Firefox Javascript Vulnerability


Once again, Javascript is the source of a new exploit that has been recently discovered on Firefox1. The vulnerability can be exploited by crafting malicious Javascript code on a Firefox 3.5 browser and leads to the execution of arbitrary code on the user’s machine. This is due to a vulnerability in the JIT engine of Firefox and affects machine running a x86, SPARC or arm architectures.

The vulnerability resolves around the return value of the escape function in the JIT engine. It’s exploited using the <font> tag. The code for the exploit is public and can be found at milw0rm. The exploit use a heap spraying technique to execute the shellcode.

A fix should be available soon, but the best solution is always to disable Javascript, although a lot of sites rely on it to operate. Another way is to use the NoScript plug-in, which let you enable and disable scripts easily according to a whitelist/blacklist system.

See also:

Mozilla Firefox Memory Corruption Vulnerability”, Secunia, July 14, 2009, accessed on 2009-07-15

Exploit 9137”, SBerry, July 13, 2009, accessed on 2009-07-15

Stopgap Fix for Critical Firefox 3.5 Security Hole”, Brian Krebs, The Washington Post, July 14, 2009, accessed on 2009-07-15

Critical JavaScript vulnerability in Firefox 3.5”, Mozilla Security Blog, July 14, 2009, accessed on 2009-07-15

1 “Mozilla Foundation tackles Firefox bug”, Nick Farell, The Inquirer, Wednesday, 15, July, 2009, accessed on 2009-07-15