Location hack

The location hack is an ugly but useful way to interact with the content scope of the page being user scripted. It does this by indirectly evaling strings within that scope.

Background

For security reasons, Greasemonkey uses XPCNativeWrappers and sandbox to isolate it from the web page. Under this system, the user script can access and manipulate the page using event listeners, the DOM API, and GM_* functions.

Sometimes the sandbox is too limiting, in which case the user script can access other parts of the page using unsafeWindow. As the name unsafeWindow implies, this can often be unsafe, and expose security holes.

In December 2005, Jesse Ruderman came up with the location hack, to be an alternative to unsafeWindow in many cases.

Basic usage: page functions

Suppose the page contains a function called pageFunc, or window.pageFunc. The user script knows this function as unsafeWindow.pageFunc.

The user script could simply call unsafeWindow.pageFunc(), but this can leak the sandbox. Instead, the user script can take advantage of javascript: URLs, which always run in the content scope. Just entering this URL into the browser's location bar does not leak a GreaseMonkey sandbox:

javascript:pageFunc();void(0)

A user script can programmaticaly navigate to this URL, to safely call the function:

location.assign( "javascript:pageFunc();void(0)" );

That, in a nutshell, is the location hack! Essentially, it is wrapping a wikipedia:bookmarklet into a user script.

It's important to add the javascript: scheme to the front, to turn it into a URL, and the ;void(0) to the end, which keeps the browser from actually navigating to this URL after it is run.

Modifying the page

The location hack can do anything a page script or bookmarklet can do, so it can modify content variables and such as easily as it can access them. For example:

location.href = "javascript:void(window.someVariable = 'someValue')";

Executing large blocks of code

Executing more than one statement can become unreadable very easily. Luckily, JavaScript can convert functions to strings, so you can use:

location.href = "javascript:(" + function() {
  // do something
} + ")()";

Even though the function is defined in the sandbox, it is not a closure of the sandbox scope. It is converted to a string and then back to a function in page scope. It cannot access anything in the sandbox scope, which is a limitation, but is also essential to making this technique secure.

Percent encoding issue

Sometimes percent-encoding the percent symbol is required. For example,

location.href = ("javascript:(" + function() {
  var n = 44;
  if(!(n%22)) alert('n is a multiple of 22');
} + ")()");

The above code will cause error because %22 is interpreted as double quotation mark. The workaround is:

location.href = "javascript:(" + encodeURI(
 function() {
  var n = 44;
  if(!(n%22)) alert('n is a multiple of 22');
 }) + ")()";

See also encodeURI().

Returning values

Functions called through the location hack cannot return data directly to the user script scope. To communicate between location hack code and regular user script code, data must be placed where the user script can see it, for example, by writing it into the DOM, or by triggering an event. A simple example:

var oldBodyTitle = document.body.title;
location.href = "javascript:void(document.body.title = pageFunc())";
var fauxReturnValue = document.body.title;
document.body.title = oldBodyTitle;

This trick, however, can be defeated by the hosting page by simply overriding the document.body.title property with a setter.

document.body.__defineSetter__("title", function(t){} );

While this does not mean a threat to the user visiting the page, it effectively prevents the userscript from accessing the returned value of the global function. A working example is available here.

However, __defineSetter__ itself can be overridden using the delete operator. A modified version of the linked user script above, using delete to ignore the setter, is available.

Function to get values of global variables

The following function can be used to access the values of global variables using var value = GM_getGlobalElement('globalVariable');. Please note that the returned value is converted to a string.

// function to get values of global variables using the "location hack"
window.GM_getGlobalElement = null;
window.getGlobalValue = function(name) {
  if (GM_getGlobalElement == null) {
    GM_getGlobalElement = document.createElement("textarea");
    GM_getGlobalElement.id = "GM_getGlobalElement";
    GM_getGlobalElement.style.visibility = "hidden";
    GM_getGlobalElement.style.display = "none";
    document.body.appendChild(GM_getGlobalElement);
  }
  location.href = "javascript:void(typeof("+ name + ")!=\"undefined\"?document.getElementById(\"GM_getGlobalElement\").value=" + name + ":null)";
  return(GM_getGlobalElement.value);
}