Mozilla has fixed a critical bug in the way the Firefox browser works with QuickTime media files.

The flaw, which was reported last week by hacker Petko Petkov, gives attackers a way to run unauthorized commands on a victim's PC. "This could be used to install malware, steal local data, or otherwise corrupt the victim's computer," Mozilla said in a security advisory published Tuesday.

A July 2007 patch was supposed to take care of this type of problem, but Petkov showed how attackers could still run commands on a victim's system by tricking a victim into opening a maliciously coded QuickTime media file.

In fact, until Apple addresses the underlying flaw in QuickTime, there still could be headaches for users, Mozilla said in its security advisory on the issue. "QuickTime Media-link files could still be used to annoy users with popup windows and dialogs until this issue is fixed in QuickTime," the advisory states.

The common security measure of disabling JavaScript does not prevent this attack, although the NoScript Firefox add-on does provide protection, Mozilla said.

"Petkov provided proof of concept code that may be easily converted into an exploit, so users should consider this a very serious issue," Mozilla's security chief, Window Snyder said in a recent blog posting. Mozilla has been able to reproduce this bug only on the Windows operating system, she added.

The flaw also affects the Internet Explorer browser, Petkov said on his blog. However, IE's security policies make the flaw less critical on Microsoft's browser, he added.

The Firefox 2.0.0.7 update was pushed out to users starting around 3 p.m. Pacific Time on Tuesday. It contains only one security update: the QuickTime fix.