This Mozilla blog reports that two "experimental" add-ons for Firefox contains Trojan code designed to hijack Windows-based PCs.
Mozilla said Thursday that the Win32.LdPinch.gen Trojan was found in v4.0 of the Sothink Web Video Downloader add-on, and the Win32.Bifrose.32.Bifrose Trojan was discovered in all versions of Master Filer. Mozilla said that both Trojans will infect the host computer once the user installs the add-ons and re-launches the browser.
"Uninstalling these add-ons does not remove the trojan from a user’s system," Mozilla said. "Users with either of these add-ons should uninstall them immediately. 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."
Mozilla said that Master Filer was downloaded from the Mozilla Add-Ons website (AMO) around 600 times between September 2009 and January 2010. Version 4.0 of Sothink Web Video Downloader was downloaded around 4,000 times between February 2008 and May 2008.
Both Firefox add-ons have now been removed from the website.
The developer admitted that its malware scanner failed when checking the Master Filer add-on during the upload process. That problem has now been fixed.
"Two additional malware detection tools have been added to the validation chain and all add-ons were rescanned, which revealed the additional Trojan in Version 4.0 of Sothink Web Video Downloader," the company said. "No other instances of malware have been discovered."
Internet Explorer was said to have giggled uncontrollably upon receipt of the news.