeHash != afterHash {
+		t.Fatalf("another workspace opting in changed this home's infra hash (%s -> %s), so its stored marker stops validating and every command fails until it is re-set-up",
+			beforeHash, afterHash)
+	}
+}

internal/sandbox/windows_offline_group_ownership_windows_test.go

@@ -0,0 +1,96 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+// The offline group's SID goes onto the persistent WFP deny filters and the
+// sandbox principal is made a member of it. Adopting a same-named group that
+// something else owns therefore cuts off every existing member's outbound
+// traffic and hands our principal that group's permissions, so an unmarked
+// group has to be refused rather than reused.
+func TestEnsureWindowsLocalGroupRefusesForeignSameNameGroup(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		status    uintptr
+		owned     bool
+		ownedErr  error
+		wantError string
+	}{
+		"foreign group with our name": {
+			status: nerrGroupExists, owned: false,
+			wantError: "not managed by zero",
+		},
+		"foreign group reported via ERROR_ALIAS_EXISTS": {
+			status: errorAliasExists, owned: false,
+			wantError: "not managed by zero",
+		},
+		"our own group on a re-run": {
+			status: nerrGroupExists, owned: true,
+		},
+		"ownership lookup fails": {
+			status: nerrGroupExists, ownedErr: errors.New("lookup refused"),
+			wantError: "lookup refused",
+		},
+		"group did not exist": {
+			status: nerrSuccess,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			prevAdd, prevOwned := addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn
+			t.Cleanup(func() {
+				addWindowsLocalGroupFn = prevAdd
+				windowsLocalGroupOwnedByZeroFn = prevOwned
+			})
+			addWindowsLocalGroupFn = func(string, string) (uintptr, error) { return testCase.status, nil }
+			lookups := 0
+			windowsLocalGroupOwnedByZeroFn = func(string, string) (bool, error) {
+				lookups++
+				return testCase.owned, testCase.ownedErr
+			}
+
+			err := ensureWindowsSandboxOfflineGroup()
+			if testCase.wantError == "" {
+				if err != nil {
+					t.Fatalf("ensureWindowsSandboxOfflineGroup: %v", err)
+				}
+				return
+			}
+			if err == nil {
+				t.Fatalf("adopted a group it should have refused (lookups=%d)", lookups)
+			}
+			if !strings.Contains(err.Error(), testCase.wantError) {
+				t.Fatalf("error = %q, want it to mention %q", err, testCase.wantError)
+			}
+		})
+	}
+}
+
+// The marker compared against is the offline group's own comment, not the
+// principals group's: two managed groups with different markers must not be
+// mistaken for each other.
+func TestEnsureWindowsLocalGroupChecksTheGroupsOwnMarker(t *testing.T) {
+	prevAdd, prevOwned := addWindowsLocalGroupFn, windowsLocalGroupOwnedByZeroFn
+	t.Cleanup(func() {
+		addWindowsLocalGroupFn = prevAdd
+		windowsLocalGroupOwnedByZeroFn = prevOwned
+	})
+	addWindowsLocalGroupFn = func(string, string) (uintptr, error) { return nerrGroupExists, nil }
+	var gotName, gotComment string
+	windowsLocalGroupOwnedByZeroFn = func(name, comment string) (bool, error) {
+		gotName, gotComment = name, comment
+		return true, nil
+	}
+	if err := ensureWindowsSandboxOfflineGroup(); err != nil {
+		t.Fatalf("ensureWindowsSandboxOfflineGroup: %v", err)
+	}
+	if gotName != windowsSandboxOfflineGroupName {
+		t.Fatalf("checked group %q, want %q", gotName, windowsSandboxOfflineGroupName)
+	}
+	if gotComment != windowsSandboxOfflineGroupComment {
+		t.Fatalf("compared marker %q, want %q", gotComment, windowsSandboxOfflineGroupComment)
+	}
+}

internal/sandbox/windows_offline_membership_windows_test.go

@@ -0,0 +1,102 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// Choosing the offline account is only half of what denies it the network: the
+// WFP filters match the offline GROUP'S SID. An account that has drifted out of
+// that group still logs on from its stored secret, and its token no longer
+// satisfies the filter condition — so a no-network profile would get full
+// egress. The command path has to re-check the membership, not trust the marker
+// setup wrote when it last succeeded.
+func TestPrincipalTokenRechecksOfflineGroupMembership(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		mode        NetworkMode
+		member      bool
+		memberErr   error
+		wantChecked bool
+		wantReached bool
+		wantErr     bool
+	}{
+		"offline principal drifted out of the group": {
+			mode: NetworkDeny, member: false,
+			wantChecked: true, wantReached: false,
+		},
+		"offline principal still a member": {
+			mode: NetworkDeny, member: true,
+			wantChecked: true, wantReached: true,
+		},
+		"membership lookup fails": {
+			mode: NetworkDeny, memberErr: errors.New("group lookup refused"),
+			wantChecked: true, wantReached: false, wantErr: true,
+		},
+		// An allow-network command uses the online principal, which is not in the
+		// offline group by design. Checking it there would refuse every approved
+		// network command.
+		"online principal is not checked": {
+			mode: NetworkAllow, member: false,
+			wantChecked: false, wantReached: true,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			prevLookup := lookupWindowsSandboxPrincipalForCommandFn
+			prevMember := windowsSandboxUserInLocalGroupFn
+			prevSecret := readWindowsSandboxSecretFn
+			prevWarn := warnWindowsSandboxOfflineMembershipMissing
+			t.Cleanup(func() {
+				lookupWindowsSandboxPrincipalForCommandFn = prevLookup
+				windowsSandboxUserInLocalGroupFn = prevMember
+				readWindowsSandboxSecretFn = prevSecret
+				warnWindowsSandboxOfflineMembershipMissing = prevWarn
+			})
+
+			sid, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+			if err != nil {
+				t.Fatal(err)
+			}
+			lookupWindowsSandboxPrincipalForCommandFn = func(string, windowsSandboxRole) (windowsSandboxIdentity, error) {
+				return windowsSandboxIdentity{Username: "zero-sbx-test", SID: sid}, nil
+			}
+			checkedGroup := ""
+			windowsSandboxUserInLocalGroupFn = func(_ string, group string) (bool, error) {
+				checkedGroup = group
+				return testCase.member, testCase.memberErr
+			}
+			secretRead := false
+			readWindowsSandboxSecretFn = func(string) (string, error) {
+				secretRead = true
+				return "", errWindowsSandboxIdentityUnavailable
+			}
+			warnWindowsSandboxOfflineMembershipMissing = func(string) {}
+
+			config := windowsSandboxTestConfig()
+			config.Env = map[string]string{windowsSandboxIdentityEnv: "1"}
+			config.PermissionProfile.Network.Mode = testCase.mode
+
+			_, ok, err := windowsSandboxPrincipalToken(config)
+			if testCase.wantErr {
+				if err == nil {
+					t.Fatal("a failed membership lookup was swallowed")
+				}
+			} else if err != nil {
+				t.Fatalf("windowsSandboxPrincipalToken: %v", err)
+			}
+			_ = ok
+			if secretRead != testCase.wantReached {
+				t.Fatalf("reached the secret read = %v, want %v (gate must short-circuit before it)", secretRead, testCase.wantReached)
+			}
+			if checked := checkedGroup != ""; checked != testCase.wantChecked {
+				t.Fatalf("membership checked = %v, want %v", checked, testCase.wantChecked)
+			}
+			if testCase.wantChecked && checkedGroup != windowsSandboxOfflineGroupName {
+				t.Fatalf("checked group %q, want %q", checkedGroup, windowsSandboxOfflineGroupName)
+			}
+		})
+	}
+}

internal/sandbox/windows_online_offline_test.go

@@ -37,9 +37,25 @@ func TestWindowsRuntimeTokenSIDs(t *testing.T) {
 // setup serves both modes (and its fingerprint is stable across modes).
 func TestBuildWindowsNetworkInfraPlanIsModeIndependent(t *testing.T) {
 	home := t.TempDir()
+	// Pinned, because the count below depends on whether this machine happens to
+	// have the offline group already. BuildWindowsNetworkInfraPlan folds that
+	// group's SID in when it resolves, so on a Windows host where an earlier
+	// elevated setup created ZeroSandboxOffline the plan legitimately carries two
+	// identity SIDs and this test failed on a correct plan. CI never saw it: the
+	// Linux and macOS jobs leave the hook nil, and a fresh Windows runner has no
+	// group yet. Stubbing it makes the assertion about the plan rather than about
+	// the machine it runs on.
+	previousHook := resolveWindowsSandboxOfflineGroupSIDHook
+	t.Cleanup(func() { resolveWindowsSandboxOfflineGroupSIDHook = previousHook })
+	resolveWindowsSandboxOfflineGroupSIDHook = nil
+
 	mk := func(mode NetworkMode) WindowsSandboxCommandConfig {
 		return WindowsSandboxCommandConfig{
-			SandboxHome:    home,
+			SandboxHome: home,
+			// Opted in, because the offline group only enters the plan for a home
+			// that asked for principals. Without this the plan is the pre-principal
+			// one and the group assertions below have nothing to find.
+			Env:            map[string]string{windowsSandboxIdentityEnv: "1"},
 			CommandCWD:     `C:\ws`,
 			WorkspaceRoots: []string{`C:\ws`},
 			PermissionProfile: PermissionProfile{
@@ -72,6 +88,40 @@ func TestBuildWindowsNetworkInfraPlanIsModeIndependent(t *testing.T) {
 	if denyPlan.IdentitySIDs[0] != offline {
 		t.Errorf("infra plan SID = %q, want offline-marker %q", denyPlan.IdentitySIDs[0], offline)
 	}
+
+	// Mode independence has to hold on a machine that already has the offline
+	// group too, which is where the pinned counts above would otherwise be
+	// asserting something about the host rather than the plan. The group SID is
+	// folded in for both modes, so the hashes must still agree; only the count
+	// changes.
+	resolveWindowsSandboxOfflineGroupSIDHook = func() (string, error) { return "S-1-5-32-9999", nil }
+	withGroupDeny, err := BuildWindowsNetworkInfraPlan(mk(NetworkDeny))
+	if err != nil {
+		t.Fatalf("deny infra plan with the group present: %v", err)
+	}
+	withGroupAllow, err := BuildWindowsNetworkInfraPlan(mk(NetworkAllow))
+	if err != nil {
+		t.Fatalf("allow infra plan with the group present: %v", err)
+	}
+	// Assert the SIDs themselves, not the count. A duplicate offline marker or an
+	// unrelated SID would satisfy a length check while meaning something quite
+	// different.
+	if len(withGroupDeny.IdentitySIDs) != 2 ||
+		withGroupDeny.IdentitySIDs[0] != offline ||
+		withGroupDeny.IdentitySIDs[1] != "S-1-5-32-9999" {
+		t.Fatalf("group present should add its SID after the offline marker, got %v (offline marker %q)", withGroupDeny.IdentitySIDs, offline)
+	}
+	groupDenyHash, _ := WindowsNetworkInfraHash(withGroupDeny)
+	groupAllowHash, _ := WindowsNetworkInfraHash(withGroupAllow)
+	if groupDenyHash != groupAllowHash || groupDenyHash == "" {
+		t.Fatalf("infra hash must stay mode-independent with the group present: deny=%q allow=%q", groupDenyHash, groupAllowHash)
+	}
+	// And it must differ from the no-group hash, which is the cross-workspace
+	// coupling called out for a maintainer decision: one workspace creating the
+	// group changes every other sandbox home's expected fingerprint.
+	if groupDenyHash == denyHash {
+		t.Error("group presence did not change the infra hash; the marker staleness this causes is a deliberate property and should be visible here")
+	}
 }
 
 // A pre-existing schema-1 capability file (no Offline SID) is upgraded in place:

internal/sandbox/windows_principal_env_windows.go

@@ -0,0 +1,67 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// windowsPrincipalIdentityEnvironment rewrites the variables that name WHO the
+// command is running as.
+//
+// The child environment starts life as the invoking user's, and the deliberate
+// sandbox redirects only replace HOME, the temp variables and the per-tool cache
+// dirs. Everything identifying the account was left alone, so a command running
+// as the principal still read USERPROFILE, APPDATA, LOCALAPPDATA, HOMEDRIVE,
+// HOMEPATH and USERNAME describing the CALLER, whose profile the principal
+// deliberately cannot open. Native tools resolve config and per-user state
+// through exactly those, so they either fail during startup or, worse, quietly
+// look somewhere they have no business reading.
+//
+// The values point into the sandbox runtime tree rather than at the principal's
+// real Windows profile. That tree is already granted to the principal and
+// already holds its caches, so the paths are writable by construction. Naming
+// the real profile would need LoadUserProfile to have run, and a variable
+// pointing at a directory that does not exist yet is a worse answer than one
+// pointing somewhere usable.
+//
+// Layered UNDER the deliberate redirects: HOME, TMPDIR, TMP and TEMP are not
+// touched here, so sandboxRuntimeEnvironment stays the single owner of those.
+func windowsPrincipalIdentityEnvironment(env map[string]string, username string, runtime *SandboxRuntime) map[string]string {
+	username = strings.TrimSpace(username)
+	if env == nil || username == "" || runtime == nil {
+		return env
+	}
+	base := strings.TrimSpace(runtime.Data)
+	if base == "" {
+		base = strings.TrimSpace(runtime.Root)
+	}
+	if base == "" {
+		// No runtime tree means no writable place to point at, and inventing one
+		// outside the grants would only move the failure. Leaving the caller's
+		// values would be worse, so name the account and stop there.
+		env["USERNAME"] = username
+		return env
+	}
+
+	profile := filepath.Join(base, "profile")
+	env["USERPROFILE"] = profile
+	env["APPDATA"] = filepath.Join(profile, "AppData", "Roaming")
+	env["LOCALAPPDATA"] = filepath.Join(profile, "AppData", "Local")
+	env["USERNAME"] = username
+	if host, err := os.Hostname(); err == nil && strings.TrimSpace(host) != "" {
+		// A local account's domain is the machine. Left as the caller's it would
+		// name a domain the principal is not a member of.
+		env["USERDOMAIN"] = host
+	}
+	// HOMEDRIVE and HOMEPATH are a split of the same location, and tools join
+	// them back together, so they have to be split from the value above rather
+	// than carried over from the caller.
+	if volume := filepath.VolumeName(profile); volume != "" {
+		env["HOMEDRIVE"] = volume
+		env["HOMEPATH"] = strings.TrimPrefix(profile, volume)
+	}
+	return env
+}

internal/sandbox/windows_principal_env_windows_test.go

@@ -0,0 +1,90 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"strings"
+	"testing"
+)
+
+// callerEnvironment is the shape the child starts from: the invoking user's
+// values, with the deliberate sandbox redirects already layered on top.
+func callerEnvironment() map[string]string {
+	return map[string]string{
+		"USERPROFILE":  `C:\Users\caller`,
+		"APPDATA":      `C:\Users\caller\AppData\Roaming`,
+		"LOCALAPPDATA": `C:\Users\caller\AppData\Local`,
+		"HOMEDRIVE":    `C:`,
+		"HOMEPATH":     `\Users\caller`,
+		"USERNAME":     "caller",
+		"USERDOMAIN":   "CALLERDOMAIN",
+		// Owned by sandboxRuntimeEnvironment, and this must not touch them.
+		"HOME": `C:\runtime\home`,
+		"TEMP": `C:\runtime\temp`,
+		"TMP":  `C:\runtime\temp`,
+	}
+}
+
+// NOTHING MAY STILL NAME THE CALLER.
+//
+// A principal is a separate account that deliberately cannot open the invoking
+// user's profile, so every variable a native tool resolves per-user state
+// through has to describe the principal instead. Left alone they pointed at the
+// caller, and tools either failed during startup or looked somewhere they had no
+// business reading.
+func TestPrincipalEnvironmentStopsNamingTheCaller(t *testing.T) {
+	runtime := &SandboxRuntime{
+		Root: `C:\runtime`,
+		Data: `C:\runtime\data`,
+	}
+	env := windowsPrincipalIdentityEnvironment(callerEnvironment(), "zero-sbx-abc123", runtime)
+
+	for _, key := range []string{"USERPROFILE", "APPDATA", "LOCALAPPDATA"} {
+		if strings.Contains(strings.ToLower(env[key]), "caller") {
+			t.Errorf("%s still points into the caller's profile: %s", key, env[key])
+		}
+		if !strings.HasPrefix(strings.ToLower(env[key]), strings.ToLower(runtime.Data)) {
+			t.Errorf("%s is not inside the runtime tree the principal can write: %s", key, env[key])
+		}
+	}
+	if strings.Contains(strings.ToLower(env["HOMEPATH"]), "caller") {
+		t.Errorf("HOMEPATH still points into the caller's profile: %s", env["HOMEPATH"])
+	}
+	if env["USERNAME"] != "zero-sbx-abc123" {
+		t.Errorf("USERNAME = %q, want the principal account", env["USERNAME"])
+	}
+	if env["USERDOMAIN"] == "CALLERDOMAIN" {
+		t.Error("USERDOMAIN still names the caller's domain, which the principal is not a member of")
+	}
+	// HOMEDRIVE and HOMEPATH are rejoined by tools, so they have to describe the
+	// same location as USERPROFILE rather than a mix of both identities.
+	if rejoined := env["HOMEDRIVE"] + env["HOMEPATH"]; rejoined != env["USERPROFILE"] {
+		t.Errorf("HOMEDRIVE+HOMEPATH = %s, want the same location as USERPROFILE %s", rejoined, env["USERPROFILE"])
+	}
+}
+
+// The deliberate redirects stay owned by sandboxRuntimeEnvironment. Clobbering
+// them here would silently move the sandbox's temp and cache handling into a
+// second place, which is how the two drift apart.
+func TestPrincipalEnvironmentLeavesTheSandboxRedirectsAlone(t *testing.T) {
+	before := callerEnvironment()
+	env := windowsPrincipalIdentityEnvironment(callerEnvironment(), "zero-sbx-abc123", &SandboxRuntime{
+		Root: `C:\runtime`,
+		Data: `C:\runtime\data`,
+	})
+	for _, key := range []string{"HOME", "TEMP", "TMP"} {
+		if env[key] != before[key] {
+			t.Errorf("%s = %q, want it left at the sandbox redirect %q", key, env[key], before[key])
+		}
+	}
+}
+
+// Without a runtime tree there is nowhere writable to point at, and inventing a
+// path outside the grants would only move the failure. Naming the account is
+// still strictly better than advertising the caller's.
+func TestPrincipalEnvironmentWithoutARuntimeTreeStillRenamesTheAccount(t *testing.T) {
+	env := windowsPrincipalIdentityEnvironment(callerEnvironment(), "zero-sbx-abc123", &SandboxRuntime{})
+	if env["USERNAME"] != "zero-sbx-abc123" {
+		t.Errorf("USERNAME = %q, want the principal account even with no runtime tree", env["USERNAME"])
+	}
+}

internal/sandbox/windows_principal_fingerprint_test.go

@@ -0,0 +1,47 @@
+package sandbox
+
+import "testing"
+
+// THE FINGERPRINT MUST DESCRIBE THE PLAN THAT ACTUALLY GETS APPLIED.
+//
+// windowsPrincipalPlanFingerprint builds a principal plan to hash into the setup
+// marker, and it was the one of three buildWindowsPrincipalACLPlan call sites
+// that did not pass DenyWrite. Apply and teardown both did. So a change to the
+// policy's deny-write paths moved the plan those two build and left the marker's
+// hash unchanged, which is the marker failing at the one job it has.
+//
+// It was not a live hole, because the capability ACLPlanHash covers the same
+// paths and moves the marker anyway. That is exactly what would have kept it
+// invisible until somebody changed the capability plan's shape.
+func TestThePrincipalFingerprintCoversDenyWrite(t *testing.T) {
+	workspace := t.TempDir()
+	base := WindowsSandboxSetupConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PrincipalOptIn: true,
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: workspace}},
+			},
+			Network: NetworkPolicy{Mode: NetworkDeny},
+		},
+	}
+
+	without, err := windowsPrincipalPlanFingerprint(base)
+	if err != nil {
+		t.Fatalf("windowsPrincipalPlanFingerprint: %v", err)
+	}
+
+	withDeny := base
+	withDeny.PermissionProfile.FileSystem.DenyWrite = []string{workspace + string('/') + "protected"}
+	changed, err := windowsPrincipalPlanFingerprint(withDeny)
+	if err != nil {
+		t.Fatalf("windowsPrincipalPlanFingerprint: %v", err)
+	}
+
+	if without == changed {
+		t.Error("adding a deny-write path did not move the principal fingerprint, so the marker cannot tell that the applied plan changed")
+	}
+}

internal/sandbox/windows_principal_jail_sids_windows_test.go

@@ -0,0 +1,87 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"strings"
+	"testing"
+)
+
+// THE ACCOUNT SID MUST NOT BE A RESTRICTING SID.
+//
+// WRITE_RESTRICTED allows a write only when the normal token AND the
+// restricting-SID list both allow it. The principal's account SID is already
+// enabled in its normal token, so listing it as a restricting SID makes the
+// second check a formality for anything granted to that account: every path
+// carrying a direct principal ACE passes both halves and is writable wherever it
+// sits, including the profile directory Windows creates on first logon outside
+// every configured write root.
+//
+// Same shape as the World SID in the restricted list (#865). A SID already
+// carried by the normal token cannot also restrict it.
+//
+// The pre-existing jail test could not catch this: it grants a GROUP
+// (WinBuiltinGuestsSid), so it exercises a configuration the product never
+// ships. This asserts the composition that actually ships.
+func TestPrincipalJailExcludesTheAccountsOwnSID(t *testing.T) {
+	accountSID := "S-1-5-21-9-9-9-1500"
+	// The account SID is planted IN the capability list on purpose. Passing a
+	// list that never contained it would make this test unfalsifiable: it would
+	// pass against any implementation, including one that appends the account SID
+	// straight back. This is the route the World SID took in #865.
+	capabilities := []string{"S-1-5-21-1-2-3-1001", accountSID, "S-1-5-21-1-2-3-1002"}
+
+	jail := windowsPrincipalJailSIDs(capabilities, accountSID)
+
+	for _, sid := range jail {
+		if strings.EqualFold(sid, accountSID) {
+			t.Fatalf("the principal's own account SID is a restricting SID, so any path with a direct principal ACE escapes the write jail: %v", jail)
+		}
+	}
+	// ...and the capability SIDs are still all present, or the principal would be
+	// jailed out of the workspace it owns and the fix would be a denial of service
+	// rather than a fix.
+	for _, want := range []string{"S-1-5-21-1-2-3-1001", "S-1-5-21-1-2-3-1002"} {
+		if !containsString(jail, want) {
+			t.Errorf("capability SID %q missing from the jail, so the principal loses write access to its own workspace: %v", want, jail)
+		}
+	}
+}
+
+// The jail must not alias the caller's slice. Appending to a shared backing
+// array would let a later append mutate the restricting set of a token already
+// being built.
+//
+// Both assertions read through to the BACKING ARRAY, which is the only place
+// aliasing is observable. An earlier version of this test appended a sentinel and
+// then ranged over the caller's slice looking for it, which can never fail:
+// append returns a new header, so the caller's length never grows and the range
+// never reaches the slot the write landed in. It passed against every
+// implementation, including one that returns the caller's slice verbatim.
+func TestPrincipalJailDoesNotAliasTheCapabilitySlice(t *testing.T) {
+	capabilities := make([]string, 2, 8)
+	capabilities[0] = "S-1-5-21-1-2-3-1001"
+	capabilities[1] = "S-1-5-21-1-2-3-1002"
+
+	jail := windowsPrincipalJailSIDs(capabilities, "S-1-5-21-9-9-9-1500")
+	if len(jail) == 0 {
+		t.Fatal("jail is empty, so neither assertion below proves anything")
+	}
+	// Shared element storage: a mutation through jail[0] would rewrite a SID in
+	// the restricting set of a token already being built.
+	if &jail[0] == &capabilities[0] {
+		t.Fatal("jail shares element storage with the caller's capability slice")
+	}
+	// Spare capacity: appending to an aliased jail writes into the caller's
+	// backing array past its length, which is invisible through the caller's own
+	// header but very visible to anything else holding a longer view of it.
+	grown := append(jail, "S-1-1-0")
+	shared := capabilities[:cap(capabilities)]
+	if shared[len(capabilities)] == "S-1-1-0" {
+		t.Fatalf("appending to the jail wrote through into the caller's backing array: jail=%v shared=%v",
+			grown, shared[:len(capabilities)+1])
+	}
+	if len(capabilities) != 2 {
+		t.Errorf("caller's slice grew to %d", len(capabilities))
+	}
+}

internal/sandbox/windows_principal_jail_windows_test.go

@@ -0,0 +1,77 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// The principal path must apply the write jail, not just hand over the account's
+// own token. A LogonUser token carries every write the account's ambient
+// memberships grant, so without this the profile's write roots are advisory.
+//
+// Driven with the process token as base because minting a real principal token
+// needs an elevated, provisioned machine; the restriction machinery under test
+// is identical either way.
+func TestRestrictWindowsTokenJailsWritesOutsideCapabilitySIDs(t *testing.T) {
+	var base windows.Token
+	desired := uint32(windows.TOKEN_DUPLICATE | windows.TOKEN_QUERY | windows.TOKEN_ASSIGN_PRIMARY |
+		windows.TOKEN_ADJUST_DEFAULT | windows.TOKEN_ADJUST_SESSIONID | windows.TOKEN_ADJUST_PRIVILEGES)
+	if err := windows.OpenProcessToken(windows.CurrentProcess(), desired, &base); err != nil {
+		t.Skipf("cannot open the process token here: %v", err)
+	}
+	defer base.Close()
+
+	// A capability SID granted nowhere near the probe directory.
+	capSID, err := windows.CreateWellKnownSid(windows.WinBuiltinGuestsSid)
+	if err != nil {
+		t.Fatal(err)
+	}
+	jailed, err := restrictWindowsTokenForCapabilitySIDs(base, []string{capSID.String()}, true)
+	if err != nil {
+		t.Skipf("cannot build a restricted token here: %v", err)
+	}
+	defer jailed.Close()
+
+	// Setup assertion: the unrestricted process can write here, so a denial below
+	// is the jail and not a broken fixture.
+	dir := t.TempDir()
+	target := filepath.Join(dir, "written.txt")
+	if err := os.WriteFile(target, []byte("probe"), 0o600); err != nil {
+		t.Fatalf("SETUP INVALID: the test process itself cannot write %s: %v", target, err)
+	}
+	if err := os.Remove(target); err != nil {
+		t.Fatal(err)
+	}
+
+	config := WindowsSandboxCommandConfig{
+		CommandCWD:     dir,
+		WorkspaceRoots: []string{dir},
+		Command:        []string{"cmd", "/c", "echo probe> " + target},
+	}
+	if _, err := runWindowsCommandAsUser(jailed, config); err != nil {
+		t.Fatalf("run under the jailed token: %v", err)
+	}
+	if _, err := os.Stat(target); err == nil {
+		t.Error("the jailed token wrote a path no capability SID covers; the write jail is not applied")
+	}
+}
+
+// parseWindowsCapabilitySIDs must reject an empty list rather than build an
+// unrestricted token, and must not leak the SIDs it already parsed on failure.
+func TestParseWindowsCapabilitySIDsRejectsEmptyAndBadInput(t *testing.T) {
+	if _, err := parseWindowsCapabilitySIDs(nil); err == nil {
+		t.Error("empty SID list accepted; that would build a token with no restriction")
+	}
+	valid, err := windows.CreateWellKnownSid(windows.WinBuiltinGuestsSid)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := parseWindowsCapabilitySIDs([]string{valid.String(), "not-a-sid"}); err == nil {
+		t.Error("an unparseable SID was accepted")
+	}
+}

internal/sandbox/windows_principal_launch_windows.go

@@ -0,0 +1,147 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// Privileges CreateProcessAsUser requires when the token belongs to a DIFFERENT
+// account than the caller.
+//
+// The documented exemption from SE_ASSIGNPRIMARYTOKEN_NAME is for a restricted
+// version of the caller's OWN primary token, which is exactly what the ordinary
+// restricted-token path passes and exactly why that path works without holding
+// anything special. A principal token comes from LogonUser against a separate
+// local account, so the exemption does not apply and both privileges are back in
+// play.
+const (
+	seAssignPrimaryTokenPrivilege = "SeAssignPrimaryTokenPrivilege"
+	seIncreaseQuotaPrivilege      = "SeIncreaseQuotaPrivilege"
+)
+
+// enableWindowsPrincipalLaunchPrivileges enables what launching another account
+// needs, and reports precisely what is missing when it cannot.
+//
+// Enabling matters on its own: a privilege present in a token but DISABLED still
+// fails the access check, and an elevated administrator typically holds
+// SeIncreaseQuotaPrivilege in exactly that state. So this is not only a
+// diagnostic, it is the step that makes the call work wherever the rights are
+// actually held.
+//
+// Where they are not held, which is the ordinary unelevated case, this turns a
+// bare "Access is denied" from deep inside process creation into a statement of
+// what the sandbox cannot do and why. That failure is otherwise indistinguishable
+// from the command itself being rejected, and it happens before the command's
+// executable is ever opened.
+func enableWindowsPrincipalLaunchPrivileges() error {
+	var token windows.Token
+	if err := windows.OpenProcessToken(
+		windows.CurrentProcess(),
+		windows.TOKEN_ADJUST_PRIVILEGES|windows.TOKEN_QUERY,
+		&token,
+	); err != nil {
+		return fmt.Errorf("open process token for principal launch: %w", err)
+	}
+	defer token.Close()
+
+	// ENUMERATE the token, do not infer from AdjustTokenPrivileges.
+	//
+	// AdjustTokenPrivileges reports a privilege the token does not hold by
+	// returning success and setting ERROR_NOT_ALL_ASSIGNED, and this binding does
+	// not surface that: it returns nil for SeTcbPrivilege on an ordinary user
+	// process, which holds nothing of the sort. A check built on its error would
+	// pass on every machine, which is worse than no check, because it would report
+	// the sandbox ready in exactly the case it cannot run.
+	held, err := windowsTokenPrivilegeLUIDs(token)
+	if err != nil {
+		return err
+	}
+	var missing []string
+	for _, name := range []string{seAssignPrimaryTokenPrivilege, seIncreaseQuotaPrivilege} {
+		luid, err := windowsPrivilegeLUID(name)
+		if err != nil {
+			return err
+		}
+		if _, ok := held[luid]; !ok {
+			missing = append(missing, name)
+			continue
+		}
+		// Held but possibly disabled, which still fails the access check. This is
+		// the case an elevated administrator lands in.
+		if err := enableWindowsTokenPrivilege(token, name); err != nil {
+			return fmt.Errorf("enable %s: %w", name, err)
+		}
+	}
+	return principalLaunchPrivilegeError(missing)
+}
+
+// principalLaunchPrivilegeError renders the refusal for a set of missing
+// privileges, and is separate from the token work so it can be driven for every
+// combination.
+//
+// It has to be, because WHICH privilege is missing depends on the account
+// running the test. An ordinary unelevated user typically holds neither, but not
+// always: this developer box holds SeAssignPrimaryTokenPrivilege and not
+// SeIncreaseQuotaPrivilege, so a test that named one of them and ran the real
+// preflight asserted a machine's configuration rather than the code. Splitting
+// the rendering out makes each case deterministic and leaves the token path with
+// only the question that genuinely varies.
+func principalLaunchPrivilegeError(missing []string) error {
+	if len(missing) == 0 {
+		return nil
+	}
+	return fmt.Errorf(
+		"launching a command as the sandbox principal needs %s, which this process does not hold. "+
+			"CreateProcessAsUser exempts only a restricted version of the caller's own token, and a principal "+
+			"is a separate account, so an ordinary unelevated process cannot start one. "+
+			"Unset %s to use the restricted-token sandbox, which needs no such privilege",
+		strings.Join(missing, " and "), windowsSandboxIdentityEnv)
+}
+
+// windowsPrivilegeLUID resolves a privilege name to its locally unique id, which
+// is how a token actually names the privileges it holds.
+func windowsPrivilegeLUID(name string) (windows.LUID, error) {
+	namePtr, err := windows.UTF16PtrFromString(name)
+	if err != nil {
+		return windows.LUID{}, fmt.Errorf("encode privilege name %s: %w", name, err)
+	}
+	var luid windows.LUID
+	if err := windows.LookupPrivilegeValue(nil, namePtr, &luid); err != nil {
+		return windows.LUID{}, fmt.Errorf("look up privilege %s: %w", name, err)
+	}
+	return luid, nil
+}
+
+// windowsTokenPrivilegeLUIDs returns every privilege the token holds, enabled or
+// not. Presence and enabled-ness are separate questions: a privilege absent from
+// this set can never be enabled, while one present but disabled only needs
+// AdjustTokenPrivileges.
+func windowsTokenPrivilegeLUIDs(token windows.Token) (map[windows.LUID]struct{}, error) {
+	var size uint32
+	// First call sizes the buffer and is expected to fail with
+	// ERROR_INSUFFICIENT_BUFFER, so its error is deliberately not checked.
+	_ = windows.GetTokenInformation(token, windows.TokenPrivileges, nil, 0, &size)
+	if size == 0 {
+		return nil, errors.New("query token privileges: zero-length result")
+	}
+	buffer := make([]byte, size)
+	if err := windows.GetTokenInformation(token, windows.TokenPrivileges, &buffer[0], size, &size); err != nil {
+		return nil, fmt.Errorf("query token privileges: %w", err)
+	}
+	privileges := (*windows.Tokenprivileges)(unsafe.Pointer(&buffer[0]))
+	if privileges.PrivilegeCount == 0 {
+		return map[windows.LUID]struct{}{}, nil
+	}
+	entries := unsafe.Slice(&privileges.Privileges[0], privileges.PrivilegeCount)
+	held := make(map[windows.LUID]struct{}, len(entries))
+	for _, entry := range entries {
+		held[entry.Luid] = struct{}{}
+	}
+	return held, nil
+}

internal/sandbox/windows_principal_launch_windows_test.go

@@ -0,0 +1,112 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"strings"
+	"testing"
+)
+
+// The launch preflight must name what is missing and what to do about it.
+//
+// Without it the failure surfaces as a bare "Access is denied" from inside
+// CreateProcessAsUser, which is indistinguishable from the sandboxed command
+// itself being rejected. It happens before the command's executable is opened,
+// so there is nothing else in the output to tell the two apart.
+//
+// WHICH privilege is missing is a property of the account running the test, not
+// of the code. An unelevated user often holds neither, but not always: the box
+// this was written on holds SeAssignPrimaryTokenPrivilege and not
+// SeIncreaseQuotaPrivilege. The previous version of this test ran the real
+// preflight and then asserted SeAssignPrimaryTokenPrivilege appeared in the
+// message, so it passed or failed on the tester's token rather than on
+// anything in the source, and it failed here for a reason that was never a
+// defect. Rendering is tested directly instead, once per combination.
+func TestPrincipalLaunchPrivilegeErrorNamesWhatIsMissing(t *testing.T) {
+	cases := []struct {
+		name    string
+		missing []string
+	}{
+		{"neither held", []string{seAssignPrimaryTokenPrivilege, seIncreaseQuotaPrivilege}},
+		{"only assign-primary-token missing", []string{seAssignPrimaryTokenPrivilege}},
+		{"only increase-quota missing", []string{seIncreaseQuotaPrivilege}},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			err := principalLaunchPrivilegeError(tc.missing)
+			if err == nil {
+				t.Fatal("a missing privilege produced no error")
+			}
+			for _, want := range tc.missing {
+				if !strings.Contains(err.Error(), want) {
+					t.Errorf("the message does not name %s, so an operator cannot act on it: %v", want, err)
+				}
+			}
+			// And it must not name one that IS held, or the operator goes looking
+			// for a privilege that was never the problem.
+			for _, other := range []string{seAssignPrimaryTokenPrivilege, seIncreaseQuotaPrivilege} {
+				if contains(tc.missing, other) {
+					continue
+				}
+				if strings.Contains(err.Error(), other) {
+					t.Errorf("the message names %s, which this case holds: %v", other, err)
+				}
+			}
+			if !strings.Contains(err.Error(), windowsSandboxIdentityEnv) {
+				t.Errorf("the message does not name the opt-out: %v", err)
+			}
+			// The point of the message is the way out. Without it the operator is
+			// told only that something is denied.
+			if !strings.Contains(err.Error(), "restricted-token sandbox") {
+				t.Errorf("the message does not offer the fallback, so it reads as a dead end: %v", err)
+			}
+		})
+	}
+}
+
+// Holding both is not an error, or the sandbox would refuse on exactly the
+// machines it is meant to run on.
+func TestPrincipalLaunchPrivilegeErrorIsNilWhenNothingIsMissing(t *testing.T) {
+	if err := principalLaunchPrivilegeError(nil); err != nil {
+		t.Errorf("no missing privileges produced an error: %v", err)
+	}
+	if err := principalLaunchPrivilegeError([]string{}); err != nil {
+		t.Errorf("an empty missing set produced an error: %v", err)
+	}
+}
+
+// The real preflight against this machine's token. The verdict legitimately
+// differs by account, so the only thing asserted is that a refusal is
+// actionable: it names at least one of the two privileges rather than failing
+// with something the operator cannot use.
+func TestPrincipalLaunchPreflightRefusalIsActionable(t *testing.T) {
+	err := enableWindowsPrincipalLaunchPrivileges()
+	if err == nil {
+		t.Log("this process holds the principal launch privileges; nothing to explain")
+		return
+	}
+	if !strings.Contains(err.Error(), seAssignPrimaryTokenPrivilege) &&
+		!strings.Contains(err.Error(), seIncreaseQuotaPrivilege) {
+		t.Errorf("the preflight refused without naming either privilege: %v", err)
+	}
+}
+
+// Repeated calls must agree. This runs once per sandboxed command, and a
+// preflight that answered differently on the second call would let a command
+// through that the first call refused.
+func TestPrincipalLaunchPreflightIsStable(t *testing.T) {
+	first := enableWindowsPrincipalLaunchPrivileges()
+	second := enableWindowsPrincipalLaunchPrivileges()
+	if (first == nil) != (second == nil) {
+		t.Fatalf("preflight verdict changed between calls: first=%v second=%v", first, second)
+	}
+}
+
+func contains(values []string, want string) bool {
+	for _, value := range values {
+		if value == want {
+			return true
+		}
+	}
+	return false
+}

internal/sandbox/windows_principal_ledger.go

@@ -0,0 +1,160 @@
+package sandbox
+
+import (
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// A record of the paths a sandbox principal was last granted ACEs on.
+//
+// applyWindowsPrincipalACLs revokes this trustee before it re-applies, but the
+// only paths it could name were the ones in the plan it was about to apply. A
+// root that LEFT the policy is absent from that plan, so its ACE survived the
+// re-setup marker validation forces and the principal kept write access the
+// current policy no longer grants — the sandbox widened as a result of being
+// tightened. Teardown repeated the same current-plan-only calculation, so it did
+// not clean the leftover either.
+//
+// Nothing on Windows can answer "which paths hold an ACE for this SID" without
+// walking every volume, so the grants have to be written down as they are made.
+//
+// Keyed by principal, beside the secret and for the same reason: one sandbox
+// home serves every workspace on the machine, so a single shared file would let
+// one workspace's setup overwrite another's record — reproducing exactly the
+// stale-ACE bug this exists to close, one level up.
+
+const windowsPrincipalACLLedgerSchemaVersion = 1
+
+const windowsPrincipalACLLedgerDirName = "windows-principal-acl"
+
+type windowsPrincipalACLLedger struct {
+	SchemaVersion int      `json:"schemaVersion"`
+	Paths         []string `json:"paths"`
+}
+
+func windowsPrincipalACLLedgerPath(sandboxHome string, username string) (string, error) {
+	if strings.TrimSpace(sandboxHome) == "" {
+		return "", errors.New("windows principal ACL ledger: empty sandbox home")
+	}
+	if strings.TrimSpace(username) == "" {
+		return "", errors.New("windows principal ACL ledger: empty principal name")
+	}
+	// The same guard the secret path applies, against a caller passing something
+	// windowsSandboxUserName did not produce.
+	if strings.ContainsAny(username, `\/:`) || strings.Contains(username, "..") {
+		return "", fmt.Errorf("windows principal ACL ledger: unsafe principal name %q", username)
+	}
+	return filepath.Join(sandboxHome, windowsPrincipalACLLedgerDirName, username+".json"), nil
+}
+
+// readWindowsPrincipalACLLedger returns the paths an earlier setup recorded for
+// this principal.
+//
+// recorded is false for every reason the record cannot be trusted — absent,
+// unreadable, malformed, or written to a schema this build does not know — and
+// not merely for "absent". Collapsing them is deliberate: there is exactly one
+// safe response to any of them, and it is the same one. The prior grant set is
+// unknown, and a principal whose grants are unknown cannot be reused. Returning
+// an error instead would invite a caller to report it and carry on with an empty
+// prior set, which is the fail-open this record exists to close — the one case
+// where the previous paths are not empty but unenumerable.
+func readWindowsPrincipalACLLedger(sandboxHome string, username string) ([]string, bool) {
+	path, err := windowsPrincipalACLLedgerPath(sandboxHome, username)
+	if err != nil {
+		return nil, false
+	}
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		return nil, false
+	}
+	var ledger windowsPrincipalACLLedger
+	if err := json.Unmarshal(contents, &ledger); err != nil {
+		return nil, false
+	}
+	if ledger.SchemaVersion != windowsPrincipalACLLedgerSchemaVersion {
+		return nil, false
+	}
+	return trimNonEmptyStrings(ledger.Paths), true
+}
+
+// writeWindowsPrincipalACLLedger records paths for this principal, replacing any
+// previous record atomically so an interrupted write cannot leave a truncated
+// file — which the reader would then treat as "no principal was ever granted
+// anything", the very state it must never guess.
+func writeWindowsPrincipalACLLedger(sandboxHome string, username string, paths []string) error {
+	path, err := windowsPrincipalACLLedgerPath(sandboxHome, username)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+		return fmt.Errorf("create windows principal ACL ledger dir: %w", err)
+	}
+	contents, err := json.MarshalIndent(windowsPrincipalACLLedger{
+		SchemaVersion: windowsPrincipalACLLedgerSchemaVersion,
+		Paths:         trimNonEmptyStrings(paths),
+	}, "", "  ")
+	if err != nil {
+		return fmt.Errorf("marshal windows principal ACL ledger: %w", err)
+	}
+	tmp, err := os.CreateTemp(filepath.Dir(path), ".windows-principal-acl-*.tmp")
+	if err != nil {
+		return fmt.Errorf("create windows principal ACL ledger temp file: %w", err)
+	}
+	tmpPath := tmp.Name()
+	if _, err := tmp.Write(contents); err != nil {
+		_ = tmp.Close()
+		_ = os.Remove(tmpPath)
+		return fmt.Errorf("write windows principal ACL ledger temp file: %w", err)
+	}
+	if err := tmp.Close(); err != nil {
+		_ = os.Remove(tmpPath)
+		return fmt.Errorf("close windows principal ACL ledger temp file: %w", err)
+	}
+	if err := os.Rename(tmpPath, path); err != nil {
+		_ = os.Remove(tmpPath)
+		return fmt.Errorf("replace windows principal ACL ledger: %w", err)
+	}
+	return nil
+}
+
+// removeWindowsPrincipalACLLedger drops the record. An absent one is not an
+// error: this runs on the teardown path, where being gone is the goal.
+func removeWindowsPrincipalACLLedger(sandboxHome string, username string) error {
+	path, err := windowsPrincipalACLLedgerPath(sandboxHome, username)
+	if err != nil {
+		return err
+	}
+	if err := os.Remove(path); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("remove windows principal ACL ledger: %w", err)
+	}
+	return nil
+}
+
+// unionWindowsPrincipalACLPaths merges path sets for revocation, keeping the
+// first spelling of each path.
+//
+// Deduplication uses the same case-insensitive key the ACL plans use, so a root
+// recorded as C:\Ws by one setup and re-granted as c:\ws by the next is one path
+// to revoke rather than two.
+func unionWindowsPrincipalACLPaths(sets ...[]string) []string {
+	seen := make(map[string]struct{})
+	union := make([]string, 0)
+	for _, set := range sets {
+		for _, path := range set {
+			key := windowsCapabilityPathKey(path)
+			if key == "" {
+				continue
+			}
+			if _, ok := seen[key]; ok {
+				continue
+			}
+			seen[key] = struct{}{}
+			union = append(union, path)
+		}
+	}
+	return union
+}

internal/sandbox/windows_principal_ledger_test.go

@@ -0,0 +1,174 @@
+package sandbox
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// The point of the record is that a LATER setup can name a root the CURRENT
+// policy no longer mentions, so the round trip has to survive the process that
+// wrote it having no memory of the paths.
+func TestPrincipalACLLedgerRoundTripsRecordedPaths(t *testing.T) {
+	home := t.TempDir()
+	paths := []string{`C:\ws\alpha`, `C:\ws\beta`, `C:\cache\runtime`}
+	if err := writeWindowsPrincipalACLLedger(home, "zerosbx01", paths); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	got, recorded := readWindowsPrincipalACLLedger(home, "zerosbx01")
+	if !recorded {
+		t.Fatal("a record this process just wrote read back as untrusted")
+	}
+	if strings.Join(got, "|") != strings.Join(paths, "|") {
+		t.Errorf("read back %v, want %v", got, paths)
+	}
+}
+
+// One sandbox home serves every workspace on the machine. If the record were a
+// single shared file, workspace B's setup would overwrite workspace A's, and A's
+// dropped roots would then be unnameable at the next re-setup — the same stale
+// ACE this record exists to revoke, produced by the record itself.
+func TestPrincipalACLLedgerIsPerPrincipal(t *testing.T) {
+	home := t.TempDir()
+	if err := writeWindowsPrincipalACLLedger(home, "zerosbx01", []string{`C:\ws\alpha`}); err != nil {
+		t.Fatalf("write alpha: %v", err)
+	}
+	if err := writeWindowsPrincipalACLLedger(home, "zerosbx02", []string{`C:\ws\beta`}); err != nil {
+		t.Fatalf("write beta: %v", err)
+	}
+	alpha, ok := readWindowsPrincipalACLLedger(home, "zerosbx01")
+	if !ok || len(alpha) != 1 || alpha[0] != `C:\ws\alpha` {
+		t.Errorf("first principal's record = %v (ok=%v); a second workspace's setup overwrote it", alpha, ok)
+	}
+}
+
+// Every untrustworthy record has to read as untrustworthy, not as "nothing was
+// ever granted". The caller retires the principal on false; treating a corrupt
+// file as an empty prior set is the fail-open.
+func TestPrincipalACLLedgerRefusesRecordsItCannotTrust(t *testing.T) {
+	for name, contents := range map[string]string{
+		"truncated mid-write":  `{"schemaVersion": 1, "pat`,
+		"not json at all":      "\x00\x01garbage",
+		"a schema from later":  `{"schemaVersion": 99, "paths": ["C:\\ws"]}`,
+		"a schema from before": `{"paths": ["C:\\ws"]}`,
+	} {
+		t.Run(name, func(t *testing.T) {
+			home := t.TempDir()
+			path, err := windowsPrincipalACLLedgerPath(home, "zerosbx01")
+			if err != nil {
+				t.Fatalf("path: %v", err)
+			}
+			if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+				t.Fatalf("mkdir: %v", err)
+			}
+			if err := os.WriteFile(path, []byte(contents), 0o600); err != nil {
+				t.Fatalf("write: %v", err)
+			}
+			if paths, recorded := readWindowsPrincipalACLLedger(home, "zerosbx01"); recorded {
+				t.Errorf("read a record it cannot interpret as trustworthy (%v); the caller would then revoke nothing", paths)
+			}
+		})
+	}
+	// And an absent one, which is the ordinary first-setup case.
+	if _, recorded := readWindowsPrincipalACLLedger(t.TempDir(), "zerosbx01"); recorded {
+		t.Error("a missing record read as trusted")
+	}
+}
+
+// A partial write must not be readable at all, which is why the file is renamed
+// into place rather than written in situ: a reader that saw half a record would
+// treat the missing half as never granted.
+func TestPrincipalACLLedgerWriteIsAtomic(t *testing.T) {
+	home := t.TempDir()
+	if err := writeWindowsPrincipalACLLedger(home, "zerosbx01", []string{`C:\ws\alpha`, `C:\ws\beta`}); err != nil {
+		t.Fatalf("first write: %v", err)
+	}
+	if err := writeWindowsPrincipalACLLedger(home, "zerosbx01", []string{`C:\ws\alpha`}); err != nil {
+		t.Fatalf("second write: %v", err)
+	}
+	path, err := windowsPrincipalACLLedgerPath(home, "zerosbx01")
+	if err != nil {
+		t.Fatalf("path: %v", err)
+	}
+	contents, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read: %v", err)
+	}
+	var ledger windowsPrincipalACLLedger
+	if err := json.Unmarshal(contents, &ledger); err != nil {
+		t.Fatalf("the replaced record did not parse: %v", err)
+	}
+	if len(ledger.Paths) != 1 {
+		t.Errorf("record = %v, want the second write to have replaced the first outright", ledger.Paths)
+	}
+	// No temp files left behind to be mistaken for a record later.
+	entries, err := os.ReadDir(filepath.Dir(path))
+	if err != nil {
+		t.Fatalf("read dir: %v", err)
+	}
+	if len(entries) != 1 {
+		t.Errorf("ledger directory holds %d entries, want just the record", len(entries))
+	}
+}
+
+// The name comes from windowsSandboxUserName, but the path builder is the last
+// thing between a caller and the filesystem, so it refuses anything that could
+// escape the directory.
+func TestPrincipalACLLedgerPathRefusesUnsafeNames(t *testing.T) {
+	for _, username := range []string{"", "  ", `..\..\evil`, "a/b", `a\b`, "c:evil", "..", "zerosbx..01"} {
+		if _, err := windowsPrincipalACLLedgerPath(`C:\home`, username); err == nil {
+			t.Errorf("accepted principal name %q", username)
+		}
+	}
+	if _, err := windowsPrincipalACLLedgerPath("", "zerosbx01"); err == nil {
+		t.Error("accepted an empty sandbox home")
+	}
+	if _, err := windowsPrincipalACLLedgerPath(`C:\home`, "zerosbx01"); err != nil {
+		t.Errorf("rejected a name windowsSandboxUserName would produce: %v", err)
+	}
+}
+
+// Removal is idempotent because teardown only cares that the record is gone,
+// and a setup that failed before writing one must not make teardown fail too.
+func TestPrincipalACLLedgerRemovalToleratesAnAbsentRecord(t *testing.T) {
+	home := t.TempDir()
+	if err := removeWindowsPrincipalACLLedger(home, "zerosbx01"); err != nil {
+		t.Fatalf("remove an absent record: %v", err)
+	}
+	if err := writeWindowsPrincipalACLLedger(home, "zerosbx01", []string{`C:\ws`}); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+	if err := removeWindowsPrincipalACLLedger(home, "zerosbx01"); err != nil {
+		t.Fatalf("remove: %v", err)
+	}
+	if _, recorded := readWindowsPrincipalACLLedger(home, "zerosbx01"); recorded {
+		t.Error("the record survived removal")
+	}
+}
+
+// The union is what setup revokes over. A path in both sets must be revoked
+// once, and a root respelled between setups — Windows opens a path whatever its
+// casing — is one path, not two.
+func TestUnionPrincipalACLPathsDedupesTheWayTheACLPlansDo(t *testing.T) {
+	union := unionWindowsPrincipalACLPaths(
+		[]string{`C:\Ws\Alpha`, `C:\ws\beta`, "  "},
+		[]string{`c:\ws\alpha`, `C:\ws\gamma`, `C:/ws/beta`},
+	)
+	if len(union) != 3 {
+		t.Fatalf("union = %v, want three distinct paths", union)
+	}
+	// The first spelling wins: revocation needs a real path, and the recorded one
+	// is the spelling that was actually granted.
+	if union[0] != `C:\Ws\Alpha` {
+		t.Errorf("union[0] = %q, want the recorded spelling kept", union[0])
+	}
+	// The recorded set comes first so a dropped root cannot be crowded out.
+	if union[1] != `C:\ws\beta` || union[2] != `C:\ws\gamma` {
+		t.Errorf("union = %v, want the recorded paths before the newly granted ones", union)
+	}
+	if got := unionWindowsPrincipalACLPaths(nil, nil); len(got) != 0 {
+		t.Errorf("union of nothing = %v, want empty", got)
+	}
+}

internal/sandbox/windows_principal_ledger_windows_test.go

@@ -0,0 +1,346 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// The finding this record exists for, end to end and against real DACLs: setup
+// grants two roots, the user removes one from their policy, setup runs again,
+// and the removed root must not still name the principal.
+//
+// Both halves go through the PRODUCTION applyWindowsPrincipalACLs rather than
+// the revoke helper, because the mechanism already worked — what did not was the
+// call site's idea of which paths to revoke. It could only name the paths of the
+// plan it was about to apply, and the dropped root is by definition absent from
+// that plan, so the re-setup marker validation forces preserved the very ACE it
+// was supposed to clear.
+func TestReSetupRevokesARootTheNarrowedPolicyDropped(t *testing.T) {
+	home := t.TempDir()
+	username := "zerosbxregression"
+	root := t.TempDir()
+	kept := filepath.Join(root, "kept")
+	dropped := filepath.Join(root, "dropped")
+	for _, dir := range []string{kept, dropped} {
+		if err := os.MkdirAll(dir, 0o700); err != nil {
+			t.Fatalf("mkdir %s: %v", dir, err)
+		}
+	}
+	// Guests: a real, resolvable trustee this process is not a member of, so the
+	// ACEs below are observable without affecting the test process.
+	principal := "S-1-5-32-546"
+
+	wide := FileSystemPolicy{
+		Kind:       FileSystemRestricted,
+		WriteRoots: []WritableRoot{{Root: kept}, {Root: dropped}},
+	}
+	if _, err := applyWindowsPrincipalACLs(home, username, principal, wide, wide.WriteRoots); err != nil {
+		t.Fatalf("first setup: %v", err)
+	}
+	if !hasACEForTrustee(t, dropped, principal) {
+		t.Fatal("precondition: the first setup should have granted the root that is about to leave the policy")
+	}
+
+	// The user narrows their policy and re-runs setup. Nothing in this call
+	// mentions the dropped root; only the record does.
+	narrow := FileSystemPolicy{
+		Kind:       FileSystemRestricted,
+		WriteRoots: []WritableRoot{{Root: kept}},
+	}
+	if _, err := applyWindowsPrincipalACLs(home, username, principal, narrow, narrow.WriteRoots); err != nil {
+		t.Fatalf("re-setup: %v", err)
+	}
+
+	if hasACEForTrustee(t, dropped, principal) {
+		t.Error("the principal kept its grant on a root the narrowed policy removed")
+	}
+	if !hasACEForTrustee(t, kept, principal) {
+		t.Error("revoking over the recorded paths also dropped the grant the narrowed policy still wants")
+	}
+	// And the record narrows with the policy, or it would accumulate every root
+	// the workspace has ever had.
+	recorded, ok := readWindowsPrincipalACLLedger(home, username)
+	if !ok {
+		t.Fatal("no record survived the re-setup")
+	}
+	if containsPathFold(recorded, dropped) {
+		t.Errorf("the record still names %q after the policy dropped it", dropped)
+	}
+}
+
+// The record is written BEFORE any DACL changes and as the union, not after and
+// as the new set. A crash between the grant and a post-hoc write would otherwise
+// leave a record missing paths this run granted, stranding them at the next
+// policy change — the same bug, one interruption away.
+func TestPrincipalACLRecordCoversTheGrantBeforeItIsMade(t *testing.T) {
+	prevApply := applyWindowsACLPlanFn
+	t.Cleanup(func() { applyWindowsACLPlanFn = prevApply })
+
+	home := t.TempDir()
+	username := "zerosbxtwophase"
+	workspace := t.TempDir()
+	stale := filepath.Join(t.TempDir(), "left-the-policy")
+	if err := writeWindowsPrincipalACLLedger(home, username, []string{stale}); err != nil {
+		t.Fatalf("seed the record: %v", err)
+	}
+
+	var atGrant []string
+	applyWindowsACLPlanFn = func(plan WindowsACLPlan) (func() error, error) {
+		if len(plan.Entries) > 0 && plan.Entries[0].Action != windowsACLRevoke {
+			atGrant, _ = readWindowsPrincipalACLLedger(home, username)
+		}
+		return func() error { return nil }, nil
+	}
+
+	filesystem := FileSystemPolicy{
+		Kind:       FileSystemRestricted,
+		WriteRoots: []WritableRoot{{Root: workspace}},
+	}
+	if _, err := applyWindowsPrincipalACLs(home, username, "S-1-5-32-546", filesystem, filesystem.WriteRoots); err != nil {
+		t.Fatalf("applyWindowsPrincipalACLs: %v", err)
+	}
+
+	if !containsPathFold(atGrant, stale) || !containsPathFold(atGrant, workspace) {
+		t.Errorf("record at grant time = %v, want the union of the recorded and newly granted paths", atGrant)
+	}
+	after, ok := readWindowsPrincipalACLLedger(home, username)
+	if !ok {
+		t.Fatal("no record after a successful setup")
+	}
+	if containsPathFold(after, stale) {
+		t.Errorf("record after = %v, want it narrowed to what is granted now", after)
+	}
+	if !containsPathFold(after, workspace) {
+		t.Errorf("record after = %v, want the granted workspace root", after)
+	}
+}
+
+// A principal from an earlier setup whose grants were never recorded is the one
+// case where the prior set is not empty but unenumerable, and carrying on with
+// it is the fail-open: revocation would then cover only what the new plan
+// happens to name. Retiring the account instead makes every ACE that cannot be
+// found name a SID Windows never reuses.
+//
+// The record is per role because the principals are: seeding one role's record
+// must retire the other and only the other. Retiring both would destroy an
+// account there is nothing wrong with, and retiring neither is the fail-open.
+func TestSetupRetiresAPrincipalWithNoRecordOfItsGrants(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		seedRoles     []windowsSandboxRole
+		identityFound bool
+		wantRetired   []windowsSandboxRole
+	}{
+		"no record and principals from an earlier setup": {
+			identityFound: true,
+			wantRetired:   []windowsSandboxRole{windowsSandboxRoleOffline, windowsSandboxRoleOnline},
+		},
+		"no record and nothing provisioned": {identityFound: false},
+		"only the offline role has a record": {
+			seedRoles:     []windowsSandboxRole{windowsSandboxRoleOffline},
+			identityFound: true,
+			wantRetired:   []windowsSandboxRole{windowsSandboxRoleOnline},
+		},
+		"both roles have a record to reconcile against": {
+			seedRoles:     []windowsSandboxRole{windowsSandboxRoleOffline, windowsSandboxRoleOnline},
+			identityFound: true,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			config := stubWindowsPrincipalSetup(t)
+			key := windowsSandboxPrincipalKey(config)
+			for _, role := range testCase.seedRoles {
+				username := windowsSandboxUserName(key, role)
+				if err := writeWindowsPrincipalACLLedger(config.SandboxHome, username, []string{`C:\ws\recorded`}); err != nil {
+					t.Fatalf("seed the %s record: %v", role, err)
+				}
+			}
+
+			lookupWindowsSandboxIdentityFn = func(_ string, role windowsSandboxRole) (windowsSandboxIdentity, error) {
+				if testCase.identityFound {
+					return windowsSandboxIdentity{Username: windowsSandboxUserName(key, role), SID: guestsSID(t)}, nil
+				}
+				return windowsSandboxIdentity{}, errWindowsSandboxIdentityUnavailable
+			}
+			var retired []windowsSandboxRole
+			removeWindowsSandboxPrincipalForSetupFn = func(_ WindowsSandboxCommandConfig, role windowsSandboxRole) error {
+				retired = append(retired, role)
+				return nil
+			}
+
+			if _, err := setupWindowsSandboxPrincipal(config); err != nil {
+				t.Fatalf("setupWindowsSandboxPrincipal: %v", err)
+			}
+			if !sameRoles(retired, testCase.wantRetired) {
+				t.Errorf("retired %v, want %v", retired, testCase.wantRetired)
+			}
+		})
+	}
+}
+
+func sameRoles(got []windowsSandboxRole, want []windowsSandboxRole) bool {
+	if len(got) != len(want) {
+		return false
+	}
+	for index := range got {
+		if got[index] != want[index] {
+			return false
+		}
+	}
+	return true
+}
+
+// A failure to retire has to fail the setup. Reporting success would leave the
+// operator believing the sandbox is provisioned while a principal whose grants
+// nobody can enumerate is still holding them.
+func TestSetupFailsWhenAnUnrecordedPrincipalCannotBeRetired(t *testing.T) {
+	config := stubWindowsPrincipalSetup(t)
+	lookupWindowsSandboxIdentityFn = func(string, windowsSandboxRole) (windowsSandboxIdentity, error) {
+		return windowsSandboxIdentity{Username: "zerosbx", SID: guestsSID(t)}, nil
+	}
+	removeWindowsSandboxPrincipalForSetupFn = func(WindowsSandboxCommandConfig, windowsSandboxRole) error {
+		return errors.New("account is in use")
+	}
+	if _, err := setupWindowsSandboxPrincipal(config); err == nil {
+		t.Fatal("setup reported success after failing to retire a principal it cannot reconcile")
+	}
+}
+
+// Teardown had the same blind spot as setup: it computed the paths to revoke
+// from the CURRENT policy, so retiring a principal cleared every ACE except the
+// one on the root that had left the policy — and then deleted the account, which
+// left that ACE naming a SID nothing could resolve to clean up later.
+func TestTeardownRevokesRecordedPathsTheCurrentPolicyNoLongerNames(t *testing.T) {
+	home := t.TempDir()
+	workspace := t.TempDir()
+	dropped := filepath.Join(t.TempDir(), "left-the-policy")
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:    home,
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: workspace}},
+			},
+		},
+	}
+	role := windowsSandboxRoleOffline
+	username := windowsSandboxUserName(windowsSandboxPrincipalKey(config), role)
+	if err := writeWindowsPrincipalACLLedger(home, username, []string{dropped}); err != nil {
+		t.Fatalf("seed the record: %v", err)
+	}
+
+	paths, err := windowsPrincipalRevocationPaths(config, "S-1-5-32-546", role)
+	if err != nil {
+		t.Fatalf("windowsPrincipalRevocationPaths: %v", err)
+	}
+
+	// The other role's revocation must not pick this up: separate accounts,
+	// separate records, and retiring one must leave the other's ACEs alone.
+	other, err := windowsPrincipalRevocationPaths(config, "S-1-5-32-546", windowsSandboxRoleOnline)
+	if err != nil {
+		t.Fatalf("windowsPrincipalRevocationPaths(online): %v", err)
+	}
+	if containsPathFold(other, dropped) {
+		t.Errorf("the online role would revoke %q, which only the offline role's record names", dropped)
+	}
+	if !containsPathFold(paths, dropped) {
+		t.Errorf("teardown would revoke %v, missing the recorded root %q the policy no longer names", paths, dropped)
+	}
+	if !containsPathFold(paths, workspace) {
+		t.Errorf("teardown would revoke %v, missing the workspace the current policy grants", paths)
+	}
+}
+
+// stubWindowsPrincipalSetup replaces every elevated call
+// setupWindowsSandboxPrincipal makes, so the decision under test is reachable on
+// a machine with nothing provisioned.
+func stubWindowsPrincipalSetup(t *testing.T) WindowsSandboxCommandConfig {
+	t.Helper()
+	home := t.TempDir()
+	workspace := t.TempDir()
+
+	prevLookup := lookupWindowsSandboxIdentityFn
+	prevRemove := removeWindowsSandboxPrincipalForSetupFn
+	prevProvision := provisionWindowsSandboxIdentityFn
+	prevGrant := grantWindowsSandboxLogonRightsFn
+	prevReset := resetWindowsSandboxUserPasswordFn
+	prevSecret := writeWindowsSandboxSecretFn
+	prevApply := applyWindowsACLPlanFn
+	prevCache := sandboxUserCacheDir
+	t.Cleanup(func() {
+		lookupWindowsSandboxIdentityFn = prevLookup
+		removeWindowsSandboxPrincipalForSetupFn = prevRemove
+		provisionWindowsSandboxIdentityFn = prevProvision
+		grantWindowsSandboxLogonRightsFn = prevGrant
+		resetWindowsSandboxUserPasswordFn = prevReset
+		writeWindowsSandboxSecretFn = prevSecret
+		applyWindowsACLPlanFn = prevApply
+		sandboxUserCacheDir = prevCache
+	})
+
+	provisionWindowsSandboxIdentityFn = func(key string, role windowsSandboxRole) (windowsSandboxIdentity, string, bool, error) {
+		return windowsSandboxIdentity{Username: windowsSandboxUserName(key, role), SID: guestsSID(t)}, "pw", true, nil
+	}
+	grantWindowsSandboxLogonRightsFn = func(*windows.SID) error { return nil }
+	resetWindowsSandboxUserPasswordFn = func(string, string) error { return nil }
+	writeWindowsSandboxSecretFn = func(string, string) error { return nil }
+	applyWindowsACLPlanFn = func(WindowsACLPlan) (func() error, error) {
+		return func() error { return nil }, nil
+	}
+	cache := t.TempDir()
+	sandboxUserCacheDir = func() (string, error) { return cache, nil }
+
+	return WindowsSandboxCommandConfig{
+		SandboxHome:    home,
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: workspace}},
+			},
+		},
+	}
+}
+
+func guestsSID(t *testing.T) *windows.SID {
+	t.Helper()
+	sid, err := windows.StringToSid("S-1-5-32-546")
+	if err != nil {
+		t.Fatalf("StringToSid: %v", err)
+	}
+	return sid
+}
+
+// containsPathFold compares the way the ACL plans do, which is the only
+// comparison that means anything here.
+//
+// Comparing the raw spellings passed CI on nothing and failed on Windows: the
+// plan builder runs every root through normalizeProfilePath, whose EvalSymlinks
+// expands the 8.3 short name GitHub's runners hand out for TEMP, so the record
+// holds C:\Users\runneradmin\... while t.TempDir() returned C:\Users\RUNNER~1\...
+// and EqualFold called two spellings of one directory different paths. A
+// developer whose TEMP has no short name never sees it.
+//
+// Production is self-consistent — both the recorded and the newly planned paths
+// go through the same normalization — so this was only ever the test being
+// naive about what "same path" means on Windows.
+func containsPathFold(paths []string, want string) bool {
+	wanted := windowsCapabilityPathKey(normalizeProfilePath(want))
+	if wanted == "" {
+		return false
+	}
+	for _, path := range paths {
+		if windowsCapabilityPathKey(normalizeProfilePath(path)) == wanted {
+			return true
+		}
+	}
+	return false
+}

internal/sandbox/windows_principal_preflight_test.go

@@ -0,0 +1,103 @@
+package sandbox
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+// PROVISIONING STAYS CLOSED WHILE NO LAUNCH MECHANISM EXISTS.
+//
+// Launching a command as a separate account needs SeAssignPrimaryTokenPrivilege
+// and SeIncreaseQuotaPrivilege. The process that needs them is the LATER,
+// ordinary command process, not setup, and nothing carries launch authority
+// across that boundary.
+//
+// The previous gate asked whether the CALLING process held the privileges,
+// which is the wrong lifetime and got the answer backwards where it mattered:
+// it refused unelevated setup, which was never dangerous, and it PASSED
+// elevated setup, which is precisely the case that lands a local account, its
+// password, its logon rights, workspace ACEs, the recovery ledger and network
+// filter state for a backend no later command can use. The gate existed to
+// prevent durable state serving something that cannot run, and it produced it.
+//
+// So the refusal must not be satisfiable by holding the privileges right now.
+func TestSetupRefusesToProvisionAPrincipalNothingCanLaunch(t *testing.T) {
+	optIn := true
+	options := WindowsSandboxSetupArgsOptions{
+		CommandCWD:     t.TempDir(),
+		SandboxHome:    t.TempDir(),
+		PrincipalOptIn: &optIn,
+	}
+
+	args, err := BuildWindowsSandboxSetupArgs(options)
+	if err == nil {
+		t.Fatalf("setup args were built for a principal nothing can launch: %v", args)
+	}
+	if !errors.Is(err, errWindowsPrincipalLaunchUnavailable) {
+		t.Errorf("the refusal is not the launch-unavailable one: %v", err)
+	}
+	for _, want := range []string{"SeAssignPrimaryTokenPrivilege", "SeIncreaseQuotaPrivilege", "provision"} {
+		if !strings.Contains(err.Error(), want) {
+			t.Errorf("the refusal does not mention %q: %v", want, err)
+		}
+	}
+	// The way forward has to be the one that works. Telling the operator to
+	// re-run elevated is the advice that produced the unusable state.
+	if strings.Contains(strings.ToLower(err.Error()), "elevated terminal") {
+		t.Errorf("the refusal still points at an elevated terminal, which does not help: %v", err)
+	}
+	if !strings.Contains(err.Error(), windowsSandboxIdentityEnv) {
+		t.Errorf("the refusal does not name the opt-out: %v", err)
+	}
+}
+
+// THE REGRESSION, STATED DIRECTLY: an elevated caller must be refused too.
+//
+// This is the case the old check let through, and it is the only one that
+// creates durable machine state. A gate that consults the current process
+// cannot express it, which is why the seam takes no token and no argument.
+func TestSetupRefusalDoesNotDependOnTheCallersPrivileges(t *testing.T) {
+	optIn := true
+	options := WindowsSandboxSetupArgsOptions{
+		CommandCWD:     t.TempDir(),
+		SandboxHome:    t.TempDir(),
+		PrincipalOptIn: &optIn,
+	}
+
+	// Whatever this process holds, twice, must give the same answer.
+	first, firstErr := BuildWindowsSandboxSetupArgs(options)
+	second, secondErr := BuildWindowsSandboxSetupArgs(options)
+	if firstErr == nil || secondErr == nil {
+		t.Fatalf("provisioning succeeded: %v / %v", first, second)
+	}
+	if firstErr.Error() != secondErr.Error() {
+		t.Errorf("the refusal is not stable across calls:\n  %v\n  %v", firstErr, secondErr)
+	}
+}
+
+// AND THE GATE IS ONLY FOR THE OPTED-IN PATH. Without the opt-in there is no
+// principal to provision, so the ordinary restricted-token sandbox, which needs
+// none of those privileges, must still set up.
+func TestSetupWithoutTheOptInIsUnaffected(t *testing.T) {
+	optOut := false
+	if _, err := BuildWindowsSandboxSetupArgs(WindowsSandboxSetupArgsOptions{
+		CommandCWD:     t.TempDir(),
+		SandboxHome:    t.TempDir(),
+		PrincipalOptIn: &optOut,
+	}); err != nil {
+		t.Errorf("the restricted-token sandbox was refused for a privilege it does not need: %v", err)
+	}
+}
+
+// allowPrincipalLaunchForTest opens the gate for tests that are about argument
+// PLUMBING rather than about whether a principal can be launched. The opt-in
+// still has to round-trip correctly for `zero doctor` and for the day a broker
+// lands, and those assertions should not be deleted just because the entry
+// point is closed today.
+func allowPrincipalLaunchForTest(t *testing.T) {
+	t.Helper()
+	previous := windowsPrincipalLaunchAvailable
+	windowsPrincipalLaunchAvailable = func() error { return nil }
+	t.Cleanup(func() { windowsPrincipalLaunchAvailable = previous })
+}

internal/sandbox/windows_read_capability_test.go

@@ -0,0 +1,135 @@
+package sandbox
+
+import "testing"
+
+// readCapabilityProfile is the shape that selects the strict token: a restricted
+// filesystem with DenyRead set. Without DenyRead the token stays
+// WRITE_RESTRICTED and reads never reach the restricted-SID check at all, so
+// every assertion below would hold vacuously.
+func readCapabilityProfile(readRoot string) PermissionProfile {
+	return PermissionProfile{
+		FileSystem: FileSystemPolicy{
+			Kind:       FileSystemRestricted,
+			WriteRoots: []WritableRoot{{Root: `C:\workspace`}},
+			ReadRoots:  []string{`C:\`, readRoot},
+			DenyRead:   []string{`C:\Users\someone\.aws\credentials`},
+		},
+		Network: NetworkPolicy{Mode: NetworkAllow},
+	}
+}
+
+// A READ ROOT MUST CARRY A RESTRICTING SID.
+//
+// On the active-principal path the command runs on a strict token, because
+// DenyRead drops WRITE_RESTRICTED and Windows then applies the restricted-SID
+// check to reads as well as writes. Read roots were granted only to the
+// principal's account SID, which windowsPrincipalJailSIDs removes from the
+// restricting set on purpose, so the normal check passed and the restricted
+// check matched nothing. The effect was not a narrower sandbox but an unusable
+// one: no read root readable, up to and including the executable being launched.
+//
+// The custom read root is the case a workspace-only assertion would miss: an SDK
+// or toolchain directory outside the workspace that the command must read.
+func TestCapabilityPlanGrantsReadRootsToARestrictingSID(t *testing.T) {
+	home := t.TempDir()
+	custom := `C:\sdk\toolchain`
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:       home,
+		CommandCWD:        `C:\workspace`,
+		WorkspaceRoots:    []string{`C:\workspace`},
+		PermissionProfile: readCapabilityProfile(custom),
+	}
+	plan, err := BuildWindowsACLPlan(config)
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	readSID, err := WindowsReadAllowSID(home)
+	if err != nil {
+		t.Fatalf("WindowsReadAllowSID: %v", err)
+	}
+	if readSID == "" {
+		t.Fatal("no read capability SID, so this test proves nothing")
+	}
+
+	for _, want := range []string{`C:\`, custom} {
+		found := false
+		for _, entry := range plan.Entries {
+			if entry.Action == WindowsACLAllowRead &&
+				windowsCapabilityPathKey(entry.Path) == windowsCapabilityPathKey(want) &&
+				entry.Capability == readSID {
+				found = true
+				break
+			}
+		}
+		if !found {
+			t.Errorf("read root %s has no allow-read entry for the read capability SID, so a strict token cannot read it", want)
+		}
+	}
+}
+
+// The read grant must not reopen the deny list.
+//
+// The read roots start at the filesystem root, so the capability that makes them
+// readable also covers every DenyRead carveout underneath. If the deny entries
+// name only the write capabilities, the carveout stays readable through the read
+// capability and DenyRead silently stops meaning anything — which is the whole
+// reason the strict token is selected in the first place.
+func TestDenyReadCoversTheReadCapabilitySID(t *testing.T) {
+	home := t.TempDir()
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:       home,
+		CommandCWD:        `C:\workspace`,
+		WorkspaceRoots:    []string{`C:\workspace`},
+		PermissionProfile: readCapabilityProfile(`C:\sdk\toolchain`),
+	}
+	plan, err := BuildWindowsACLPlan(config)
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	readSID, err := WindowsReadAllowSID(home)
+	if err != nil {
+		t.Fatalf("WindowsReadAllowSID: %v", err)
+	}
+
+	denied := false
+	sawDeny := false
+	for _, entry := range plan.Entries {
+		if entry.Action != WindowsACLDenyRead {
+			continue
+		}
+		sawDeny = true
+		if entry.Capability == readSID {
+			denied = true
+		}
+	}
+	if !sawDeny {
+		t.Fatal("plan has no deny-read entries at all, so this test proves nothing")
+	}
+	if !denied {
+		t.Error("no deny-read entry names the read capability SID, so every DenyRead path stays readable through the read grant")
+	}
+}
+
+// Without DenyRead the token stays WRITE_RESTRICTED, reads skip the restricted
+// check, and the read grant would be ACEs that buy nothing — on roots as broad
+// as the filesystem root. Both halves of the protocol decide this from the
+// profile alone, so setup and the command cannot disagree about whether the
+// entries exist.
+func TestReadCapabilityIsAbsentWithoutDenyRead(t *testing.T) {
+	profile := readCapabilityProfile(`C:\sdk\toolchain`)
+	profile.FileSystem.DenyRead = nil
+	plan, err := BuildWindowsACLPlan(WindowsSandboxCommandConfig{
+		SandboxHome:       t.TempDir(),
+		CommandCWD:        `C:\workspace`,
+		WorkspaceRoots:    []string{`C:\workspace`},
+		PermissionProfile: profile,
+	})
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	for _, entry := range plan.Entries {
+		if entry.Action == WindowsACLAllowRead {
+			t.Fatalf("allow-read entry %s present without DenyRead, where reads are never restricted", entry.Path)
+		}
+	}
+}

internal/sandbox/windows_restricted_sid_read_test.go

@@ -0,0 +1,134 @@
+package sandbox
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// THE STRICT TOKEN MUST CARRY THE GRANT ITS READ ROOTS NAME.
+//
+// A profile with DenyRead drops WRITE_RESTRICTED, which puts reads under the
+// restricted-SID check as well. Everyone used to satisfy that check, and taking
+// it away without putting the read capability in its place leaves a token that
+// cannot open cmd.exe: the command dies at launch with an access denial that
+// says nothing about the profile.
+//
+// The ACL plan grants this same capability on every read root, so the two halves
+// have to be decided together. This pins the token half; the plan half is pinned
+// by the plan tests, and the kernel behaviour by the impersonation test beside
+// this one.
+func TestTheStrictTokenCarriesTheReadCapability(t *testing.T) {
+	home := t.TempDir()
+	want, err := WindowsReadAllowSID(home)
+	if err != nil {
+		t.Fatalf("SETUP INVALID: no read capability for %s: %v", home, err)
+	}
+
+	base := []string{"S-1-15-3-1024-workspace"}
+	got, err := windowsRestrictedTokenSIDsForProfile(base, home, false)
+	if err != nil {
+		t.Fatalf("windowsRestrictedTokenSIDsForProfile: %v", err)
+	}
+	if !carriesSID(got, want) {
+		t.Fatalf("the strict token's restricted SIDs %v omit the read capability %s, so the command cannot open its own executable", got, want)
+	}
+	if !carriesSID(got, base[0]) {
+		t.Fatalf("the workspace capability was dropped: %v", got)
+	}
+}
+
+// And the WRITE_RESTRICTED token does not get it: reads are already unrestricted
+// there, so adding a SID would widen the write jail for nothing.
+func TestTheWriteRestrictedTokenDoesNotCarryTheReadCapability(t *testing.T) {
+	home := t.TempDir()
+	readSID, err := WindowsReadAllowSID(home)
+	if err != nil {
+		t.Fatalf("SETUP INVALID: no read capability for %s: %v", home, err)
+	}
+	got, err := windowsRestrictedTokenSIDsForProfile([]string{"S-1-15-3-1024-workspace"}, home, true)
+	if err != nil {
+		t.Fatalf("windowsRestrictedTokenSIDsForProfile: %v", err)
+	}
+	if carriesSID(got, readSID) {
+		t.Fatalf("the WRITE_RESTRICTED token carries the read capability %s, widening the write jail to every read root", readSID)
+	}
+}
+
+// Not containsSID: #886 adds a package-level helper by that name, and the two
+// branches would stop compiling the moment both land.
+func carriesSID(values []string, want string) bool {
+	for _, value := range values {
+		if strings.EqualFold(value, want) {
+			return true
+		}
+	}
+	return false
+}
+
+// THE PLAN AND THE TOKEN DECIDE THE READ CAPABILITY SEPARATELY.
+//
+// The ACL plan asks whether the profile configures DenyRead; the runner asks
+// whether the token keeps WRITE_RESTRICTED. Both are read off the same field
+// today, in two files, with nothing tying them together. Drift either way is
+// silent and bad: a token carrying a SID no DACL names cannot open its own
+// executable, and a plan granting a SID no token carries leaves reads confined
+// by nothing.
+//
+// So this pins the equivalence rather than either half.
+func TestThePlanAndTheTokenAgreeOnTheReadCapability(t *testing.T) {
+	for _, testCase := range []struct {
+		name     string
+		denyRead []string
+	}{
+		{name: "with denyRead", denyRead: []string{filepath.Join("secrets")}},
+		{name: "without denyRead"},
+	} {
+		t.Run(testCase.name, func(t *testing.T) {
+			workspace := t.TempDir()
+			config := WindowsSandboxCommandConfig{
+				SandboxHome:    t.TempDir(),
+				CommandCWD:     workspace,
+				WorkspaceRoots: []string{workspace},
+				PermissionProfile: PermissionProfile{
+					FileSystem: FileSystemPolicy{
+						Kind:       FileSystemRestricted,
+						WriteRoots: []WritableRoot{{Root: workspace}},
+						ReadRoots:  []string{workspace},
+						DenyRead:   prefixEach(workspace, testCase.denyRead),
+					},
+				},
+			}
+
+			planned, err := windowsReadAllowCapabilitySID(config)
+			if err != nil {
+				t.Fatalf("windowsReadAllowCapabilitySID: %v", err)
+			}
+			writeRestricted := len(config.PermissionProfile.FileSystem.DenyRead) == 0
+			tokenSIDs, err := windowsRestrictedTokenSIDsForProfile(nil, config.SandboxHome, writeRestricted)
+			if err != nil {
+				t.Fatalf("windowsRestrictedTokenSIDsForProfile: %v", err)
+			}
+
+			inPlan := planned != ""
+			inToken := len(tokenSIDs) > 0
+			if inPlan != inToken {
+				t.Fatalf("the plan grants the read capability = %t but the token carries it = %t; one side confines reads the other side never checks", inPlan, inToken)
+			}
+			if inPlan && !carriesSID(tokenSIDs, planned) {
+				t.Fatalf("the plan grants %s but the token carries %v, so the command cannot read what setup allowed", planned, tokenSIDs)
+			}
+		})
+	}
+}
+
+func prefixEach(root string, relatives []string) []string {
+	if len(relatives) == 0 {
+		return nil
+	}
+	out := make([]string, 0, len(relatives))
+	for _, relative := range relatives {
+		out = append(out, filepath.Join(root, relative))
+	}
+	return out
+}

internal/sandbox/windows_role_inventory_windows_test.go

@@ -0,0 +1,68 @@
+//go:build windows
+
+package sandbox
+
+import "testing"
+
+func containsRole(roles []windowsSandboxRole, want windowsSandboxRole) bool {
+	for _, role := range roles {
+		if role == want {
+			return true
+		}
+	}
+	return false
+}
+
+// THE LEGACY ROLE BELONGS TO EXACTLY ONE OF THE TWO INVENTORIES.
+//
+// Both directions are real failures that have nearly happened here.
+//
+// In the retirable list: teardown retires legacy, and the is-it-still-installed
+// guard used to ask only the live pair. An upgraded machine still holding the
+// untagged pre-split account was therefore reported clean, and the opted-out
+// marker claimed a teardown that had not happened.
+//
+// Out of the live list: provisioning legacy would create that pre-split account
+// fresh on every setup of a machine that had already been upgraded past it.
+func TestTheLegacyRoleIsRetirableButNeverProvisioned(t *testing.T) {
+	if containsRole(windowsSandboxLiveRoles, windowsSandboxRoleLegacy) {
+		t.Error("the legacy role is in the provisioning inventory; setup would recreate the pre-split account on every run")
+	}
+	if !containsRole(windowsSandboxRetirableRoles, windowsSandboxRoleLegacy) {
+		t.Error("the legacy role is not in the retirable inventory; teardown and the installed check would both miss the pre-split account")
+	}
+	for _, role := range windowsSandboxLiveRoles {
+		if !containsRole(windowsSandboxRetirableRoles, role) {
+			t.Errorf("live role %q is not retirable, so teardown would leave it behind", role)
+		}
+	}
+}
+
+// And the installed check consults the retirable inventory, so a machine still
+// holding only the legacy account is not reported clean.
+func TestPrincipalIsInstalledSeesTheLegacyAccount(t *testing.T) {
+	prev := lookupWindowsSandboxIdentityFn
+	t.Cleanup(func() { lookupWindowsSandboxIdentityFn = prev })
+
+	var asked []windowsSandboxRole
+	lookupWindowsSandboxIdentityFn = func(_ string, role windowsSandboxRole) (windowsSandboxIdentity, error) {
+		asked = append(asked, role)
+		if role == windowsSandboxRoleLegacy {
+			return windowsSandboxIdentity{}, nil
+		}
+		return windowsSandboxIdentity{}, errWindowsSandboxIdentityUnavailable
+	}
+
+	workspace := t.TempDir()
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+	}
+	if !windowsSandboxPrincipalIsInstalled(config) {
+		t.Errorf("a machine still holding the legacy account was reported clean; roles asked: %v", asked)
+	}
+	if !containsRole(asked, windowsSandboxRoleLegacy) {
+		t.Errorf("the legacy role was never looked up: %v", asked)
+	}
+}

internal/sandbox/windows_role_test.go

@@ -0,0 +1,26 @@
+package sandbox
+
+import "testing"
+
+// Doctor and the runtime must name the SAME principal.
+//
+// The helper this replaces existed to be that single rule and said so, and the
+// drift it was meant to prevent happened anyway: dual-role provisioning made the
+// runtime use an offline principal under NetworkDeny while doctor still reported
+// the old restricted-token standdown, so an operator was told their reads were
+// unconfined while they were confined. Both sides now call this, and the test
+// pins the mapping rather than either caller's copy of it.
+func TestPrincipalRoleForNetwork(t *testing.T) {
+	for mode, want := range map[NetworkMode]string{
+		NetworkAllow: "online",
+		NetworkDeny:  "offline",
+		// Unset and unrecognized lose the network rather than keeping it, matching
+		// what the runtime does with a mode it does not know.
+		"":           "offline",
+		"not-a-mode": "offline",
+	} {
+		if got := WindowsSandboxPrincipalRoleForNetwork(mode); got != want {
+			t.Errorf("role for %q = %q, want %q", mode, got, want)
+		}
+	}
+}

internal/sandbox/windows_runner.go

@@ -173,6 +173,12 @@ type WindowsSandboxCommandConfig struct {
 	Env               map[string]string
 	SandboxLevel      WindowsSandboxLevel
 	Command           []string
+	// CallerSID names the Windows user whose sandbox principal this config plans
+	// against. Empty — the case for every ordinary command — means the process
+	// running it, which is already the invoking user. It is set only by the
+	// elevated setup half, which runs as somebody else under over-the-shoulder UAC
+	// and would otherwise provision an account the caller can never find.
+	CallerSID string
 }
 
 func BuildWindowsSandboxCommandArgs(options WindowsSandboxCommandArgsOptions) ([]string, error) {
@@ -336,13 +342,27 @@ func windowsRestrictedTokenCommandPlan(execRequest SandboxExecutionRequest, poli
 		level = WindowsSandboxLevelUnelevated
 	}
 	args, err := BuildWindowsSandboxCommandArgs(WindowsSandboxCommandArgsOptions{
-		SandboxHome:       sandboxHome,
-		CommandCWD:        spec.Dir,
-		WorkspaceRoots:    []string{execRequest.WorkspaceRoot},
-		PermissionProfile: execRequest.PermissionProfile,
-		Env:               childEnv,
-		SandboxLevel:      level,
-		Command:           append([]string{spec.Name}, spec.Args...),
+		SandboxHome:    sandboxHome,
+		CommandCWD:     spec.Dir,
+		WorkspaceRoots: []string{execRequest.WorkspaceRoot},
+		// Augmented HERE, in the parent, because this is the last process whose
+		// TEMP is the operator's.
+		//
+		// The runner cannot derive the runtime candidates itself: the child env it
+		// runs with has TEMP and TMP pointed at the sandbox runtime temp, so
+		// os.TempDir() inside it returns the redirected value and the temp-derived
+		// candidate comes out rooted under the runtime tree instead of under the
+		// real temp. Setup, whose TEMP is untouched, derived the other spelling —
+		// same number of roots, different paths — and every command died on
+		// "permission roots or deny lists changed" with two 12-entry plans that
+		// disagreed.
+		PermissionProfile: windowsSandboxProfileWithRuntime(
+			execRequest.PermissionProfile,
+			[]string{execRequest.WorkspaceRoot},
+		),
+		Env:          childEnv,
+		SandboxLevel: level,
+		Command:      append([]string{spec.Name}, spec.Args...),
 	})
 	if err != nil {
 		return CommandPlan{}, err
@@ -442,6 +462,20 @@ type WindowsCapabilitySIDs struct {
 	// commands include it and are blocked; online (approved) commands omit it and
 	// reach the network — both still write-jailed by the capability SIDs.
 	Offline string `json:"offline,omitempty"`
+	// ReadAllow carries the principal's READ grants, and exists because a strict
+	// (non-WRITE_RESTRICTED) token runs the restricted-SID check over reads too.
+	//
+	// A profile with DenyRead drops WRITE_RESTRICTED, so every read has to satisfy
+	// the restricting set as well as the normal token. Read roots were granted
+	// only to the principal's own account SID, which the write jail deliberately
+	// keeps OUT of that set, so the normal check passed and the restricted check
+	// matched nothing: the whole disk became unreadable, down to the executable
+	// the command was trying to start.
+	//
+	// Synthetic and held by nobody. An ACE naming it grants access only to a token
+	// the sandbox itself composed, and restricting SIDs can only narrow a token,
+	// never widen it, so publishing this grant on the read roots hands out nothing.
+	ReadAllow string `json:"readAllow,omitempty"`
 }
 
 func ResolveWindowsSandboxHome(env map[string]string) (string, error) {
@@ -478,8 +512,13 @@ func LoadOrCreateWindowsCapabilitySIDs(sandboxHome string) (WindowsCapabilitySID
 			// Back-compat: an older (schema 1) file has no offline-marker SID.
 			// Mint one and persist so the setup helper and the runner agree on a
 			// single value for the WFP filter scope across processes.
-			if caps.Offline == "" {
-				caps.Offline = randomWindowsCapabilitySID()
+			if caps.Offline == "" || caps.ReadAllow == "" {
+				if caps.Offline == "" {
+					caps.Offline = randomWindowsCapabilitySID()
+				}
+				if caps.ReadAllow == "" {
+					caps.ReadAllow = randomWindowsCapabilitySID()
+				}
 				caps.SchemaVersion = windowsCapabilitySIDSchemaVersion
 				if err := saveWindowsCapabilitySIDs(path, caps); err != nil {
 					return WindowsCapabilitySIDs{}, err
@@ -494,6 +533,7 @@ func LoadOrCreateWindowsCapabilitySIDs(sandboxHome string) (WindowsCapabilitySID
 		SchemaVersion:      windowsCapabilitySIDSchemaVersion,
 		ReadOnly:           randomWindowsCapabilitySID(),
 		Offline:            randomWindowsCapabilitySID(),
+		ReadAllow:          randomWindowsCapabilitySID(),
 		WorkspaceByRoot:    map[string]string{},
 		WritableRootByPath: map[string]string{},
 	}
@@ -686,3 +726,44 @@ func randomWindowsCapabilitySID() string {
 	}
 	return fmt.Sprintf("S-1-5-21-%d-%d-%d-%d", words[0], words[1], words[2], words[3])
 }
+
+// windowsRestrictedTokenSIDsForProfile adds the read capability to a restricted
+// token's SID list when the profile selected the strict token.
+//
+// WRITE_RESTRICTED scopes the restricted-SID check to writes. Without it the
+// check applies to reads as well, so the token needs a restricted SID that the
+// read roots' DACLs name or the command cannot open its own executable. It used
+// to be Everyone, which is a key to every object whose DACL grants Everyone
+// write and so handed back the write jail (#869). BuildWindowsACLPlan grants
+// this capability on every read root and denies it on every DenyRead path, so
+// the read allowance and the restriction are the same decision.
+//
+// Only elevated setup can grant it on the volume root the production profile
+// seeds; the unelevated tier refuses a DenyRead profile up front for that
+// reason, so nothing reaches here expecting a grant nobody applied.
+func windowsRestrictedTokenSIDsForProfile(tokenSIDs []string, sandboxHome string, writeRestricted bool) ([]string, error) {
+	if writeRestricted {
+		return tokenSIDs, nil
+	}
+	readSID, err := WindowsReadAllowSID(sandboxHome)
+	if err != nil {
+		return nil, fmt.Errorf("resolve the sandbox read capability: %w", err)
+	}
+	return append(tokenSIDs, readSID), nil
+}
+
+// WindowsReadAllowSID returns the sandbox home's read-capability SID, minting and
+// persisting it on first use. Both halves of the setup protocol ask for it: the
+// capability ACL plan grants it on every read root, and the principal's strict
+// token carries it as a restricting SID, so the read allow-list and the
+// restriction come from one value instead of two that can drift.
+func WindowsReadAllowSID(sandboxHome string) (string, error) {
+	caps, err := LoadOrCreateWindowsCapabilitySIDs(sandboxHome)
+	if err != nil {
+		return "", err
+	}
+	if strings.TrimSpace(caps.ReadAllow) == "" {
+		return "", errors.New("windows sandbox read-capability SID is missing")
+	}
+	return caps.ReadAllow, nil
+}

internal/sandbox/windows_runtime_ace_windows.go

@@ -0,0 +1,99 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"fmt"
+	"os"
+	"strings"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// verifyWindowsRuntimeRootCapability confirms the runtime roots this command is
+// about to write to still carry the capability ACE setup gave them.
+//
+// THE MARKER ATTESTS TO A PLAN, NOT TO AN OBJECT. cleanupSandboxRuntimeRoots
+// removes inactive sibling workspace roots on an age and count policy. When that
+// workspace runs again, command-side preparation recreates the SAME deterministic
+// pathname as the ordinary caller, and the new directory inherits from its
+// parent: it does not carry the capability SID ACE that elevated setup applied to
+// the object that used to be there. ValidateWindowsSandboxSetupMarker still
+// passes, because it fingerprints pathnames and actions rather than the identity
+// of the ACL-bearing object, so the restricted child launched and then failed its
+// cache, temp and package-cache writes with a bare ACCESS_DENIED and nothing
+// pointing at setup.
+//
+// Cleanup treats these directories as disposable cache state; setup and its
+// marker treat their DACL as durable provisioned state. Checking the ACE here is
+// what reconciles the two: the command refuses with the remedy instead of
+// launching into a tree it cannot write.
+func verifyWindowsRuntimeRootCapability(config WindowsSandboxCommandConfig) error {
+	runtime := config.PermissionProfile.Runtime
+	if runtime == nil {
+		return nil
+	}
+	root := strings.TrimSpace(runtime.Root)
+	if root == "" {
+		return nil
+	}
+	sid, err := windowsCapabilitySIDForWriteRoot(config, root)
+	if err != nil {
+		return fmt.Errorf("resolve the sandbox runtime capability for %s: %w", root, err)
+	}
+	present, err := windowsPathGrantsCapability(root, sid)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return fmt.Errorf("the sandbox runtime root %s does not exist, so setup's provisioning is gone — run `zero sandbox setup` from an elevated (Administrator) terminal", root)
+		}
+		return fmt.Errorf("inspect the sandbox runtime root %s: %w", root, err)
+	}
+	if !present {
+		return fmt.Errorf("the sandbox runtime root %s no longer carries the capability grant setup applied to it, "+
+			"which happens when the directory was reclaimed and recreated by an ordinary run; "+
+			"every sandboxed write into it would fail with access denied — run `zero sandbox setup` from an elevated (Administrator) terminal", root)
+	}
+	return nil
+}
+
+// windowsPathGrantsCapability reports whether path's DACL carries an allow ACE
+// for sid. Read off the object, because the whole point is that the pathname
+// says nothing about which object now answers to it.
+func windowsPathGrantsCapability(path string, sid string) (bool, error) {
+	wanted, err := windows.StringToSid(sid)
+	if err != nil {
+		return false, fmt.Errorf("parse the capability SID %q: %w", sid, err)
+	}
+	handle, _, err := openWindowsACLTarget(path)
+	if err != nil {
+		return false, err
+	}
+	defer func() { _ = windows.CloseHandle(handle) }()
+	descriptor, err := windows.GetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		return false, err
+	}
+	dacl, _, err := descriptor.DACL()
+	if err != nil || dacl == nil {
+		return false, err
+	}
+	// GetAce hands back a generic header that is reinterpreted here, which is
+	// sound only for the fixed-layout ACE types: an object ACE carries Flags and
+	// two GUIDs ahead of the trustee, so SidStart would land mid-structure. A type
+	// this does not recognise is skipped rather than decoded into a nonsense SID.
+	for index := uint32(0); index < uint32(dacl.AceCount); index++ {
+		var ace *windows.ACCESS_ALLOWED_ACE
+		if err := windows.GetAce(dacl, index, &ace); err != nil {
+			continue
+		}
+		if ace.Header.AceType != windows.ACCESS_ALLOWED_ACE_TYPE {
+			continue
+		}
+		aceSID := (*windows.SID)(unsafe.Pointer(&ace.SidStart))
+		if aceSID.Equals(wanted) {
+			return true, nil
+		}
+	}
+	return false, nil
+}

internal/sandbox/windows_runtime_reclaim_windows_test.go

@@ -0,0 +1,104 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// CLEANUP AND SETUP DISAGREE ABOUT WHO OWNS THE RUNTIME DIRECTORY.
+//
+// cleanupSandboxRuntimeRoots reclaims inactive sibling workspace roots on an age
+// and count policy, treating them as disposable cache state. Setup and its marker
+// treat their DACL as durable provisioned state. When the reclaimed workspace runs
+// again, command-side preparation recreates the SAME deterministic pathname as
+// the ordinary caller, and the new directory inherits from its parent without the
+// capability SID ACE elevated setup applied to the object that used to be there.
+//
+// The marker still validates, because it fingerprints pathnames and actions
+// rather than the identity of the ACL-bearing object. So the restricted child
+// launched and then failed its cache, temp and package-cache writes with a bare
+// access-denied and nothing pointing at setup.
+//
+// Driven by applying a real plan, reclaiming the directory the way cleanup does,
+// recreating the pathname the way an ordinary run does, and asking the check the
+// command now makes. DACL edits on a user-owned directory need no Administrator
+// rights, so this runs unelevated.
+//
+// Not covered here: an actual write performed by a real capability token, which
+// needs a provisioned sandbox this box cannot create. The ACE presence is the
+// fact the command consumes, and it is what this pins.
+func TestRuntimeRootReclaimedAndRecreatedLosesItsCapability(t *testing.T) {
+	cacheRoot := t.TempDir()
+	previous := sandboxUserCacheDir
+	t.Cleanup(func() { sandboxUserCacheDir = previous })
+	sandboxUserCacheDir = func() (string, error) { return cacheRoot, nil }
+
+	workspace := t.TempDir()
+	runtimeRoot, ok := deterministicSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(cacheRoot))
+	if !ok {
+		t.Fatalf("SETUP INVALID: no deterministic runtime root for %s", workspace)
+	}
+	if err := os.MkdirAll(runtimeRoot, 0o700); err != nil {
+		t.Fatal(err)
+	}
+
+	config := WindowsSandboxCommandConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PermissionProfile: PermissionProfile{
+			Runtime: &SandboxRuntime{Root: runtimeRoot},
+			FileSystem: FileSystemPolicy{
+				Kind: FileSystemRestricted,
+				WriteRoots: []WritableRoot{
+					{Root: workspace},
+					{Root: runtimeRoot},
+				},
+			},
+		},
+	}
+
+	plan, err := BuildWindowsACLPlan(config)
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	if _, err := applyWindowsACLPlan(plan); err != nil {
+		t.Skipf("cannot apply an ACL plan here: %v", err)
+	}
+
+	// A provisioned machine passes, or the assertion below would be satisfied by a
+	// check that refuses unconditionally.
+	if err := verifyWindowsRuntimeRootCapability(config); err != nil {
+		t.Fatalf("SETUP INVALID: a freshly provisioned runtime root was rejected: %v", err)
+	}
+
+	// Cleanup reclaims it, and the next ordinary run recreates the same pathname.
+	if err := os.RemoveAll(runtimeRoot); err != nil {
+		t.Fatalf("reclaim the runtime root the way cleanup does: %v", err)
+	}
+	if err := os.MkdirAll(runtimeRoot, 0o700); err != nil {
+		t.Fatalf("recreate the pathname the way an ordinary run does: %v", err)
+	}
+
+	err = verifyWindowsRuntimeRootCapability(config)
+	if err == nil {
+		t.Fatal("a recreated runtime root with no capability grant was accepted; the command would launch and then fail every sandboxed write with nothing explaining why")
+	}
+	message := err.Error()
+	if !strings.Contains(message, runtimeRoot) {
+		t.Errorf("the refusal does not name the root: %v", err)
+	}
+	if !strings.Contains(strings.ToLower(message), "sandbox setup") {
+		t.Errorf("the refusal does not point at the remedy: %v", err)
+	}
+
+	// And the marker on its own still says everything is fine, which is exactly
+	// why the object has to be checked separately.
+	if _, statErr := os.Stat(filepath.Dir(runtimeRoot)); statErr != nil {
+		t.Fatalf("the runtime parent vanished: %v", statErr)
+	}
+}

internal/sandbox/windows_secret_removal_windows_test.go

@@ -0,0 +1,63 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// A secret this account cannot delete must be distinguishable from one that
+// simply would not delete.
+//
+// Teardown carries on past the first and aborts on the second. Before the
+// distinction existed, a secret owned by another administrator's setup stopped
+// teardown at its very first step, leaving the account, its logon rights, its
+// ACEs and its ledger all installed. The operator asked for removal, got an
+// error, and kept a working principal.
+func TestRemovingAnUndeletableSecretIsDistinguishable(t *testing.T) {
+	// A non-empty directory standing in for a secret: os.Remove refuses it, which
+	// gives a real removal failure without needing a second administrator.
+	path := filepath.Join(t.TempDir(), "secret")
+	if err := os.Mkdir(path, 0o700); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	if err := os.WriteFile(filepath.Join(path, "occupant"), []byte("x"), 0o600); err != nil {
+		t.Fatalf("seed occupant: %v", err)
+	}
+
+	err := removeWindowsSandboxSecret(path)
+	if err == nil {
+		t.Fatal("a secret that could not be removed reported success")
+	}
+	// This is NOT the foreign-owner case, so teardown must still treat it as
+	// fatal rather than shrugging and carrying on.
+	if errors.Is(err, errWindowsSandboxSecretNotOurs) {
+		t.Errorf("an ordinary removal failure was classified as a foreign owner: %v", err)
+	}
+}
+
+// Absence is success. Teardown runs on machines that never provisioned a
+// principal, and re-running it must converge rather than fail.
+func TestRemovingAMissingSecretIsNotAnError(t *testing.T) {
+	if err := removeWindowsSandboxSecret(filepath.Join(t.TempDir(), "never-existed")); err != nil {
+		t.Fatalf("removing a missing secret reported an error: %v", err)
+	}
+}
+
+// The ordinary case still works, so the tolerance above did not turn removal
+// into a no-op.
+func TestRemovingOurOwnSecretDeletesIt(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "secret")
+	if err := os.WriteFile(path, []byte("x"), 0o600); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	if err := removeWindowsSandboxSecret(path); err != nil {
+		t.Fatalf("removeWindowsSandboxSecret: %v", err)
+	}
+	if _, err := os.Stat(path); !errors.Is(err, os.ErrNotExist) {
+		t.Errorf("the secret survived removal: stat err = %v", err)
+	}
+}

internal/sandbox/windows_setup.go

@@ -11,17 +11,121 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+
+	"github.com/Gitlawb/zero/internal/peermsg"
 )
 
 const WindowsSandboxSetupName = "zero-windows-sandbox-setup.exe"
 
-const windowsSandboxSetupMarkerSchemaVersion = 4
+// Bumped to 7 when the single principal was split into offline and online role
+// accounts. THE NAMES CHANGED, so a marker written by the previous version is
+// not evidence that this version's setup has run: it validates by comparing
+// serialized plans and hashes, and neither names an account. Left at 6, an
+// upgraded machine kept a valid marker, setup was never re-run, and the runner
+// found neither "zero-sbx-d<key>" nor "zero-sbx-n<key>" and fell back to the
+// restricted-token backend with no read confinement, silently.
+//
+// Bumping it makes that installation report as out of date, which is what sends
+// the operator back through elevated setup. Setup then provisions both roles and
+// retires the untagged predecessor (see removeWindowsSandboxPrincipalsForSetup).
+const windowsSandboxSetupMarkerSchemaVersion = 7
+
+// windowsSandboxIdentityEnv opts a machine into the principal backend while it
+// is still experimental. Provisioning is inert without it, so an existing
+// install keeps the restricted-token behaviour until someone turns this on.
+//
+// Lives here, beside the setup protocol rather than beside the Windows-only
+// runtime, because the opt-in is part of that protocol: it has to be readable on
+// every platform so the setup args and the marker can carry it.
+const windowsSandboxIdentityEnv = "ZERO_WINDOWS_SANDBOX_IDENTITY"
+
+// windowsSandboxIdentityEnabled reports whether the principal backend is opted
+// into. An explicit entry in env is authoritative; otherwise the process
+// environment decides.
+func windowsSandboxIdentityEnabled(env map[string]string) bool {
+	if value, ok := env[windowsSandboxIdentityEnv]; ok {
+		return strings.TrimSpace(value) == "1"
+	}
+	return strings.TrimSpace(os.Getenv(windowsSandboxIdentityEnv)) == "1"
+}
+
+// WindowsSandboxPrincipalOptIn resolves the principal opt-in for callers outside
+// this package (the `zero sandbox setup` CLI and `zero doctor`), so both sides
+// of the setup protocol read the opt-in the same way. Pass nil to consult the
+// current process environment.
+func WindowsSandboxPrincipalOptIn(env map[string]string) bool {
+	return windowsSandboxIdentityEnabled(env)
+}
+
+func windowsSandboxPrincipalOptInValue(optIn bool) string {
+	if optIn {
+		return "1"
+	}
+	return "0"
+}
 
 type WindowsSandboxSetupArgsOptions struct {
 	SandboxHome       string
 	CommandCWD        string
 	WorkspaceRoots    []string
 	PermissionProfile PermissionProfile
+	// PrincipalOptIn is the caller's principal opt-in, serialized into the setup
+	// args. Elevated setup runs in its own process — a UAC-elevated one whose
+	// environment is not the caller's — so it must be told the value rather than
+	// left to sample an environment nobody set.
+	//
+	// Tri-state on purpose. nil means "this caller did not resolve the opt-in",
+	// and BuildWindowsSandboxSetupArgs then resolves it from the environment of
+	// the process building the args — which is the caller's own process, the one
+	// place where the ambient value is the value the operator typed. A plain bool
+	// could not say that: its zero value asserts "opted out", so every caller that
+	// simply did not know about this field would serialize `--sandbox-principal 0`
+	// while the command half still resolved the opt-in from its environment. Under
+	// a machine-wide opt-in the two halves would then disagree and marker
+	// validation would refuse every command — the same silent-disagreement bug
+	// this flag exists to remove, re-created one layer up.
+	//
+	// Set it only to override the environment (a caller holding a command's Env
+	// map, or a test pinning a value); leave it nil to mean "whatever this shell
+	// says", which is what `zero sandbox setup` and `zero doctor` want.
+	PrincipalOptIn *bool
+	// CallerSID identifies the Windows user setup was invoked BY, serialized into
+	// the args for the same reason PrincipalOptIn is: elevated setup runs in its
+	// own process, and under over-the-shoulder UAC (or `runas`) that process
+	// belongs to a DIFFERENT administrator than the caller.
+	//
+	// Everything a principal needs is scoped to the invoking user — the account
+	// name, its ACL ledger and its DPAPI secret all key off that identity — so a
+	// helper that sampled its own token would provision an account for the
+	// administrator who typed the credentials while every later command looked for
+	// one named after the ordinary user, found nothing, fell back to the restricted
+	// token, and left an admin-keyed account nothing would ever reclaim.
+	//
+	// Empty means "resolve from the process building the args", which IS the
+	// caller: these args are built before the UAC boundary is crossed.
+	CallerSID string
+}
+
+// callerSID resolves the invoking user, in the caller's own process, before the
+// args cross the UAC boundary. Empty when the token cannot be read (and on
+// non-Windows builds), which leaves the elevated helper on its pre-existing
+// behaviour of sampling its own token rather than inventing an identity.
+func (options WindowsSandboxSetupArgsOptions) callerSID() string {
+	if sid := strings.TrimSpace(options.CallerSID); sid != "" {
+		return sid
+	}
+	return windowsCurrentUserSID()
+}
+
+// principalOptIn resolves the tri-state. It runs inside
+// BuildWindowsSandboxSetupArgs, i.e. in the caller's process, before the args
+// cross the UAC boundary — so an unset caller still ships an explicit 0|1 that
+// the elevated helper can trust.
+func (options WindowsSandboxSetupArgsOptions) principalOptIn() bool {
+	if options.PrincipalOptIn != nil {
+		return *options.PrincipalOptIn
+	}
+	return windowsSandboxIdentityEnabled(nil)
 }
 
 type WindowsSandboxSetupConfig struct {
@@ -29,6 +133,11 @@ type WindowsSandboxSetupConfig struct {
 	CommandCWD        string
 	WorkspaceRoots    []string
 	PermissionProfile PermissionProfile
+	PrincipalOptIn    bool
+	// CallerSID is the invoking user this setup is provisioning FOR. See
+	// WindowsSandboxSetupArgsOptions.CallerSID. Empty means the caller did not
+	// say, and the helper falls back to its own token.
+	CallerSID string
 }
 
 type WindowsSandboxSetupMarker struct {
@@ -43,6 +152,21 @@ type WindowsSandboxSetupMarker struct {
 	NetworkInfraHash string `json:"networkInfraHash"`
 	OfflineFilterSID string `json:"offlineFilterSid"`
 	NetworkFilters   int    `json:"networkFilters"`
+	// PrincipalOptIn records whether the run that wrote this marker provisioned a
+	// sandbox principal. Without it the two halves each sampled their own
+	// environment and could disagree silently — see
+	// ValidateWindowsSandboxSetupMarker.
+	PrincipalOptIn bool `json:"principalOptIn"`
+	// PrincipalPlanHash fingerprints the PRINCIPAL ACL plan, which ACLPlanHash
+	// above does not cover: that one hashes BuildWindowsACLPlan, the
+	// capability-SID plan, while principal grants are built separately by
+	// buildWindowsPrincipalACLPlan from the same profile.
+	//
+	// Without it, narrowing or removing a principal read root left setup looking
+	// current, so the old AllowRead ACEs stayed on disk with nothing to notice
+	// they no longer matched the policy. Empty when the principal backend is not
+	// opted into, which keeps the marker stable for the default install.
+	PrincipalPlanHash string `json:"principalPlanHash,omitempty"`
 }
 
 func WindowsSandboxSetupMarkerPath(sandboxHome string) string {
@@ -66,6 +190,43 @@ func BuildWindowsSandboxSetupArgs(options WindowsSandboxSetupArgsOptions) ([]str
 	if len(workspaceRoots) == 0 {
 		workspaceRoots = []string{commandCWD}
 	}
+	// THE PRINCIPAL BACKEND HAS NO LAUNCH PATH YET, SO IT IS NOT PROVISIONED.
+	//
+	// Launching a command as a separate account needs SeAssignPrimaryTokenPrivilege
+	// and SeIncreaseQuotaPrivilege. An ordinary unelevated process holds neither,
+	// and the command runs LATER, from the caller, not from setup. So the
+	// advertised lifecycle -- provision once from an elevated terminal, then run
+	// ordinary commands -- cannot execute: the elevated setup process holds the
+	// privileges and the process that needs them does not.
+	//
+	// An earlier version of this check tested the CALLING process's token, which
+	// is the wrong lifetime. It refused unelevated setup, which was never the
+	// dangerous case, and PASSED elevated setup, which is exactly the case that
+	// creates a local account, its password, its logon-right assignments,
+	// workspace ACEs, the recovery ledger and network filter state for a backend
+	// no later command can use. Durable machine state serving something that
+	// cannot run is the outcome the check existed to prevent, and it produced it.
+	//
+	// Requiring every sandboxed command to run elevated would make the privileges
+	// line up, and is deliberately not done: it is a worse boundary than the
+	// restricted token this would replace, and it is not the lifecycle the feature
+	// is for. Until a reviewed launch mechanism exists -- a bootstrap or an
+	// authenticated broker that carries launch authority across the UAC boundary
+	// -- provisioning stays unavailable rather than half-working.
+	//
+	// Everything below the launch path (the plans, the ledger, the ACL and filter
+	// work) still builds and is still tested; it is the provisioning entry point
+	// that is closed. See #662, which this PR is foundation for and does not
+	// close.
+	if options.principalOptIn() {
+		if err := windowsPrincipalLaunchAvailable(); err != nil {
+			return nil, fmt.Errorf("refusing to provision a sandbox principal: %w", err)
+		}
+	}
+	// Augmented here, in the caller's shell, before the args cross into the
+	// elevated helper. Same reason the opt-in and caller SID are: the value has to
+	// be resolved where the environment is the operator's.
+	options.PermissionProfile = WindowsSandboxProfileWithRuntimeRoots(options.PermissionProfile, workspaceRoots)
 	profileJSON, err := json.Marshal(options.PermissionProfile)
 	if err != nil {
 		return nil, fmt.Errorf("marshal windows sandbox setup permission profile: %w", err)
@@ -74,6 +235,16 @@ func BuildWindowsSandboxSetupArgs(options WindowsSandboxSetupArgsOptions) ([]str
 		"--sandbox-home", sandboxHome,
 		"--command-cwd", commandCWD,
 		"--permission-profile", string(profileJSON),
+		// Always explicit, never omitted-means-false: the elevated helper must be
+		// able to tell "the caller wants no principal" from "an older caller said
+		// nothing", and only the first of those is safe to run silently.
+		"--sandbox-principal", windowsSandboxPrincipalOptInValue(options.principalOptIn()),
+	}
+	// Omitted rather than sent empty when the caller's identity cannot be read:
+	// absence means "this caller did not say", which the helper answers by
+	// sampling its own token exactly as it did before this flag existed.
+	if callerSID := options.callerSID(); callerSID != "" {
+		args = append(args, "--caller-sid", callerSID)
 	}
 	for _, root := range workspaceRoots {
 		args = append(args, "--workspace-root", root)
@@ -117,6 +288,31 @@ func ParseWindowsSandboxSetupArgs(args []string) (WindowsSandboxSetupConfig, err
 			}
 			profileJSON = strings.TrimSpace(value)
 			index = next
+		case "--sandbox-principal":
+			value, next, err := nextWindowsSandboxFlagValue(args, index)
+			if err != nil {
+				return WindowsSandboxSetupConfig{}, err
+			}
+			switch strings.TrimSpace(value) {
+			case "1":
+				config.PrincipalOptIn = true
+			case "0":
+				config.PrincipalOptIn = false
+			default:
+				// Refused rather than treated as off: a value this helper cannot read
+				// is a caller it does not understand, and guessing "no principal"
+				// there would provision a weaker sandbox than the caller asked for
+				// while reporting success.
+				return WindowsSandboxSetupConfig{}, fmt.Errorf("invalid --sandbox-principal %q, want 0 or 1", value)
+			}
+			index = next
+		case "--caller-sid":
+			value, next, err := nextWindowsSandboxFlagValue(args, index)
+			if err != nil {
+				return WindowsSandboxSetupConfig{}, err
+			}
+			config.CallerSID = strings.TrimSpace(value)
+			index = next
 		default:
 			return WindowsSandboxSetupConfig{}, fmt.Errorf("unknown windows sandbox setup flag %q", arg)
 		}
@@ -148,22 +344,49 @@ func RunWindowsSandboxSetup(args []string, stderr io.Writer) int {
 	return runWindowsSandboxSetup(config, stderr)
 }
 
+// commandConfig is the command-shaped view the setup half plans against. Its Env
+// carries the opt-in the caller serialized into the setup args, so every
+// downstream windowsSandboxIdentityEnabled call — the gate that decides whether
+// elevated setup provisions a principal at all — reads the caller's intent
+// rather than sampling the elevated helper's own environment, which UAC does not
+// inherit from the shell the user typed in.
 func (config WindowsSandboxSetupConfig) commandConfig() WindowsSandboxCommandConfig {
 	return WindowsSandboxCommandConfig{
-		SandboxHome:       config.SandboxHome,
-		CommandCWD:        config.CommandCWD,
-		WorkspaceRoots:    cloneStrings(config.WorkspaceRoots),
+		SandboxHome:    config.SandboxHome,
+		CommandCWD:     config.CommandCWD,
+		WorkspaceRoots: cloneStrings(config.WorkspaceRoots),
+		// Taken as given. This computes the hash BOTH halves compare, and it runs
+		// in whichever process is asking — including the command runner, whose TEMP
+		// points at the sandbox runtime temp. Deriving the candidates here made the
+		// answer depend on the caller's environment, which is the one thing a
+		// shared fingerprint cannot afford. WindowsSandboxProfileWithRuntimeRoots
+		// is applied by the callers whose TEMP is still the operator's.
 		PermissionProfile: config.PermissionProfile,
+		Env:               map[string]string{windowsSandboxIdentityEnv: windowsSandboxPrincipalOptInValue(config.PrincipalOptIn)},
 		SandboxLevel:      WindowsSandboxLevelRestrictedToken,
+		// Carried through for the same reason as the opt-in above: every principal
+		// name, ledger and secret path the setup half plans is derived from the
+		// invoking user, and that is the caller's identity, not this process's.
+		CallerSID: config.CallerSID,
 	}
 }
 
+// WindowsSandboxSetupConfigFromCommand is how a command asks "was setup run for
+// what I need?". It carries the command's own opt-in so marker validation can
+// compare it against what setup actually provisioned.
 func WindowsSandboxSetupConfigFromCommand(config WindowsSandboxCommandConfig) WindowsSandboxSetupConfig {
 	return WindowsSandboxSetupConfig{
-		SandboxHome:       config.SandboxHome,
-		CommandCWD:        config.CommandCWD,
-		WorkspaceRoots:    cloneStrings(config.WorkspaceRoots),
+		SandboxHome:    config.SandboxHome,
+		CommandCWD:     config.CommandCWD,
+		WorkspaceRoots: cloneStrings(config.WorkspaceRoots),
+		// NOT augmented here. This runs inside the command runner, whose TEMP and
+		// TMP are the sandbox runtime's, so deriving the temp-based candidate here
+		// yields a path no other process agrees on. The parent already folded the
+		// candidates into the profile it serialized, back when TEMP was still the
+		// operator's, so take the profile as given.
 		PermissionProfile: config.PermissionProfile,
+		PrincipalOptIn:    windowsSandboxIdentityEnabled(config.Env),
+		CallerSID:         config.CallerSID,
 	}
 }
 
@@ -191,16 +414,63 @@ func BuildWindowsSandboxSetupMarker(config WindowsSandboxSetupConfig) (WindowsSa
 	if len(infraPlan.IdentitySIDs) > 0 {
 		offlineSID = infraPlan.IdentitySIDs[0]
 	}
+	principalHash, err := windowsPrincipalPlanFingerprint(config)
+	if err != nil {
+		return WindowsSandboxSetupMarker{}, err
+	}
 	return WindowsSandboxSetupMarker{
-		SchemaVersion:    windowsSandboxSetupMarkerSchemaVersion,
-		ACLPlanHash:      hash,
-		ACLPlanEntries:   len(plan.Entries),
-		NetworkInfraHash: infraHash,
-		OfflineFilterSID: offlineSID,
-		NetworkFilters:   len(infraPlan.Filters),
+		SchemaVersion:     windowsSandboxSetupMarkerSchemaVersion,
+		ACLPlanHash:       hash,
+		ACLPlanEntries:    len(plan.Entries),
+		NetworkInfraHash:  infraHash,
+		OfflineFilterSID:  offlineSID,
+		NetworkFilters:    len(infraPlan.Filters),
+		PrincipalOptIn:    config.PrincipalOptIn,
+		PrincipalPlanHash: principalHash,
 	}, nil
 }
 
+// windowsPrincipalPlanFingerprint hashes the principal ACL plan so a change to
+// principal read or write roots invalidates setup.
+//
+// The SID is a fixed placeholder rather than the real principal's, deliberately.
+// The account is recreated with a fresh SID on reprovision, so hashing the real
+// one would make the fingerprint change every time the account is rebuilt even
+// though the GRANTED PATHS are identical, and every command would then rerun
+// setup. What must invalidate the marker is the set of paths and actions, which
+// is exactly what this captures.
+//
+// Returns empty when the principal backend is not opted into, so the default
+// install's marker is unchanged.
+func windowsPrincipalPlanFingerprint(config WindowsSandboxSetupConfig) (string, error) {
+	if !config.PrincipalOptIn {
+		return "", nil
+	}
+	filesystem := config.commandConfig().PermissionProfile.FileSystem
+	// EVERY FIELD apply and teardown pass, or the fingerprint describes a
+	// different plan than the one that gets applied. This omitted DenyWrite while
+	// both buildWindowsPrincipalACLPlan call sites in windows_identity_runtime_windows.go
+	// passed it, so a change to the policy's deny-write paths moved the applied
+	// plan and left this hash where it was. The capability ACLPlanHash happens to
+	// cover the same paths today, which is what kept it from being a live hole
+	// and also what would have kept it invisible.
+	plan, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: windowsPrincipalFingerprintSID,
+		WriteRoots:   filesystem.WriteRoots,
+		ReadRoots:    filesystem.ReadRoots,
+		DenyRead:     filesystem.DenyRead,
+		DenyWrite:    filesystem.DenyWrite,
+	})
+	if err != nil {
+		return "", fmt.Errorf("fingerprint windows principal ACL plan: %w", err)
+	}
+	return WindowsACLPlanHash(plan)
+}
+
+// windowsPrincipalFingerprintSID is a placeholder trustee used only for hashing.
+// It never reaches an ACE.
+const windowsPrincipalFingerprintSID = "S-1-0-0"
+
 func WriteWindowsSandboxSetupMarker(config WindowsSandboxSetupConfig) (WindowsSandboxSetupMarker, error) {
 	marker, err := BuildWindowsSandboxSetupMarker(config)
 	if err != nil {
@@ -255,8 +525,56 @@ func ValidateWindowsSandboxSetupMarker(config WindowsSandboxSetupConfig) error {
 	if actual.SchemaVersion != expected.SchemaVersion {
 		return fmt.Errorf("windows sandbox setup is out of date: schema %d, want %d", actual.SchemaVersion, expected.SchemaVersion)
 	}
+	// The two halves of the protocol run in different processes, so they can
+	// disagree about the principal opt-in. Refuse the command rather than pick a
+	// winner.
+	//
+	// The direction that matters is the first one: the opt-in is on, setup never
+	// provisioned an account, and the runtime's lookup declines with a nil error —
+	// so without this the command runs on the restricted token, which does not
+	// confine reads, while the operator believes a principal is isolating them.
+	// A sandbox that is weaker than advertised has to be loud.
+	//
+	// The reverse is refused too. It is not the dangerous direction — the command
+	// gets the well-worn restricted token it asked for — but setup did create a
+	// local account and grant it ACEs on the workspace, and letting commands run
+	// as if that had not happened leaves nothing to reconcile it. Both directions
+	// clear the same way: run `zero sandbox setup` again with the environment you
+	// actually want.
+	if actual.PrincipalOptIn != expected.PrincipalOptIn {
+		if expected.PrincipalOptIn {
+			return fmt.Errorf("windows sandbox setup is out of date: %s=1 asks for a sandbox principal, but setup provisioned none — "+
+				"re-run `zero sandbox setup` from an elevated (Administrator) terminal with %s=1, or unset it to use the restricted-token sandbox",
+				windowsSandboxIdentityEnv, windowsSandboxIdentityEnv)
+		}
+		// "principals", plural: a workspace gets an offline and an online account,
+		// and the opt-out path retires the legacy one alongside them. The singular
+		// described the pre-split world and understated what the re-run does.
+		return fmt.Errorf("windows sandbox setup is out of date: setup provisioned sandbox principals, but %s is not set for this command — "+
+			"set %s=1, or re-run `zero sandbox setup` from an elevated (Administrator) terminal without it to retire them",
+			windowsSandboxIdentityEnv, windowsSandboxIdentityEnv)
+	}
 	if actual.ACLPlanHash != expected.ACLPlanHash || actual.ACLPlanEntries != expected.ACLPlanEntries {
-		return errors.New("windows sandbox setup is out of date: permission roots or deny lists changed")
+		// Say WHAT disagrees. The bare "permission roots or deny lists changed"
+		// left an operator with a command that refuses to run and no way to tell
+		// which side is wrong, or even whether the marker belongs to this
+		// workspace at all. The entry counts separate the two shapes this takes:
+		// equal counts mean the same roots spelled differently, unequal counts
+		// mean one side has roots the other has never heard of.
+		return fmt.Errorf("windows sandbox setup is out of date: permission roots or deny lists changed "+
+			"(marker %s has %d entries, hash %s; this command expects %d entries, hash %s) — "+
+			"re-run `zero sandbox setup` from an elevated (Administrator) terminal",
+			path, actual.ACLPlanEntries, shortWindowsACLPlanHash(actual.ACLPlanHash),
+			expected.ACLPlanEntries, shortWindowsACLPlanHash(expected.ACLPlanHash))
+	}
+	// The capability-SID plan above and the principal plan are built separately
+	// from the same profile, so the hash above does not cover principal grants.
+	// Without this check, removing a principal read root left setup looking
+	// current and the stale AllowRead ACE in place, which is the opposite of
+	// what narrowing a policy is supposed to do.
+	if actual.PrincipalPlanHash != expected.PrincipalPlanHash {
+		return errors.New("windows sandbox setup is out of date: sandbox principal grants changed — " +
+			"re-run `zero sandbox setup` from an elevated (Administrator) terminal so the old grants are revoked")
 	}
 	// Mode-agnostic: validate the provisioned infrastructure, never the
 	// per-command network mode — so an approved (allow) network command and an
@@ -306,3 +624,234 @@ func canonicalWindowsACLEntries(entries []WindowsACLEntry) []WindowsACLEntry {
 	})
 	return out
 }
+
+// windowsSandboxRuntimeCandidates returns every runtime root setup provisions.
+//
+// BOTH candidates, not the one this process would select. sandboxRuntimeRootFor
+// prefers the cache-derived root and falls back to the temp-derived one when the
+// first would land inside the workspace or its lease cannot be taken, and that
+// choice is made per process. Setup that granted only its own choice left the
+// other unprovisioned, so a command that fell back wrote to a tree with no ACE
+// on it. Both are deterministic now, so setup can cover both and command
+// selection lands on a provisioned root either way.
+func windowsSandboxRuntimeCandidates(workspaceRoots []string) []string {
+	workspaceRoot := ""
+	for _, candidate := range workspaceRoots {
+		if trimmed := strings.TrimSpace(candidate); trimmed != "" {
+			workspaceRoot = canonicalSandboxWorkspaceRoot(trimmed)
+			break
+		}
+	}
+	if workspaceRoot == "" || workspaceRoot == "." {
+		return nil
+	}
+	var roots []string
+	if cacheRoot, err := sandboxUserCacheDir(); err == nil {
+		if cacheRoot = canonicalSandboxWorkspaceRoot(cacheRoot); cacheRoot != "" && cacheRoot != "." {
+			if root, ok := deterministicSandboxRuntimeRoot(workspaceRoot, cacheRoot); ok {
+				roots = append(roots, root)
+			}
+		}
+	}
+	if root, err := fallbackSandboxRuntimeRoot(workspaceRoot); err == nil {
+		roots = append(roots, root)
+	}
+	return roots
+}
+
+// windowsSandboxProfileWithRuntime adds the runtime candidates as write roots.
+//
+// Applied on BOTH sides of the setup protocol, which is the whole point. The
+// marker fingerprints the capability ACL plan built from this profile, while
+// every command reaches the Windows runner having already had
+// permissionProfileWithRuntime append the root it selected. Setup fingerprinted
+// the bare profile and the command presented an augmented one, so a marker
+// written seconds earlier was rejected with "permission roots or deny lists
+// changed" and no command could run at all.
+//
+// Adding the full candidate set on both sides makes the two hashes agree without
+// the command having to know which root setup happened to pick, and it puts the
+// runtime roots into the CAPABILITY plan as well. That second part matters since
+// the principal command runs on a WRITE_RESTRICTED token restricted to the
+// capability SIDs: a runtime root carrying only the principal ACE satisfies the
+// normal token and fails the restricted check, so cache and temp writes were
+// denied even once the marker agreed.
+func windowsSandboxProfileWithRuntime(profile PermissionProfile, workspaceRoots []string) PermissionProfile {
+	candidates := windowsSandboxRuntimeCandidates(workspaceRoots)
+	if len(candidates) == 0 {
+		return profile
+	}
+	existing := make(map[string]struct{}, len(profile.FileSystem.WriteRoots))
+	for _, root := range profile.FileSystem.WriteRoots {
+		existing[windowsCapabilityPathKey(root.Root)] = struct{}{}
+	}
+	writeRoots := append([]WritableRoot{}, profile.FileSystem.WriteRoots...)
+	for _, candidate := range candidates {
+		if _, ok := existing[windowsCapabilityPathKey(candidate)]; ok {
+			continue
+		}
+		existing[windowsCapabilityPathKey(candidate)] = struct{}{}
+		writeRoots = append(writeRoots, WritableRoot{Root: candidate})
+	}
+	profile.FileSystem.WriteRoots = writeRoots
+	return profile
+}
+
+// ensureWindowsSandboxRuntimeCandidates creates every runtime root setup grants.
+//
+// Paired with windowsSandboxProfileWithRuntime: that function puts the candidates
+// into the ACL plan, and this one makes them exist. Splitting the two is what
+// broke elevated setup once already, because the capability plan refuses to
+// materialize a write root and fails the whole run on a path that is merely
+// absent. Whenever one of these grows a candidate, so must the other.
+//
+// Called on the setup side only. A command must never create these: setup is the
+// gate that decides which trees the sandbox may write to, and a command that
+// created its own root would be granting itself one.
+func ensureWindowsSandboxRuntimeCandidates(workspaceRoots []string) error {
+	for _, root := range windowsSandboxRuntimeCandidates(workspaceRoots) {
+		if err := ensureRuntimeCandidateDir(root); err != nil {
+			return fmt.Errorf("create sandbox runtime root %s: %w", root, err)
+		}
+	}
+	return nil
+}
+
+// ensureRuntimeCandidateDir creates one candidate without following a reparse
+// point into anything Zero does not own.
+//
+// VALIDATION BEFORE THE PRIVILEGED MUTATION, NOT AFTER IT. This runs as
+// Administrator, and os.MkdirAll follows links: the invoking user can prepare
+// either candidate's ancestry, so a junction planted at a not-yet-created
+// component below the cache directory, or at the fallback anchor, made elevated
+// setup create the tree beneath a redirected, Administrator-writable target. The
+// ACL applier's own no-follow check runs later and correctly spots the
+// redirection, but by then the privileged create has already happened, outside
+// the applier's rollback boundary. A leaf check after MkdirAll would leave the
+// same gap.
+//
+// EnsurePrivateDir is the descent that does not have it: handle-relative,
+// no-follow, refusing any component that is a link and any leaf this user does
+// not own. It is given a PHYSICAL base first, because it walks from the volume
+// root and a redirected cache or TEMP above the owned tail is the operator's
+// business, not ours -- the same pairing the fallback anchor uses, and the same
+// mistake that refused every fallback on macOS when the parent was left
+// unresolved.
+func ensureRuntimeCandidateDir(root string) error {
+	base, ok := runtimeCandidateBase(root)
+	if !ok {
+		return fmt.Errorf("candidate %s does not sit beneath a known base, so its trust boundary cannot be established", root)
+	}
+	tail, err := filepath.Rel(base, root)
+	if err != nil {
+		return fmt.Errorf("locate %s beneath %s: %w", root, base, err)
+	}
+	// A resolve failure keeps the unresolved base deliberately: EnsurePrivateDir
+	// is still the fail-closed check, and refusing there names the real component
+	// rather than hiding it behind a resolver error.
+	physical := base
+	if resolved, resolveErr := physicalDir(base); resolveErr == nil && resolved != "" {
+		physical = resolved
+	}
+	return peermsg.EnsurePrivateDir(filepath.Join(physical, tail))
+}
+
+// runtimeCandidateBase returns the ancestor of a candidate that belongs to the
+// operator rather than to Zero. Everything below it was created by us and has
+// no business being a link.
+func runtimeCandidateBase(root string) (string, bool) {
+	// The fallback candidate is <temp>/zero-runtime-<tag>/v1/<digest>, and the
+	// anchor's own parent is the operator's temp directory.
+	if isFallbackRuntimeRoot(root) {
+		return filepath.Dir(fallbackRuntimeAnchor()), true
+	}
+	// The cache candidate is <cacheRoot>/zero/runtime/v1/<digest>.
+	if cacheRoot, err := sandboxUserCacheDir(); err == nil {
+		if cleaned := canonicalSandboxWorkspaceRoot(cacheRoot); cleaned != "" && cleaned != "." && pathWithinRoot(cleaned, root) {
+			return cleaned, true
+		}
+	}
+	return "", false
+}
+
+// shortWindowsACLPlanHash trims a plan hash for a human-facing error. Twelve hex
+// characters is plenty to tell two plans apart by eye, and the full 64 buries the
+// rest of the message.
+func shortWindowsACLPlanHash(hash string) string {
+	hash = strings.TrimSpace(hash)
+	if hash == "" {
+		return "(none)"
+	}
+	if len(hash) > 12 {
+		return hash[:12]
+	}
+	return hash
+}
+
+// WindowsSandboxProfileWithRuntimeRoots folds the sandbox runtime roots into a
+// permission profile, for callers that build a setup config outside this package.
+//
+// Call it ONLY from a process whose TEMP and TMP are the operator's. The
+// temp-derived candidate reads os.TempDir(), and the sandbox points those
+// variables at its own runtime temp for anything it launches, so a process on the
+// far side of that redirection derives a path no other process agrees on. The
+// command runner is exactly such a process: it takes the profile it is handed.
+func WindowsSandboxProfileWithRuntimeRoots(profile PermissionProfile, workspaceRoots []string) PermissionProfile {
+	return windowsSandboxProfileWithRuntime(profile, workspaceRoots)
+}
+
+// WindowsSandboxPrincipalRoleForNetwork names the principal a command in this
+// network mode runs as.
+//
+// Exported and portable so `zero doctor` and the Windows runtime answer from ONE
+// rule. The helper this replaces existed for that reason and said so, and the
+// thing it was protecting against happened anyway once dual-role provisioning
+// changed the behaviour under NetworkDeny: the runtime started using an offline
+// principal while doctor still reported the old restricted-token standdown, so
+// operators were told their reads were unconfined when they were confined.
+// windowsSandboxRoleForNetwork delegates here rather than repeating the switch.
+//
+// Anything that is not an EXACT allow is offline: a mode this does not recognise
+// should lose the network, not keep it.
+//
+// Deliberately not normalized first. NormalizeNetworkMode case-folds, so routing
+// through it made "ALLOW" select the online principal where the runtime required
+// an exact match and failed closed to offline. Sharing one rule is only an
+// improvement if it shares the STRICTER one; a shared rule that quietly widened
+// the runtime would be worse than the drift it fixes.
+func WindowsSandboxPrincipalRoleForNetwork(network NetworkMode) string {
+	if network == NetworkAllow {
+		return "online"
+	}
+	return "offline"
+}
+
+// errWindowsPrincipalLaunchUnavailable is why provisioning refuses.
+//
+// Stated as one sentence about the MECHANISM rather than about the caller's
+// token, because the caller's token was never the question: the process that
+// needs the launch privileges is a later, ordinary one, and no supported path
+// carries launch authority to it. A message about the current process invited
+// the operator to re-run from an elevated terminal, which passes such a check
+// and still cannot run a command.
+var errWindowsPrincipalLaunchUnavailable = errors.New(
+	"launching a command as a separate account needs SeAssignPrimaryTokenPrivilege and SeIncreaseQuotaPrivilege, " +
+		"which an ordinary command process does not hold; elevated setup cannot grant them to it, and no bootstrap " +
+		"or broker exists yet to carry that authority across. The restricted-token sandbox is unaffected: unset " +
+		windowsSandboxIdentityEnv + " to use it",
+)
+
+// windowsPrincipalLaunchAvailable reports whether a provisioned principal could
+// actually be launched by the process that will run the command.
+//
+// ONE SEAM, DEFAULTING TO UNAVAILABLE. The answer does not depend on the token
+// of whoever calls setup, so this takes no argument: it is a statement about
+// which mechanisms exist, not about the current process. When a bootstrap or
+// broker lands, this is the single place that learns to say yes, and the plans,
+// ledger and ACL work below it are already built and tested against it.
+//
+// It stays a var so the argument-plumbing tests can exercise the opt-in path
+// without depending on a capability no machine currently has.
+var windowsPrincipalLaunchAvailable = func() error {
+	return errWindowsPrincipalLaunchUnavailable
+}

internal/sandbox/windows_setup_caller_windows_test.go

@@ -0,0 +1,284 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"bytes"
+	"errors"
+	"os"
+	"strings"
+	"testing"
+)
+
+// windowsSetupSeams stubs every external effect of the setup transaction so a
+// test can drive its control flow on an ordinary machine. Each seam defaults to
+// a benign success, and a test overrides only the one it is about.
+type windowsSetupSeams struct {
+	elevated bool
+	// principalStillInstalled is what the account lookup reports AFTER retirement
+	// ran. It is the whole of the opt-out branch's fatality rule, so it is stated
+	// separately from retireErr rather than inferred from it.
+	principalStillInstalled bool
+	aclRollback             func() error
+	retireErr               error
+	provisionErr            error
+	networkErr              error
+	markerErr               error
+	rollbackCalled          *bool
+	markerWritten           *bool
+}
+
+func withWindowsSetupSeams(t *testing.T, seams windowsSetupSeams) {
+	t.Helper()
+
+	originalElevated := windowsProcessIsElevatedFn
+	originalLock := acquireWindowsSandboxSetupLockFn
+	originalACL := applyWindowsACLPlanFn
+	originalPrincipal := setupWindowsSandboxPrincipalFn
+	originalRetire := removeWindowsSandboxPrincipalsForSetupFn
+	originalGroupSID := resolveWindowsSandboxOfflineGroupSIDHook
+	originalLookup := lookupWindowsSandboxIdentityFn
+	originalNetwork := applyWindowsNetworkPlanFn
+	originalMarker := writeWindowsSandboxSetupMarkerFn
+	t.Cleanup(func() {
+		lookupWindowsSandboxIdentityFn = originalLookup
+		windowsProcessIsElevatedFn = originalElevated
+		acquireWindowsSandboxSetupLockFn = originalLock
+		applyWindowsACLPlanFn = originalACL
+		setupWindowsSandboxPrincipalFn = originalPrincipal
+		removeWindowsSandboxPrincipalsForSetupFn = originalRetire
+		resolveWindowsSandboxOfflineGroupSIDHook = originalGroupSID
+		applyWindowsNetworkPlanFn = originalNetwork
+		writeWindowsSandboxSetupMarkerFn = originalMarker
+	})
+
+	windowsProcessIsElevatedFn = func() bool { return seams.elevated }
+	// A zero lock is safe to release: release() tolerates a nil handle and
+	// runtime.UnlockOSThread is a no-op when the thread was never pinned.
+	acquireWindowsSandboxSetupLockFn = func(string) (*windowsSandboxSetupLock, error) {
+		return &windowsSandboxSetupLock{}, nil
+	}
+	applyWindowsACLPlanFn = func(WindowsACLPlan) (func() error, error) {
+		return func() error {
+			if seams.rollbackCalled != nil {
+				*seams.rollbackCalled = true
+			}
+			if seams.aclRollback != nil {
+				return seams.aclRollback()
+			}
+			return nil
+		}, nil
+	}
+	setupWindowsSandboxPrincipalFn = func(WindowsSandboxCommandConfig) (func() error, error) {
+		if seams.provisionErr != nil {
+			return nil, seams.provisionErr
+		}
+		return func() error { return nil }, nil
+	}
+	removeWindowsSandboxPrincipalsForSetupFn = func(WindowsSandboxCommandConfig) error {
+		return seams.retireErr
+	}
+	// Dual-role setup asserts the block filters name the offline group, which a
+	// stubbed provisioning never creates.
+	resolveWindowsSandboxOfflineGroupSIDHook = func() (string, error) { return "S-1-5-32-999", nil }
+	lookupWindowsSandboxIdentityFn = func(string, windowsSandboxRole) (windowsSandboxIdentity, error) {
+		if seams.principalStillInstalled {
+			return windowsSandboxIdentity{Username: "zero-sbx-stub"}, nil
+		}
+		return windowsSandboxIdentity{}, errWindowsSandboxIdentityUnavailable
+	}
+	applyWindowsNetworkPlanFn = func(WindowsNetworkPlan) error { return seams.networkErr }
+	writeWindowsSandboxSetupMarkerFn = func(config WindowsSandboxSetupConfig) (WindowsSandboxSetupMarker, error) {
+		if seams.markerWritten != nil {
+			*seams.markerWritten = true
+		}
+		if seams.markerErr != nil {
+			return WindowsSandboxSetupMarker{}, seams.markerErr
+		}
+		return BuildWindowsSandboxSetupMarker(config)
+	}
+}
+
+func windowsSetupTestConfig(t *testing.T, optIn bool) WindowsSandboxSetupConfig {
+	t.Helper()
+	root := t.TempDir()
+	return WindowsSandboxSetupConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     root,
+		WorkspaceRoots: []string{root},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: root}}},
+			Network:    NetworkPolicy{Mode: NetworkDeny},
+		},
+		PrincipalOptIn: optIn,
+	}
+}
+
+// windowsForeignSID is a well-formed account SID that no test machine's own
+// process can be running as.
+const windowsForeignSID = "S-1-5-21-1111111111-2222222222-3333333333-1001"
+
+// The finding: opting out printed the retirement failure and carried on to write
+// an opted-out marker and exit 0.
+//
+// That is the one outcome this branch must never produce. Teardown is
+// idempotent, so a failure here means a real account, secret, logon right, ACE
+// set and ledger are still installed — and the marker now says none of them
+// exist, so nothing afterwards will ever look for them again.
+//
+// Asserting on the marker as well as the exit code is deliberate: an exit code
+// alone would still pass if the marker had already been written, and the marker
+// is what makes the residue permanent.
+func TestOptOutRetirementFailureFailsSetupAndWritesNoMarker(t *testing.T) {
+	markerWritten := false
+	rollbackCalled := false
+	withWindowsSetupSeams(t, windowsSetupSeams{
+		elevated:                true,
+		retireErr:               errors.New("sandbox principal zero-sbx-abc could not be deleted"),
+		principalStillInstalled: true,
+		markerWritten:           &markerWritten,
+		rollbackCalled:          &rollbackCalled,
+	})
+
+	var stderr bytes.Buffer
+	config := windowsSetupTestConfig(t, false)
+	if code := runWindowsSandboxSetup(config, &stderr); code == 0 {
+		t.Fatal("setup reported success after failing to retire an existing sandbox principal, so the operator is told the machine is clean while the account is still installed")
+	}
+	if markerWritten {
+		t.Error("setup wrote an opted-out marker after the retirement failed, which is what makes the leftover principal permanently invisible")
+	}
+	if !rollbackCalled {
+		t.Error("setup failed without undoing the ACL grants it had already applied")
+	}
+	if !strings.Contains(stderr.String(), "could not be deleted") {
+		t.Errorf("the underlying retirement error must reach the operator, got %q", stderr.String())
+	}
+	if _, err := os.Stat(WindowsSandboxSetupMarkerPath(config.SandboxHome)); !os.IsNotExist(err) {
+		t.Errorf("no marker file should exist on disk after a failed setup, stat error = %v", err)
+	}
+}
+
+// The other side of that rule, and a regression I introduced fixing the first
+// one: failing on ANY retirement error is wrong.
+//
+// removeWindowsSandboxPrincipalForSetup deliberately carries on past a secret
+// file owned by another administrator's setup (errWindowsSandboxSecretNotOurs
+// goes into secretErr and is reported at the end via errors.Join) so one
+// unlinkable file cannot strand the account, its logon rights and its ACEs. The
+// account is therefore already gone when that error is returned, and treating it
+// as fatal made `zero sandbox setup` fail permanently on such a machine, taking
+// the DEFAULT restricted-token sandbox down with it over inert residue.
+//
+// Setup must complete, and must still say what was left behind.
+func TestOptOutSucceedsWhenOnlyInertResidueSurvivesRetirement(t *testing.T) {
+	markerWritten := false
+	rollbackCalled := false
+	withWindowsSetupSeams(t, windowsSetupSeams{
+		elevated: true,
+		// What teardown reports when the only leftover is another administrator's
+		// secret file: the account, its rights, its ACEs and its ledger are gone.
+		retireErr:               errWindowsSandboxSecretNotOurs,
+		principalStillInstalled: false,
+		markerWritten:           &markerWritten,
+		rollbackCalled:          &rollbackCalled,
+	})
+
+	var stderr bytes.Buffer
+	if code := runWindowsSandboxSetup(windowsSetupTestConfig(t, false), &stderr); code != 0 {
+		t.Fatalf("setup failed over residue left behind by an already-retired principal, which makes the default sandbox unsetupable on this machine: %s", stderr.String())
+	}
+	if !markerWritten {
+		t.Error("setup that completed must write its marker")
+	}
+	if rollbackCalled {
+		t.Error("setup rolled back its ACL work over residue it had already reported as survivable")
+	}
+	if !strings.Contains(stderr.String(), "residue") {
+		t.Errorf("the leftover must still be named for whoever cleans it up, got %q", stderr.String())
+	}
+}
+
+// The opt-out path still succeeds when retirement succeeds. Without this the
+// test above would pass against a branch that always failed.
+func TestOptOutSucceedsWhenRetirementSucceeds(t *testing.T) {
+	markerWritten := false
+	withWindowsSetupSeams(t, windowsSetupSeams{elevated: true, markerWritten: &markerWritten})
+
+	var stderr bytes.Buffer
+	if code := runWindowsSandboxSetup(windowsSetupTestConfig(t, false), &stderr); code != 0 {
+		t.Fatalf("opting out with nothing to retire must succeed, got %d: %s", code, stderr.String())
+	}
+	if !markerWritten {
+		t.Error("a successful setup must write its marker")
+	}
+}
+
+// The other half of the caller-identity finding: a principal provisioned by an
+// administrator who is not the caller can never be used, because its password is
+// sealed to the user that stores it. Refuse before anything is created rather
+// than leave an account, a secret and a grant nobody can reach.
+func TestPrincipalSetupRefusesWhenElevatedAsAnotherUser(t *testing.T) {
+	markerWritten := false
+	withWindowsSetupSeams(t, windowsSetupSeams{elevated: true, markerWritten: &markerWritten})
+
+	config := windowsSetupTestConfig(t, true)
+	config.CallerSID = windowsForeignSID
+	if current := windowsCurrentUserSID(); current == "" || strings.EqualFold(current, config.CallerSID) {
+		t.Skipf("cannot distinguish the caller from this process (current SID %q)", current)
+	}
+
+	var stderr bytes.Buffer
+	if code := runWindowsSandboxSetup(config, &stderr); code == 0 {
+		t.Fatal("setup provisioned a sandbox principal for a caller whose secret it cannot seal, so every later command would find the account and fail to unseal its password")
+	}
+	if markerWritten {
+		t.Error("setup wrote a marker for a principal it refused to provision")
+	}
+	if !strings.Contains(stderr.String(), "different Windows user") {
+		t.Errorf("the refusal must say what is wrong and how to clear it, got %q", stderr.String())
+	}
+}
+
+// The refusal is scoped to the principal opt-in. The default restricted-token
+// sandbox stores no secret and names no account after the user, so it works
+// perfectly well across this boundary and must keep doing so.
+func TestRestrictedTokenSetupIsUnaffectedByACrossUserElevation(t *testing.T) {
+	withWindowsSetupSeams(t, windowsSetupSeams{elevated: true})
+
+	config := windowsSetupTestConfig(t, false)
+	config.CallerSID = windowsForeignSID
+
+	var stderr bytes.Buffer
+	if code := runWindowsSandboxSetup(config, &stderr); code != 0 {
+		t.Fatalf("the restricted-token sandbox does not depend on the invoking user and must still set up, got %d: %s", code, stderr.String())
+	}
+}
+
+// An unknown caller SID is treated as a match. That is the pre-existing
+// behaviour for an older caller or a token query that failed, and refusing on
+// absence would break setup on machines where nothing is wrong.
+func TestPrincipalSetupProceedsWhenTheCallerIsUnknown(t *testing.T) {
+	withWindowsSetupSeams(t, windowsSetupSeams{elevated: true})
+
+	var stderr bytes.Buffer
+	if code := runWindowsSandboxSetup(windowsSetupTestConfig(t, true), &stderr); code != 0 {
+		t.Fatalf("an unstated caller must not block setup, got %d: %s", code, stderr.String())
+	}
+}
+
+// The account name, its ledger and its ACEs are all derived from this key, and
+// under over-the-shoulder UAC the elevated helper is a different user than the
+// caller. Keying off the caller is what makes setup provision the account the
+// caller's next command will actually look for.
+func TestPrincipalKeyFollowsTheCallerNotTheCurrentProcess(t *testing.T) {
+	roots := []string{`C:\ws\shared`}
+	self := windowsSandboxPrincipalKey(WindowsSandboxCommandConfig{WorkspaceRoots: roots})
+	caller := windowsSandboxPrincipalKey(WindowsSandboxCommandConfig{WorkspaceRoots: roots, CallerSID: windowsForeignSID})
+	if self == caller {
+		t.Fatal("the principal key ignored the caller SID, so elevated setup would provision an account named after the administrator who typed the credentials")
+	}
+	if again := windowsSandboxPrincipalKey(WindowsSandboxCommandConfig{WorkspaceRoots: roots, CallerSID: windowsForeignSID}); again != caller {
+		t.Error("the caller-scoped key is not stable, so setup and the command half would derive different accounts")
+	}
+}

internal/sandbox/windows_setup_lock_windows.go

@@ -0,0 +1,193 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+	"time"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// Serializing elevated setup across processes.
+//
+// WHY THIS EXISTS. Setup is not one write, it is a transaction: it rotates the
+// principal's password, stores the secret, read-modify-writes the ACL ledger,
+// installs WFP filters and finally writes the marker. Two setups for the same
+// workspace running at once interleave those steps, and the interleavings are
+// not merely untidy. The account ends up with process B's password while the
+// stored secret is process A's, so every later command fails to log on. The
+// ledger's read-modify-write loses one process's path set, so ACEs that should
+// have been revoked stay on disk with nothing recording them.
+//
+// Atomic individual writes do not help. Each step being all-or-nothing is
+// orthogonal to two processes interleaving BETWEEN steps, which is the actual
+// failure. The lock has to span the whole transaction.
+//
+// Worth recording: the reference implementation this was compared against has
+// the identical bug. Its password rotation and secret write hold no lock at
+// all, its named mutex guards only an unrelated read-ACL-only mode, and the
+// singleflight that looks like a fix is process-local and gives no cross-process
+// guarantee. There was nothing to port, so this is built.
+
+// windowsSetupLockTimeout bounds the wait. Setup is interactive and elevated,
+// so a minute is generous for a legitimate concurrent run and short enough that
+// a stuck holder gets reported rather than hung on. A var rather than a const
+// only so a contention test does not have to wait a real minute to prove the
+// two setups exclude each other.
+var windowsSetupLockTimeout = 60 * time.Second
+
+// windowsSetupLockSDDL grants full control to Administrators and SYSTEM and
+// nobody else, with the DACL protected so no inherited ACE widens it.
+//
+// Load-bearing rather than tidy. A sandbox principal able to open this object
+// could hold it and stall every future setup, or create it first and squat the
+// name so real setup believes it holds a lock it does not. Only accounts that
+// could already run setup may touch it.
+//
+// A var so a test can exercise the mutex wait path without elevation. Setup
+// itself always runs elevated, so unelevated callers cannot even open the
+// object under this DACL, which would make the contention test unrunnable
+// anywhere it matters. TestSetupLockIsAdministratorsOnly pins the production
+// value so loosening it needs an explicit change here.
+var windowsSetupLockSDDL = "D:P(A;;GA;;;BA)(A;;GA;;;SY)"
+
+const (
+	// windowsWaitTimeout is WAIT_TIMEOUT. x/sys exports WAIT_ABANDONED,
+	// WAIT_OBJECT_0 and WAIT_FAILED at this version but not this one.
+	windowsWaitTimeout = 0x00000102
+
+	// windowsSetupLockKeyChars keeps the object name short while staying
+	// collision-free in practice: the key is already a hash, so a prefix of it
+	// distinguishes workspaces.
+	windowsSetupLockKeyChars = 32
+)
+
+// windowsSandboxSetupLock is a held cross-process setup lock. Release it once.
+type windowsSandboxSetupLock struct {
+	handle   windows.Handle
+	released bool
+	// abandoned records that the previous holder died without releasing, which
+	// means it stopped somewhere inside the transaction.
+	abandoned bool
+}
+
+// windowsSandboxSetupLockName derives the object name for a workspace.
+//
+// Per-workspace rather than machine-wide because every artifact the lock
+// protects is per-workspace: this account, this secret, this ledger. A global
+// lock would serialize unrelated workspaces for no reason.
+//
+// Global rather than Local because the protected state is not session-scoped.
+// The local account, its LSA logon rights and the WFP filters are machine
+// state, so two setups in different logon sessions must still exclude each
+// other. Creating a Global object needs SeCreateGlobalPrivilege, which the
+// elevated setup helper has and which is the only caller.
+func windowsSandboxSetupLockName(workspaceKey string) string {
+	key := strings.Map(func(r rune) rune {
+		switch {
+		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
+			return r
+		case r >= 'A' && r <= 'Z':
+			return r + ('a' - 'A')
+		default:
+			return -1
+		}
+	}, workspaceKey)
+	if key == "" {
+		key = "default"
+	}
+	if len(key) > windowsSetupLockKeyChars {
+		key = key[:windowsSetupLockKeyChars]
+	}
+	return `Global\ZeroSandboxSetup-` + key
+}
+
+// acquireWindowsSandboxSetupLock blocks until this process owns the setup lock
+// for a workspace, or fails with an actionable error.
+//
+// The OS thread is pinned for the lock's lifetime. Win32 mutex ownership is
+// thread-affine, so a goroutine that migrated between the wait and the release
+// would call ReleaseMutex from a thread that does not own the object, leaving
+// the mutex held until the process exits. That is a deadlock for every later
+// setup on the machine, so the pinning is not optional.
+func acquireWindowsSandboxSetupLock(workspaceKey string) (*windowsSandboxSetupLock, error) {
+	descriptor, err := windows.SecurityDescriptorFromString(windowsSetupLockSDDL)
+	if err != nil {
+		return nil, fmt.Errorf("build the sandbox setup lock security descriptor: %w", err)
+	}
+	attributes := windows.SecurityAttributes{SecurityDescriptor: descriptor}
+	attributes.Length = uint32(unsafe.Sizeof(attributes))
+
+	name := windowsSandboxSetupLockName(workspaceKey)
+	namePtr, err := windows.UTF16PtrFromString(name)
+	if err != nil {
+		return nil, fmt.Errorf("encode the sandbox setup lock name: %w", err)
+	}
+	// initialOwner false, then wait explicitly. Asking for ownership at creation
+	// would take the lock only when this process created the object, so the
+	// create and open paths would differ and only one of them would ever wait.
+	handle, err := windows.CreateMutex(&attributes, false, namePtr)
+	if handle == 0 {
+		return nil, fmt.Errorf("open the sandbox setup lock %s: %w", name, err)
+	}
+	runtime.LockOSThread()
+
+	lock := &windowsSandboxSetupLock{handle: handle}
+	state, waitErr := windows.WaitForSingleObject(handle, uint32(windowsSetupLockTimeout/time.Millisecond))
+	switch state {
+	case windows.WAIT_OBJECT_0:
+		return lock, nil
+	case windows.WAIT_ABANDONED:
+		// The previous holder died inside the transaction. We own the mutex now.
+		// Proceeding is right rather than refusing: setup is idempotent and
+		// re-running it is exactly how a half-finished transaction is repaired,
+		// whereas refusing would leave the machine stuck in that half state with
+		// no way forward.
+		lock.abandoned = true
+		return lock, nil
+	case windowsWaitTimeout:
+		lock.releaseThreadAndHandle()
+		return nil, fmt.Errorf("another `zero sandbox setup` is already running for this workspace and did not finish within %s; "+
+			"wait for it to finish, or look for a stuck elevated setup process before re-running", windowsSetupLockTimeout)
+	default:
+		lock.releaseThreadAndHandle()
+		if waitErr != nil {
+			return nil, fmt.Errorf("wait for the sandbox setup lock %s: %w", name, waitErr)
+		}
+		return nil, fmt.Errorf("wait for the sandbox setup lock %s returned an unexpected state %#x", name, state)
+	}
+}
+
+// Abandoned reports that the previous holder crashed mid-transaction, so this
+// run is repairing a half-finished setup rather than starting a clean one.
+func (lock *windowsSandboxSetupLock) Abandoned() bool {
+	return lock != nil && lock.abandoned
+}
+
+// release drops ownership and unpins the thread. Safe to call twice, so a defer
+// and an explicit call cannot double-release.
+func (lock *windowsSandboxSetupLock) release() {
+	if lock == nil || lock.released {
+		return
+	}
+	lock.released = true
+	// Release before closing. Closing a still-held mutex leaves it abandoned for
+	// the next waiter, which is recoverable but reports a crash that never
+	// happened.
+	_ = windows.ReleaseMutex(lock.handle)
+	lock.releaseThreadAndHandle()
+}
+
+// releaseThreadAndHandle unpins the thread and drops the handle without
+// releasing ownership, for the paths that never acquired it.
+func (lock *windowsSandboxSetupLock) releaseThreadAndHandle() {
+	if lock.handle != 0 {
+		_ = windows.CloseHandle(lock.handle)
+		lock.handle = 0
+	}
+	runtime.UnlockOSThread()
+}

internal/sandbox/windows_setup_lock_windows_test.go

@@ -0,0 +1,214 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+	"time"
+)
+
+// A unique key per test run, so a leftover object from an earlier run cannot
+// make these pass or fail for the wrong reason.
+func lockTestKey(t *testing.T) string {
+	t.Helper()
+	return fmt.Sprintf("t%dx%d", time.Now().UnixNano(), len(t.Name()))
+}
+
+// The production DACL is Administrators only, so an unelevated test process
+// cannot OPEN an existing lock and never reaches the mutex wait at all. These
+// tests are about the wait, so they relax the DACL for themselves.
+// TestSetupLockIsAdministratorsOnly pins the real value.
+func withOpenableLock(t *testing.T) {
+	t.Helper()
+	previous := windowsSetupLockSDDL
+	windowsSetupLockSDDL = "D:P(A;;GA;;;WD)"
+	t.Cleanup(func() { windowsSetupLockSDDL = previous })
+}
+
+func withShortLockTimeout(t *testing.T, d time.Duration) {
+	t.Helper()
+	previous := windowsSetupLockTimeout
+	windowsSetupLockTimeout = d
+	t.Cleanup(func() { windowsSetupLockTimeout = previous })
+}
+
+// THE POINT OF THE WHOLE FILE. A second setup for the same workspace must not
+// get in while the first holds the lock.
+//
+// Without this, two elevated setups interleave: the account ends up with one
+// process's password while the stored secret is the other's, and every later
+// command fails to log on. The second acquisition runs on its own goroutine
+// because Win32 mutex ownership is per THREAD and acquire pins the thread, so a
+// same-thread re-entry would succeed recursively and prove nothing.
+func TestSecondSetupIsExcludedWhileTheFirstHoldsTheLock(t *testing.T) {
+	withShortLockTimeout(t, 300*time.Millisecond)
+	withOpenableLock(t)
+	key := lockTestKey(t)
+
+	first, err := acquireWindowsSandboxSetupLock(key)
+	if err != nil {
+		t.Fatalf("first acquisition: %v", err)
+	}
+	defer first.release()
+
+	result := make(chan error, 1)
+	go func() {
+		second, err := acquireWindowsSandboxSetupLock(key)
+		if err == nil {
+			second.release()
+		}
+		result <- err
+	}()
+
+	select {
+	case err := <-result:
+		if err == nil {
+			t.Fatal("a second setup acquired the lock while the first still held it, so the transaction is not serialized")
+		}
+		// It has to say what to do, since the operator's next question is always
+		// whether to wait or to go looking for a stuck process.
+		if !strings.Contains(err.Error(), "already running") {
+			t.Errorf("contention error does not explain itself: %v", err)
+		}
+	case <-time.After(10 * time.Second):
+		t.Fatal("the second acquisition neither succeeded nor timed out, so the bounded wait is not bounded")
+	}
+}
+
+// Releasing must actually hand the lock on. A lock that excludes forever is a
+// worse failure than one that never excluded: the machine could never be set up
+// again without a reboot.
+func TestReleasingTheLockLetsTheNextSetupIn(t *testing.T) {
+	withShortLockTimeout(t, 5*time.Second)
+	withOpenableLock(t)
+	key := lockTestKey(t)
+
+	first, err := acquireWindowsSandboxSetupLock(key)
+	if err != nil {
+		t.Fatalf("first acquisition: %v", err)
+	}
+	first.release()
+
+	result := make(chan error, 1)
+	go func() {
+		second, err := acquireWindowsSandboxSetupLock(key)
+		if err == nil {
+			second.release()
+		}
+		result <- err
+	}()
+
+	select {
+	case err := <-result:
+		if err != nil {
+			t.Fatalf("the lock was not released, so no later setup could ever run: %v", err)
+		}
+	case <-time.After(15 * time.Second):
+		t.Fatal("acquiring a released lock hung")
+	}
+}
+
+// Different workspaces must not block each other. A machine-wide lock would
+// serialize unrelated setups for no reason.
+func TestDifferentWorkspacesDoNotExcludeEachOther(t *testing.T) {
+	withShortLockTimeout(t, 5*time.Second)
+	withOpenableLock(t)
+	base := lockTestKey(t)
+
+	first, err := acquireWindowsSandboxSetupLock(base + "alpha")
+	if err != nil {
+		t.Fatalf("first workspace: %v", err)
+	}
+	defer first.release()
+
+	result := make(chan error, 1)
+	go func() {
+		second, err := acquireWindowsSandboxSetupLock(base + "beta")
+		if err == nil {
+			second.release()
+		}
+		result <- err
+	}()
+
+	select {
+	case err := <-result:
+		if err != nil {
+			t.Fatalf("a second workspace was blocked by an unrelated one: %v", err)
+		}
+	case <-time.After(15 * time.Second):
+		t.Fatal("a second workspace hung behind an unrelated lock")
+	}
+}
+
+// Release must be safe to call twice, since it is both deferred and reachable
+// explicitly. A double ReleaseMutex would fail and, worse, a double
+// UnlockOSThread panics the runtime.
+func TestReleasingTwiceIsSafe(t *testing.T) {
+	withShortLockTimeout(t, 5*time.Second)
+	withOpenableLock(t)
+	lock, err := acquireWindowsSandboxSetupLock(lockTestKey(t))
+	if err != nil {
+		t.Fatalf("acquire: %v", err)
+	}
+	lock.release()
+	lock.release()
+}
+
+// The object name has to be per workspace and stable, since that is what makes
+// the exclusion scoped correctly rather than machine-wide or per-run.
+func TestLockNameIsPerWorkspaceAndStable(t *testing.T) {
+	first := windowsSandboxSetupLockName("workspace-one")
+	again := windowsSandboxSetupLockName("workspace-one")
+	other := windowsSandboxSetupLockName("workspace-two")
+
+	if first != again {
+		t.Errorf("the same workspace derived two names, %q then %q", first, again)
+	}
+	if first == other {
+		t.Errorf("two workspaces share the lock name %q, so unrelated setups would serialize", first)
+	}
+	// Global rather than Local: the account, its logon rights and the WFP
+	// filters are machine state, so setups in different logon sessions must
+	// still exclude each other.
+	if !strings.HasPrefix(first, `Global\`) {
+		t.Errorf("lock name %q is not in the Global namespace, so it would not span logon sessions", first)
+	}
+	// A backslash past the namespace prefix is rejected by the object manager,
+	// so a key that smuggled one in would fail at CreateMutex.
+	if strings.Contains(strings.TrimPrefix(first, `Global\`), `\`) {
+		t.Errorf("lock name %q contains a separator past its namespace prefix", first)
+	}
+}
+
+// The production lock must stay reachable only by accounts that could already
+// run setup. A sandbox principal able to open it could hold it and stall every
+// future setup, or squat the name so real setup believes it holds a lock it
+// does not.
+func TestSetupLockIsAdministratorsOnly(t *testing.T) {
+	// BA is Administrators, SY is SYSTEM, and the leading P protects the DACL so
+	// no inherited ACE can widen it.
+	for _, want := range []string{"D:P", "(A;;GA;;;BA)", "(A;;GA;;;SY)"} {
+		if !strings.Contains(windowsSetupLockSDDL, want) {
+			t.Errorf("lock SDDL %q is missing %q", windowsSetupLockSDDL, want)
+		}
+	}
+	// WD is Everyone. Granting it here would let any sandboxed process stall
+	// setup for the whole machine.
+	if strings.Contains(windowsSetupLockSDDL, ";WD)") {
+		t.Errorf("lock SDDL %q grants Everyone, so any process could hold the setup lock", windowsSetupLockSDDL)
+	}
+}
+
+// An empty or unusable key must still produce a valid name rather than a bare
+// prefix, which every such workspace would then share.
+func TestLockNameHandlesAnEmptyKey(t *testing.T) {
+	name := windowsSandboxSetupLockName("")
+	if name == `Global\ZeroSandboxSetup-` {
+		t.Fatal("an empty key produced a bare prefix, so every such workspace would share one lock")
+	}
+	if !strings.HasPrefix(name, `Global\ZeroSandboxSetup-`) {
+		t.Errorf("unexpected name %q", name)
+	}
+}

internal/sandbox/windows_setup_other.go

@@ -11,3 +11,9 @@ func runWindowsSandboxSetup(config WindowsSandboxSetupConfig, stderr io.Writer)
 	fmt.Fprintln(stderr, WindowsSandboxSetupName+": Windows sandbox setup is only available on Windows")
 	return 1
 }
+
+// windowsCurrentUserSID has no meaning off Windows. Returning empty keeps
+// BuildWindowsSandboxSetupArgs from emitting a --caller-sid nobody could
+// interpret, which is also what a Windows caller does when its own token cannot
+// be read.
+func windowsCurrentUserSID() string { return "" }

internal/sandbox/windows_setup_principal_fingerprint_test.go

@@ -0,0 +1,112 @@
+package sandbox
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func principalFingerprintConfig(sandboxHome string, readRoots []string) WindowsSandboxSetupConfig {
+	workspace := filepath.FromSlash("/ws/project")
+	return WindowsSandboxSetupConfig{
+		SandboxHome:    sandboxHome,
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PrincipalOptIn: true,
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: workspace}},
+				ReadRoots:  readRoots,
+			},
+		},
+	}
+}
+
+// Narrowing a principal's read roots MUST invalidate setup.
+//
+// ACLPlanHash covers BuildWindowsACLPlan, the capability-SID plan. Principal
+// grants are built separately by buildWindowsPrincipalACLPlan from the same
+// profile, so before this the marker was blind to them: remove a read root and
+// setup still looked current while the AllowRead ACE stayed on disk. Narrowing
+// a policy has to be able to take access away.
+func TestSetupMarkerInvalidatesWhenPrincipalReadRootsShrink(t *testing.T) {
+	home := t.TempDir()
+	wide, err := BuildWindowsSandboxSetupMarker(principalFingerprintConfig(home, []string{
+		filepath.FromSlash("/ws/project"),
+		filepath.FromSlash("/ws/extra-read"),
+	}))
+	if err != nil {
+		t.Fatalf("build wide marker: %v", err)
+	}
+	narrow, err := BuildWindowsSandboxSetupMarker(principalFingerprintConfig(home, []string{
+		filepath.FromSlash("/ws/project"),
+	}))
+	if err != nil {
+		t.Fatalf("build narrow marker: %v", err)
+	}
+
+	if wide.PrincipalPlanHash == "" {
+		t.Fatal("no principal fingerprint recorded while opted in, so principal grants are unfingerprinted")
+	}
+	if wide.PrincipalPlanHash == narrow.PrincipalPlanHash {
+		t.Error("dropping a principal read root did not change the fingerprint, so stale AllowRead ACEs survive a narrowed policy")
+	}
+}
+
+// A stale principal fingerprint must be refused through the real validator,
+// which reads the marker off disk, with a message that says what to do.
+func TestValidateRefusesAChangedPrincipalFingerprint(t *testing.T) {
+	home := t.TempDir()
+	config := principalFingerprintConfig(home, []string{filepath.FromSlash("/ws/project")})
+	if _, err := WriteWindowsSandboxSetupMarker(config); err != nil {
+		t.Fatalf("write marker: %v", err)
+	}
+	if err := ValidateWindowsSandboxSetupMarker(config); err != nil {
+		t.Fatalf("a freshly written marker did not validate, so this test cannot isolate the fingerprint: %v", err)
+	}
+
+	// Rewrite only the principal fingerprint, the way a policy change would.
+	path := WindowsSandboxSetupMarkerPath(home)
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatalf("read marker: %v", err)
+	}
+	var marker map[string]any
+	if err := json.Unmarshal(raw, &marker); err != nil {
+		t.Fatalf("parse marker: %v", err)
+	}
+	marker["principalPlanHash"] = "stale-hash-from-an-earlier-policy"
+	rewritten, err := json.Marshal(marker)
+	if err != nil {
+		t.Fatalf("marshal marker: %v", err)
+	}
+	if err := os.WriteFile(path, rewritten, 0o600); err != nil {
+		t.Fatalf("rewrite marker: %v", err)
+	}
+
+	err = ValidateWindowsSandboxSetupMarker(config)
+	if err == nil {
+		t.Fatal("a marker whose principal grants no longer match the policy was accepted")
+	}
+	if !strings.Contains(err.Error(), "principal grants changed") {
+		t.Errorf("refused for the wrong reason: %v", err)
+	}
+}
+
+// The default install must be untouched: opted out means no fingerprint, so the
+// marker does not churn for the overwhelming majority of users.
+func TestSetupMarkerHasNoPrincipalFingerprintWhenOptedOut(t *testing.T) {
+	config := principalFingerprintConfig(t.TempDir(), []string{filepath.FromSlash("/ws/project")})
+	config.PrincipalOptIn = false
+
+	marker, err := BuildWindowsSandboxSetupMarker(config)
+	if err != nil {
+		t.Fatalf("build marker: %v", err)
+	}
+	if marker.PrincipalPlanHash != "" {
+		t.Errorf("principal fingerprint %q recorded while opted out", marker.PrincipalPlanHash)
+	}
+}

internal/sandbox/windows_setup_runtime_root_test.go

@@ -0,0 +1,240 @@
+package sandbox
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+// runtimeRootTestConfig is the shape every command reaches the Windows runner
+// with: a restricted filesystem rooted at the workspace, which is what makes the
+// runtime root necessary in the first place.
+func runtimeRootTestConfig(t *testing.T) WindowsSandboxCommandConfig {
+	t.Helper()
+	// REDIRECT THE AMBIENT CACHE PRODUCER, not only the obvious inputs.
+	//
+	// The workspace and sandbox home below are test-owned, but
+	// windowsSandboxRuntimeCandidates still derives a candidate from
+	// sandboxUserCacheDir, and the runtime-root test creates every candidate and
+	// registers os.RemoveAll cleanup for them. That reaches into the real user
+	// cache: it fails outright on a read-only home, and on an ordinary developer
+	// or CI account it creates and then deletes a path outside the test's
+	// storage. The workspace hash makes a collision unlikely; it does not make
+	// somebody else's directory test-owned.
+	cache := t.TempDir()
+	previousCache := sandboxUserCacheDir
+	sandboxUserCacheDir = func() (string, error) { return cache, nil }
+	t.Cleanup(func() { sandboxUserCacheDir = previousCache })
+
+	workspace := t.TempDir()
+	return WindowsSandboxCommandConfig{
+		SandboxHome:    t.TempDir(),
+		CommandCWD:     workspace,
+		WorkspaceRoots: []string{workspace},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{
+				Kind:       FileSystemRestricted,
+				WriteRoots: []WritableRoot{{Root: workspace}},
+			},
+			Network: NetworkPolicy{Mode: NetworkDeny},
+		},
+	}
+}
+
+// A marker written by a fresh setup must accept the ordinary command, and the
+// ordinary command is the RUNTIME-AUGMENTED one: Engine.run calls
+// permissionProfileWithRuntime before the Windows runner ever sees the profile,
+// so the profile presented at validation always carries the selected runtime
+// root as an extra write root.
+//
+// Setup used to fingerprint the bare profile. The extra write root changed the
+// ACL plan, the plan hash changed with it, and validation rejected a marker
+// written seconds earlier with "permission roots or deny lists changed" — so on
+// a restricted filesystem no command could run at all, including the very
+// command that had just been set up for.
+//
+// Asserted for BOTH candidates because which one a process selects is not fixed:
+// sandboxRuntimeRootFor prefers the cache-derived root and falls back to the
+// temp-derived one, and a marker that only accepts the preferred root bricks
+// every machine that falls back.
+func TestWindowsSandboxSetupMarkerAcceptsRuntimeAugmentedCommand(t *testing.T) {
+	config := runtimeRootTestConfig(t)
+	// The setup half, as BuildWindowsSandboxSetupArgs prepares it in the
+	// operator's shell before the elevated helper ever runs.
+	setup := WindowsSandboxSetupConfigFromCommand(config)
+	setup.PermissionProfile = WindowsSandboxProfileWithRuntimeRoots(setup.PermissionProfile, config.WorkspaceRoots)
+	if _, err := WriteWindowsSandboxSetupMarker(setup); err != nil {
+		t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
+	}
+
+	candidates := windowsSandboxRuntimeCandidates(config.WorkspaceRoots)
+	if len(candidates) == 0 {
+		t.Fatal("windowsSandboxRuntimeCandidates returned none, so this test proves nothing")
+	}
+	for _, candidate := range candidates {
+		augmented := config
+		// The command half, in the same order the real path builds it: the engine
+		// appends the SELECTED root, then the Windows plan folds in the candidate
+		// set before serializing the profile to the runner.
+		augmented.PermissionProfile = WindowsSandboxProfileWithRuntimeRoots(
+			permissionProfileWithRuntime(config.PermissionProfile, SandboxRuntime{Root: candidate}),
+			config.WorkspaceRoots,
+		)
+		err := ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(augmented))
+		if err != nil {
+			t.Fatalf("ValidateWindowsSandboxSetupMarker with runtime root %s: %v", candidate, err)
+		}
+	}
+
+	// THE RUNNER'S TEMP IS NOT THE OPERATOR'S.
+	//
+	// sandboxRuntimeEnvironment points TMPDIR/TMP/TEMP at the sandbox runtime
+	// temp for everything the sandbox launches, and the command runner inherits
+	// that env. While the runner derived the candidate set itself, os.TempDir()
+	// there returned the redirected value, so it produced a temp-derived root
+	// under the runtime tree while setup produced one under the real temp: two
+	// plans with the SAME entry count and different hashes, and every sandboxed
+	// command refused to run with "permission roots or deny lists changed".
+	//
+	// Validation must not move when that variable does.
+	// Augmented FIRST, standing in for the parent, whose TEMP is still real.
+	runner := config
+	runner.PermissionProfile = WindowsSandboxProfileWithRuntimeRoots(
+		permissionProfileWithRuntime(config.PermissionProfile, SandboxRuntime{Root: candidates[0]}),
+		config.WorkspaceRoots,
+	)
+	// Only THEN does the environment become the runner's. Anything downstream of
+	// this line that re-derives a runtime root gets the redirected answer, which
+	// is precisely the defect: validation has to be settled before here.
+	t.Setenv("TEMP", t.TempDir())
+	t.Setenv("TMP", os.Getenv("TEMP"))
+	if err := ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(runner)); err != nil {
+		t.Fatalf("ValidateWindowsSandboxSetupMarker with a redirected TEMP: %v", err)
+	}
+
+	// The guard has to still bite, or the test above passes for the wrong reason
+	// — a validator that accepts everything would satisfy it too.
+	changed := config
+	changed.PermissionProfile.FileSystem.DenyRead = []string{`C:\workspace\secret`}
+	if err := ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(changed)); err == nil {
+		t.Fatal("ValidateWindowsSandboxSetupMarker accepted a changed deny list, so it no longer detects drift")
+	} else if !strings.Contains(err.Error(), "out of date") {
+		t.Fatalf("ValidateWindowsSandboxSetupMarker changed error = %v, want out of date", err)
+	}
+}
+
+// The runtime root needs BOTH sides of the write-restricted grant.
+//
+// A principal command runs on a token restricted to the capability SIDs, and a
+// WRITE_RESTRICTED token grants a write only when the normal token check AND the
+// restricting-SID check both pass. The runtime root used to be appended to the
+// principal plan alone, so it carried the account ACE and no capability ACE: the
+// normal check passed, the restricted check found nothing, and every cache and
+// temp write was denied even once the marker agreed.
+//
+// This asserts the capability side, which is the side that was missing. Both
+// candidates again, for the same reason as above.
+func TestWindowsSandboxRuntimeRootsAreInTheCapabilityPlan(t *testing.T) {
+	config := runtimeRootTestConfig(t)
+	candidates := windowsSandboxRuntimeCandidates(config.WorkspaceRoots)
+	if len(candidates) == 0 {
+		t.Fatal("windowsSandboxRuntimeCandidates returned none, so this test proves nothing")
+	}
+
+	setup := WindowsSandboxSetupConfigFromCommand(config)
+	setup.PermissionProfile = WindowsSandboxProfileWithRuntimeRoots(setup.PermissionProfile, config.WorkspaceRoots)
+	plan, err := BuildWindowsACLPlan(setup.commandConfig())
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	granted := make(map[string]struct{}, len(plan.Entries))
+	for _, entry := range plan.Entries {
+		granted[windowsCapabilityPathKey(entry.Path)] = struct{}{}
+	}
+	for _, candidate := range candidates {
+		if _, ok := granted[windowsCapabilityPathKey(candidate)]; !ok {
+			t.Fatalf("capability ACL plan has no entry for runtime root %s; writes there fail the restricting-SID check", candidate)
+		}
+	}
+}
+
+// EVERY write root the capability plan grants must exist by the time setup
+// applies it.
+//
+// The capability plan deliberately refuses to materialize a write root, so a
+// granted path that is merely absent fails the entire setup run with
+//
+//	windows ACL target does not exist: ...\zero\runtime\v1\<hash>
+//
+// which is exactly what an elevated run hit: the runtime candidates were added
+// to the plan while only the selected one was ever created, and `zero sandbox
+// setup` stopped working altogether. The two halves are separate functions, so
+// nothing but this test stops one from growing a candidate without the other.
+func TestWindowsSandboxSetupProvisionsEveryGrantedWriteRoot(t *testing.T) {
+	config := runtimeRootTestConfig(t)
+	candidates := windowsSandboxRuntimeCandidates(config.WorkspaceRoots)
+	if len(candidates) == 0 {
+		t.Fatal("windowsSandboxRuntimeCandidates returned none, so this test proves nothing")
+	}
+	// The workspace is unique per run, and the roots are derived from it, so these
+	// cannot pre-exist. Assert that rather than trust it: if they did, the test
+	// would pass with the provisioning step deleted.
+	for _, candidate := range candidates {
+		if _, err := os.Stat(candidate); err == nil {
+			t.Fatalf("runtime root %s already exists before provisioning, so this test proves nothing", candidate)
+		}
+		t.Cleanup(func() { _ = os.RemoveAll(candidate) })
+	}
+
+	if err := ensureWindowsSandboxRuntimeCandidates(config.WorkspaceRoots); err != nil {
+		t.Fatalf("ensureWindowsSandboxRuntimeCandidates: %v", err)
+	}
+
+	setup := WindowsSandboxSetupConfigFromCommand(config)
+	setup.PermissionProfile = WindowsSandboxProfileWithRuntimeRoots(setup.PermissionProfile, config.WorkspaceRoots)
+	plan, err := BuildWindowsACLPlan(setup.commandConfig())
+	if err != nil {
+		t.Fatalf("BuildWindowsACLPlan: %v", err)
+	}
+	for _, entry := range plan.Entries {
+		if entry.Action != WindowsACLAllowWrite || entry.Materialize {
+			continue
+		}
+		if _, err := os.Stat(entry.Path); err != nil {
+			t.Errorf("capability plan grants write on %s but nothing created it, so setup fails with "+
+				"\"windows ACL target does not exist\": %v", entry.Path, err)
+		}
+	}
+}
+
+// Both candidates are pure functions of the workspace root. Setup provisions the
+// set and a later command selects from it in a different process, so a candidate
+// that varied per process (a random or time-seeded fallback) would be granted by
+// setup and never selected, or selected and never granted.
+func TestWindowsSandboxRuntimeCandidatesAreDeterministic(t *testing.T) {
+	config := runtimeRootTestConfig(t)
+	first := windowsSandboxRuntimeCandidates(config.WorkspaceRoots)
+	if len(first) == 0 {
+		t.Fatal("windowsSandboxRuntimeCandidates returned none, so this test proves nothing")
+	}
+	second := windowsSandboxRuntimeCandidates(config.WorkspaceRoots)
+	if len(first) != len(second) {
+		t.Fatalf("candidate count = %d then %d, want stable", len(first), len(second))
+	}
+	for i := range first {
+		if first[i] != second[i] {
+			t.Fatalf("candidate %d = %q then %q, want stable", i, first[i], second[i])
+		}
+	}
+
+	other := runtimeRootTestConfig(t)
+	otherCandidates := windowsSandboxRuntimeCandidates(other.WorkspaceRoots)
+	for _, candidate := range otherCandidates {
+		for _, mine := range first {
+			if candidate == mine {
+				t.Fatalf("workspaces %s and %s share runtime root %s, so one workspace's grant covers the other",
+					config.CommandCWD, other.CommandCWD, candidate)
+			}
+		}
+	}
+}

internal/sandbox/windows_setup_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -156,6 +157,178 @@ func TestWindowsSandboxSetupMarkerRejectsOldSchema(t *testing.T) {
 	}
 }
 
+// The principal opt-in has to travel in the setup args, because the elevated
+// half runs in its own process: a UAC-elevated helper does not inherit the
+// environment of the shell that asked for setup. Sampling the ambient
+// environment there let the two halves disagree, so the serialized value must
+// win over the environment in BOTH directions.
+func TestWindowsSandboxSetupPrincipalOptInSurvivesElevatedEnvironment(t *testing.T) {
+	allowPrincipalLaunchForTest(t)
+	profile := PermissionProfile{
+		FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: `C:\workspace`}}},
+		Network:    NetworkPolicy{Mode: NetworkAllow},
+	}
+	testCases := []struct {
+		name       string
+		optIn      bool
+		ambientEnv string
+	}{
+		// The reported case: the caller's shell opted in, the elevated helper's
+		// environment has nothing. Without the serialized value setup provisions no
+		// principal and every later command silently falls back.
+		{name: "opted in, elevated environment empty", optIn: true, ambientEnv: ""},
+		// The mirror: the elevated helper happens to have a machine-wide opt-in the
+		// caller did not ask for. Setup must not create an account on its own say-so.
+		{name: "opted out, elevated environment opted in", optIn: false, ambientEnv: "1"},
+	}
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			t.Setenv(windowsSandboxIdentityEnv, testCase.ambientEnv)
+			optIn := testCase.optIn
+			args, err := BuildWindowsSandboxSetupArgs(WindowsSandboxSetupArgsOptions{
+				SandboxHome:    t.TempDir(),
+				CommandCWD:     `C:\workspace`,
+				WorkspaceRoots: []string{`C:\workspace`},
+				// The parent folds in the runtime roots before the runner sees the profile,
+				// so a command half that skips it no longer matches the setup half.
+				PermissionProfile: WindowsSandboxProfileWithRuntimeRoots(profile, []string{`C:\workspace`}),
+				PrincipalOptIn:    &optIn,
+			})
+			if err != nil {
+				t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)
+			}
+			config, err := ParseWindowsSandboxSetupArgs(args)
+			if err != nil {
+				t.Fatalf("ParseWindowsSandboxSetupArgs: %v", err)
+			}
+			if config.PrincipalOptIn != testCase.optIn {
+				t.Fatalf("parsed PrincipalOptIn = %v, want %v", config.PrincipalOptIn, testCase.optIn)
+			}
+			// This is the value the elevated setup gate actually reads before it
+			// decides to provision an account.
+			if got := windowsSandboxIdentityEnabled(config.commandConfig().Env); got != testCase.optIn {
+				t.Fatalf("elevated setup opt-in = %v, want %v (ambient %s=%q must not decide)",
+					got, testCase.optIn, windowsSandboxIdentityEnv, testCase.ambientEnv)
+			}
+		})
+	}
+}
+
+// A setup helper that cannot read the opt-in must refuse rather than default to
+// "no principal": provisioning less than the caller asked for and reporting
+// success is the silent downgrade this protocol exists to prevent.
+func TestParseWindowsSandboxSetupArgsRejectsUnreadablePrincipalOptIn(t *testing.T) {
+	allowPrincipalLaunchForTest(t)
+	optIn := true
+	args, err := BuildWindowsSandboxSetupArgs(WindowsSandboxSetupArgsOptions{
+		SandboxHome:       t.TempDir(),
+		CommandCWD:        `C:\workspace`,
+		WorkspaceRoots:    []string{`C:\workspace`},
+		PermissionProfile: PermissionProfile{FileSystem: FileSystemPolicy{Kind: FileSystemRestricted}, Network: NetworkPolicy{Mode: NetworkDeny}},
+		PrincipalOptIn:    &optIn,
+	})
+	if err != nil {
+		t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)
+	}
+	for index, arg := range args {
+		if arg == "--sandbox-principal" {
+			args[index+1] = "yes"
+		}
+	}
+	if _, err := ParseWindowsSandboxSetupArgs(args); err == nil || !strings.Contains(err.Error(), "--sandbox-principal") {
+		t.Fatalf("ParseWindowsSandboxSetupArgs error = %v, want rejection of the unreadable opt-in", err)
+	}
+}
+
+// Setup and the commands that follow it run in separate processes, so they can
+// disagree about the opt-in. The marker records what setup provisioned and the
+// command refuses on a mismatch — most of all when the command opted in and
+// setup did not, because the runtime's principal lookup declines with a nil
+// error and the command would otherwise run on the read-unconfined
+// restricted-token backend while the operator believes a principal is isolating
+// it.
+func TestWindowsSandboxSetupMarkerRejectsPrincipalOptInMismatch(t *testing.T) {
+	// Neutral ambient environment: the disagreement under test is between the two
+	// recorded intents, not between either of them and this process.
+	t.Setenv(windowsSandboxIdentityEnv, "")
+	profile := PermissionProfile{
+		FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: `C:\workspace`}}},
+		Network:    NetworkPolicy{Mode: NetworkAllow},
+	}
+	command := func(home string, env map[string]string) WindowsSandboxCommandConfig {
+		return WindowsSandboxCommandConfig{
+			SandboxHome:    home,
+			CommandCWD:     `C:\workspace`,
+			WorkspaceRoots: []string{`C:\workspace`},
+			// The parent folds in the runtime roots before the runner sees the profile,
+			// so a command half that skips it no longer matches the setup half.
+			PermissionProfile: WindowsSandboxProfileWithRuntimeRoots(profile, []string{`C:\workspace`}),
+			Env:               env,
+			SandboxLevel:      WindowsSandboxLevelRestrictedToken,
+			Command:           []string{"cmd.exe", "/c", "echo"},
+		}
+	}
+	testCases := []struct {
+		name       string
+		setupOptIn bool
+		commandEnv map[string]string
+		wantError  string
+	}{
+		{
+			name:       "command opts in, setup provisioned no principal",
+			setupOptIn: false,
+			commandEnv: map[string]string{windowsSandboxIdentityEnv: "1"},
+			wantError:  "asks for a sandbox principal, but setup provisioned none",
+		},
+		{
+			name:       "setup provisioned a principal, command opts out",
+			setupOptIn: true,
+			commandEnv: map[string]string{windowsSandboxIdentityEnv: "0"},
+			wantError:  "setup provisioned sandbox principals",
+		},
+	}
+	for _, testCase := range testCases {
+		t.Run(testCase.name, func(t *testing.T) {
+			home := t.TempDir()
+			setupConfig := WindowsSandboxSetupConfig{
+				SandboxHome:    home,
+				CommandCWD:     `C:\workspace`,
+				WorkspaceRoots: []string{`C:\workspace`},
+				// The parent folds in the runtime roots before the runner sees the profile,
+				// so a command half that skips it no longer matches the setup half.
+				PermissionProfile: WindowsSandboxProfileWithRuntimeRoots(profile, []string{`C:\workspace`}),
+				PrincipalOptIn:    testCase.setupOptIn,
+			}
+			marker, err := WriteWindowsSandboxSetupMarker(setupConfig)
+			if err != nil {
+				t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
+			}
+			// Assert the setup half recorded what it was told before trusting what
+			// the command half makes of it.
+			if marker.PrincipalOptIn != testCase.setupOptIn {
+				t.Fatalf("marker PrincipalOptIn = %v, want %v", marker.PrincipalOptIn, testCase.setupOptIn)
+			}
+			// An agreeing command still validates, so the refusal below is about the
+			// disagreement and not about the marker being unusable.
+			agreeing := command(home, map[string]string{windowsSandboxIdentityEnv: windowsSandboxPrincipalOptInValue(testCase.setupOptIn)})
+			if err := ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(agreeing)); err != nil {
+				t.Fatalf("agreeing command must validate against its own setup: %v", err)
+			}
+
+			err = ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(command(home, testCase.commandEnv)))
+			if err == nil {
+				t.Fatalf("disagreeing command validated the marker, want refusal")
+			}
+			if !strings.Contains(err.Error(), testCase.wantError) {
+				t.Fatalf("validate error = %v, want it to contain %q", err, testCase.wantError)
+			}
+			if !strings.Contains(err.Error(), "zero sandbox setup") {
+				t.Fatalf("validate error = %v, want the remedy to name `zero sandbox setup`", err)
+			}
+		})
+	}
+}
+
 func TestWindowsSandboxSetupConfigFromCommandPreservesProfileInputs(t *testing.T) {
 	command := WindowsSandboxCommandConfig{
 		SandboxHome:    t.TempDir(),
@@ -181,6 +354,111 @@ func TestWindowsSandboxSetupConfigFromCommandPreservesProfileInputs(t *testing.T
 	}
 }
 
+// Serializing the opt-in makes it a field every caller of
+// BuildWindowsSandboxSetupArgs could get wrong, so the field is a tri-state and
+// its unset meaning is load-bearing: "consult the environment", never "opted
+// out". The command half still resolves the opt-in from the process environment
+// when its own Env carries no explicit entry, so if an unset setup caller
+// asserted false instead, the two halves would disagree under a machine-wide
+// opt-in and marker validation would refuse every command — safe, but it bricks
+// the caller, and it re-creates the very disagreement this flag removes. That is
+// exactly what the existing smoke callers
+// (runner_windows_integration_test.go:43 and :52) do: they never set the field.
+//
+// This test runs on every GOOS and pins the unset default in both ambient
+// states, so getting it backwards is a test failure here rather than a surprise
+// on a real elevated machine.
+func TestWindowsSandboxSetupArgsUnsetPrincipalOptInConsultsEnvironment(t *testing.T) {
+	allowPrincipalLaunchForTest(t)
+	profile := PermissionProfile{
+		FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: `C:\workspace`}}},
+		Network:    NetworkPolicy{Mode: NetworkAllow},
+	}
+	// The command half as an ambient caller declares it: no explicit entry in Env,
+	// so it resolves the opt-in from the environment.
+	command := func(home string) WindowsSandboxCommandConfig {
+		return WindowsSandboxCommandConfig{
+			SandboxHome:    home,
+			CommandCWD:     `C:\workspace`,
+			WorkspaceRoots: []string{`C:\workspace`},
+			// The parent folds in the runtime roots before the runner sees the profile,
+			// so a command half that skips it no longer matches the setup half.
+			PermissionProfile: WindowsSandboxProfileWithRuntimeRoots(profile, []string{`C:\workspace`}),
+			SandboxLevel:      WindowsSandboxLevelRestrictedToken,
+			Command:           []string{"cmd.exe", "/c", "echo"},
+		}
+	}
+	// setupMarkerFor runs the full caller path — build args, cross the (simulated)
+	// UAC boundary by re-parsing them, write the marker — so what is asserted is
+	// what an elevated helper would actually have provisioned.
+	setupMarkerFor := func(t *testing.T, home string, optIn *bool) WindowsSandboxSetupMarker {
+		t.Helper()
+		args, err := BuildWindowsSandboxSetupArgs(WindowsSandboxSetupArgsOptions{
+			SandboxHome:    home,
+			CommandCWD:     `C:\workspace`,
+			WorkspaceRoots: []string{`C:\workspace`},
+			// Deliberately NOT pre-augmented. BuildWindowsSandboxSetupArgs folds the
+			// runtime roots in itself, and passing them in here would hide it from
+			// this test if it ever stopped.
+			PermissionProfile: profile,
+			PrincipalOptIn:    optIn,
+		})
+		if err != nil {
+			t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)
+		}
+		config, err := ParseWindowsSandboxSetupArgs(args)
+		if err != nil {
+			t.Fatalf("ParseWindowsSandboxSetupArgs: %v", err)
+		}
+		marker, err := WriteWindowsSandboxSetupMarker(config)
+		if err != nil {
+			t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
+		}
+		return marker
+	}
+
+	for _, ambient := range []string{"1", ""} {
+		name := "machine-wide opt-in"
+		if ambient == "" {
+			name = "no opt-in"
+		}
+		t.Run(name, func(t *testing.T) {
+			t.Setenv(windowsSandboxIdentityEnv, ambient)
+			want := ambient == "1"
+
+			// An unset caller must provision what the environment says. Assert the
+			// setup half recorded that before trusting the agreement below: a marker
+			// that recorded the wrong thing could still "agree" if the command half
+			// were broken in the same direction.
+			home := t.TempDir()
+			marker := setupMarkerFor(t, home, nil)
+			if marker.PrincipalOptIn != want {
+				t.Fatalf("unset caller recorded PrincipalOptIn = %v, want %v (ambient %s=%q decides)",
+					marker.PrincipalOptIn, want, windowsSandboxIdentityEnv, ambient)
+			}
+			if err := ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(command(home))); err != nil {
+				t.Fatalf("an unset caller must agree with the ambient command half: %v", err)
+			}
+
+			// And an explicit value still overrides the environment in both
+			// directions, or the tri-state would have no third state.
+			override := !want
+			overrideHome := t.TempDir()
+			overrideMarker := setupMarkerFor(t, overrideHome, &override)
+			if overrideMarker.PrincipalOptIn != override {
+				t.Fatalf("explicit caller recorded PrincipalOptIn = %v, want %v", overrideMarker.PrincipalOptIn, override)
+			}
+			err := ValidateWindowsSandboxSetupMarker(WindowsSandboxSetupConfigFromCommand(command(overrideHome)))
+			if err == nil {
+				t.Fatalf("an explicit opt-in of %v validated against an ambient command half of %v, want refusal", override, want)
+			}
+			if !strings.Contains(err.Error(), windowsSandboxIdentityEnv) {
+				t.Fatalf("validate error = %v, want it to name %s", err, windowsSandboxIdentityEnv)
+			}
+		})
+	}
+}
+
 func TestWindowsACLPlanHashIsStableAcrossEntryOrder(t *testing.T) {
 	left, err := WindowsACLPlanHash(WindowsACLPlan{Entries: []WindowsACLEntry{
 		{Action: WindowsACLDenyRead, Path: `C:\workspace\secret`, Capability: "S-1-5-21-3", Materialize: true},
@@ -202,3 +480,79 @@ func TestWindowsACLPlanHashIsStableAcrossEntryOrder(t *testing.T) {
 		t.Fatalf("ACL plan hashes differ: %q vs %q", left, right)
 	}
 }
+
+// The caller's identity has to survive the UAC boundary for the same reason the
+// opt-in does: elevated setup is a separate process, and under over-the-shoulder
+// elevation it belongs to a different administrator. Every principal name, its
+// ACL ledger and its DPAPI secret are scoped to the invoking user, so a helper
+// left to sample its own token provisions for the wrong one.
+//
+// Runs on every GOOS. The identity is resolved before the boundary, so what is
+// pinned here is the transport, which is where it would silently go missing.
+func TestWindowsSandboxSetupArgsCarryTheCallerIdentity(t *testing.T) {
+	profile := PermissionProfile{
+		FileSystem: FileSystemPolicy{Kind: FileSystemRestricted, WriteRoots: []WritableRoot{{Root: `C:\workspace`}}},
+		Network:    NetworkPolicy{Mode: NetworkDeny},
+	}
+	const callerSID = "S-1-5-21-1111111111-2222222222-3333333333-1001"
+	args, err := BuildWindowsSandboxSetupArgs(WindowsSandboxSetupArgsOptions{
+		SandboxHome:    `C:\home`,
+		CommandCWD:     `C:\workspace`,
+		WorkspaceRoots: []string{`C:\workspace`},
+		// The parent folds in the runtime roots before the runner sees the profile,
+		// so a command half that skips it no longer matches the setup half.
+		PermissionProfile: WindowsSandboxProfileWithRuntimeRoots(profile, []string{`C:\workspace`}),
+		CallerSID:         callerSID,
+	})
+	if err != nil {
+		t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)
+	}
+	config, err := ParseWindowsSandboxSetupArgs(args)
+	if err != nil {
+		t.Fatalf("ParseWindowsSandboxSetupArgs: %v", err)
+	}
+	if config.CallerSID != callerSID {
+		t.Fatalf("the caller identity did not survive the setup args: got %q, want %q", config.CallerSID, callerSID)
+	}
+	// And on to the command-shaped view the setup half actually plans against —
+	// the only place it changes what gets provisioned.
+	if got := config.commandConfig().CallerSID; got != callerSID {
+		t.Errorf("commandConfig dropped the caller identity: got %q, want %q", got, callerSID)
+	}
+}
+
+// A caller that cannot state its identity must not send an empty flag. Absence
+// means "this caller did not say", which the helper answers by sampling its own
+// token exactly as it did before the flag existed; an empty value would have to
+// be guessed at instead.
+func TestWindowsSandboxSetupArgsOmitAnUnknownCallerIdentity(t *testing.T) {
+	args, err := BuildWindowsSandboxSetupArgs(WindowsSandboxSetupArgsOptions{
+		SandboxHome:    `C:\home`,
+		CommandCWD:     `C:\workspace`,
+		WorkspaceRoots: []string{`C:\workspace`},
+		PermissionProfile: PermissionProfile{
+			FileSystem: FileSystemPolicy{Kind: FileSystemRestricted},
+			Network:    NetworkPolicy{Mode: NetworkDeny},
+		},
+		// Left unset. On non-Windows builds windowsCurrentUserSID also returns
+		// empty, which is the same case a Windows caller hits when its token query
+		// fails.
+	})
+	if err != nil {
+		t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)
+	}
+	if runtime.GOOS != "windows" {
+		for _, arg := range args {
+			if arg == "--caller-sid" {
+				t.Fatalf("an unresolvable caller identity was still serialized: %v", args)
+			}
+		}
+	}
+	config, err := ParseWindowsSandboxSetupArgs(args)
+	if err != nil {
+		t.Fatalf("ParseWindowsSandboxSetupArgs: %v", err)
+	}
+	if runtime.GOOS != "windows" && config.CallerSID != "" {
+		t.Errorf("CallerSID = %q, want empty so the helper falls back to its own token", config.CallerSID)
+	}
+}

internal/sandbox/windows_setup_windows.go

@@ -3,39 +3,188 @@
 package sandbox
 
 import (
+	"errors"
 	"fmt"
 	"io"
+	"strings"
 
 	"golang.org/x/sys/windows"
 )
 
+// Seams for the setup transaction's external effects.
+//
+// Every step below needs something a test machine cannot be asked for: an
+// elevated token, a machine-global mutex, real DACLs, the WFP engine, a local
+// account. That left the ORDER of those steps and the handling of each failure
+// — which is where this function's bugs actually live — with no coverage at all.
+// Each var defaults to its production function and is only ever reassigned by a
+// test. removeWindowsSandboxPrincipalForSetupFn and applyWindowsACLPlanFn
+// already existed in windows_identity_windows.go for the same reason; the rest
+// follow them, and the ACL call below now goes through that one so a single stub
+// covers both plans.
+var (
+	windowsProcessIsElevatedFn       = windowsProcessIsElevated
+	acquireWindowsSandboxSetupLockFn = acquireWindowsSandboxSetupLock
+	applyWindowsNetworkPlanFn        = applyWindowsNetworkPlan
+	setupWindowsSandboxPrincipalFn   = setupWindowsSandboxPrincipal
+	writeWindowsSandboxSetupMarkerFn = WriteWindowsSandboxSetupMarker
+)
+
 func runWindowsSandboxSetup(config WindowsSandboxSetupConfig, stderr io.Writer) int {
 	// Applying the WFP network filters and workspace ACLs requires Administrator
 	// rights; without them WFP fails deep inside with a raw ACCESS_DENIED (0x5).
 	// Check up front and return an actionable message instead.
-	if !windowsProcessIsElevated() {
+	if !windowsProcessIsElevatedFn() {
 		fmt.Fprintln(stderr, WindowsSandboxSetupName+": Administrator rights are required. Re-run `zero sandbox setup` from an elevated (Run as administrator) terminal.")
 		return 1
 	}
+	// Serialize the WHOLE transaction, not the individual writes.
+	//
+	// Setup rotates the principal's password, stores the secret,
+	// read-modify-writes the ACL ledger, installs WFP filters and writes the
+	// marker. Two setups for this workspace at once interleave those steps: the
+	// account ends up with one process's password while the stored secret is the
+	// other's, so every later command fails to log on, and the ledger's
+	// read-modify-write silently loses a path set. Each write being individually
+	// atomic does nothing about interleaving BETWEEN writes, which is the actual
+	// failure, so the lock is taken here and held to the end.
+	//
+	// Immediately after the elevation check, because acquiring a Global object
+	// needs the rights that check just confirmed.
+	lock, err := acquireWindowsSandboxSetupLockFn(windowsSandboxWorkspaceKey(config.WorkspaceRoots))
+	if err != nil {
+		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+		return 1
+	}
+	defer lock.release()
+	if lock.Abandoned() {
+		// The previous holder died somewhere inside the transaction, so this
+		// machine may be half set up. Setup is idempotent and re-running it is
+		// the repair, but say so: a silent recovery hides that something crashed
+		// while holding the machine's sandbox state open.
+		fmt.Fprintf(stderr, "%s: a previous setup for this workspace exited without finishing; re-running to repair it.\n",
+			WindowsSandboxSetupName)
+	}
+
+	// Before anything is provisioned: a principal this helper could not hand back
+	// to the caller must not be created at all.
+	if config.PrincipalOptIn {
+		if err := assertWindowsSetupRunsAsCaller(config.CallerSID); err != nil {
+			fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+			return 1
+		}
+	}
+
+	// Create the runtime roots BEFORE building the plan that grants them.
+	//
+	// The capability plan deliberately does not materialize a write root: an
+	// entry whose path is absent is a typo or a stale config, and inventing the
+	// tree would silently grant write on a directory nobody asked for. So every
+	// write root in the plan has to exist by the time it is applied, and the
+	// runtime roots are the only ones setup owns rather than receives.
+	//
+	// Both of them, not the one this process would select. Setup grants the whole
+	// candidate set so a later command that falls back still lands on a
+	// provisioned tree, and granting a path setup never created fails the entire
+	// run with "windows ACL target does not exist".
+	if err := ensureWindowsSandboxRuntimeCandidates(config.WorkspaceRoots); err != nil {
+		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+		return 1
+	}
+
 	plan, err := BuildWindowsACLPlan(config.commandConfig())
 	if err != nil {
 		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
 		return 1
 	}
-	// Always provision the mode-INDEPENDENT infrastructure: the outbound block
-	// filters scoped to the offline-marker SID. Runtime gates network per command
-	// by whether the token carries that SID, so one setup serves both modes.
+	rollback, err := applyWindowsACLPlanFn(plan)
+	if err != nil {
+		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+		return 1
+	}
+	// Provision this workspace's sandbox principal, when opted in. A principal is
+	// a separate local account, so it is created only on an explicit opt-in: it
+	// is visible in `net user`, and account creation is exactly the kind of thing
+	// endpoint protection and enterprise policy object to. Without the opt-in the
+	// capability-SID backend above is the whole of setup, unchanged.
+	if windowsSandboxIdentityEnabled(config.commandConfig().Env) {
+		principalRollback, err := setupWindowsSandboxPrincipalFn(config.commandConfig())
+		if err != nil {
+			if rollbackErr := rollback(); rollbackErr != nil {
+				fmt.Fprintf(stderr, "%s: %v; rollback failed: %v\n", WindowsSandboxSetupName, err, rollbackErr)
+				return 1
+			}
+			fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+			return 1
+		}
+		// Fold the principal into the existing rollback so every later failure
+		// path undoes it too, rather than each one having to remember.
+		aclRollback := rollback
+		rollback = func() error {
+			principalErr := principalRollback()
+			aclErr := aclRollback()
+			if principalErr != nil {
+				return principalErr
+			}
+			return aclErr
+		}
+	} else if err := removeWindowsSandboxPrincipalsForSetupFn(config.commandConfig()); err != nil {
+		// Opting out has to actually retire the principal, because that is what
+		// we tell people it does. ValidateWindowsSandboxSetupMarker sends an
+		// operator here in as many words: re-run setup from an elevated terminal
+		// without the opt-in to retire the principal. Until now this branch did
+		// not exist, so the marker flipped to opted-out while the account, its
+		// secret, its logon rights, its ACEs and its ledger all stayed exactly
+		// where they were. The instruction was a lie, and the leftovers were
+		// invisible, because nothing afterwards looks for a principal it believes
+		// was never provisioned.
+		//
+		// Fatal only when the ACCOUNT survived, which is a narrower rule than
+		// either of the two this branch has had.
+		//
+		// It first printed and carried on, which was wrong: a marker that says "no
+		// principal" while the account is still installed is a lie nothing
+		// afterwards can detect, because nothing looks for a principal it believes
+		// was never provisioned.
+		//
+		// Then it failed on any error at all, which was wrong in the other
+		// direction. removeWindowsSandboxPrincipalForSetup deliberately continues
+		// past a secret file owned by ANOTHER administrator's setup and reports it
+		// at the end, so that one unlinkable file does not strand the account, its
+		// logon rights and its ACEs. Treating that report as fatal made the whole
+		// default sandbox unsetupable on such a machine, permanently, over inert
+		// residue: the account is already gone and the leftover is a file with a
+		// DACL we do not own.
+		//
+		// So the question is not whether teardown reported a problem, it is
+		// whether the thing the marker is about to deny is still there. Asked of
+		// the account rather than inferred from the error, because the error is
+		// a join and its parts are not separable at this distance.
+		if windowsSandboxPrincipalIsInstalled(config.commandConfig()) {
+			if rollbackErr := rollback(); rollbackErr != nil {
+				fmt.Fprintf(stderr, "%s: opted out, but the existing sandbox principal is still installed: %v; rollback failed: %v\n",
+					WindowsSandboxSetupName, err, rollbackErr)
+				return 1
+			}
+			fmt.Fprintf(stderr, "%s: opted out, but the existing sandbox principal is still installed: %v\n",
+				WindowsSandboxSetupName, err)
+			return 1
+		}
+		// Retired, with something inert left over. Named rather than swallowed:
+		// whoever has to clean it up needs to know it is there.
+		fmt.Fprintf(stderr, "%s: opted out; the sandbox principal was retired, but some of its residue could not be removed: %v\n",
+			WindowsSandboxSetupName, err)
+	}
+	// Built AFTER any principal provisioning, not before. The block filters are
+	// keyed to the offline group as well as the offline-marker SID, and that group
+	// is created as part of provisioning: planning first would install filters
+	// that name only the marker, leaving every offline principal with an open
+	// network while looking correctly set up.
+	//
+	// Mode-INDEPENDENT by design. Which identity a command runs under is what
+	// selects allow or deny, so one setup serves both modes.
 	networkPlan, err := BuildWindowsNetworkInfraPlan(config.commandConfig())
 	if err != nil {
-		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
-		return 1
-	}
-	rollback, err := applyWindowsACLPlan(plan)
-	if err != nil {
-		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
-		return 1
-	}
-	if err := applyWindowsNetworkPlan(networkPlan); err != nil {
 		if rollbackErr := rollback(); rollbackErr != nil {
 			fmt.Fprintf(stderr, "%s: %v; rollback failed: %v\n", WindowsSandboxSetupName, err, rollbackErr)
 			return 1
@@ -43,7 +192,47 @@ func runWindowsSandboxSetup(config WindowsSandboxSetupConfig, stderr io.Writer)
 		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
 		return 1
 	}
-	if _, err := WriteWindowsSandboxSetupMarker(config); err != nil {
+	// Refuse to install filters that would not cover the principals just
+	// provisioned. The ordering above is what makes them cover it, and nothing at
+	// this call site shows that, so assert it rather than trusting it to survive
+	// a later refactor. Installing anyway would leave a machine reporting a
+	// successful setup while every offline principal had an open network.
+	// Gated on the SAME opt-in that decides whether principals were provisioned
+	// at all (line 36). The offline group is created inside provisioning, so on
+	// an opt-out machine it legitimately does not exist, and asserting there
+	// would refuse a setup that has nothing to get wrong.
+	if err := assertWindowsNetworkPlanCoversOfflineGroup(networkPlan, resolveWindowsSandboxOfflineGroupSIDHook,
+		windowsSandboxIdentityEnabled(config.commandConfig().Env)); err != nil {
+		if rollbackErr := rollback(); rollbackErr != nil {
+			fmt.Fprintf(stderr, "%s: %v; rollback failed: %v\n", WindowsSandboxSetupName, err, rollbackErr)
+			return 1
+		}
+		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+		return 1
+	}
+	// Install the machine's plan, not this home's. The filters are global and are
+	// replaced wholesale on every setup, so an opted-out run here would otherwise
+	// strip the offline group SID that another workspace's principals depend on
+	// and hand them egress under a NetworkDeny profile. networkPlan itself is left
+	// alone because it is what this home's marker fingerprints.
+	planToInstall, planErr := WindowsNetworkPlanForApply(networkPlan, resolveWindowsSandboxOfflineGroupSIDHook)
+	if planErr != nil {
+		if rollbackErr := rollback(); rollbackErr != nil {
+			fmt.Fprintf(stderr, "%s: %v; rollback failed: %v\n", WindowsSandboxSetupName, planErr, rollbackErr)
+			return 1
+		}
+		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+planErr.Error())
+		return 1
+	}
+	if err := applyWindowsNetworkPlanFn(planToInstall); err != nil {
+		if rollbackErr := rollback(); rollbackErr != nil {
+			fmt.Fprintf(stderr, "%s: %v; rollback failed: %v\n", WindowsSandboxSetupName, err, rollbackErr)
+			return 1
+		}
+		fmt.Fprintln(stderr, WindowsSandboxSetupName+": "+err.Error())
+		return 1
+	}
+	if _, err := writeWindowsSandboxSetupMarkerFn(config); err != nil {
 		if rollbackErr := rollback(); rollbackErr != nil {
 			fmt.Fprintf(stderr, "%s: %v; rollback failed: %v\n", WindowsSandboxSetupName, err, rollbackErr)
 			return 1
@@ -54,6 +243,94 @@ func runWindowsSandboxSetup(config WindowsSandboxSetupConfig, stderr io.Writer)
 	return 0
 }
 
+// windowsSandboxPrincipalIsInstalled reports whether this workspace's principal
+// account still resolves.
+//
+// Fails CLOSED. Only an outright "not provisioned" counts as gone; a name that
+// resolves to something unexpected, or a lookup that fails for its own reasons,
+// is treated as still installed. The consumer uses this to decide whether an
+// opted-out marker would be a lie, and the expensive mistake there is claiming a
+// principal is gone when nobody actually checked.
+// A workspace now has TWO principals, and either one still installed makes the
+// answer yes. Checking a single role would let an opted-out marker claim the
+// workspace is clean while the other account is still on the machine, which is
+// the exact lie this guard exists to prevent.
+func windowsSandboxPrincipalIsInstalled(config WindowsSandboxCommandConfig) bool {
+	// EVERY RETIRABLE ROLE, not just the live pair. Teardown retires legacy too,
+	// so an upgraded machine still holding the untagged pre-split account would
+	// otherwise be reported clean here while that account was still installed,
+	// and the opted-out marker would be exactly the lie this guard prevents.
+	key := windowsSandboxPrincipalKey(config)
+	for _, role := range windowsSandboxRetirableRoles {
+		if _, err := lookupWindowsSandboxIdentityFn(key, role); !errors.Is(err, errWindowsSandboxIdentityUnavailable) {
+			return true
+		}
+	}
+	return false
+}
+
+// assertWindowsSetupRunsAsCaller refuses to provision a sandbox principal when
+// this elevated helper belongs to a different Windows user than the caller that
+// launched it.
+//
+// A principal is unusable across that boundary, and not for a reason more
+// plumbing can fix. Its password is sealed with CryptProtectData, which derives
+// its key from the CALLING user, so a blob written here is decryptable only by
+// this administrator; the caller's next command would find the account, find the
+// secret file, and fail to unseal it. Threading the caller's SID through the
+// setup args fixes the account NAME, the ledger and the ACEs — everything that
+// is merely named after the invoking user — but it cannot re-key DPAPI for a
+// user whose token this process does not hold.
+//
+// So the answer is to say so before anything exists, rather than provision an
+// account, a secret and a grant that resolve to a sandbox nobody can log into.
+//
+// Same-account elevation is unaffected: a UAC consent prompt splits the caller's
+// token but leaves the user SID identical, which is the ordinary path. An
+// unknown caller SID (an older caller, or a token query that failed) is treated
+// as a match: that is the pre-existing behaviour, and refusing on absence would
+// break setup on machines where nothing is wrong.
+//
+// What this does NOT do, stated plainly because the name suggests otherwise:
+// it cannot fire on any path Zero itself takes. Zero never elevates the helper.
+// runSandboxSetupHelper (internal/cli/app.go) is a plain exec.Command with no
+// token work, and runWindowsSandboxSetup refuses unless the process is ALREADY
+// elevated, so the operator supplies elevation by opening an elevated terminal.
+// Both halves then run in that terminal: BuildWindowsSandboxSetupArgs resolves
+// the caller SID from the very process that later becomes the helper, so the two
+// are equal by construction and this returns nil every time.
+//
+// It is kept rather than deleted because it is a correctness assertion, not a
+// no-op. It becomes load-bearing the moment anyone adds a ShellExecute "runas"
+// path, which is the natural way this command grows, and it already covers a
+// helper .exe invoked directly with hand-written args. What it must not do is be
+// mistaken for a live defence against over-the-shoulder elevation today.
+//
+// The residual hazard that boundary was supposed to describe is real but has a
+// different shape: an operator who elevates a terminal as a DIFFERENT
+// administrator runs both halves as that administrator, so setup provisions into
+// THAT account's sandbox home. Their own unelevated session resolves its own
+// home, finds no marker, and is told to run setup. Loud, not silent, and not a
+// weakened sandbox.
+//
+// Scoped to the opt-in by its caller, deliberately. The default restricted-token
+// sandbox stores no secret and names no account after the user, so it works
+// perfectly well across this boundary and must keep doing so.
+func assertWindowsSetupRunsAsCaller(callerSID string) error {
+	caller := strings.TrimSpace(callerSID)
+	if caller == "" {
+		return nil
+	}
+	current := windowsCurrentUserSID()
+	if current == "" || strings.EqualFold(current, caller) {
+		return nil
+	}
+	return fmt.Errorf("this elevated setup is running as a different Windows user (%s) than the one that started it (%s), "+
+		"and a sandbox principal's password is sealed to the user that stores it, so the account provisioned here could never be used. "+
+		"Re-run `zero sandbox setup` from a terminal elevated as your own account, or unset %s to use the restricted-token sandbox",
+		current, caller, windowsSandboxIdentityEnv)
+}
+
 // windowsProcessIsElevated reports whether the current process runs with an
 // elevated (Administrator) token. On any error obtaining the token it returns
 // true so the setup proceeds and surfaces the real WFP/ACL error rather than a

internal/sandbox/windows_stale_ace_windows_test.go

@@ -0,0 +1,274 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+)
+
+// applyWindowsACLPlan merges into the existing DACL, so narrowing a policy and
+// re-running setup used to leave the wider ACEs in place next to the new ones.
+// The principal kept access the current policy no longer grants — a silent
+// widening of the sandbox produced by tightening it.
+func TestRevokeDropsStalePrincipalACEsBeforeReapply(t *testing.T) {
+	root := t.TempDir()
+	kept := filepath.Join(root, "kept")
+	dropped := filepath.Join(root, "dropped")
+	for _, dir := range []string{kept, dropped} {
+		if err := os.MkdirAll(dir, 0o700); err != nil {
+			t.Fatalf("mkdir %s: %v", dir, err)
+		}
+	}
+	// Guests: a real, resolvable trustee that the test process is not a member
+	// of, so the ACEs below are observable without affecting this process.
+	principal := "S-1-5-32-546"
+
+	// First setup: both roots writable.
+	wide, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: principal,
+		WriteRoots:   []WritableRoot{{Root: kept}, {Root: dropped}},
+	})
+	if err != nil {
+		t.Fatalf("wide plan: %v", err)
+	}
+	if _, err := applyWindowsACLPlan(wide); err != nil {
+		t.Fatalf("apply wide plan: %v", err)
+	}
+	if !hasACEForTrustee(t, dropped, principal) {
+		t.Fatal("precondition: the wide plan should have granted the root it is about to lose")
+	}
+
+	// Policy narrows: "dropped" is no longer a write root.
+	narrow, err := buildWindowsPrincipalACLPlan(windowsPrincipalACLInput{
+		PrincipalSID: principal,
+		WriteRoots:   []WritableRoot{{Root: kept}},
+	})
+	if err != nil {
+		t.Fatalf("narrow plan: %v", err)
+	}
+	// Revocation has to cover the paths the OLD plan touched, not just the new
+	// one — the whole point is the path that left the policy.
+	if _, err := revokeWindowsPrincipalACEs(principal, windowsACLPlanPaths(wide)); err != nil {
+		t.Fatalf("revoke: %v", err)
+	}
+	if _, err := applyWindowsACLPlan(narrow); err != nil {
+		t.Fatalf("apply narrow plan: %v", err)
+	}
+
+	if hasACEForTrustee(t, dropped, principal) {
+		t.Error("principal kept its grant on a root the narrowed policy removed")
+	}
+	if !hasACEForTrustee(t, kept, principal) {
+		t.Error("revocation also dropped the grant the narrowed policy still wants")
+	}
+}
+
+// Revoking a path that was never created is cleanup with nothing to clean, not
+// an error — setup would otherwise fail on any carveout git has not made yet.
+func TestRevokeIgnoresPathsThatDoNotExist(t *testing.T) {
+	missing := filepath.Join(t.TempDir(), "never-created")
+	if _, err := revokeWindowsPrincipalACEs("S-1-5-32-546", []string{missing}); err != nil {
+		t.Fatalf("revoke over a missing path: %v", err)
+	}
+}
+
+func hasACEForTrustee(t *testing.T, path string, trustee string) bool {
+	t.Helper()
+	descriptor, err := windows.GetNamedSecurityInfo(path, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION)
+	if err != nil {
+		t.Fatalf("GetNamedSecurityInfo(%s): %v", path, err)
+	}
+	dacl, _, err := descriptor.DACL()
+	if err != nil {
+		t.Fatalf("DACL(%s): %v", path, err)
+	}
+	if dacl == nil {
+		return false
+	}
+	want, err := windows.StringToSid(trustee)
+	if err != nil {
+		t.Fatalf("StringToSid(%s): %v", trustee, err)
+	}
+	// Deny ACEs count here as much as allow ACEs: revocation is by trustee and
+	// drops both, so an assertion that only saw allows would call a leftover
+	// deny "revoked".
+	for index := uint32(0); index < uint32(dacl.AceCount); index++ {
+		var header *windows.ACE_HEADER
+		if err := windows.GetAce(dacl, index, (**windows.ACCESS_ALLOWED_ACE)(unsafe.Pointer(&header))); err != nil {
+			continue
+		}
+		ace := (*windows.ACCESS_ALLOWED_ACE)(unsafe.Pointer(header))
+		switch ace.Header.AceType {
+		case windows.ACCESS_ALLOWED_ACE_TYPE, windows.ACCESS_DENIED_ACE_TYPE:
+		default:
+			continue
+		}
+		if (*windows.SID)(unsafe.Pointer(&ace.SidStart)).Equals(want) {
+			return true
+		}
+	}
+	return false
+}
+
+// The mechanism working is not the same as the production path using it. This
+// pins the call site and its ORDER: revocation is only worth anything if it
+// runs before the plan that re-adds the current grants.
+func TestApplyPrincipalACLsRevokesBeforeApplying(t *testing.T) {
+	prevApply := applyWindowsACLPlanFn
+	t.Cleanup(func() { applyWindowsACLPlanFn = prevApply })
+
+	var actions []WindowsACLAction
+	applyWindowsACLPlanFn = func(plan WindowsACLPlan) (func() error, error) {
+		if len(plan.Entries) > 0 {
+			actions = append(actions, plan.Entries[0].Action)
+		}
+		return func() error { return nil }, nil
+	}
+
+	workspace := t.TempDir()
+	filesystem := FileSystemPolicy{
+		Kind:       FileSystemRestricted,
+		WriteRoots: []WritableRoot{{Root: workspace}},
+	}
+	if _, err := applyWindowsPrincipalACLs(t.TempDir(), "zero-sbx-test", "S-1-5-32-546", filesystem, filesystem.WriteRoots); err != nil {
+		t.Fatalf("applyWindowsPrincipalACLs: %v", err)
+	}
+
+	if len(actions) != 2 {
+		t.Fatalf("saw %d ACL plans (%v), want a revocation then the grants", len(actions), actions)
+	}
+	if actions[0] != windowsACLRevoke {
+		t.Errorf("first plan was %q, want the trustee revocation to go first", actions[0])
+	}
+	if actions[1] == windowsACLRevoke {
+		t.Error("second plan was another revocation; the current grants were never applied")
+	}
+}
+
+// The revocation's rollback has to be returned, not discarded.
+//
+// Discarding it was justified on the grounds that the only failure path from
+// applyWindowsPrincipalACLs removes the principal outright, so restoring ACEs
+// for a doomed account would be pointless. That holds only for a principal the
+// run CREATED. #812 keeps an ADOPTED principal alive on failure rather than
+// destroying a working account someone else provisioned — and then the discarded
+// snapshot left it logged-on and unable to reach its own workspace, with its
+// previous ACEs revoked and the new ones rolled back.
+func TestApplyPrincipalACLsRollbackRestoresTheRevokedACEs(t *testing.T) {
+	prevApply := applyWindowsACLPlanFn
+	t.Cleanup(func() { applyWindowsACLPlanFn = prevApply })
+
+	var applied []WindowsACLAction
+	var reverted []WindowsACLAction
+	applyWindowsACLPlanFn = func(plan WindowsACLPlan) (func() error, error) {
+		action := WindowsACLAction("")
+		if len(plan.Entries) > 0 {
+			action = plan.Entries[0].Action
+		}
+		applied = append(applied, action)
+		return func() error {
+			reverted = append(reverted, action)
+			return nil
+		}, nil
+	}
+
+	workspace := t.TempDir()
+	filesystem := FileSystemPolicy{
+		Kind:       FileSystemRestricted,
+		WriteRoots: []WritableRoot{{Root: workspace}},
+	}
+	rollback, err := applyWindowsPrincipalACLs(t.TempDir(), "zero-sbx-test", "S-1-5-32-546", filesystem, filesystem.WriteRoots)
+	if err != nil {
+		t.Fatalf("applyWindowsPrincipalACLs: %v", err)
+	}
+	if len(applied) != 2 || applied[0] != windowsACLRevoke {
+		t.Fatalf("applied %v, want a revocation then the grants", applied)
+	}
+	if err := rollback(); err != nil {
+		t.Fatalf("rollback: %v", err)
+	}
+
+	// Both halves must unwind, and in reverse order: the grant comes off first,
+	// then the ACEs the revocation removed go back.
+	if len(reverted) != 2 {
+		t.Fatalf("rollback reverted %v, want both the grant and the revocation", reverted)
+	}
+	if reverted[0] == windowsACLRevoke {
+		t.Error("rollback undid the revocation before the grant; the grant would survive")
+	}
+	if reverted[1] != windowsACLRevoke {
+		t.Errorf("rollback never restored the revoked ACEs, got %v", reverted)
+	}
+}
+
+// THE LEDGER IS PART OF THE ROLLBACK, and nothing pinned it.
+//
+// applyWindowsPrincipalACLs narrows the ledger before the whole setup
+// transaction has succeeded. If network setup or the marker write then fails,
+// rollback restores the DACLs, and the ledger has to come back with them: a
+// ledger that still records only the narrowed path set leaves the restored
+// grants unnamed, so later cleanup cannot find them.
+//
+// The neighbouring test asserts only the ORDER of the two ACL reverts and never
+// reads the ledger afterwards, so the restore could be deleted and the suite
+// would stay green.
+func TestApplyPrincipalACLsRollbackRestoresTheLedger(t *testing.T) {
+	prevApply := applyWindowsACLPlanFn
+	t.Cleanup(func() { applyWindowsACLPlanFn = prevApply })
+	applyWindowsACLPlanFn = func(WindowsACLPlan) (func() error, error) {
+		return func() error { return nil }, nil
+	}
+
+	sandboxHome := t.TempDir()
+	const username = "zero-sbx-test"
+
+	// A wider path set than the new plan will record, standing in for a previous
+	// policy's grants.
+	stale := []string{filepath.Join(t.TempDir(), "previously-granted")}
+	if err := writeWindowsPrincipalACLLedger(sandboxHome, username, stale); err != nil {
+		t.Fatalf("seed the ledger: %v", err)
+	}
+
+	workspace := t.TempDir()
+	filesystem := FileSystemPolicy{
+		Kind:       FileSystemRestricted,
+		WriteRoots: []WritableRoot{{Root: workspace}},
+	}
+	rollback, err := applyWindowsPrincipalACLs(sandboxHome, username, "S-1-5-32-546", filesystem, filesystem.WriteRoots)
+	if err != nil {
+		t.Fatalf("applyWindowsPrincipalACLs: %v", err)
+	}
+
+	narrowed, ok := readWindowsPrincipalACLLedger(sandboxHome, username)
+	if !ok {
+		t.Fatal("no ledger after apply")
+	}
+	if slicesContainsString(narrowed, stale[0]) {
+		t.Fatalf("the ledger still names the old path after apply, so this test is not exercising the restore: %v", narrowed)
+	}
+
+	if err := rollback(); err != nil {
+		t.Fatalf("rollback: %v", err)
+	}
+	restored, ok := readWindowsPrincipalACLLedger(sandboxHome, username)
+	if !ok {
+		t.Fatal("no ledger after rollback")
+	}
+	if !slicesContainsString(restored, stale[0]) {
+		t.Errorf("rollback restored the DACLs but left the narrowed ledger %v; the path %q it put back is now unnamed, so cleanup cannot find it", restored, stale[0])
+	}
+}
+
+func slicesContainsString(values []string, want string) bool {
+	for _, value := range values {
+		if value == want {
+			return true
+		}
+	}
+	return false
+}

internal/sandbox/windows_stale_secret_windows_test.go

@@ -0,0 +1,99 @@
+//go:build windows
+
+package sandbox
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"golang.org/x/sys/windows"
+)
+
+// "Absent beats stale" is the invariant the rollback's secret removal exists to
+// keep: the command path treats a missing secret as not-provisioned and falls
+// back, while a stale one fails the logon and reports a broken sandbox.
+//
+// So when the removal itself fails after a password rotation, the invariant was
+// NOT restored — a credential for a password that no longer works is still on
+// disk. Swallowing that error claims otherwise, and the operator finds out on
+// the next command instead of from the setup that broke it.
+func TestProvisionSurfacesFailedStaleSecretCleanup(t *testing.T) {
+	for name, testCase := range map[string]struct {
+		rotate     bool
+		removeErr  error
+		wantInErr  string
+		wantRemove bool
+	}{
+		"rotation happened and the stale secret cannot be removed": {
+			rotate: true, removeErr: errors.New("access is denied"),
+			wantRemove: true, wantInErr: "stale and could not be removed",
+		},
+		"rotation happened and cleanup succeeds": {
+			rotate: true, wantRemove: true,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			prevProvision := provisionWindowsSandboxIdentityFn
+			prevRemove := removeWindowsSandboxSecretFn
+			prevReset := resetWindowsSandboxUserPasswordFn
+			prevGrant := grantWindowsSandboxLogonRightsFn
+			prevWrite := writeWindowsSandboxSecretFn
+			t.Cleanup(func() {
+				provisionWindowsSandboxIdentityFn = prevProvision
+				removeWindowsSandboxSecretFn = prevRemove
+				resetWindowsSandboxUserPasswordFn = prevReset
+				grantWindowsSandboxLogonRightsFn = prevGrant
+				writeWindowsSandboxSecretFn = prevWrite
+			})
+
+			sid, err := windows.CreateWellKnownSid(windows.WinLocalSystemSid)
+			if err != nil {
+				t.Fatal(err)
+			}
+			// created=false so the run ADOPTS an account and rotation applies.
+			provisionWindowsSandboxIdentityFn = func(string, windowsSandboxRole) (windowsSandboxIdentity, string, bool, error) {
+				return windowsSandboxIdentity{Username: "zero-sbx-test", SID: sid}, "pw", false, nil
+			}
+			grantWindowsSandboxLogonRightsFn = func(*windows.SID) error { return nil }
+			removed := false
+			removeWindowsSandboxSecretFn = func(string) error {
+				removed = true
+				return testCase.removeErr
+			}
+			// Rotate, then fail immediately after so undo runs with rotated=true.
+			resetWindowsSandboxUserPasswordFn = func(string, string) error {
+				if testCase.rotate {
+					return nil
+				}
+				return errors.New("no rotation")
+			}
+
+			// The only step after rotation; failing it is what drives undo with
+			// rotated=true, which is the state the invariant is about.
+			writeWindowsSandboxSecretFn = func(string, string) error {
+				return errors.New("secret store refused")
+			}
+
+			config := WindowsSandboxCommandConfig{
+				SandboxHome:    t.TempDir(),
+				CommandCWD:     `C:\ws`,
+				WorkspaceRoots: []string{`C:\ws`},
+			}
+			_, _, err = provisionWindowsSandboxPrincipalForSetup(config, windowsSandboxRoleOffline)
+
+			if removed != testCase.wantRemove {
+				t.Fatalf("stale secret removal attempted = %v, want %v", removed, testCase.wantRemove)
+			}
+			if testCase.wantInErr == "" {
+				return
+			}
+			if err == nil {
+				t.Fatal("a failed stale-secret cleanup was swallowed")
+			}
+			if !strings.Contains(err.Error(), testCase.wantInErr) {
+				t.Fatalf("error = %q, want it to mention %q", err, testCase.wantInErr)
+			}
+		})
+	}
+}