Mozilla has warned that the Vietnamese language pack of Firefox 2 was compromised as a result of a viral infection.
The language pack did not contain a virus itself, but code that resulted in users seeing unwanted ads. More malicious action might have been possible as a result of the security flaw, the probable result of a developer's machine getting infected rather than a deliberate backdoor.
Everyone who downloaded the most recent Vietnamese language pack since February 18 this year got an infected copy, Mozilla warns. In total there's been 16,667 total downloads of the Vietnamese language pack since November 2007, so the total number of affected users is probably in the low hundreds rather than the thousands.
Mozilla said it does virus scans at upload time, but the virus scanner did not catch this issue until several months after the upload. It said it would revamp its testing procedure following the incident, the first of its kind to affect browser software.
A new Vietnamese language pack will be available shortly. In the meantime, Vietnamese language pack users are advised to disable this package using the add-ons dialogue on the Tools menu.