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MS06-042 Internet Explorer Crash is Exploitable

August 23rd, 2006

On August 8th Microsoft released MS06-042 which was a cumulative update for Internet Explorer. Over the course of a few days after the release of this patch various Internet Explorer users and businesses started to experience Internet Explorer crashing problems when viewing certain websites.

Later on August 11th Microsoft created a knowledge base article which talked about problems with the MS06-042 patch and how Internet Explorer could crash when viewing some web pages that used compression. This Microsoft KB article referenced a patch, which could be requested through Microsoft Product Support Services, that would fix the “crashing” bug. There was further discussion about the extent of the crashes and widespread nature of the bug on places such as SANS and various patch and IT mailing lists. Because of the widespread discussions and number of people experiencing the Internet Explorer crash various security researchers, including eEye, decided to investigate as a lot of times crashes can be exploitable.

We have since found that indeed the reason that people are experiencing Internet Explorer browser crashes is certain websites, that use HTTP 1.1 compression (as stated by Microsoft ), are causing a non-malicious buffer overflow to occur within Internet Explorer. After investigating and confirming that indeed this is an exploitable condition we are alerting people to the true severity of these “crashing” problems that people are experiencing, so that they can take the appropriate mitigation steps as need be.

This information is already known in various research circles and also with exploit writers. So it is important that IT administrators understand the true threat of this problem that this is not simply a crashing bug as Microsoft has been incorrectly misrepresenting it but in fact that it is an exploitable security bug. Researchers and exploit developers know this, therefore it is extremely important that IT administrators are told what really is going on.

Windows 2000 IE6 SP1 Systems: Patch: Microsoft created and released a non-public patch on August 11th.

Windows XP SP1 IE6 SP1 Systems: Patch: The best way to protect your XP systems is to upgrade to Windows XP SP2 as it is protect against this vulnerability. Also support for XP SP1 ends in October and there are huge security benefits to XP SP2 so hopefully your already migrated to it. If you are not however and you are stuck on XP SP1 then you can use the Microsoft Knowledge base patch which was released on August 11th through the PSS process.

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