The problem is caused by a buffer overflow in legacy Netscape code still included in the browser for animating GIF images, Chris Hofmann, director of engineering for Mozilla, said. Similar memory problems have affected Mozilla's browsers and Microsoft's Internet Explorer in the past. A malicious attacker could exploit them by creating carefully crafted image files that, when viewed by a victim in a browser, execute a program and compromise the system.
The flaw was discovered by Internet Security Systems, a network protection company, and patched before the public learned of the issue, Hofmann said.
"We are staying ahead and being proactive in fixing the code," he said. "The deciding factor, in this case, was the potential for this: It's a little easier for hackers to turn it into an exploit that could be dangerous."
The Mozilla Foundation released version 1.02 of Firefox on Wednesday to fix the problem and asked that all users to download and apply the patch.
Recently published data has prompted questions about the security of Firefox. Security technology provider Symantec said in this week's Internet Threat Report that during the second half of last year, 21 vulnerabilities affected Mozilla browsers and 13 flaws affected Internet Explorer.
However, only seven of the flaws in Firefox were considered "highly severe," compared with nine in Internet Explorer.
Mozilla's Hofmann pointed to the data as a positive indication that the developers were doing a good job of securing the Firefox code.
"As the data shows, the flaws are of lesser severity," he said. "The kinds of things the Microsoft's browser is vulnerable to is much more worrisome."
On Tuesday, Mozilla president Mitchell Baker predicted that Firefox won't suffer nearly as many security flaws as Internet Explorer and that the increasing popularity of the open-source browser won't change that.
"Microsoft has a proven track record with Internet Explorer," Microsoft said in statement. "We continue to make significant investments in Internet Explorer, including Windows XP Service Pack 2, which features a much stronger security infrastructure to help thwart malware attacks, block suspicious content and eliminate many common spoofing attempts. In addition, Internet Explorer 7 will be a major upgrade that will focus on security."
Mozilla is currently reviewing the roughly 2 million lines of code that makes up the Firefox browser to find similar vulnerabilities to those patched Wednesday. Last August, the organization offered a bounty to anyone who finds significant flaws in the software. The developers are looking with particular intensity at the legacy code that remains in the browser.
"Most of the things that we are looking at and fixing are potential exploits that no one has figured out how to exploit yet," Hofmann said.