Security

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

Mark Pilgrim originally described a security flaw with this design, on July 19th 2005, while Greasemonkey was at version 0.3.4. Greasemonkey version 0.3.5 was immediately released, with all GM_* functions disabled, to plug the security hole.

To fix the security flaw, XPCNativeWrappers, 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.