diff --git a/addon-sdk/source/lib/sdk/content/sandbox.js b/addon-sdk/source/lib/sdk/content/sandbox.js
index e8c9a1533c6..7fb88838848 100644
--- a/addon-sdk/source/lib/sdk/content/sandbox.js
+++ b/addon-sdk/source/lib/sdk/content/sandbox.js
@@ -38,6 +38,12 @@ const metadata = require('@loader/options').metadata;
 const permissions = (metadata && metadata['permissions']) || {};
 const EXPANDED_PRINCIPALS = permissions['cross-domain-content'] || [];
 
+const waiveSecurityMembrane = !!permissions['unsafe-content-script'];
+
+const nsIScriptSecurityManager = Ci.nsIScriptSecurityManager;
+const secMan = Cc["@mozilla.org/scriptsecuritymanager;1"].
+  getService(Ci.nsIScriptSecurityManager);
+
 const JS_VERSION = '1.8';
 
 const WorkerSandbox = Class({
@@ -96,8 +102,10 @@ const WorkerSandbox = Class({
     this.emit = this.emit.bind(this);
     this.emitSync = this.emitSync.bind(this);
 
-    // Eventually use expanded principal sandbox feature, if some are given.
-    //
+    // Use expanded principal for content-script if the content is a
+    // regular web content for better isolation.
+    // (This behavior can be turned off for now with the unsafe-content-script
+    // flag to give addon developers time for making the necessary changes)
     // But prevent it when the Worker isn't used for a content script but for
     // injecting `addon` object into a Panel, Widget, ... scope.
     // That's because:
@@ -110,12 +118,17 @@ const WorkerSandbox = Class({
     // domain principal.
     let principals = window;
     let wantGlobalProperties = [];
-    if (EXPANDED_PRINCIPALS.length > 0 && !requiresAddonGlobal(worker)) {
-      principals = EXPANDED_PRINCIPALS.concat(window);
-      // We have to replace XHR constructor of the content document
-      // with a custom cross origin one, automagically added by platform code:
-      delete proto.XMLHttpRequest;
-      wantGlobalProperties.push('XMLHttpRequest');
+    let isSystemPrincipal = secMan.isSystemPrincipal(
+      window.document.nodePrincipal);
+    if (!isSystemPrincipal && !requiresAddonGlobal(worker)) {
+      if (EXPANDED_PRINCIPALS.length > 0) {
+        // We have to replace XHR constructor of the content document
+        // with a custom cross origin one, automagically added by platform code:
+        delete proto.XMLHttpRequest;
+        wantGlobalProperties.push('XMLHttpRequest');
+      }
+      if (!waiveSecurityMembrane)
+        principals = EXPANDED_PRINCIPALS.concat(window);
     }
 
     // Instantiate trusted code in another Sandbox in order to prevent content
@@ -129,6 +142,7 @@ const WorkerSandbox = Class({
       sandboxPrototype: proto,
       wantXrays: true,
       wantGlobalProperties: wantGlobalProperties,
+      wantExportHelpers: !waiveSecurityMembrane,
       sameZoneAs: window,
       metadata: {
         SDKContentScript: true,
diff --git a/addon-sdk/source/test/addons/unsafe-content-script/main.js b/addon-sdk/source/test/addons/unsafe-content-script/main.js
new file mode 100644
index 00000000000..81a2359af4b
--- /dev/null
+++ b/addon-sdk/source/test/addons/unsafe-content-script/main.js
@@ -0,0 +1,61 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+const { create: makeFrame } = require("sdk/frame/utils");
+const { window } = require("sdk/addon/window");
+const { Loader } = require('sdk/test/loader');
+const loader = Loader(module);
+const Worker = loader.require("sdk/content/worker").Worker;
+
+exports.testMembranelessMode = function(assert, done) {
+
+  let url = "data:text/html;charset=utf-8," + encodeURIComponent(
+    '<script>' +
+    'function runTest() {' +
+    '  assert(fuu.bar == 42, "Content-script objects should be accessible to content with' +
+    '         the unsafe-content-script flag on.");' +
+    '}' +
+    '</script>'
+  );
+
+  let element = makeFrame(window.document, {
+    nodeName: "iframe",
+    type: "content",
+    allowJavascript: true,
+    allowPlugins: true,
+    allowAuth: true,
+    uri: url
+  });
+
+  element.addEventListener("DOMContentLoaded", onDOMReady, false);
+
+  function onDOMReady() {
+    let worker = Worker({
+      window: element.contentWindow,
+      contentScript:
+        'new ' + function () {
+          var assert = function assert(v, msg) {
+            self.port.emit("assert", { assertion: v, msg: msg });
+          }
+          var done = function done() {
+            self.port.emit("done");
+          }
+          window.wrappedJSObject.fuu = { bar: 42 };
+          window.wrappedJSObject.assert = assert;
+          window.wrappedJSObject.runTest();
+          done();
+        }
+    });
+    worker.port.on("done", function () {
+      element.parentNode.removeChild(element);
+      done();
+    });
+    worker.port.on("assert", function (data) {
+      assert.ok(data.assertion, data.msg);
+    });
+  }
+};
+
+require("sdk/test/runner").runTestsFromModule(module);
diff --git a/addon-sdk/source/test/addons/unsafe-content-script/package.json b/addon-sdk/source/test/addons/unsafe-content-script/package.json
new file mode 100644
index 00000000000..72c5b512f81
--- /dev/null
+++ b/addon-sdk/source/test/addons/unsafe-content-script/package.json
@@ -0,0 +1,6 @@
+{
+  "id": "content-permissions",
+  "permissions": {
+    "unsafe-content-script": true
+  }
+}
diff --git a/addon-sdk/source/test/test-content-script.js b/addon-sdk/source/test/test-content-script.js
index 38acfc1ca16..7bfa3123c91 100644
--- a/addon-sdk/source/test/test-content-script.js
+++ b/addon-sdk/source/test/test-content-script.js
@@ -842,4 +842,30 @@ exports["test MutationObvserver"] = createProxyTest(html, function (helper) {
 
 });
 
+let html = '<script>' +
+  'var accessCheck = function() {' +
+  '  assert(true, "exporting function works");' +
+  '  try{' +
+  '    exportedObj.prop;' +
+  '    assert(false, "content should not have access to content-script");' +
+  '  } catch(e) {' +
+  '    assert(e.toString().indexOf("Permission denied") != -1,' +
+  '           "content should not have access to content-script");' +
+  '  }' +
+  '}</script>';
+exports["test nsEp for content-script"] = createProxyTest(html, function (helper) {
+
+  helper.createWorker(
+    'let glob = this; new ' + function ContentScriptScope() {
+
+      exportFunction(assert, unsafeWindow, { defineAs: "assert" });
+      window.wrappedJSObject.assert(true, "assert exported");
+      window.wrappedJSObject.exportedObj = { prop: 42 };
+      window.wrappedJSObject.accessCheck();
+      done();
+    }
+  );
+
+});
+
 require("test").run(exports);