--- a/testing/marionette/evaluate.js
+++ b/testing/marionette/evaluate.js
@@ -107,17 +107,20 @@ evaluate.sandbox = function(sb, script, 
     unloadHandler = () => reject(
         new JavaScriptError("Document was unloaded during execution"));
 
     // wrap in function
     if (!opts.directInject) {
       sb[CALLBACK] = sb[COMPLETE];
       sb[ARGUMENTS] = Cu.cloneInto(args, sb, {wrapReflectors: true});
 
-      script = `${ARGUMENTS}.push(${CALLBACK});` +
+      // callback function made private
+      // so that introspection is possible
+      // on the arguments object
+      script = `${ARGUMENTS}.push(rv => ${CALLBACK}(rv));` +
           `(function() { ${script} }).apply(null, ${ARGUMENTS})`;
 
       // marionetteScriptFinished is not WebDriver conformant,
       // hence it is only exposed to immutable sandboxes
       if (opts.sandboxName) {
         sb[MARIONETTE_SCRIPT_FINISHED] = sb[CALLBACK];
       }
     }

--- a/testing/marionette/harness/marionette/tests/unit/test_execute_async_script.py
+++ b/testing/marionette/harness/marionette/tests/unit/test_execute_async_script.py
@@ -106,16 +106,29 @@ marionetteScriptFinished(4);
 
         self.marionette.execute_async_script("global.barfoo = [arguments[0], arguments[1]];"
                                              "marionetteScriptFinished()",
                                              script_args=[42, 23], new_sandbox=False)
         self.assertEqual(self.marionette.execute_async_script(
             "marionetteScriptFinished(global.barfoo);", new_sandbox=False),
                          [42, 23])
 
+    # Functions defined in higher privilege scopes, such as the privileged
+    # content frame script listener.js runs in, cannot be accessed from
+    # content.  This tests that it is possible to introspect the objects on
+    # `arguments` without getting permission defined errors.  This is made
+    # possible because the last argument is always the callback/complete
+    # function.
+    #
+    # See bug 1290966.
+    def test_introspection_of_arguments(self):
+        self.marionette.execute_async_script(
+            "arguments[0].cheese; __webDriverCallback();",
+            script_args=[], sandbox=None)
+
 
 class TestExecuteAsyncChrome(TestExecuteAsyncContent):
     def setUp(self):
         super(TestExecuteAsyncChrome, self).setUp()
         self.marionette.set_context("chrome")
 
     def test_execute_async_unload(self):
         pass