Mozilla came out with an advisory yesterday warning users of compromised files in the Vietnamese language pack for Firefox 2. This was not the work of a malicious hacker or intentional booby-trapping of the files by the author but the result of a careless internal virus infection.

The author of the add-on was accidently infected and every help file (*.xhtml) in the Vietnamese language pack for Firefox was modified by the virus and appended with a script. Any user who installed this language pack would have malicious ads displayed in their browser and could have potentially being infected with other exploits.

The script linked to hxxp://js.k0102.com/[Removed].asp (currently offline) - a remote website based in China. The offending script in the compromised help pages have since been removed by the Mozilla developers.

According to Mozilla’s blog, anyone who downloaded the most recent Vietnamese language pack for Firefox 2 since February 18, 2008 would have potentially got an infected copy. The exact number of compromised downloads cannot be ascertained, but since this affected only users who downloaded the Vietnamese language pack, the numbers could be limited.

When contacted, the Mozilla developers were quick to respond and provided us a copy of the compromised files.

McAfee users are pro-actively detected against this threat. The malicious HTML pages are already detected as the W32/Fujacks!htm virus with the 5174 DAT files that were released way back in 29th November 2007.