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Security researchers have disclosed a zero-day vulnerability in the latest version of Firefox that gives miscreants complete control of Windows-based computers when the Mozilla browser visits a booby-trapped website.

The vulnerability resides in the way Firefox handles uniform resource identifiers, the protocols that allow the browser to access software and other resources located on a PC. The browser fails to properly vet at least five different URIs, a flaw that could allow an attacker to install malware on a PC simply by convincing a victim to click on a doctored link.

The problem is the result of the wealth of poorly documented URIs that countless third-party applications add to the Windows registry. Browsers are frequently ill-prepared to screen URI handlers for security exploits, and even when they are, related applications are often unable to parse the restrictions - or are prone to their own security vulnerabilities, said Billy Rios, one of the researchers who discovered the flaw.

With each registered URI, a PC's attack surface increases.

"It's a perfect storm where the browsers aren't really doing their job in terms of sanitizing the URIs and applications that are registering these URIs aren't really able to properly handle the parameters that are being passed to it from the browser," Rios told El Reg.

Mozilla's security team is aware of the flaw, which was reported on the Billy (BK) Rios blog, and is working on a patch, a spokesman for the open-source organization said.

An episode that played out over the past few weeks demonstrates just how difficult it is to tackle the problem. Last week, Mozilla patched a hole in Firefox that allowed the browser to accept malicious URIs that were passed from IE. Microsoft security officials argued that validation is the job of the application receiving the input and said flaw rested solely in Firefox.

Window Snyder, Mozilla's chief security officer, disagreed, arguing that IE should screen the URI before passing it along and warned that other Windows applications could also be vulnerable.

To further complicate matters, Snyder later acknowledged that Firefox could indeed be used to pass off a bad URI, proving there is plenty of culpability when it comes to the way browsers and other applications work with the protocol.

The latest Firefox flaw is particularly serious because it can be exposed anywhere a person is vulnerable to a cross-site scripting attack. That includes documents or emails containing malicious links or, potentially, forums containing hidden scripts. The vulnerability is also present in Netscape Navigator 9, and the Mozilla browser.

As is often the case with Firefox vulnerabilities, a browser extension called NoScript can mitigate the exploit. Indeed, we had to disable it in order to get proof of concept links (at the bottom of this page) to work. ®

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