Vupen rates the vulnerability, which it confirmed on IE8 running on Windows XP SP3, as critical. The vulnerability also affects IE8 running on Windows 7, Windows Server 2008 SP2 and R2, Windows Vista SP2, and Windows Server 2003 SP2. No patch has been released.
According to Vupen, "this issue is caused by a use-after-free error within the 'mshtml.dll' library when handling circular references between JScript objects and Document Object Model (DOM) objects, which could allow remote attackers to execute arbitrary code via a specially crafted Web page." Attackers could exploit the vulnerability to take control of a targeted system.
In a follow-up tweet, Vupen noted that in verifying the vulnerability, "reproducing was/is hard." The observation is pertinent, since Google's Zalewski released a timeline asserting that he first informed Microsoft of vulnerabilities discovered by his fuzzing engine in 2008, and alerted it to this specific bug in July 2010.
Microsoft, however, said that the vulnerabilities identified by Zalewski only appeared with the third version of his fuzzing tool, which was released in December. At that point, Microsoft requested that Zalewski delay the release of his fuzzing tool to give it time to address the vulnerability.
"After reviewing the new version of the tool and the crash report, we requested that Zalewski hold the public release of the new version of the tool and information on the specific vulnerability found in December until we could investigate further," said Jerry Bryant, group manager for trustworthy computing response communications at Microsoft. "We specifically told Zalewski we were fine with him publishing the two versions of the tool reported in July."
Zalewski declined to hold the release of the latest version of his code-fuzzing tool, asserting that Chinese security researchers -- or attackers -- had been probing for the precise vulnerability discovered by the tool. His actions drew criticism from Bryant, who accused him of amplifying the related risk.