+      // Only allow some "about:" pages to have access to contentaccessible

+      // "chrome://branding/" assets. Otherwise web pages could easily and

+      // consistently detect the differences between channels when their

+      // branding differs. See tor-browser#43308 and tor-browser#42319.

+      // NOTE: The same assets under the alternative URI

+      // "resource:///chrome/browser/content/branding/" should already be

+      // inaccessible to web content, so we only add a condition for the chrome

+      // path.

+      if (targetScheme.EqualsLiteral("chrome")) {

+        nsAutoCString targetHost;

+        rv = aTargetBaseURI->GetHost(targetHost);

+        NS_ENSURE_SUCCESS(rv, rv);

+        if (targetHost.EqualsLiteral("branding")) {

+          // Disallow any Principal whose scheme is not "about", or is a

+          // contentaccessible "about" URI ("about:blank" or "about:srcdoc").

+          // NOTE: "about:blank" and "about:srcdoc" would be unexpected here

+          // since such a document spawned by a web document should inherit the

+          // same Principal URI. I.e. they would be "http:" or "https:" schemes.

+          // But we add this condition for extra assurances.

+          // NOTE: Documents with null Principals, like "about:blank" typed by

+          // the user, would also be excluded since the Principal URI would be

+          // "moz-nullprincipal:".

+          if (!aSourceBaseURI->SchemeIs("about") ||

+              NS_IsContentAccessibleAboutURI(aSourceBaseURI)) {

+            return NS_ERROR_DOM_BAD_URI;

+          }

+          // Also exclude "about:reader" from accessing branding assets. I.e. if

+          // a web page includes `<img src="chrome://branding/..." />` we do not

+          // want it to render within "about:reader" either.

+          // Though it is unknown whether the information within "about:reader"

+          // would be exploitable by a web page, we also want to exclude

+          // "about:reader" for consistency: if it does not display in the

+          // original web page, it should not display in "about:reader" either.

+          nsAutoCString sourcePath;

+          rv = aSourceBaseURI->GetFilePath(sourcePath);

+          NS_ENSURE_SUCCESS(rv, rv);

+          if (sourcePath.EqualsLiteral("reader")) {

+            return NS_ERROR_DOM_BAD_URI;

+          }

+        }

+      }

+

+    // Do not allow web pages to link to about:logo, which varies between

+    // channels. See tor-browser#43308.

+    // Moreover, it exposes firefox-specific branding.