JITter Bug

Bugs are a fact of life in the technology world, and the Open Source community is no exception. What is exceptional, however, is the open way these vulnerabilities are handled, as the developers behind Mozilla's Firefox browser have aptly demonstrated.

Two weeks ago, Mozilla was celebrating the triumphant release of the much-delayed Firefox 3.5. The browser brings its users a pantheon of new features, with perhaps the most celebrated being the TraceMonkey JavaScript engine, said to provide speed enhancements twice as fast as Firefox 3.0 and up to ten times that of Firefox 2.0.

One element of the acclaimed performance booster is giving its developers something of a headache this week, however. The first zero-day exploit for Firefox 3.5 was revealed publicly on Monday, in the form of a vulnerability in the browser's Just-in-time compiler. Unlike older methods of execution, which interpret the bytecode created from the browser's source code, a Just-in-time compiler transforms the bytecode into native machine code just before executing it, resulting in significant performance improvements. Attackers can utilize the vulnerability to execute malicious code on the user's system by luring them to a website containing the exploit code.

A patch for the exploit has yet to be released, though Firefox developers are on the case. Mozilla has indicated that once developers have prepared and tested the patch, it will be pushed out to users via the normal update channels. Linux users may wish to make special note of the update — because it was released so recently, users are likely to have installed Firefox 3.5 manually rather than via their distribution's repositories, and thus may not receive updates in the manner they are accustomed to.

Pending a final patch, Mozilla is recommending that users disable the JIT through the about:config dialog in order to circumvent the exploit. After entering about:config in the browser's address bar and clicking through the "This may void your warranty" screen, locate the javascript.options.jit.content setting via the filter box and set its value to false.

Developers stress that this is only a temporary fix, and as it will result in significantly decreased browser performance, should be returned to its original setting as soon as the patch is installed. Users uncomfortable with altering about:config settings can achieve the same result by running the browser in Safe Mode, though this will result in additional components being disabled.