The problem lies in the way Firefox handles Web links that are overly long and contain dashes, security researcher Tom Ferris said in an interview via instant messaging last Thursday.
He posted an advisory and a proof of concept to the Full Disclosure security mailing list and to his Security Protocols Web site.
The security vulnerability is a buffer overflow flaw that "allows for an attacker to remotely execute arbitrary code" on a vulnerable PC, Ferris said. An attacker could host a Web site containing the malicious code to exploit the flaw, he said. Though his proof of concept only crashes Firefox, Ferris claims he has been able to tweak it to run code.
Buffer overflows are a commonly exploited security problem. They occur when a program allows data to be written beyond the allocated end of a buffer in memory. A computer can be made to execute potentially malicious code by feeding in extra data that is designed to flood the buffer.
Ferris reported the bug to the Mozilla Foundation on Sunday, intending to go through the organisation's bug-reporting process, he said. However, in an example of the uneasy alliance between security researchers and software makers, he decided to publicly disclose the flaw after a run-in with Mozilla staff, he said.
Mozilla, which coordinates the development of Firefox and distributes the software, on Friday confirmed the bug but said the scope of the flaw is still under investigation. The organisation said it received the bug report on Tuesday, not Sunday.
"We believe there is a buffer overflow issue," said Mike Schroepfer, director of engineering at Mozilla. "We are still determining whether it is exploitable in an attack."
Users are currently not at risk because there are no known attacks that take advantage of the flaw, Schroepfer said. Mozilla is working on a fix that will be released with an upcoming version of Firefox, he said.
Mozilla is unhappy with the disclosure of the flaw. "We'd like to make sure that by the time something goes public, we have a solution for the users," Schroepfer said.
Since the debut of Firefox 1.0 in November, usage of the open-source browser has grown. Security has been a main selling point for Firefox over Microsoft's Internet Explorer, which has begun to see its market share dip slightly -- for the first time in years.
However, Firefox has had its own security woes. Several serious holes in the browser have been plugged since its official release, and experts have said that safe Web browsers don't exist.
The public bug disclosure comes just as Mozilla released the first beta of Firefox 1.5. The final release of the next Firefox update, which includes security enhancements, is due by year's end, according to the Firefox road map.
Ferris has found bugs in Microsoft software before, including a yet-unpatched flaw in Internet Explorer that Microsoft still has under investigation.
Earlier this month Microsoft credited Ferris with reporting a bug in a Windows feature called Remote Desktop Protocol that could allow an attacker to remotely restart Windows systems.
Both of these have already been patched. Given that this issue affect Mozilla too, new packages of it are available as well.