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Details concerning a potentially serious security vulnerability in fully patched versions of Microsoft's Internet Explorer have been leaked to people in China, a researcher warned over the weekend.

Michal Zalewski, a security researcher at Google, blogged that data concerning at least one “clearly exploitable crash” in the Microsoft browser was inadvertently disclosed to people who were using a Chinese IP address. Details about the bug, which resides in the mshtml.dll component, were stored on a server that had accidentally been indexed by Google, Zalewski wrote elsewhere. On December 30, detailed search queries showed that the sensitive information, in addition to files for an unpublished security tool, had been retrieved by the unknown party.

“This pattern is very strongly indicative of an independent discovery of the same fault condition in MSIE by unrelated means,” Zalewski wrote. “Other explanations for this pair of consecutive searches seem extremely unlikely.”

The bug leads to arbitrary crashes in the EIP, or extended instruction pointer, of machines running the Microsoft browser. Zalewski said the flaw “is pretty much fully attacker-controlled.” It was uncovered using cross_fuzz, a security tool the researcher developed in his spare time more than two years ago to identify potential security vulnerabilities in IE, Firefox, and other browsers. Since its release, the tool has helped to identify nearly 100 various browser bugs.

A statement attributed to Jerry Bryant, group manager in Microsoft's Response Communications, said company researchers are working to reproduce the crash to see if the underlying vulnerability can be exploited by malicious hackers.

“At this point, we're not aware of any exploits or attacks for the reported issue and are continuing to investigate and monitor the threat environment for any changes,” Bryant said.

Zalewski provided this account of his communications with Microsoft, which started in May 2008. In it, he claims that on December 21, Microsoft researcher David Ross “confirms being able to reproduce crashes locally right away.”

Zalewski said that Microsoft researchers asked him to delay the release of cross_fuzz until they had more time to investigate the crashes. He published his warning on New Year's Day, after he learned that the crash logs and related files had been downloaded.

“These search queries are looking for information on two MSHTML.DLL functions – BreakAASpecial and BreakCircularMemoryReferences – that are unique to the stack signature of this vulnerability, and had *absolutely* no other mentions on the internet at that time,” he said. ®

Update

Zalewski has updated his post to make clear that he believes the individuals who accessed cross_fuzz data already knew of the IE zero day.

"The pattern is very strongly indicative of an independent discovery of the same vulnerability in MSIE with unrelated tools, eventually leading the discoverer to my site," he wrote. "Other explanations for this pair of consecutive searches seem extremely unlikely."

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