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Warning: The contents of this page are not accurate in reference to Greasemonkey 4.0.
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This page is a description of [[Greasemonkey]]'s security model. For tips you can apply directly to your scripts, see '''[[Security tips]]'''.

== Overview ==

[[Version history|Historically]], [[Greasemonkey]] would inject a [[user script]] into a page by creating a <code><script></code> tag with the [[user script]] contents inline, and appending it to the content page's DOM.

Mark Pilgrim originally [https://web.archive.org/web/20150602052709/http://mozdev.org/pipermail/greasemonkey/2005-July/004022.html described a security flaw] with this design, on July 19th 2005, while [[Greasemonkey]] was at [[version]] 0.3.4.
Essentially, the issue was that Greasemonkey scripts are given special permissions that the rest of the JavaScript running on the web page is not. For example, Greasemonkey scripts contained their own GM.xmlHttpRequest object which, unlike a normal XMLHttpRequest object, could access any local files one one's computer or make arbitrary requests to arbitrary sites without regard for the same origin policy that typically applies to XMLHttpRequest.

Unfortunately, because Greasemonkey scripts were injected directly into the page using a script tag, these objects with special permissions could be called by a script sent by the website.
In other words, if you ran a Greasemonkey script on a site, the site's own JavaScript could access all the files on your computer!

[[Greasemonkey]] [[version]] 0.3.5 was immediately released, with all [[API reference|GM_* functions]] disabled, to plug the security hole.

To fix the security flaw, [[XPCNativeWrapper]]s, a new feature of the then-in-development Firefox 1.5, were used to isolate privileged [[user script]] code from insecure content pages.
Certain other changes were made, including restrictions on the [[GM_xmlhttpRequest]] method, to disallow access to local files.

== unsafeWindow ==

Wrapping the [[user script]] environment this way creates a [[sandbox]].
This sandbox introduces many side effects and limitations.
To allow maximum flexibility for [[user script]] authors, the <code>[[unsafeWindow]]</code> property was added in to the sandbox.

The <code>window</code> object functions as the global scope in JavaScript.
For [[user script]]s, this global window option is in fact a "deep wrapper" of the content window.
The content window can be accessed by [[user script]]s, but only indirectly through the wrapper.
The <code>unsafeWindow</code> property is a direct line to the actual content window.

Use of the <code>unsafeWindow</code> property should be avoided whenever possible.
Its use has the potential to open up all the original security holes that introducing the [[XPCNativeWrapper]]s fixed.
When a [[user script]] relies on the <code>unsafeWindow</code> property, it should be included only on trusted pages, and even then is not guaranteed to be safe.

== See Also ==

* [[Greasemonkey access violation]]
* [[Greasemonkey Manual:Environment]]
* [http://it.slashdot.org/article.pl?sid=05/07/19/143241 Slashdot: Firefox Greasemonkey Extension Security Problem]

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[[Category:Security|*]]