Two experimental add-ons, Version 4.0 of Sothink Web Video Downloader and all versions of Master Filer were found to contain Trojan code aimed at Windows users, Mozilla announced today.

Version 4.0 of Sothink Web Video Downloader contained Win32.LdPinch.gen, and Master Filer contained Win32.Bifrose.32.Bifrose Trojan. Both add-ons have been disabled on AMO.

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. Uninstalling these add-ons does not remove the trojan from a user?s system. Users with either of these add-ons should uninstall them immediately, Mozilla says. Since uninstalling these extensions does not remove the trojan from a user?s system, an antivirus program should be used to scan and remove any infections.

This vulnerability is known to affect Firefox on Windows only, if either Master Filer or Version 4.0 of Sothink Web Video Downloader are installed. Versions of Sothink Web Video Downloader greater than 4.0 are not infected. Master Filer was downloaded approximately 600 times between September 2009 and January 2010, according to Mozilla. Version 4.0 of Sothink Web Video Downloader was downloaded approximately 4,000 times between February 2008 and May 2008. Master Filer was removed from AMO on January 25, 2010 and Version 4.0 of Sothink Web Video Downloader was removed from AMO on February 2, 2010.

AMO performs a malware check on all add-ons uploaded to the Mozilla's Add-on site, and blocks add-ons that are detected as such. This scanning tool failed to detect the Trojan in Master Filer.