fix(mcp): protect discovered OAuth endpoints

internal/mcp/oauth.go

@@ -63,8 +63,9 @@ func tokenToStored(t oauth.Token) StoredToken {
 }
 
 // OAuthConfig describes how to authenticate to a remote MCP server using OAuth.
-// Endpoints may be discovered from the server's metadata document; explicit
-// values here override or fill in anything discovery cannot provide.
+// Endpoints may be discovered through RFC 9728 protected-resource metadata and
+// RFC 8414 authorization-server metadata; explicit values override or fill in
+// anything discovery cannot provide.
 type OAuthConfig struct {
 	ClientID              string
 	ClientSecret          string
@@ -85,6 +86,11 @@ type authServerMetadata struct {
 	TokenEndpoint         string   `json:"token_endpoint"`
 	RegistrationEndpoint  string   `json:"registration_endpoint"`
 	ScopesSupported       []string `json:"scopes_supported"`
+
+	protectedResourceURL         string
+	protectAuthorizationEndpoint bool
+	protectTokenEndpoint         bool
+	protectRegistrationEndpoint  bool
 }
 
 // pkceParams holds a PKCE verifier/challenge pair.
@@ -120,7 +126,7 @@ type authorizationFlow struct {
 // discoverAuthorizationServer fetches the RFC 8414 authorization server metadata
 // at the well-known path under baseURL, via the shared engine.
 func discoverAuthorizationServer(ctx context.Context, client *http.Client, baseURL string) (authServerMetadata, error) {
-	meta, err := oauth.DiscoverAuthorizationServer(ctx, client, baseURL)
+	meta, err := oauth.DiscoverAuthorizationServer(ctx, withoutDiscoveryRedirects(client), baseURL)
 	if err != nil {
 		return authServerMetadata{}, err
 	}
@@ -133,10 +139,9 @@ func discoverAuthorizationServer(ctx context.Context, client *http.Client, baseU
 	}, nil
 }
 
-// validateResolvedEndpoints applies the shared https/loopback endpoint rule to
-// every non-empty endpoint in the resolved metadata — configured or discovered
-// alike — so discovery metadata can never downgrade an MCP login to an insecure
-// or attacker-controlled authorization, token, or registration endpoint.
+// validateResolvedEndpoints applies the shared https/loopback scheme rule to
+// every non-empty endpoint in the resolved metadata. Protected-resource
+// discovery adds its stricter provenance-aware network rule below.
 func validateResolvedEndpoints(metadata authServerMetadata) error {
 	for _, ep := range []struct{ kind, url string }{
 		{"authorization", metadata.AuthorizationEndpoint},
@@ -153,9 +158,43 @@ func validateResolvedEndpoints(metadata authServerMetadata) error {
 	return nil
 }
 
-// resolveAuthorizationServer discovers metadata and applies explicit config
-// overrides. Configured endpoints take precedence over discovered ones, and act
-// as a fallback when discovery fails or omits a value.
+func validateProtectedDiscoveredEndpoints(ctx context.Context, metadata authServerMetadata) error {
+	if metadata.protectedResourceURL == "" {
+		return nil
+	}
+	policy, err := newOAuthDiscoveryNetworkPolicy(metadata.protectedResourceURL)
+	if err != nil {
+		return err
+	}
+	for _, endpoint := range []struct {
+		kind      string
+		url       string
+		protected bool
+	}{
+		{kind: "authorization", url: metadata.AuthorizationEndpoint, protected: metadata.protectAuthorizationEndpoint},
+		{kind: "token", url: metadata.TokenEndpoint, protected: metadata.protectTokenEndpoint},
+		{kind: "registration", url: metadata.RegistrationEndpoint, protected: metadata.protectRegistrationEndpoint},
+	} {
+		if !endpoint.protected || strings.TrimSpace(endpoint.url) == "" {
+			continue
+		}
+		parsed, parseErr := url.Parse(endpoint.url)
+		if parseErr != nil || parsed.Hostname() == "" {
+			return fmt.Errorf("mcp oauth: %s endpoint: %w", endpoint.kind, errUnsafeOAuthDiscoveryTarget)
+		}
+		if err := policy.validateLiteralHost(parsed.Hostname()); err != nil {
+			return fmt.Errorf("mcp oauth: %s endpoint: %w", endpoint.kind, err)
+		}
+		if _, err := policy.resolve(ctx, parsed.Hostname()); err != nil {
+			return fmt.Errorf("mcp oauth: %s endpoint: %w", endpoint.kind, err)
+		}
+	}
+	return nil
+}
+
+// resolveAuthorizationServer follows protected-resource metadata when the MCP
+// endpoint advertises it, falls back to direct authorization-server discovery
+// for legacy providers, then applies explicit config overrides.
 func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseURL string, cfg OAuthConfig) (authServerMetadata, error) {
 	// When the config supplies both the authorization and token endpoints
 	// directly, skip network discovery entirely: there is nothing to discover,
@@ -174,17 +213,40 @@ func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseUR
 		return metadata, nil
 	}
 
+	discoveryClient := withoutDiscoveryRedirects(client)
 	discoveryBase := strings.TrimSpace(cfg.IssuerURL)
+	protectedResourceDiscovery := false
 	if discoveryBase == "" {
-		discoveryBase = baseURL
+		issuer, found, protectedErr := discoverProtectedResourceAuthorizationServer(ctx, discoveryClient, baseURL)
+		if protectedErr != nil {
+			return authServerMetadata{}, protectedErr
+		}
+		if found {
+			discoveryBase = issuer
+			protectedResourceDiscovery = true
+			discoveryClient, protectedErr = newAdvertisedOAuthDiscoveryClient(discoveryClient, baseURL)
+			if protectedErr != nil {
+				return authServerMetadata{}, protectedErr
+			}
+		} else {
+			discoveryBase = baseURL
+		}
 	}
 
-	metadata, err := discoverAuthorizationServer(ctx, client, discoveryBase)
+	metadata, err := discoverAuthorizationServer(ctx, discoveryClient, discoveryBase)
 	if err != nil {
 		// Discovery failures are non-fatal when the config supplies the endpoints
 		// directly; otherwise surface the discovery error.
 		metadata = authServerMetadata{}
 	}
+	if protectedResourceDiscovery {
+		if strings.TrimSpace(metadata.Issuer) == "" {
+			return authServerMetadata{}, errors.New("mcp oauth: authorization server metadata is missing its required issuer")
+		}
+		if metadata.Issuer != discoveryBase {
+			return authServerMetadata{}, errors.New("mcp oauth: authorization server metadata issuer does not match protected resource metadata")
+		}
+	}
 
 	if endpoint := strings.TrimSpace(cfg.AuthorizationEndpoint); endpoint != "" {
 		metadata.AuthorizationEndpoint = endpoint
@@ -195,6 +257,12 @@ func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseUR
 	if endpoint := strings.TrimSpace(cfg.RegistrationEndpoint); endpoint != "" {
 		metadata.RegistrationEndpoint = endpoint
 	}
+	if protectedResourceDiscovery {
+		metadata.protectedResourceURL = strings.TrimSpace(baseURL)
+		metadata.protectAuthorizationEndpoint = strings.TrimSpace(cfg.AuthorizationEndpoint) == ""
+		metadata.protectTokenEndpoint = strings.TrimSpace(cfg.TokenEndpoint) == ""
+		metadata.protectRegistrationEndpoint = strings.TrimSpace(cfg.RegistrationEndpoint) == ""
+	}
 
 	if strings.TrimSpace(metadata.AuthorizationEndpoint) == "" {
 		return authServerMetadata{}, errors.New("no authorization endpoint discovered or configured")
@@ -205,6 +273,9 @@ func resolveAuthorizationServer(ctx context.Context, client *http.Client, baseUR
 	if err := validateResolvedEndpoints(metadata); err != nil {
 		return authServerMetadata{}, err
 	}
+	if err := validateProtectedDiscoveredEndpoints(ctx, metadata); err != nil {
+		return authServerMetadata{}, err
+	}
 	return metadata, nil
 }
 
@@ -363,10 +434,25 @@ func Login(ctx context.Context, options LoginOptions) (StoredToken, error) {
 	}
 	defer listener.Close()
 	redirectURI := fmt.Sprintf("http://%s/callback", listener.Addr().String())
+	protectedEndpointClient := httpClient
+	if metadata.protectRegistrationEndpoint || metadata.protectTokenEndpoint {
+		protectedEndpointClient, err = newAdvertisedOAuthDiscoveryClient(httpClient, metadata.protectedResourceURL)
+		if err != nil {
+			return StoredToken{}, err
+		}
+	}
+	tokenClient := httpClient
+	if metadata.protectTokenEndpoint {
+		tokenClient = protectedEndpointClient
+	}
 
 	if strings.TrimSpace(cfg.ClientID) == "" {
 		if registration := strings.TrimSpace(metadata.RegistrationEndpoint); registration != "" {
-			clientID, clientSecret, regErr := registerClient(loginCtx, httpClient, registration, redirectURI, cfg.Scopes)
+			registrationClient := httpClient
+			if metadata.protectRegistrationEndpoint {
+				registrationClient = protectedEndpointClient
+			}
+			clientID, clientSecret, regErr := registerClient(loginCtx, registrationClient, registration, redirectURI, cfg.Scopes)
 			if regErr != nil {
 				return StoredToken{}, regErr
 			}
@@ -390,7 +476,7 @@ func Login(ctx context.Context, options LoginOptions) (StoredToken, error) {
 	}
 
 	flow := &authorizationFlow{
-		httpClient: httpClient,
+		httpClient: tokenClient,
 		metadata:   metadata,
 		config:     cfg,
 		pkce:       pkce,

internal/mcp/oauth_discovery_network.go

@@ -0,0 +1,290 @@
+package mcp
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"fmt"
+	"net"
+	"net/http"
+	"net/netip"
+	"net/url"
+	"strings"
+)
+
+var errUnsafeOAuthDiscoveryTarget = errors.New("mcp oauth: refused non-public server-advertised discovery target")
+
+var nonPublicOAuthDiscoveryPrefixes = []netip.Prefix{
+	netip.MustParsePrefix("0.0.0.0/8"),
+	netip.MustParsePrefix("10.0.0.0/8"),
+	netip.MustParsePrefix("100.64.0.0/10"),
+	netip.MustParsePrefix("127.0.0.0/8"),
+	netip.MustParsePrefix("169.254.0.0/16"),
+	netip.MustParsePrefix("172.16.0.0/12"),
+	netip.MustParsePrefix("192.0.0.0/24"),
+	netip.MustParsePrefix("192.0.2.0/24"),
+	netip.MustParsePrefix("192.88.99.0/24"),
+	netip.MustParsePrefix("192.168.0.0/16"),
+	netip.MustParsePrefix("198.18.0.0/15"),
+	netip.MustParsePrefix("198.51.100.0/24"),
+	netip.MustParsePrefix("203.0.113.0/24"),
+	netip.MustParsePrefix("224.0.0.0/4"),
+	netip.MustParsePrefix("240.0.0.0/4"),
+	netip.MustParsePrefix("::/128"),
+	netip.MustParsePrefix("::1/128"),
+	netip.MustParsePrefix("100::/64"),
+	netip.MustParsePrefix("2001:2::/48"),
+	netip.MustParsePrefix("2001:db8::/32"),
+	netip.MustParsePrefix("fc00::/7"),
+	netip.MustParsePrefix("fe80::/10"),
+	netip.MustParsePrefix("ff00::/8"),
+}
+
+type oauthDiscoveryNetworkPolicy struct {
+	allowLoopback bool
+	lookupNetIP   func(context.Context, string, string) ([]netip.Addr, error)
+}
+
+type advertisedOAuthDiscoveryTransport struct {
+	base   http.RoundTripper
+	policy oauthDiscoveryNetworkPolicy
+}
+
+// newAdvertisedOAuthDiscoveryClient binds server-advertised discovery to the
+// public network. Loopback targets remain available only when the operator
+// explicitly configured a loopback MCP resource.
+func newAdvertisedOAuthDiscoveryClient(client *http.Client, resourceURL string) (*http.Client, error) {
+	policy, err := newOAuthDiscoveryNetworkPolicy(resourceURL)
+	if err != nil {
+		return nil, err
+	}
+	copied := withoutDiscoveryRedirects(client)
+	transport := copied.Transport
+	if transport == nil {
+		transport = http.DefaultTransport
+	}
+	copied.Transport = advertisedOAuthDiscoveryTransport{base: transport, policy: policy}
+	return copied, nil
+}
+
+func newOAuthDiscoveryNetworkPolicy(resourceURL string) (oauthDiscoveryNetworkPolicy, error) {
+	parsed, err := url.Parse(strings.TrimSpace(resourceURL))
+	if err != nil || parsed.Hostname() == "" {
+		return oauthDiscoveryNetworkPolicy{}, errors.New("mcp oauth: invalid MCP resource URL for discovery policy")
+	}
+	return oauthDiscoveryNetworkPolicy{allowLoopback: isExplicitOAuthLoopbackHost(parsed.Hostname())}, nil
+}
+
+func validateAdvertisedOAuthDiscoveryURL(resourceURL string, targetURL string) error {
+	policy, err := newOAuthDiscoveryNetworkPolicy(resourceURL)
+	if err != nil {
+		return err
+	}
+	parsed, err := url.Parse(targetURL)
+	if err != nil || parsed.Hostname() == "" {
+		return errUnsafeOAuthDiscoveryTarget
+	}
+	return policy.validateLiteralHost(parsed.Hostname())
+}
+
+func (transport advertisedOAuthDiscoveryTransport) RoundTrip(request *http.Request) (*http.Response, error) {
+	if request == nil || request.URL == nil {
+		return nil, errors.New("mcp oauth: discovery request has no URL")
+	}
+	if err := transport.policy.validateLiteralHost(request.URL.Hostname()); err != nil {
+		return nil, err
+	}
+
+	baseTransport, ok := transport.base.(*http.Transport)
+	if !ok {
+		return nil, errors.New("mcp oauth: cannot enforce discovery network policy with a custom HTTP transport")
+	}
+	addresses, err := transport.policy.resolve(request.Context(), request.URL.Hostname())
+	if err != nil {
+		return nil, err
+	}
+	var lastErr error
+	for index, address := range addresses {
+		attempt := request
+		if index > 0 && request.Body != nil {
+			if request.GetBody == nil {
+				break
+			}
+			body, bodyErr := request.GetBody()
+			if bodyErr != nil {
+				return nil, bodyErr
+			}
+			attempt = request.Clone(request.Context())
+			attempt.Body = body
+		}
+		response, roundTripErr := roundTripPinnedOAuthDiscovery(attempt, baseTransport, address)
+		if roundTripErr == nil {
+			return response, nil
+		}
+		lastErr = roundTripErr
+	}
+	if lastErr == nil {
+		lastErr = errUnsafeOAuthDiscoveryTarget
+	}
+	return nil, lastErr
+}
+
+func roundTripPinnedOAuthDiscovery(request *http.Request, base *http.Transport, address netip.Addr) (*http.Response, error) {
+	pinnedRequest := request.Clone(request.Context())
+	pinnedURL := *request.URL
+	originalHost := request.URL.Hostname()
+	port := request.URL.Port()
+	if port == "" {
+		switch strings.ToLower(request.URL.Scheme) {
+		case "http":
+			port = "80"
+		case "https":
+			port = "443"
+		default:
+			return nil, errUnsafeOAuthDiscoveryTarget
+		}
+	}
+	pinnedURL.Host = net.JoinHostPort(address.String(), port)
+	pinnedRequest.URL = &pinnedURL
+	if pinnedRequest.Host == "" {
+		pinnedRequest.Host = request.URL.Host
+	}
+
+	pinnedTransport := base.Clone()
+	pinnedTransport.DisableKeepAlives = true
+	var proxyURL *url.URL
+	if base.Proxy != nil {
+		var err error
+		proxyURL, err = base.Proxy(request)
+		if err != nil {
+			return nil, fmt.Errorf("mcp oauth: resolve discovery proxy: %w", err)
+		}
+		if proxyURL == nil {
+			pinnedTransport.Proxy = nil
+		} else {
+			pinnedTransport.Proxy = http.ProxyURL(proxyURL)
+		}
+	}
+	if pinnedTransport.TLSClientConfig == nil {
+		pinnedTransport.TLSClientConfig = &tls.Config{ServerName: originalHost} //nolint:gosec // standard verification remains enabled
+	} else {
+		pinnedTransport.TLSClientConfig = pinnedTransport.TLSClientConfig.Clone()
+		pinnedTransport.TLSClientConfig.ServerName = originalHost
+	}
+	// A custom TLS dialer must not resolve the advertised hostname again after
+	// the policy has pinned it. For an HTTPS proxy, however, that proxy remains
+	// the TLS first hop and the transport sends CONNECT to the pinned target.
+	dialContext := pinnedTransport.DialContext
+	if dialContext == nil {
+		dialContext = (&net.Dialer{}).DialContext
+	}
+	pinnedAddress := net.JoinHostPort(address.String(), port)
+	targetTLSConfig := pinnedTransport.TLSClientConfig
+	pinnedTransport.DialTLSContext = func(ctx context.Context, network string, address string) (net.Conn, error) {
+		if proxyURL != nil && strings.EqualFold(proxyURL.Scheme, "https") {
+			proxyTLSConfig := targetTLSConfig.Clone()
+			proxyTLSConfig.ServerName = proxyURL.Hostname()
+			return dialOAuthDiscoveryTLS(ctx, dialContext, network, address, proxyTLSConfig)
+		}
+		return dialOAuthDiscoveryTLS(ctx, dialContext, network, pinnedAddress, targetTLSConfig)
+	}
+	return pinnedTransport.RoundTrip(pinnedRequest)
+}
+
+func dialOAuthDiscoveryTLS(ctx context.Context, dialContext func(context.Context, string, string) (net.Conn, error), network string, address string, config *tls.Config) (net.Conn, error) {
+	connection, err := dialContext(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+	tlsConnection := tls.Client(connection, config)
+	if err := tlsConnection.HandshakeContext(ctx); err != nil {
+		connection.Close()
+		return nil, err
+	}
+	return tlsConnection, nil
+}
+
+func (policy oauthDiscoveryNetworkPolicy) resolve(ctx context.Context, host string) ([]netip.Addr, error) {
+	if address, err := netip.ParseAddr(strings.Trim(host, "[]")); err == nil {
+		address = address.Unmap()
+		if err := policy.validateAddress(address); err != nil {
+			return nil, err
+		}
+		return []netip.Addr{address}, nil
+	}
+
+	lookupNetIP := policy.lookupNetIP
+	if lookupNetIP == nil {
+		lookupNetIP = net.DefaultResolver.LookupNetIP
+	}
+	addresses, err := lookupNetIP(ctx, "ip", host)
+	if err != nil {
+		return nil, errors.New("mcp oauth: resolve server-advertised discovery target failed")
+	}
+	if len(addresses) == 0 {
+		return nil, errors.New("mcp oauth: server-advertised discovery target resolved to no addresses")
+	}
+	explicitLoopback := isExplicitOAuthLoopbackHost(host)
+	resolved := make([]netip.Addr, 0, len(addresses))
+	for _, address := range addresses {
+		address = address.Unmap()
+		if explicitLoopback && !address.IsLoopback() {
+			return nil, errUnsafeOAuthDiscoveryTarget
+		}
+		if err := policy.validateAddress(address); err != nil {
+			return nil, err
+		}
+		resolved = append(resolved, address)
+	}
+	return resolved, nil
+}
+
+func (policy oauthDiscoveryNetworkPolicy) validateLiteralHost(host string) error {
+	normalized := strings.ToLower(strings.TrimSuffix(strings.Trim(host, "[]"), "."))
+	if normalized == "" {
+		return errUnsafeOAuthDiscoveryTarget
+	}
+	if isCloudMetadataHostname(normalized) {
+		return errUnsafeOAuthDiscoveryTarget
+	}
+	if isExplicitOAuthLoopbackHost(normalized) && !policy.allowLoopback {
+		return errUnsafeOAuthDiscoveryTarget
+	}
+	if address, err := netip.ParseAddr(normalized); err == nil {
+		return policy.validateAddress(address.Unmap())
+	}
+	return nil
+}
+
+func (policy oauthDiscoveryNetworkPolicy) validateAddress(address netip.Addr) error {
+	if policy.allowLoopback && address.IsLoopback() {
+		return nil
+	}
+	if !address.IsValid() || !address.IsGlobalUnicast() || address.IsPrivate() {
+		return errUnsafeOAuthDiscoveryTarget
+	}
+	for _, prefix := range nonPublicOAuthDiscoveryPrefixes {
+		if prefix.Contains(address) {
+			return errUnsafeOAuthDiscoveryTarget
+		}
+	}
+	return nil
+}
+
+func isExplicitOAuthLoopbackHost(host string) bool {
+	normalized := strings.ToLower(strings.TrimSuffix(strings.Trim(host, "[]"), "."))
+	if normalized == "localhost" || strings.HasSuffix(normalized, ".localhost") {
+		return true
+	}
+	address, err := netip.ParseAddr(normalized)
+	return err == nil && address.Unmap().IsLoopback()
+}
+
+func isCloudMetadataHostname(host string) bool {
+	switch strings.ToLower(strings.TrimSuffix(host, ".")) {
+	case "metadata", "metadata.google.internal", "metadata.azure.internal":
+		return true
+	default:
+		return false
+	}
+}

internal/mcp/oauth_discovery_network_test.go

@@ -0,0 +1,180 @@
+package mcp
+
+import (
+	"bufio"
+	"context"
+	"errors"
+	"io"
+	"net"
+	"net/http"
+	"net/netip"
+	"net/url"
+	"strings"
+	"sync/atomic"
+	"testing"
+)
+
+func TestOAuthDiscoveryNetworkPolicyRejectsNonPublicAddresses(t *testing.T) {
+	policy := oauthDiscoveryNetworkPolicy{}
+	for _, value := range []string{
+		"127.0.0.1",
+		"10.0.0.1",
+		"169.254.169.254",
+		"192.0.2.10",
+		"224.0.0.1",
+		"::1",
+		"fc00::1",
+		"fe80::1",
+		"2001:db8::1",
+	} {
+		t.Run(value, func(t *testing.T) {
+			if err := policy.validateAddress(netip.MustParseAddr(value)); err == nil {
+				t.Fatalf("address %s should be rejected", value)
+			}
+		})
+	}
+	for _, value := range []string{"8.8.8.8", "2606:4700:4700::1111"} {
+		if err := policy.validateAddress(netip.MustParseAddr(value)); err != nil {
+			t.Fatalf("public address %s rejected: %v", value, err)
+		}
+	}
+	loopbackPolicy := oauthDiscoveryNetworkPolicy{allowLoopback: true}
+	if err := loopbackPolicy.validateAddress(netip.MustParseAddr("127.0.0.1")); err != nil {
+		t.Fatalf("explicit loopback resource should allow loopback discovery: %v", err)
+	}
+	if err := loopbackPolicy.validateAddress(netip.MustParseAddr("10.0.0.1")); err == nil {
+		t.Fatal("loopback development must not permit arbitrary private targets")
+	}
+}
+
+func TestProtectedDiscoveryRejectsNonPublicOAuthEndpoints(t *testing.T) {
+	for _, endpoint := range []struct {
+		name     string
+		metadata authServerMetadata
+	}{
+		{
+			name: "authorization",
+			metadata: authServerMetadata{
+				AuthorizationEndpoint:        "http://127.0.0.1/authorize",
+				protectAuthorizationEndpoint: true,
+			},
+		},
+		{
+			name: "token",
+			metadata: authServerMetadata{
+				TokenEndpoint:        "https://10.0.0.1/token",
+				protectTokenEndpoint: true,
+			},
+		},
+		{
+			name: "registration",
+			metadata: authServerMetadata{
+				RegistrationEndpoint:        "https://169.254.169.254/register",
+				protectRegistrationEndpoint: true,
+			},
+		},
+	} {
+		t.Run(endpoint.name, func(t *testing.T) {
+			endpoint.metadata.protectedResourceURL = "https://mcp.example/mcp"
+			if err := validateProtectedDiscoveredEndpoints(context.Background(), endpoint.metadata); err == nil {
+				t.Fatalf("protected %s endpoint should be rejected", endpoint.name)
+			}
+		})
+	}
+}
+
+func TestAdvertisedOAuthDiscoveryRejectsHostnameResolvingPrivateAtUseTime(t *testing.T) {
+	var dialed atomic.Bool
+	policy := oauthDiscoveryNetworkPolicy{
+		lookupNetIP: func(context.Context, string, string) ([]netip.Addr, error) {
+			return []netip.Addr{netip.MustParseAddr("10.0.0.12")}, nil
+		},
+	}
+	client := &http.Client{Transport: advertisedOAuthDiscoveryTransport{
+		base: &http.Transport{DialContext: func(context.Context, string, string) (net.Conn, error) {
+			dialed.Store(true)
+			return nil, io.EOF
+		}},
+		policy: policy,
+	}}
+
+	_, err := client.Get("https://auth.example/.well-known/oauth-authorization-server")
+	if err == nil || !strings.Contains(err.Error(), errUnsafeOAuthDiscoveryTarget.Error()) {
+		t.Fatalf("error = %v, want private-resolution rejection", err)
+	}
+	if dialed.Load() {
+		t.Fatal("private resolved address reached the dialer")
+	}
+}
+
+func TestAdvertisedOAuthDiscoveryPinsResolvedAddress(t *testing.T) {
+	const publicAddress = "93.184.216.34"
+	var dialedAddress string
+	policy := oauthDiscoveryNetworkPolicy{
+		lookupNetIP: func(context.Context, string, string) ([]netip.Addr, error) {
+			return []netip.Addr{netip.MustParseAddr(publicAddress)}, nil
+		},
+	}
+	baseTransport := &http.Transport{DialContext: func(_ context.Context, _, address string) (net.Conn, error) {
+		dialedAddress = address
+		clientConn, serverConn := net.Pipe()
+		go func() {
+			defer serverConn.Close()
+			reader := bufio.NewReader(serverConn)
+			for {
+				line, err := reader.ReadString('\n')
+				if err != nil || line == "\r\n" {
+					break
+				}
+			}
+			_, _ = io.WriteString(serverConn, "HTTP/1.1 200 OK\r\nContent-Length: 2\r\nConnection: close\r\n\r\n{}")
+		}()
+		return clientConn, nil
+	}}
+	client := &http.Client{Transport: advertisedOAuthDiscoveryTransport{base: baseTransport, policy: policy}}
+
+	response, err := client.Get("http://auth.example/metadata")
+	if err != nil {
+		t.Fatalf("GET pinned discovery target: %v", err)
+	}
+	response.Body.Close()
+	if dialedAddress != net.JoinHostPort(publicAddress, "80") {
+		t.Fatalf("dialed %q, want resolved address", dialedAddress)
+	}
+}
+
+func TestPinnedOAuthDiscoveryPreservesHTTPSProxyFirstHop(t *testing.T) {
+	for _, test := range []struct {
+		name     string
+		proxy    string
+		firstHop string
+	}{
+		{name: "HTTP", proxy: "http://proxy.example:8080", firstHop: "proxy.example:8080"},
+		{name: "HTTPS", proxy: "https://proxy.example:8443", firstHop: "proxy.example:8443"},
+		{name: "SOCKS", proxy: "socks5://proxy.example:1080", firstHop: "proxy.example:1080"},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			proxyURL, err := url.Parse(test.proxy)
+			if err != nil {
+				t.Fatalf("parse proxy URL: %v", err)
+			}
+			var dialedAddress string
+			baseTransport := &http.Transport{
+				Proxy: http.ProxyURL(proxyURL),
+				DialContext: func(_ context.Context, _, address string) (net.Conn, error) {
+					dialedAddress = address
+					return nil, errors.New("stop after observing first hop")
+				},
+			}
+			request, err := http.NewRequestWithContext(context.Background(), http.MethodGet, "https://auth.example/metadata", nil)
+			if err != nil {
+				t.Fatalf("create request: %v", err)
+			}
+
+			_, _ = roundTripPinnedOAuthDiscovery(request, baseTransport, netip.MustParseAddr("93.184.216.34"))
+			if dialedAddress != test.firstHop {
+				t.Fatalf("dialed %q, want %s proxy first hop", dialedAddress, test.name)
+			}
+		})
+	}
+}

internal/mcp/oauth_protected_resource.go

@@ -0,0 +1,233 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+
+	"github.com/Gitlawb/zero/internal/oauth"
+)
+
+const protectedResourceMetadataLimit = 1 << 20
+
+type protectedResourceMetadata struct {
+	Resource             string   `json:"resource"`
+	AuthorizationServers []string `json:"authorization_servers"`
+}
+
+// withoutDiscoveryRedirects returns a shallow client copy that exposes 3xx
+// responses to the caller. Every discovered URL is validated before use, so a
+// redirect must never replace that validated target with an unchecked one.
+func withoutDiscoveryRedirects(client *http.Client) *http.Client {
+	if client == nil {
+		client = http.DefaultClient
+	}
+	copied := *client
+	// Discovery requests are deliberately unauthenticated. A caller-supplied
+	// jar must not turn a plain metadata GET into a credential-bearing request.
+	copied.Jar = nil
+	copied.CheckRedirect = func(*http.Request, []*http.Request) error {
+		return http.ErrUseLastResponse
+	}
+	return &copied
+}
+
+// discoverProtectedResourceAuthorizationServer follows the RFC 9728 URL
+// advertised by a protected MCP resource. found is false when the endpoint does
+// not advertise protected-resource metadata, allowing legacy direct discovery.
+func discoverProtectedResourceAuthorizationServer(ctx context.Context, client *http.Client, resourceURL string) (issuer string, found bool, err error) {
+	resourceURL = strings.TrimSpace(resourceURL)
+	client = withoutDiscoveryRedirects(client)
+	metadataURL, found, err := probeProtectedResourceMetadataURL(ctx, client, resourceURL)
+	if err != nil || !found {
+		return "", found, err
+	}
+	if err := validateResourceMetadataOrigin(resourceURL, metadataURL); err != nil {
+		return "", true, err
+	}
+	client, err = newAdvertisedOAuthDiscoveryClient(client, resourceURL)
+	if err != nil {
+		return "", true, err
+	}
+	metadata, err := fetchProtectedResourceMetadata(ctx, client, metadataURL)
+	if err != nil {
+		return "", true, err
+	}
+	if metadata.Resource != resourceURL {
+		return "", true, errors.New("mcp oauth: protected resource metadata resource does not match the MCP server URL")
+	}
+	if len(metadata.AuthorizationServers) == 0 {
+		return "", true, errors.New("mcp oauth: protected resource metadata has no authorization_servers")
+	}
+	issuer = metadata.AuthorizationServers[0]
+	if strings.TrimSpace(issuer) == "" {
+		return "", true, errors.New("mcp oauth: protected resource metadata has an empty authorization server")
+	}
+	if issuer != strings.TrimSpace(issuer) {
+		return "", true, errors.New("mcp oauth: protected resource authorization server must not contain surrounding whitespace")
+	}
+	if err := oauth.ValidateEndpointURL(issuer); err != nil {
+		return "", true, fmt.Errorf("mcp oauth: protected resource authorization server: %w", err)
+	}
+	if err := validateAdvertisedOAuthDiscoveryURL(resourceURL, issuer); err != nil {
+		return "", true, err
+	}
+	return issuer, true, nil
+}
+
+func validateResourceMetadataOrigin(resourceURL string, metadataURL string) error {
+	resource, resourceErr := url.Parse(resourceURL)
+	metadata, metadataErr := url.Parse(metadataURL)
+	if resourceErr != nil || metadataErr != nil || !sameMCPOrigin(resource, metadata) {
+		return errors.New("mcp oauth: protected resource metadata URL must be same-origin with the MCP server URL")
+	}
+	return nil
+}
+
+func probeProtectedResourceMetadataURL(ctx context.Context, client *http.Client, resourceURL string) (string, bool, error) {
+	if client == nil {
+		client = http.DefaultClient
+	}
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, strings.TrimSpace(resourceURL), nil)
+	if err != nil {
+		return "", false, errors.New("mcp oauth: invalid MCP resource URL")
+	}
+	request.Header.Set("Accept", "application/json, text/event-stream")
+	request.Header.Set("MCP-Protocol-Version", defaultProtocolVersion)
+	response, err := client.Do(request)
+	if err != nil {
+		// A resource that cannot be probed may still support the legacy direct
+		// authorization-server discovery flow.
+		return "", false, nil
+	}
+	defer response.Body.Close()
+	if response.StatusCode != http.StatusUnauthorized {
+		return "", false, nil
+	}
+	metadataURL, found, err := parseResourceMetadataURL(response.Header.Values("WWW-Authenticate"))
+	if err != nil {
+		return "", false, fmt.Errorf("mcp oauth: invalid WWW-Authenticate resource_metadata parameter: %w", err)
+	}
+	if !found {
+		return "", false, nil
+	}
+	if err := oauth.ValidateEndpointURL(metadataURL); err != nil {
+		return "", false, fmt.Errorf("mcp oauth: protected resource metadata URL: %w", err)
+	}
+	return metadataURL, true, nil
+}
+
+func fetchProtectedResourceMetadata(ctx context.Context, client *http.Client, metadataURL string) (protectedResourceMetadata, error) {
+	if client == nil {
+		client = http.DefaultClient
+	}
+	request, err := http.NewRequestWithContext(ctx, http.MethodGet, metadataURL, nil)
+	if err != nil {
+		return protectedResourceMetadata{}, errors.New("mcp oauth: invalid protected resource metadata URL")
+	}
+	request.Header.Set("Accept", "application/json")
+	response, err := client.Do(request)
+	if err != nil {
+		return protectedResourceMetadata{}, errors.New("mcp oauth: fetch protected resource metadata failed")
+	}
+	defer response.Body.Close()
+	if response.StatusCode < http.StatusOK || response.StatusCode >= http.StatusMultipleChoices {
+		return protectedResourceMetadata{}, fmt.Errorf("mcp oauth: protected resource metadata returned HTTP %d", response.StatusCode)
+	}
+	var metadata protectedResourceMetadata
+	if err := json.NewDecoder(io.LimitReader(response.Body, protectedResourceMetadataLimit)).Decode(&metadata); err != nil {
+		return protectedResourceMetadata{}, errors.New("mcp oauth: decode protected resource metadata failed")
+	}
+	return metadata, nil
+}
+
+func parseResourceMetadataURL(headerValues []string) (string, bool, error) {
+	var selected string
+	for _, headerValue := range headerValues {
+		for index := 0; index < len(headerValue); {
+			if headerValue[index] == '"' {
+				_, next, err := readAuthQuotedString(headerValue, index)
+				if err != nil {
+					// Malformed parameters for another authentication scheme do
+					// not suppress a valid challenge in a separate header value.
+					break
+				}
+				index = next
+				continue
+			}
+			if !isAuthTokenByte(headerValue[index]) {
+				index++
+				continue
+			}
+			start := index
+			for index < len(headerValue) && isAuthTokenByte(headerValue[index]) {
+				index++
+			}
+			name := headerValue[start:index]
+			for index < len(headerValue) && (headerValue[index] == ' ' || headerValue[index] == '\t') {
+				index++
+			}
+			if index >= len(headerValue) || headerValue[index] != '=' {
+				continue
+			}
+			index++
+			for index < len(headerValue) && (headerValue[index] == ' ' || headerValue[index] == '\t') {
+				index++
+			}
+			if !strings.EqualFold(name, "resource_metadata") {
+				continue
+			}
+			if index >= len(headerValue) || headerValue[index] != '"' {
+				return "", false, errors.New("resource_metadata must be a quoted URL")
+			}
+			value, next, err := readAuthQuotedString(headerValue, index)
+			if err != nil {
+				return "", false, err
+			}
+			value = strings.TrimSpace(value)
+			if value == "" {
+				return "", false, errors.New("resource_metadata URL is empty")
+			}
+			if selected != "" && selected != value {
+				return "", false, errors.New("conflicting resource_metadata URLs")
+			}
+			selected = value
+			index = next
+		}
+	}
+	return selected, selected != "", nil
+}
+
+func readAuthQuotedString(value string, quote int) (string, int, error) {
+	var decoded strings.Builder
+	for index := quote + 1; index < len(value); index++ {
+		switch value[index] {
+		case '"':
+			return decoded.String(), index + 1, nil
+		case '\\':
+			index++
+			if index >= len(value) {
+				return "", 0, errors.New("unterminated quoted resource_metadata URL")
+			}
+			decoded.WriteByte(value[index])
+		default:
+			if value[index] < 0x20 || value[index] == 0x7f {
+				return "", 0, errors.New("resource_metadata URL contains a control character")
+			}
+			decoded.WriteByte(value[index])
+		}
+	}
+	return "", 0, errors.New("unterminated quoted resource_metadata URL")
+}
+
+func isAuthTokenByte(value byte) bool {
+	if value >= 'a' && value <= 'z' || value >= 'A' && value <= 'Z' || value >= '0' && value <= '9' {
+		return true
+	}
+	return strings.ContainsRune("!#$%&'*+-.^_`|~", rune(value))
+}

internal/mcp/oauth_protected_resource_test.go

@@ -0,0 +1,742 @@
+package mcp
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"net/http/cookiejar"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"sync/atomic"
+	"testing"
+	"time"
+)
+
+func TestParseResourceMetadataURL(t *testing.T) {
+	got, found, err := parseResourceMetadataURL([]string{
+		`Basic realm="resource_metadata=not-a-parameter"`,
+		`Bearer realm="OAuth", scope="read", Resource_Metadata="https://mcp.example/resource-metadata"`,
+	})
+	if err != nil || !found {
+		t.Fatalf("parseResourceMetadataURL() found=%v err=%v", found, err)
+	}
+	if got != "https://mcp.example/resource-metadata" {
+		t.Fatalf("resource metadata URL = %q", got)
+	}
+
+	got, found, err = parseResourceMetadataURL([]string{
+		`Basic realm="unterminated`,
+		`Bearer resource_metadata="https://mcp.example/fallback-metadata"`,
+	})
+	if err != nil || !found || got != "https://mcp.example/fallback-metadata" {
+		t.Fatalf("unrelated malformed challenge blocked discovery: got=%q found=%v err=%v", got, found, err)
+	}
+
+	for name, headers := range map[string][]string{
+		"unquoted":     {`Bearer resource_metadata=https://mcp.example/metadata`},
+		"unterminated": {`Bearer resource_metadata="https://mcp.example/metadata`},
+		"conflicting": {
+			`Bearer resource_metadata="https://mcp.example/one"`,
+			`DPoP resource_metadata="https://mcp.example/two"`,
+		},
+	} {
+		t.Run(name, func(t *testing.T) {
+			if _, _, err := parseResourceMetadataURL(headers); err == nil {
+				t.Fatal("expected malformed resource_metadata rejection")
+			}
+		})
+	}
+}
+
+func TestProtectedResourceDiscoveryRejectsInvalidMetadata(t *testing.T) {
+	for _, test := range []struct {
+		name      string
+		challenge func(base string) string
+		metadata  func(base string) string
+		wantError string
+	}{
+		{
+			name:      "malformed challenge",
+			challenge: func(string) string { return `Bearer resource_metadata="unterminated` },
+			wantError: "WWW-Authenticate",
+		},
+		{
+			name:      "insecure metadata URL",
+			challenge: func(string) string { return `Bearer resource_metadata="http://metadata.example/resource"` },
+			wantError: "metadata URL",
+		},
+		{
+			name:      "invalid JSON",
+			challenge: func(base string) string { return `Bearer resource_metadata="` + base + `/metadata"` },
+			metadata:  func(string) string { return `{` },
+			wantError: "decode protected resource metadata",
+		},
+		{
+			name:      "resource mismatch",
+			challenge: func(base string) string { return `Bearer resource_metadata="` + base + `/metadata"` },
+			metadata: func(base string) string {
+				return `{"resource":"` + base + `/other","authorization_servers":["https://auth.example"]}`
+			},
+			wantError: "does not match",
+		},
+		{
+			name:      "resource leading whitespace mismatch",
+			challenge: func(base string) string { return `Bearer resource_metadata="` + base + `/metadata"` },
+			metadata: func(base string) string {
+				return `{"resource":" ` + base + `/mcp","authorization_servers":["https://auth.example"]}`
+			},
+			wantError: "does not match",
+		},
+		{
+			name:      "resource trailing whitespace mismatch",
+			challenge: func(base string) string { return `Bearer resource_metadata="` + base + `/metadata"` },
+			metadata: func(base string) string {
+				return `{"resource":"` + base + `/mcp ","authorization_servers":["https://auth.example"]}`
+			},
+			wantError: "does not match",
+		},
+		{
+			name:      "authorization servers missing",
+			challenge: func(base string) string { return `Bearer resource_metadata="` + base + `/metadata"` },
+			metadata: func(base string) string {
+				return `{"resource":"` + base + `/mcp"}`
+			},
+			wantError: "authorization_servers",
+		},
+		{
+			name:      "insecure authorization server",
+			challenge: func(base string) string { return `Bearer resource_metadata="` + base + `/metadata"` },
+			metadata: func(base string) string {
+				return `{"resource":"` + base + `/mcp","authorization_servers":["http://auth.example"]}`
+			},
+			wantError: "authorization server",
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				base := "http://" + r.Host
+				switch r.URL.Path {
+				case "/mcp":
+					w.Header().Set("WWW-Authenticate", test.challenge(base))
+					w.WriteHeader(http.StatusUnauthorized)
+				case "/metadata":
+					if test.metadata == nil {
+						http.NotFound(w, r)
+						return
+					}
+					_, _ = w.Write([]byte(test.metadata(base)))
+				default:
+					http.NotFound(w, r)
+				}
+			}))
+			defer server.Close()
+
+			_, _, err := discoverProtectedResourceAuthorizationServer(context.Background(), server.Client(), server.URL+"/mcp")
+			if err == nil || !strings.Contains(err.Error(), test.wantError) {
+				t.Fatalf("error = %v, want %q", err, test.wantError)
+			}
+		})
+	}
+}
+
+func TestProtectedResourceProbeSendsNoCredentials(t *testing.T) {
+	var authorization, cookie string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		authorization = r.Header.Get("Authorization")
+		cookie = r.Header.Get("Cookie")
+		w.WriteHeader(http.StatusNotFound)
+	}))
+	defer server.Close()
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		t.Fatalf("create cookie jar: %v", err)
+	}
+	serverURL, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parse server URL: %v", err)
+	}
+	jar.SetCookies(serverURL, []*http.Cookie{{Name: "session", Value: "secret"}})
+	client := server.Client()
+	client.Jar = jar
+
+	_, found, err := discoverProtectedResourceAuthorizationServer(context.Background(), client, server.URL+"/mcp")
+	if err != nil || found {
+		t.Fatalf("discovery found=%v err=%v", found, err)
+	}
+	if authorization != "" || cookie != "" {
+		t.Fatalf("probe sent credentials: authorization=%q cookie=%q", authorization, cookie)
+	}
+}
+
+func TestDirectAuthorizationServerDiscoverySendsNoCookies(t *testing.T) {
+	var cookie string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		cookie = r.Header.Get("Cookie")
+		base := "http://" + r.Host
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"issuer":                 base,
+			"authorization_endpoint": base + "/authorize",
+			"token_endpoint":         base + "/token",
+		})
+	}))
+	defer server.Close()
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		t.Fatalf("create cookie jar: %v", err)
+	}
+	serverURL, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parse server URL: %v", err)
+	}
+	jar.SetCookies(serverURL, []*http.Cookie{{Name: "session", Value: "secret"}})
+	client := server.Client()
+	client.Jar = jar
+
+	if _, err := resolveAuthorizationServer(context.Background(), client, server.URL, OAuthConfig{}); err != nil {
+		t.Fatalf("resolveAuthorizationServer() error = %v", err)
+	}
+	if cookie != "" {
+		t.Fatalf("direct discovery sent cookie %q", cookie)
+	}
+}
+
+func TestProtectedResourceDiscoveryRejectsAdvertisedLoopbackMetadata(t *testing.T) {
+	var metadataHits atomic.Int64
+	client := &http.Client{Transport: roundTripperFunc(func(request *http.Request) (*http.Response, error) {
+		response := &http.Response{
+			StatusCode: http.StatusOK,
+			Header:     make(http.Header),
+			Body:       io.NopCloser(strings.NewReader("")),
+			Request:    request,
+		}
+		switch request.URL.String() {
+		case "https://mcp.example/mcp":
+			response.StatusCode = http.StatusUnauthorized
+			response.Header.Set("WWW-Authenticate", `Bearer resource_metadata="http://127.0.0.1/metadata"`)
+		case "http://127.0.0.1/metadata":
+			metadataHits.Add(1)
+			response.Body = io.NopCloser(strings.NewReader(`{"resource":"https://mcp.example/mcp","authorization_servers":["https://auth.example"]}`))
+		default:
+			response.StatusCode = http.StatusNotFound
+		}
+		return response, nil
+	})}
+
+	_, _, err := discoverProtectedResourceAuthorizationServer(context.Background(), client, "https://mcp.example/mcp")
+	if err == nil {
+		t.Fatal("advertised loopback metadata URL should be rejected")
+	}
+	if metadataHits.Load() != 0 {
+		t.Fatalf("loopback metadata target was fetched %d time(s)", metadataHits.Load())
+	}
+}
+
+func TestProtectedResourceDiscoveryRejectsAdvertisedLoopbackIssuer(t *testing.T) {
+	var issuerHits atomic.Int64
+	client := &http.Client{Transport: roundTripperFunc(func(request *http.Request) (*http.Response, error) {
+		response := &http.Response{
+			StatusCode: http.StatusOK,
+			Header:     make(http.Header),
+			Body:       io.NopCloser(strings.NewReader("")),
+			Request:    request,
+		}
+		switch request.URL.String() {
+		case "https://mcp.example/mcp":
+			response.StatusCode = http.StatusUnauthorized
+			response.Header.Set("WWW-Authenticate", `Bearer resource_metadata="https://mcp.example/metadata"`)
+		case "https://mcp.example/metadata":
+			response.Body = io.NopCloser(strings.NewReader(`{"resource":"https://mcp.example/mcp","authorization_servers":["http://127.0.0.1"]}`))
+		case "http://127.0.0.1/.well-known/oauth-authorization-server":
+			issuerHits.Add(1)
+			response.Body = io.NopCloser(strings.NewReader(`{"issuer":"http://127.0.0.1","authorization_endpoint":"http://127.0.0.1/authorize","token_endpoint":"http://127.0.0.1/token"}`))
+		default:
+			response.StatusCode = http.StatusNotFound
+		}
+		return response, nil
+	})}
+
+	_, err := resolveAuthorizationServer(context.Background(), client, "https://mcp.example/mcp", OAuthConfig{})
+	if err == nil {
+		t.Fatal("advertised loopback issuer should be rejected")
+	}
+	if issuerHits.Load() != 0 {
+		t.Fatalf("loopback issuer target was fetched %d time(s)", issuerHits.Load())
+	}
+}
+
+func TestProtectedResourceDiscoveryRejectsAdvertisedPrivateTargets(t *testing.T) {
+	for _, target := range []string{
+		"http://10.0.0.8/metadata",
+		"https://169.254.169.254/metadata",
+		"https://metadata.google.internal/metadata",
+	} {
+		t.Run(target, func(t *testing.T) {
+			if err := validateAdvertisedOAuthDiscoveryURL("https://mcp.example/mcp", target); err == nil {
+				t.Fatalf("advertised target %q should be rejected", target)
+			}
+		})
+	}
+}
+
+func TestProtectedDiscoveryPathsSendNoCookies(t *testing.T) {
+	var probeCookie, resourceCookie, issuerCookie string
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			probeCookie = r.Header.Get("Cookie")
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/metadata":
+			resourceCookie = r.Header.Get("Cookie")
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              base + "/mcp",
+				"authorization_servers": []string{base},
+			})
+		case "/.well-known/oauth-authorization-server":
+			issuerCookie = r.Header.Get("Cookie")
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"issuer":                 base,
+				"authorization_endpoint": base + "/authorize",
+				"token_endpoint":         base + "/token",
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+	jar, err := cookiejar.New(nil)
+	if err != nil {
+		t.Fatalf("create cookie jar: %v", err)
+	}
+	serverURL, err := url.Parse(server.URL)
+	if err != nil {
+		t.Fatalf("parse server URL: %v", err)
+	}
+	jar.SetCookies(serverURL, []*http.Cookie{{Name: "session", Value: "secret"}})
+	client := server.Client()
+	client.Jar = jar
+
+	if _, err := resolveAuthorizationServer(context.Background(), client, server.URL+"/mcp", OAuthConfig{}); err != nil {
+		t.Fatalf("resolveAuthorizationServer() error = %v", err)
+	}
+	if probeCookie != "" || resourceCookie != "" || issuerCookie != "" {
+		t.Fatalf("discovery sent cookies: probe=%q resource=%q issuer=%q", probeCookie, resourceCookie, issuerCookie)
+	}
+}
+
+func TestProtectedDiscoveryPreservesExplicitEndpointOverride(t *testing.T) {
+	fixture := newPublicProtectedOAuthFixture(t, "", "https://93.184.216.35/token", "", "")
+	explicitToken := "http://127.0.0.1:19432/token"
+	metadata, err := resolveAuthorizationServer(context.Background(), fixture.client, fixture.resourceURL, OAuthConfig{TokenEndpoint: explicitToken})
+	if err != nil {
+		t.Fatalf("resolve with explicit endpoint override: %v", err)
+	}
+	if metadata.TokenEndpoint != explicitToken || metadata.protectTokenEndpoint {
+		t.Fatalf("explicit token override lost provenance: endpoint=%q protected=%v", metadata.TokenEndpoint, metadata.protectTokenEndpoint)
+	}
+}
+
+func TestProtectedDiscoveryRejectsLoopbackRegistrationEndpointBeforeUse(t *testing.T) {
+	var registrationHits atomic.Int64
+	registration := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		registrationHits.Add(1)
+		_ = json.NewEncoder(w).Encode(map[string]any{"client_id": "registered-client"})
+	}))
+	defer registration.Close()
+	fixture := newPublicProtectedOAuthFixture(t, registration.URL+"/register", "https://93.184.216.35/token", "", "")
+
+	_, err := Login(context.Background(), LoginOptions{
+		ServerName: "registration-ssrf",
+		ServerURL:  fixture.resourceURL,
+		HTTPClient: fixture.client,
+		OpenBrowser: func(string) error {
+			return errors.New("authorization browser should not open")
+		},
+		Timeout: 5 * time.Second,
+	})
+	if err == nil {
+		t.Fatal("protected discovery should reject a loopback registration endpoint")
+	}
+	if registrationHits.Load() != 0 {
+		t.Fatalf("loopback registration endpoint received %d request(s)", registrationHits.Load())
+	}
+}
+
+func TestProtectedDiscoveryRejectsLoopbackTokenEndpointBeforeUse(t *testing.T) {
+	var tokenHits atomic.Int64
+	tokenServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		tokenHits.Add(1)
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"access_token": "unexpected",
+			"token_type":   "Bearer",
+		})
+	}))
+	defer tokenServer.Close()
+	fixture := newPublicProtectedOAuthFixture(t, "", tokenServer.URL+"/token", "", "")
+
+	_, err := Login(context.Background(), LoginOptions{
+		ServerName: "token-ssrf",
+		ServerURL:  fixture.resourceURL,
+		Config:     OAuthConfig{ClientID: "configured-client"},
+		HTTPClient: fixture.client,
+		OpenBrowser: func(authURL string) error {
+			parsed, parseErr := url.Parse(authURL)
+			if parseErr != nil {
+				return parseErr
+			}
+			callbackURL := parsed.Query().Get("redirect_uri") + "?code=code&state=" + url.QueryEscape(parsed.Query().Get("state"))
+			go func() {
+				for attempt := 0; attempt < 20; attempt++ {
+					response, requestErr := http.Get(callbackURL)
+					if requestErr == nil {
+						response.Body.Close()
+						return
+					}
+					time.Sleep(25 * time.Millisecond)
+				}
+			}()
+			return nil
+		},
+		Timeout: 5 * time.Second,
+	})
+	if err == nil {
+		t.Fatal("protected discovery should reject a loopback token endpoint")
+	}
+	if tokenHits.Load() != 0 {
+		t.Fatalf("loopback token endpoint received %d request(s)", tokenHits.Load())
+	}
+}
+
+func TestProtectedDiscoveryRegistrationRefusesPublicToPrivateRedirect(t *testing.T) {
+	var victimHits atomic.Int64
+	victim := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		victimHits.Add(1)
+		_ = json.NewEncoder(w).Encode(map[string]any{"client_id": "redirected-client"})
+	}))
+	defer victim.Close()
+	fixture := newPublicProtectedOAuthFixture(t, "", "https://93.184.216.35/token", victim.URL+"/register", "")
+
+	_, err := Login(context.Background(), LoginOptions{
+		ServerName: "registration-redirect",
+		ServerURL:  fixture.resourceURL,
+		HTTPClient: fixture.client,
+		OpenBrowser: func(string) error {
+			return errors.New("authorization browser should not open")
+		},
+		Timeout: 5 * time.Second,
+	})
+	if err == nil {
+		t.Fatal("protected registration redirect should be refused")
+	}
+	if victimHits.Load() != 0 {
+		t.Fatalf("registration redirect reached private victim %d time(s)", victimHits.Load())
+	}
+}
+
+func TestProtectedDiscoveryTokenRefusesPublicToPrivateRedirect(t *testing.T) {
+	var victimHits atomic.Int64
+	victim := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		victimHits.Add(1)
+		_ = json.NewEncoder(w).Encode(map[string]any{"access_token": "redirected-token", "token_type": "Bearer"})
+	}))
+	defer victim.Close()
+	fixture := newPublicProtectedOAuthFixture(t, "", "", "", victim.URL+"/token")
+
+	_, err := Login(context.Background(), LoginOptions{
+		ServerName: "token-redirect",
+		ServerURL:  fixture.resourceURL,
+		Config:     OAuthConfig{ClientID: "configured-client"},
+		HTTPClient: fixture.client,
+		OpenBrowser: func(authURL string) error {
+			parsed, parseErr := url.Parse(authURL)
+			if parseErr != nil {
+				return parseErr
+			}
+			callbackURL := parsed.Query().Get("redirect_uri") + "?code=code&state=" + url.QueryEscape(parsed.Query().Get("state"))
+			go func() {
+				for attempt := 0; attempt < 20; attempt++ {
+					response, requestErr := http.Get(callbackURL)
+					if requestErr == nil {
+						response.Body.Close()
+						return
+					}
+					time.Sleep(25 * time.Millisecond)
+				}
+			}()
+			return nil
+		},
+		Timeout: 5 * time.Second,
+	})
+	if err == nil {
+		t.Fatal("protected token redirect should be refused")
+	}
+	if victimHits.Load() != 0 {
+		t.Fatalf("token redirect reached private victim %d time(s)", victimHits.Load())
+	}
+}
+
+type publicProtectedOAuthFixture struct {
+	client      *http.Client
+	resourceURL string
+}
+
+func newPublicProtectedOAuthFixture(t *testing.T, registrationEndpoint string, tokenEndpoint string, registrationRedirect string, tokenRedirect string) publicProtectedOAuthFixture {
+	t.Helper()
+	var resourceOrigin, issuerOrigin string
+	gateway := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		resourceHost := strings.TrimPrefix(resourceOrigin, "https://")
+		issuerHost := strings.TrimPrefix(issuerOrigin, "https://")
+		switch {
+		case r.Host == resourceHost && r.URL.Path == "/mcp":
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+resourceOrigin+`/metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case r.Host == resourceHost && r.URL.Path == "/metadata":
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              resourceOrigin + "/mcp",
+				"authorization_servers": []string{issuerOrigin},
+			})
+		case r.Host == issuerHost && r.URL.Path == "/.well-known/oauth-authorization-server":
+			metadataRegistration := registrationEndpoint
+			metadataToken := tokenEndpoint
+			if registrationRedirect != "" {
+				metadataRegistration = issuerOrigin + "/register"
+			}
+			if tokenRedirect != "" {
+				metadataToken = issuerOrigin + "/token"
+			}
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"issuer":                 issuerOrigin,
+				"authorization_endpoint": issuerOrigin + "/authorize",
+				"token_endpoint":         metadataToken,
+				"registration_endpoint":  metadataRegistration,
+			})
+		case r.Host == issuerHost && r.URL.Path == "/register" && registrationRedirect != "":
+			http.Redirect(w, r, registrationRedirect, http.StatusTemporaryRedirect)
+		case r.Host == issuerHost && r.URL.Path == "/token" && tokenRedirect != "":
+			http.Redirect(w, r, tokenRedirect, http.StatusTemporaryRedirect)
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	t.Cleanup(gateway.Close)
+	port := gateway.Listener.Addr().(*net.TCPAddr).Port
+	resourceOrigin = fmt.Sprintf("https://93.184.216.34:%d", port)
+	issuerOrigin = fmt.Sprintf("https://93.184.216.35:%d", port)
+
+	transport := gateway.Client().Transport.(*http.Transport).Clone()
+	transport.TLSClientConfig = transport.TLSClientConfig.Clone()
+	transport.TLSClientConfig.InsecureSkipVerify = true //nolint:gosec // hermetic public-address routing fixture
+	dialer := &net.Dialer{}
+	gatewayAddress := gateway.Listener.Addr().String()
+	transport.DialContext = func(ctx context.Context, network string, address string) (net.Conn, error) {
+		host, _, err := net.SplitHostPort(address)
+		if err == nil && (host == "93.184.216.34" || host == "93.184.216.35") {
+			return dialer.DialContext(ctx, network, gatewayAddress)
+		}
+		return dialer.DialContext(ctx, network, address)
+	}
+	return publicProtectedOAuthFixture{
+		client:      &http.Client{Transport: transport},
+		resourceURL: resourceOrigin + "/mcp",
+	}
+}
+
+func TestProtectedResourceProbeDoesNotFollowRedirects(t *testing.T) {
+	var redirectedHits atomic.Int64
+	redirected := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		redirectedHits.Add(1)
+		w.WriteHeader(http.StatusUnauthorized)
+	}))
+	defer redirected.Close()
+	source := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, redirected.URL+"/mcp", http.StatusFound)
+	}))
+	defer source.Close()
+
+	_, found, err := discoverProtectedResourceAuthorizationServer(context.Background(), source.Client(), source.URL+"/mcp")
+	if err != nil || found {
+		t.Fatalf("redirected probe found=%v err=%v", found, err)
+	}
+	if redirectedHits.Load() != 0 {
+		t.Fatalf("probe followed redirect %d time(s)", redirectedHits.Load())
+	}
+}
+
+func TestProtectedResourceMetadataDoesNotFollowRedirects(t *testing.T) {
+	var redirectedHits atomic.Int64
+	redirected := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		redirectedHits.Add(1)
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"resource":              "https://attacker.example/mcp",
+			"authorization_servers": []string{"https://attacker.example"},
+		})
+	}))
+	defer redirected.Close()
+	source := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/metadata":
+			http.Redirect(w, r, redirected.URL+"/metadata", http.StatusFound)
+		}
+	}))
+	defer source.Close()
+
+	_, _, err := discoverProtectedResourceAuthorizationServer(context.Background(), source.Client(), source.URL+"/mcp")
+	if err == nil || !strings.Contains(err.Error(), "HTTP 302") {
+		t.Fatalf("error = %v, want redirect refusal", err)
+	}
+	if redirectedHits.Load() != 0 {
+		t.Fatalf("metadata fetch followed redirect %d time(s)", redirectedHits.Load())
+	}
+}
+
+func TestResolveAuthorizationServerRejectsProtectedIssuerMismatch(t *testing.T) {
+	authorizationServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"issuer":                 "https://unexpected.example",
+			"authorization_endpoint": base + "/authorize",
+			"token_endpoint":         base + "/token",
+		})
+	}))
+	defer authorizationServer.Close()
+	resourceServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/metadata":
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              base + "/mcp",
+				"authorization_servers": []string{authorizationServer.URL},
+			})
+		}
+	}))
+	defer resourceServer.Close()
+
+	_, err := resolveAuthorizationServer(context.Background(), resourceServer.Client(), resourceServer.URL+"/mcp", OAuthConfig{})
+	if err == nil || !strings.Contains(err.Error(), "issuer does not match") {
+		t.Fatalf("error = %v, want issuer mismatch", err)
+	}
+}
+
+func TestResolveAuthorizationServerRejectsMissingProtectedIssuer(t *testing.T) {
+	authorizationServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"authorization_endpoint": base + "/authorize",
+			"token_endpoint":         base + "/token",
+		})
+	}))
+	defer authorizationServer.Close()
+	resourceServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/metadata":
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              base + "/mcp",
+				"authorization_servers": []string{authorizationServer.URL},
+			})
+		}
+	}))
+	defer resourceServer.Close()
+
+	_, err := resolveAuthorizationServer(context.Background(), resourceServer.Client(), resourceServer.URL+"/mcp", OAuthConfig{})
+	if err == nil || !strings.Contains(err.Error(), "issuer") {
+		t.Fatalf("error = %v, want missing issuer rejection", err)
+	}
+}
+
+func TestResolveAuthorizationServerRejectsWhitespaceAlteredProtectedIssuer(t *testing.T) {
+	for _, alter := range []struct {
+		name  string
+		apply func(string) string
+	}{
+		{name: "leading", apply: func(value string) string { return " " + value }},
+		{name: "trailing", apply: func(value string) string { return value + " " }},
+	} {
+		t.Run(alter.name, func(t *testing.T) {
+			authorizationServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				base := "http://" + r.Host
+				_ = json.NewEncoder(w).Encode(map[string]any{
+					"issuer":                 alter.apply(base),
+					"authorization_endpoint": base + "/authorize",
+					"token_endpoint":         base + "/token",
+				})
+			}))
+			defer authorizationServer.Close()
+			resourceServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				base := "http://" + r.Host
+				switch r.URL.Path {
+				case "/mcp":
+					w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/metadata"`)
+					w.WriteHeader(http.StatusUnauthorized)
+				case "/metadata":
+					_ = json.NewEncoder(w).Encode(map[string]any{
+						"resource":              base + "/mcp",
+						"authorization_servers": []string{authorizationServer.URL},
+					})
+				}
+			}))
+			defer resourceServer.Close()
+
+			_, err := resolveAuthorizationServer(context.Background(), resourceServer.Client(), resourceServer.URL+"/mcp", OAuthConfig{})
+			if err == nil || !strings.Contains(err.Error(), "issuer does not match") {
+				t.Fatalf("error = %v, want whitespace issuer mismatch", err)
+			}
+		})
+	}
+}
+
+func TestAuthorizationServerMetadataDoesNotFollowRedirects(t *testing.T) {
+	var redirectedHits atomic.Int64
+	redirected := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		redirectedHits.Add(1)
+		base := "http://" + r.Host
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"issuer":                 base,
+			"authorization_endpoint": base + "/authorize",
+			"token_endpoint":         base + "/token",
+		})
+	}))
+	defer redirected.Close()
+	authorizationServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, redirected.URL+"/.well-known/oauth-authorization-server", http.StatusFound)
+	}))
+	defer authorizationServer.Close()
+	resourceServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/metadata":
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              base + "/mcp",
+				"authorization_servers": []string{authorizationServer.URL},
+			})
+		}
+	}))
+	defer resourceServer.Close()
+
+	_, err := resolveAuthorizationServer(context.Background(), resourceServer.Client(), resourceServer.URL+"/mcp", OAuthConfig{})
+	if err == nil {
+		t.Fatal("authorization metadata redirect should fail")
+	}
+	if redirectedHits.Load() != 0 {
+		t.Fatalf("authorization metadata fetch followed redirect %d time(s)", redirectedHits.Load())
+	}
+}

internal/mcp/oauth_test.go

@@ -10,6 +10,7 @@ import (
 	"net/http/httptest"
 	"net/url"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -145,6 +146,130 @@ func TestResolveEndpointsConfigOverridesDiscovery(t *testing.T) {
 	}
 }
 
+func TestResolveAuthorizationServerUsesProtectedResourceMetadata(t *testing.T) {
+	var authorizationMetadataHits atomic.Int64
+	authorizationServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/.well-known/oauth-authorization-server" {
+			http.NotFound(w, r)
+			return
+		}
+		authorizationMetadataHits.Add(1)
+		base := "http://" + r.Host
+		_ = json.NewEncoder(w).Encode(map[string]any{
+			"issuer":                 base,
+			"authorization_endpoint": base + "/authorize",
+			"token_endpoint":         base + "/token",
+			"registration_endpoint":  base + "/register",
+		})
+	}))
+	defer authorizationServer.Close()
+
+	var resourceProbeHits, resourceMetadataHits atomic.Int64
+	resourceServer := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/mcp":
+			resourceProbeHits.Add(1)
+			metadataURL := "http://" + r.Host + "/resource-metadata"
+			w.Header().Set("WWW-Authenticate", `Bearer realm="OAuth", resource_metadata="`+metadataURL+`"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/resource-metadata":
+			resourceMetadataHits.Add(1)
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              "http://" + r.Host + "/mcp",
+				"authorization_servers": []string{authorizationServer.URL},
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer resourceServer.Close()
+
+	metadata, err := resolveAuthorizationServer(context.Background(), resourceServer.Client(), resourceServer.URL+"/mcp", OAuthConfig{})
+	if err != nil {
+		t.Fatalf("resolveAuthorizationServer() error = %v", err)
+	}
+	if metadata.Issuer != authorizationServer.URL {
+		t.Fatalf("issuer = %q, want %q", metadata.Issuer, authorizationServer.URL)
+	}
+	if metadata.AuthorizationEndpoint != authorizationServer.URL+"/authorize" || metadata.TokenEndpoint != authorizationServer.URL+"/token" {
+		t.Fatalf("resolved metadata = %#v", metadata)
+	}
+	if resourceProbeHits.Load() != 1 || resourceMetadataHits.Load() != 1 || authorizationMetadataHits.Load() != 1 {
+		t.Fatalf("discovery hits = probe:%d resource:%d authorization:%d", resourceProbeHits.Load(), resourceMetadataHits.Load(), authorizationMetadataHits.Load())
+	}
+}
+
+func TestResolveAuthorizationServerFallsBackToDirectDiscovery(t *testing.T) {
+	var resourceProbeHits, directDiscoveryHits atomic.Int64
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		switch r.URL.Path {
+		case "/mcp":
+			resourceProbeHits.Add(1)
+			w.Header().Set("Content-Type", "text/event-stream")
+			w.WriteHeader(http.StatusOK)
+			if flusher, ok := w.(http.Flusher); ok {
+				flusher.Flush()
+			}
+			<-r.Context().Done()
+		case "/.well-known/oauth-authorization-server/mcp":
+			directDiscoveryHits.Add(1)
+			base := "http://" + r.Host
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"issuer":                 base + "/mcp",
+				"authorization_endpoint": base + "/authorize",
+				"token_endpoint":         base + "/token",
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+
+	metadata, err := resolveAuthorizationServer(context.Background(), server.Client(), server.URL+"/mcp", OAuthConfig{})
+	if err != nil {
+		t.Fatalf("resolveAuthorizationServer() error = %v", err)
+	}
+	if metadata.AuthorizationEndpoint != server.URL+"/authorize" || metadata.TokenEndpoint != server.URL+"/token" {
+		t.Fatalf("resolved metadata = %#v", metadata)
+	}
+	if resourceProbeHits.Load() != 1 || directDiscoveryHits.Load() != 1 {
+		t.Fatalf("discovery hits = probe:%d direct:%d", resourceProbeHits.Load(), directDiscoveryHits.Load())
+	}
+}
+
+func TestResolveAuthorizationServerIssuerOverrideSkipsResourceProbe(t *testing.T) {
+	var resourceProbeHits, issuerDiscoveryHits atomic.Int64
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			resourceProbeHits.Add(1)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/.well-known/oauth-authorization-server/issuer":
+			issuerDiscoveryHits.Add(1)
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"issuer":                 base + "/issuer",
+				"authorization_endpoint": base + "/authorize",
+				"token_endpoint":         base + "/token",
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+
+	metadata, err := resolveAuthorizationServer(context.Background(), server.Client(), server.URL+"/mcp", OAuthConfig{IssuerURL: server.URL + "/issuer"})
+	if err != nil {
+		t.Fatalf("resolveAuthorizationServer() error = %v", err)
+	}
+	if metadata.AuthorizationEndpoint != server.URL+"/authorize" || metadata.TokenEndpoint != server.URL+"/token" {
+		t.Fatalf("resolved metadata = %#v", metadata)
+	}
+	if resourceProbeHits.Load() != 0 || issuerDiscoveryHits.Load() != 1 {
+		t.Fatalf("discovery hits = resource:%d issuer:%d", resourceProbeHits.Load(), issuerDiscoveryHits.Load())
+	}
+}
+
 func TestPKCEChallengeIsS256OfVerifier(t *testing.T) {
 	pkce, err := newPKCE()
 	if err != nil {
@@ -448,3 +573,98 @@ func TestFullFlowStoresTokens(t *testing.T) {
 		t.Fatalf("login token = %#v", result)
 	}
 }
+
+func TestFullFlowUsesProtectedResourceDiscovery(t *testing.T) {
+	var resourceProbeHits, resourceMetadataHits, authorizationMetadataHits, tokenHits atomic.Int64
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		base := "http://" + r.Host
+		switch r.URL.Path {
+		case "/mcp":
+			resourceProbeHits.Add(1)
+			if r.Header.Get("Authorization") != "" || r.Header.Get("Cookie") != "" {
+				t.Error("resource probe sent credentials")
+			}
+			w.Header().Set("WWW-Authenticate", `Bearer resource_metadata="`+base+`/resource-metadata"`)
+			w.WriteHeader(http.StatusUnauthorized)
+		case "/resource-metadata":
+			resourceMetadataHits.Add(1)
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"resource":              base + "/mcp",
+				"authorization_servers": []string{base},
+			})
+		case "/.well-known/oauth-authorization-server":
+			authorizationMetadataHits.Add(1)
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"issuer":                 base,
+				"authorization_endpoint": base + "/authorize",
+				"token_endpoint":         base + "/token",
+			})
+		case "/token":
+			tokenHits.Add(1)
+			if err := r.ParseForm(); err != nil {
+				t.Errorf("parse token form: %v", err)
+			}
+			if r.Form.Get("code") != "authorization-code-secret" || r.Form.Get("code_verifier") == "" {
+				t.Error("token exchange omitted the authorization code or PKCE verifier")
+			}
+			_ = json.NewEncoder(w).Encode(map[string]any{
+				"access_token":  "access-token-secret",
+				"refresh_token": "refresh-token-secret",
+				"token_type":    "Bearer",
+				"expires_in":    3600,
+			})
+		default:
+			http.NotFound(w, r)
+		}
+	}))
+	defer server.Close()
+
+	opener := func(authURL string) error {
+		parsed, err := url.Parse(authURL)
+		if err != nil {
+			return err
+		}
+		if parsed.Path != "/authorize" || parsed.Query().Get("code_challenge_method") != "S256" || parsed.Query().Get("code_challenge") == "" {
+			return errors.New("authorization request omitted the discovered endpoint or PKCE challenge")
+		}
+		if parsed.Query().Get("code") != "" || parsed.Query().Get("code_verifier") != "" {
+			return errors.New("authorization request exposed an authorization code or PKCE verifier")
+		}
+		state := parsed.Query().Get("state")
+		redirect := parsed.Query().Get("redirect_uri")
+		go func() {
+			callbackURL := redirect + "?code=authorization-code-secret&state=" + url.QueryEscape(state)
+			for attempt := 0; attempt < 20; attempt++ {
+				response, requestErr := http.Get(callbackURL)
+				if requestErr == nil {
+					response.Body.Close()
+					return
+				}
+				time.Sleep(50 * time.Millisecond)
+			}
+		}()
+		return nil
+	}
+
+	token, err := Login(context.Background(), LoginOptions{
+		ServerName: "notion-style",
+		ServerURL:  server.URL + "/mcp",
+		Config: OAuthConfig{
+			ClientID: "client-123",
+			Scopes:   []string{"read"},
+		},
+		HTTPClient:  server.Client(),
+		OpenBrowser: opener,
+		Timeout:     5 * time.Second,
+		Now:         time.Now,
+	})
+	if err != nil {
+		t.Fatalf("Login() error = %v", err)
+	}
+	if token.AccessToken != "access-token-secret" || token.RefreshToken != "refresh-token-secret" {
+		t.Fatal("login did not return the issued tokens")
+	}
+	if resourceProbeHits.Load() != 1 || resourceMetadataHits.Load() != 1 || authorizationMetadataHits.Load() != 1 || tokenHits.Load() != 1 {
+		t.Fatalf("flow hits = probe:%d resource:%d authorization:%d token:%d", resourceProbeHits.Load(), resourceMetadataHits.Load(), authorizationMetadataHits.Load(), tokenHits.Load())
+	}
+}