Mozilla on Tuesday warned users that a password-stealing add-on slipped into Firefox's extension gallery more than a month ago had been downloaded nearly 2,000 times before it was detected.

The malicious "Mozilla Sniffer" add-on was yanked from Mozilla's servers Monday, and added to the Firefox "blocklist," a last-resort defense that uninstalls potentially-dangerous browser extensions from users' machines.

Mozilla also notified users of a critical security vulnerability in another add-on, the popular "CoolPreviews," which currently sits at No. 21 on the Firefox most-downloaded list, saying it had temporarily yanked that plug-in, too.

The Mozilla Sniffer add-on was submitted to the Firefox Add-ons site June 6, Mozilla announced in a blog post yesterday.

"It was discovered that this add-on contains code that intercepts login data submitted to any website, and sends this data to a remote location," Mozilla confirmed. "Anybody who has installed this add-on should change their passwords as soon as possible."

Mozilla pulled the Sniffer add-on July 12 after it found out about the plug-in's extracurricular activities, then added it to the blocklist. "All current [Mozilla Sniffer] users should receive an uninstall notification within a day or so," the company said. According to Mozilla's count, the malicious extension had been downloaded about 1,800 times in the last five weeks, and had 334 active users when it was dumped.

Mozilla Sniffer was isolated in the experimental portion of the Add-ons site, where new add-ons are kept until they undergo a public review process. To install experimental add-ons, Firefox users must view and accept an additional warning.

The situation with CoolPreviews was different. That add-on, which is downloaded about 77,000 times each week, contained a critical bug that could have been used by hackers to hijack computers.

"The vulnerability can be triggered using a specially crafted hyperlink," Mozilla explained. "If the user hovers the cursor over this link, the preview function executes remote JavaScript code with local chrome privileges, giving the attacking script control over the host computer."

CoolPreviews is billed by its maker, Cooliris, as a Firefox extension that displays previews of Web pages when users pause the mouse pointer over any link.

CoolPreviews 3.01 and earlier editions included the vulnerability; Mozilla disabled the buggy versions, then posted an update from Cooliris the following day. The revised CoolPreviews, version 3.1.0625, was first offered to Firefox users June 25.

Proof-of-concept attack code that exploits CoolPreviews 3.01 and earlier had been published on a Japanese-language blog, but Mozilla said it knew of no in-the-wild exploits.

As of Tuesday, approximately 177,000 users were running a vulnerable CoolPreviews add-on, said Mozilla, which said that it would add the bug-containing editions to the blocklist "very soon."

The Sniffer incident was not the first time Mozilla has missed a malicious add-on.

In May 2008, the company acknowledged that a worm that had gone unnoticed in Firefox's Vietnamese language add-on for months. In February 2010, it warned users that the Sothink Web Video Downloader 4.0 and all versions of Master Filer were infected with a Trojan horse that could spread to the host PC.

Mozilla admitted that under its current practice, add-ons are only scanned for known malware, but said it is trying to come up with a plan to review add-on source code to catch the future Sniffers of the world. "We're already working on implementing a new security model for addons.mozilla.org that will require all add-ons to be code-reviewed before they are discoverable in the site," the company said.

More information about how Mozilla plans to review add-on code can be found in a proposal that was first brought up in September 2009, but has not yet been implemented.

Mozilla said it would kick off code review as part of revamp of its Developer Tools before the end of this quarter.

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer or subscribe to Gregg's RSS feed . His e-mail address is gkeizer@ix.netcom.com.