Difference between revisions of "UnsafeWindow"

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Revision as of 14:26, 22 May 2007

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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.




User scripts can use this object to access "custom" properties--variable and functions defined in the page--set by the web page. This is done by bypassing Greasemonkey's XPCNativeWrapper-based security model. The unsafeWindow object is shorthand for window.wrappedJSObject; it is the raw window object inside the XPCNativeWrapper provided by the Greasemonkey sandbox.

Use of unsafeWindow is insecure, and it should be avoided whenever possible. User scripts absolutely should not use unsafeWindow if they are executed for arbitrary web pages, such as those with @include *. 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.


unsafeWindow.SomeVarInPage = "Testing";
var oldFunction = unsafeWindow.SomeFunctionInPage;
unsafeWindow.SomeFunctionInPage = function(text) {
  alert("Hijacked! Argument was " + text + ".");
  return oldFunction(text);

Alternatives to unsafeWindow


Event listeners never need to be created on unsafeWindow. Rather than using unsafeWindow.onclick = function(event) { ... }, use:

window.addEventListener('click', function(event) {
}, false);

addEventListener at MDC

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 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.