UnsafeWindow: Difference between revisions

From GreaseSpot Wiki
Jump to navigationJump to search
Line 133: Line 133:
Place this line at the very beginning of the script to inject the entire script into the page  
Place this line at the very beginning of the script to inject the entire script into the page  
using the location hack. Like Attach Method 1, this will give the script access to variables
using the location hack. Like Attach Method 1, this will give the script access to variables
on the page, but notaccess to Greasemonkey API methods.
on the page, but not access to Greasemonkey API methods.

Revision as of 23:25, 24 April 2009


This command can open certain security holes in your user script, and it is recommended to use this command sparingly.

Please be sure to read the entire article and understand it before using it in a script.

Greasemonkey Manual
Using Greasemonkey
Installing Scripts
Monkey Menu
Getting Help
User Script Authoring


This API object allows a User script to access "custom" properties--variable and functions defined in the page--set by the web page. The unsafeWindow object is shorthand for window.wrappedJSObject. It is the raw window object inside the XPCNativeWrapper provided by the Greasemonkey sandbox.


unsafeWindow bypasses Greasemonkey's XPCNativeWrapper-based security model, which exists to make sure that malicious web pages cannot alter objects in such a way as to make greasemonkey scripts (which execute with more privileges than ordinary Javascript running in a web page) do things that their authors or users did not intend. User scripts should therefore avoid calling or in any other way depending on any properties on unsafeWindow - especally if if they are executed for arbitrary web pages, such as those with @include *, where the page authors may have subverted the environment in this way.

User script authors are strongly encouraged to learn how XPCNativeWrappers work, and how to perform the desired function within their security context, instead of using unsafeWindow to break out.

Examples | Alternatives to unsafeWindow | Notes



Value: Object
Returns: Variant
Compatibility: Greasemonkey 0.5b+



unsafeWindow.SomeVarInPage = "Testing";
var oldFunction = unsafeWindow.SomeFunctionInPage;
unsafeWindow.SomeFunctionInPage = function(text) {
  alert("Hijacked! Argument was " + text + ".");
  return oldFunction(text);
For issues with GM_getValue, GM_setValue and GM_xmlhttpRequest, see see 0.7.20080121.0_compatibility.
  function() {
    var redirectURL = window.name;

var sGetter = document.createElement("script");
sGetter.type = "text/javascript";
sGetter.innerHTML = 
  "function uXHR(url) {" +
  "  var xhr = new XMLHttpRequest();" +
  "  xhr.onreadystatechange = function() { " +
  "    if (xhr.status == 301 || xhr.status == 302) {" +
  "      window.name = xhr.getResponseHeader('Location');" +
  "      document.title = document.title;" +
  "    }" +
  "  };" +
  "  xhr.open('HEAD', url, true);" +
  "  xhr.send(null);" +




Alternatives to unsafeWindow

Events | Functions defined in the page | Attach script to page


Event listeners never need to be created on unsafeWindow. Rather than using
unsafeWindow.onclick = function(event) { /* some code */ };


window.addEventListener("click", function(event) { /* some code */ }, false);
See also addEventListener at MDC

top | back

Functions defined in the page

If a user script must execute a page function, it can use the location hack to call it safely. This involves setting location.href to a javascript: URL, which is like using a bookmarklet. For example:
location.href = "javascript:void(pageFunc(123));";
Larger blocks of code independent of the Greasemonkey context/APIs can also be executed this way:
location.href = "javascript:(" + encodeURI(uneval(function() { /* some code */ })) + ")();";
This code will run in the page context without leaking the sandbox. This code is completely separate from the rest of the script scope, sometimes limiting its usefulness. For example, data cannot be returned by the function.
Another drawback is that this technique is rather ugly. Still, it is preferred over unsafeWindow.

top | back

Attach script to page

Attach Method 1

function myScript() {
  for (var x in document) {
    // some code with x
  // some code

// attach script to page; script can therefore reference variables on the page, but likewise
// cannot use Greasemonkey API methods

document.body.appendChild(document.createElement("script")).innerHTML="(" + myScript + ")()";

top | back

Attach Method 2

Place this line at the very beginning of the script to inject the entire script into the page using the location hack. Like Attach Method 1, this will give the script access to variables on the page, but not access to Greasemonkey API methods.


top | back

Attach Method 3

This way is interesting for those who want:

  • execute the init() function NOT in GM address space
  • inject multiple css hacks
  • inject multiple .js files (even from different domains)
/* Filename: hello-world.user.js */

  Define single/multiple script(s) to inject
      - Schemes of file:// type, absolute or implied, are not supported with this method due to
        browser security restrictions.
      - Schemes of http:// or https:// must be present

      - This method currently works only on Windows but NOT Linux NOR OS X
        See pitfall 1 below

var scripts = [

var script;
for (i in scripts) {
  script = document.createElement("script");
  script.src = scripts[i];
  script.type = "text/javascript";

  function(event) {
    location.href = "javascript:void(init());";
function inject_css() {
  document.title += ' dynamically modified version';
  GM_addStyle("body { color:white; background-color:black } img { border:0 }");
  Filename: http://localhost:8080/hello-injecting.js

function init() {
  alert('Hello, world!');

top | back

Attach Method 3 Pitfall 1
The method of hosting hello-injecting.js on a local and remote web server was tested on:
Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv: Gecko/2009011912 Firefox/3.0.6
Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv: Gecko/2009021906 Firefox/3.0.7
and returned Component failure code with the following error:
Error: Component returned failure code: 0x805e000a [nsIDOMLocation.href] = <unknown>
Source file: file://~/.mozilla/firefox/{randomseed}.default/extensions/{class-id}/components/greasemonkey.js
Line: 377
Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv: Gecko/2009032608 Firefox/3.0.8
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv: Gecko/2009032608 Firefox/3.0.8
and failed to execute init() with the following error:
Error: init is not defined
Source file: javascript:void(init());
Line: 1