Mozilla warned Wednesday that a malicious program inserted adware code into a Firefox plugin that has been downloaded thousands of times over the past three months.

Because of a virus infection, the Vietnamese language pack for Firefox 2 was polluted with adware, Mozilla security chief Window Snyder said in a blog posting. "Everyone who downloaded the most recent Vietnamese language pack since February 18, 2008 got an infected copy," she wrote. "Mozilla does virus scans at upload time but the virus scanner did not catch this issue until several months after the upload."

Mozilla is now going to add additional scans of its software to prevent this kind of thing from happening in the future.

The malware in the language pack is from the Xorer Trojan, according to discussion on Mozilla's Bugzilla developer Web site, which indicates that Mozilla developers first discovered the issue on Tuesday.

Mozilla missed the code during its initial scan because antivirus vendors had not yet added detection for Xorer into their products. Antivirus vendor Panda Security first detected Xorer on Feb. 28, 10 days after the infected plugin was published. Firefox developers have now scanned all of their plugins.

The open-source browser maker does not know how many people were infected with the adware, but the plugin was downloaded more than 1,200 times in the past week and has been downloaded 16,667 times since November.