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In this case, more researchers were discovering that the issue could be exploited, Maiffret said. Because attackers were likely learning of the issue as well, he decided to publicise the fact that the flaw could be used to compromise systems.

"The cat's out of the bag," Maiffret said. "When there are multiple threads on the patch crashing systems on multiple security mailing lists, then you know that people are going to investigate this. And multiple researchers have already reported this to Microsoft.

The software giant originally declined to comment on the issue, but referred SecurityFocus to its security Weblog, posted last week, which confirmed that a bug introduced by the cumulative update crashed Internet Explorer and pledged to bring out a patch on Tuesday.

"Since MS06-042 resolves a number of security vulnerabilities we recommend customers continue to deploy the update, but we do plan to revise only the IE6SP1 update and re-release the bulletin with more information by 22 August for all IE6SP1 customers," Mike Reavey, operations manager for the Microsoft Security Research Center, stated on the group's blog.

A company spokesperson originally could not confirm that the flaw also could be used to exploit vulnerable Windows systems.

The software giant posted a more in-depth article on the problem on 11 August and has a hotfix available from its product group. Users can also turn off support for HTTP 1.1 in Internet Explorer as a temporary workaround. Using an alternative browser, such as Mozilla's Firefox or Opera's eponymous browser, also eliminates any threat from the flaw.

The incident may undo a great deal of the work that Microsoft has done to convince users to trust its software updates and install them by default.

Under its nearly five-year-old Trustworthy Computing Initiative, the company has fought to increase the number of users that apply patches automatically. In the past, some network administrators have waited to test, or hear the community reaction to, Microsoft's latest patch. A year ago, the company refused to release a fix rather than push out a software update that could cause problems.

Introducing a security issue in its latest set of patches undermines network administrators' confidence in Microsoft's software updates, said Johannes Ullrich, chief technology officer for the SANS Institute's Internet Storm Centre.

"The entire danger of this particular issue is that this is an important patch to apply, but if they cause your applications to not work, network administrator won't deploy patches," Ullrich said.

The Internet Storm Centre had a couple of dozen reports of users encountering crashes after the August cumulative update. That's the most complaints reported to the group following a Microsoft patch, Ullrich said. The security expert could not confirm that the flaw in the latest patch could be used by attackers to compromise systems.

Yet, the security expert agreed with Microsoft's advice. Patching systems with the latest software update is still important.

"Given that there were people exploiting some of the flaws fixed by this patch, users should apply the fix," he said. "You have to somehow put other defenses in place to defend against other issues.

UPDATE: The article was updated Tuesday at 3pm PST with Microsoft's confirmation of the exploitability of the flaw, Microsoft comment on the disclosure and eEye's response to that comment. Microsoft had originally been given a chance to comment on the disclosure and declined.

This article originally appeared in Security Focus.

Copyright © 2006, SecurityFocus

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