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A serious vulnerability that causes Internet Explorer to launch Firefox and execute a malicious payload is sparking debate about exactly who is responsible for the flaw.

The vulnerability, which was widely reported on security blogs, allows an attacker to remotely execute malicious code on a machine that is running IE but also has the Mozilla browser installed. By luring an IE user to a malevolently crafted site, the attacker can cause Firefox to execute the code without first vetting it for security.

"We recommend that people use Firefox and as always take care when browsing unknown websites," she wrote.

For its part, Microsoft representatives said company researchers have "investigated the claim of a vulnerability in Internet Explorer and found that this is not a vulnerability in a Microsoft product." Jesper Johansson, a former senior security strategist for Microsoft, similarly argues that "most definitely" the problem isn't caused by IE.

"Firefox fails to properly validate the parameters, and any fix will have to come from Mozilla, not Microsoft," he wrote in a blog entry.

A proof of concept exploit found here uses IE to hand off maliciously-scripted code to a Firefox handler known as "firefoxurl." Handlers, which also include strings such as "ftp" and "aim," are found in the address bar and in many cases can be used to get Firefox to carry out certain actions.

Roger Thompson, CTO of Exploit Prevention Labs, says Microsoft shares culpability because IE fails to properly validate the input before passing it along.

"I think it's an IE issue mostly, because if you access the exploit directly with Firefox, FF warns you that something bad is happening and advises you to not do it," he said in an instant message. ®

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