A serious security flaw has been discovered in Greasemonkey, a widely used extension to the Mozilla Firefox browser.

The bug is the third annoyance affecting Firefox users in the space of only a few days, following a successful attack on the Spread Firefox marketing site, and a Firefox update that broke many third-party extensions.

Greasemonkey is an extension, or add-on, to Firefox that allows users to customise the sites they view using powerful scripting tools. The problem is that certain of Greasemonkey's functions are exposed in an insecure way, allowing them to be exploited by a malicious site.

Specifically, a site could read the contents of specific files on a user's system, or could list the contents of a user's hard drive, according to researchers. The bug affects any operating system running the extension. The bug is only exploitable if Greasemonkey is configured to run scripts on the malicious Web site, for example if the extension is set up to run on all sites the user browses.

"Running a Greasemonkey script on a site can expose the contents of every file on your local hard drive to that site," wrote Mark Pilgrim, who discovered the flaw, in an email to the Greasemonkey mailing list this week. If users have configured the extension to run their scripts on any site they visit, this "can expose the contents of every file on your local hard drive to every site you visit".

Greasemonkey's developer advised users to upgrade to the most recent version, 0.3.5, which fixes the problem by disabling the tool's more advanced features. The bug affects all previous versions, researchers said.

Firefox doesn't have an equivalent of ActiveX, the component of Microsoft Internet Explorer that allows Web sites to run powerful scripts on a user's system, which is often used in attacks and in spyware. Firefox extensions can be as powerful as the developer likes, but cannot automatically install themselves as can ActiveX controls.