Mozilla claimed that first Firefox 1.5 security vulnerability was not as critical as initially perceived, but a patch will be available to fix it early next year.

Mike Schroepfer, vice president of engineering for Mozilla said that the company planned to repair the hole in the latest version of thebrowser when it releases its next regularly scheduled stability build of Firefox in late January or early February.

"Based on the information we have, it's a low-severity issue, but we will address it anyway," he said.

Packet Storm Security has released code that takes advantage of the vulnerability, which can cause a buffer overflow and put users at risk of a denial of service attack when running the browser, according to a posting by independent security consultant John Bambenek on the SANS Internet Storm Center website.

The vulnerability is in the browser's history.dat file, which stores a user's history of sites visited. It can be exploited by crafting an abnormally long URL with perhaps a few million characters. If a user navigates to a site that exploits the hole, "it will crash the browser each time it is started after going to such a page," Bambenek wrote in the posting.

Security research company Secunia Thursday gave the bug a rating of "not critical" on its website.

Schroepfer said that a team of volunteers and Mozilla engineers could not discover a denial-of-service problem using the proof of concept code from Packetstorm. "We have no independent confirmation that it crashes, not for lack of trying," he said.

Schroepfer said that when engineers tried to recreate the problem, the browser worked sluggishly and took an exceptionally long time to load a web page, but only after the browser was closed and then restarted after the first security breach. However, the browser will not crash. "Eventually, it will process and will start properly," he said.

Furthermore, Schroepfer said it would be extremely rare for someone to visit a site that exploits the hole during a typical browsing session. "You'd have to browse to a malicious site, one that someone spent the time to create to cause people harm," he said. "Then you'd have to browse there, and the next time your browser would take longer to start."

Even if a user does encounter a malicious website that exploits the hole, clearing out the browser's history will remedy the problem, Schroepfer added.