Mozilla Corp. yesterday warned users about a worm that slipped into Firefox's Vietnamese language add-on and went undetected for months.

The malware-infected file has been pulled from Mozilla's servers.

"The Vietnamese language pack for Firefox 2 contains inserted code to load remote content," Window Snyder, Mozilla's chief security executive, confirmed in a post to the company's blog on Wednesday. "Everyone who downloaded the most recent Vietnamese language pack since Feb. 18, 2008, got an infected copy."

According to Snyder, the download count for the add-on since last November has been 16,667. "So we anticipate the impact on users to be limited," she said.