Hackers: Firefox has JavaScript flaw

The open source Firefox Web browser is critically flawed in the way it handles JavaScript, two hackers said on Saturday afternoon.

An attacker could commandeer a computer running the browser simply by crafting a Web page that contains some malicious JavaScript code, Mischa Spiegelmock and Andrew Wbeelsoi said in a presentation at the ToorCon hacker conference in San Diego. The flaw affects Firefox on Windows, Apple's Mac OS X and Linux, they said.

"Internet Explorer, everybody knows, is not very secure. But Firefox is also fairly insecure," said Spiegelmock, who in everyday life works at blog company SixApart. He detailed the flaw, showing a slide that displayed key parts of the attack code needed to exploit it.

The flaw is specific to Firefox's implementation of JavaScript, a 10-year-old scripting language widely used on the Web. In particular, various programming tricks can cause a stack overflow error, Spiegelmock said. The implementation is a "complete mess," he said. "It is impossible to patch."

The JavaScript issue appears to be a real vulnerability, Window Snyder, Mozilla's security chief, said after watching a video of the presentation Saturday night. "What they are describing might be a variation on an old attack," she said. "We're going to do some investigating."

Snyder said she isn't happy with the disclosure and release of an apparent exploit during the presentation. "It looks like they had enough information in their slide for an attacker to reproduce it," she said. "I think it is unfortunate because it puts users at risk, but that seems to be their goal."

At the same time, the presentation probably gives Mozilla enough data to fix the apparent flaw, Snyder said. However, because the possible flaw appears to be in the part of the browser that deals with JavaScript, addressing it might be tougher than the average patch, she added. "If it is in the JavaScript virtual machine, it is not going to be a quick fix," Snyder said.

The hackers claim they know of about 30 unpatched Firefox flaws. They don't plan to disclose them, instead holding on to the bugs.

Jesse Ruderman, a Mozilla security staffer, attended the presentation and was called up on the stage with the two hackers. He attempted to persuade the presenters to responsibly disclose flaws via Mozilla's bug bounty program instead of using them for malicious purposes such as creating networks of hijacked PCs, called botnets.

"I do hope you guys change your minds and decide to report the holes to us and take away $500 per vulnerability instead of using them for botnets," Ruderman said.

The two hackers laughed off the comment. "It is a double-edged sword, but what we're doing is really for the greater good of the Internet, we're setting up communication networks for black hats," Wbeelsoi said.