The patch protects against exploitation of a serious flaw by disabling the browser feature that contains the vulnerability.
The downloadable fix protects against attacks that take advantage of a new, unpatched flaw that could let attackers secretly run malicious software on users' PCs. The flaw was disclosed late Thursday by security researcher Tom Ferris, sending Mozilla staff into damage-control mode.
The problem has to do with the way the Firefox and Mozilla browsers handle International Domain Names, or IDNs, said Mike Schroepfer, director of engineering at Mozilla. IDNs are domain names that use local language characters. The fix disables support for such Web addresses, he said.
"This is a temporary work-around just to deal with the immediate issue," Schroepfer said. "We're working on a future release in which we will actually fix the problem and re-enable the IDN feature." Switching off IDN support impacts a subset of Firefox and Mozilla users who actually use such special domain names, he said.
Though there is no known attack that takes advantage of the flaw, Mozilla advises Firefox and Mozilla users to disable IDN. "Luckily we do not have any known use of this exploit, but it is fairly critical if there were to be (an attack), so this is a recommended download," Schroepfer said.
Mozilla expects to fix the vulnerability in beta 2 of Firefox 1.5, the next release of the open-source Web browser. Beta 2 is due Oct. 5 and the final release of 1.5 is expected by year's end, Schroepfer said.
In addition to the downloadable fix, Mozilla on its Web site also offers instructions to manually disable IDN: Type "about:config" in the address bar, hit Enter; type "network.enableIDN" in the filter toolbar, hit Enter; right-click the "network.enableIDN" item and select Toggle to change value to false.
IDNs have caused trouble for Mozilla in the past. A Firefox security update in February fixed a flaw that would allow domain spoofing using the special domain names. A spoofed link would seem to be a legitimate address, but instead of taking the victim to the trusted site, the link would lead to a phony Web site.
Though vulnerabilities in Microsoft's Internet Explorer have been the focus of much of the concern, other browsers also have had their fair share of flaws. Security has been a main selling point for Firefox over IE, 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.