Difference between revisions of "Location hack"

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The '''location hack''' is an ugly but useful way to interact with the content scope of the page being [[user script]]ed. It does this by indirectly [http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Global_Functions:eval evaling] strings within that scope.
The location hack is an older technique for crossing the barrier between the user script scope and the content window scope, when it exists.
The previous version of this page may be read [http://wiki.greasespot.net/index.php?title=Location_hack&oldid=7202 via history].


== Background ==
In Firefox version 39.0.3 Mozilla patched a known [https://www.mozilla.org/en-US/security/advisories/mfsa2015-78/ security vulnerability].
As a side effect, this broke the location hack.


For [[security]] reasons, [[Greasemonkey]] uses [[XPCNativeWrapper]]s and a [[sandbox]] to isolate user scripts from the web page.
Thankfully in modern versions of Firefox and Greasemonkey, the location hack is no longer necessary.
Under this system, the user script can access and manipulate the page using event listeners, the DOM API, and [[API reference|GM_* functions]].
Read about how to execute code in the content scope, like previously possible with the location hack, at [[Content Script Injection]].
 
Sometimes the sandbox is too limiting, in which case the user script can access other parts of the page using [[unsafeWindow]].
As the name ''unsafe''Window implies, this can often be unsafe, and expose security holes.
 
In December 2005, Jesse Ruderman [http://www.squarefree.com/2005/12/01/updated-greasemonkey-scripts/ 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 <code>pageFunc</code>, or <code>window.pageFunc</code>.
The user script knows this function as <code>unsafeWindow.pageFunc</code>.
 
The user script could simply call <code>unsafeWindow.pageFunc()</code>, but this can leak the sandbox.
Instead, the user script can take advantage of <code>javascript:</code> 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|bookmarklet]] into a user script.
 
It's important to add the <code>javascript:</code> scheme to the front, to turn it into a URL, and the <code>;void(0)</code> 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.replace("javascript:void(window.someVariable = 'someValue')");
 
'''Note:''' The reason why you use location.replace and not location.href is so that your script does not change the url history and break the back button on the browser.
 
== 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.replace("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.
 
== Double Slash Comment issue ==
 
As of Firefox 17.0, you can no longer use // (double slash) comments within the location hack. For example,
 
location.replace("javascript:(" + function() {
  // foo
} + ")()");
 
The above code will cause a syntax error. The workaround is to use /* */ (slash star or multiline) comments instead:
 
location.replace("javascript:(" + function() {
  /* foo */
} + ")()");
 
== Percent encoding issue ==
 
Sometimes percent-encoding the percent symbol is required. For example,
 
location.replace("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.replace("javascript:(" + encodeURI(
  function() {
  var n = 44;
  if(!(n%22)) alert('n is a multiple of 22');
  }) + ")()");
 
See also [http://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference:Global_Functions:encodeURI encodeURI()].
 
== Returning values ==
 
The location hack is really handy for passing values to the content scope, or to call functions defined there.
It is not, however, capable of directly reading a variable or value returned from a function.
Furthermore, it is run asynchronously, much like <code>setTimeout()</code>, so you cannot immediately rely on side effects.
(If you use the location hack to, for example, store a value in the DOM and then attempt to read it, it will only be available at some other later point in time.)
For reading javascript values from the content scope inside the sandbox, see [[Reading Content Globals]] for a reliable example.
 
== See Also ==
 
* [[Reading Content Globals]]
* [[Content Script Injection]]


[[Category:Coding Tips:Interacting With The Page]]
[[Category:Coding Tips:Interacting With The Page]]

Latest revision as of 15:54, 23 September 2015

The location hack is an older technique for crossing the barrier between the user script scope and the content window scope, when it exists. The previous version of this page may be read via history.

In Firefox version 39.0.3 Mozilla patched a known security vulnerability. As a side effect, this broke the location hack.

Thankfully in modern versions of Firefox and Greasemonkey, the location hack is no longer necessary. Read about how to execute code in the content scope, like previously possible with the location hack, at Content Script Injection.