Bug 1735746 - Add a pref and limit external protocol sandbox blocking to Nightly. r=ckerschb,farre

Depends on D141131

Differential Revision: https://phabricator.services.mozilla.com/D141132

modules/libpref/init/StaticPrefList.yaml

@@ -1916,6 +1916,12 @@
   value: true
   mirror: always
 
+# Block sandboxed BrowsingContexts from navigating to external protocols.
+- name: dom.block_external_protocol_navigation_from_sandbox
+  type: bool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+
 # Block Insecure downloads from Secure Origins
 - name: dom.block_download_insecure
   type: bool

uriloader/exthandler/nsExternalHelperAppService.cpp

@@ -16,6 +16,7 @@
 #include "mozilla/dom/CanonicalBrowsingContext.h"
 #include "mozilla/dom/WindowGlobalParent.h"
 #include "mozilla/RandomNum.h"
+#include "mozilla/StaticPrefs_dom.h"
 #include "mozilla/StaticPrefs_security.h"
 #include "mozilla/StaticPtr.h"
 #include "nsXULAppAPI.h"
@@ -1099,6 +1100,10 @@ nsresult nsExternalHelperAppService::EscapeURI(nsIURI* aURI, nsIURI** aResult) {
 bool ExternalProtocolIsBlockedBySandbox(
     BrowsingContext* aBrowsingContext,
     const bool aHasValidUserGestureActivation) {
+  if (!StaticPrefs::dom_block_external_protocol_navigation_from_sandbox()) {
+    return false;
+  }
+
   if (!aBrowsingContext || aBrowsingContext->IsTop()) {
     return false;
   }