Mozilla overlooked malware-laced Firefox add-ons

Feels like the first time. But it's not

Two Firefox add-ons available for months on Mozilla's website infected users with malware that stole passwords and opened a backdoor on Windows machines, the open-source browser maker has confirmed.

The add-ons, available on an experimental section of Mozilla's official add-on download site carried trojans that have been detected since 2008 by commercial anti-virus products. And yet they weren't removed until late January and earlier this week because a scanning tool used to vet add-ons during upload failed to catch the malicious files.

"If a user installs one of these infected add-ons, the trojan would be executed when Firefox starts and the host computer would be infected by the trojan," a note on Mozilla's add-on blog stated. "Uninstalling these add-ons does not remove the trojan from a user's system."

Instead, infected users will need to thoroughly scan their machines with an anti-virus program. Or better yet, use multiple scanners, or simply reinstall the operating system to be on the safe side.

This isn't the first time Mozilla has served malware-laced add-ons to its loyal base of users. In May 2008, a Vietnamese language pack for Firefox 2 contained a viral infection that resulted in users seeing unwanted ads. The add-on was downloaded almost 17,000 times before it was pulled.

In the most recent case, version 4 of the Sothink Web Video Downloader add-on installed a password sniffer dubbed Win32.LdPinch.gen and was downloaded about 4,000 times between February 2008 and May 2008. A separate add-on called Master Filer was laced with a backdoor trojan known as Win32.Bifrose that was downloaded 600 times between September 2009 and January of this year.

Mozilla removed Master Filer on January 25 and nixed Sothink on Tuesday.

The blog post said Mozilla added two new scanners to its validation chain. It was this change that allowed the organization to detect version 4 of the Sothink Web Video Downloader.

Versions greater the 4.0 of the video downloader add-on were not infected, Mozilla's blog post stated. Both infections affected only Windows users of the open-source browser. ®