"The use-after-free vulnerability is triggered when handling circular memory references," according to the US-CERT alert posted today. The bug resides in the mshtml.dll library.
Google's Michel Zalewski discovered the bug while testing a fuzzer tool, cross_fuzz, he had written for browsers and had released on Jan. 1. That's when he discovered an accidental leak of the fuzzer's whereabouts online showed "third parties in China" apparently also know about an unpatched and exploitable bug he found in IE with the fuzzer. The leak occurred when one of cross_fuzz's developers, who was working on crashes in the open-source WebKit browser engine used in Chrome and Safari, inadvertently leaked the address of the fuzzer in one of the crash traces that was uploaded. That made the fuzzer's directory, as well as the IE test results from the fuzzer, indexed by GoogleBot, he says.
Zalewski saw that an IP address in China queried keywords included in one of the indexed cross_fuzz files -- specifically two DLL functions associated with and unique to the zero-day IE flaw he found with the fuzzer -- now revealed as the use-after-free bug.
Researchers at VUPEN Security have tested and confirmed that the flaw exists in IE 8 running on XP SP3. It also affects Windows 7, Windows Server 2008 SP2, Windows Server 2008 R2, Vista Service Pack 2, Windows Server 2003 Service Pack 2, and Windows XP Service Pack 3.
VUPEN rated the flaw as "critical." "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," according to VUPEN.
There is no workaround or patch for the flaw as yet. Microsoft says it's investigating it. "On January 1, 2011, Microsoft confirmed that the issue was a potentially exploitable vulnerability. We are currently not aware of any reliable proof of concept code that would indicate attackers have determined how to exploit this issue," says Jerry Bryant, group manager for response communications at Microsoft.
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