MS06-042 Related Internet Explorer 'Crash' is Exploitable Date: August 22, 2006 Severity: High Systems Affected: Windows 2000 with IE6 SP1 and MS06-042 hotfix installed Windows XP SP1 with IE6 SP1 and MS06-042 hotfix installed Overview: On August 8th Microsoft released MS06-042 which was a cumulative update for Internet Explorer[1]. 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[2]. 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[3]. 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[4]. 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 compression (as stated by Microsoft[5]), 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. Prevention: Windows 2000 IE6 SP1 Systems Patch: Microsoft created and released a non-public patch on August 11th. You can find out more about this patch here: http://support.microsoft.com/?kbid=923762. This patch can only currently be obtained through the Microsoft PSS process. However, Microsoft does plan to eventually release a public patch through Windows Update etc... Workaround: Disable HTTP1.1 functionality as outlined by Microsoft in their knowledge base article: http://support.microsoft.com/?kbid=923762. Please review the caveats of doing this as outlined by Microsoft. 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 protected against this vulnerability. Also support for XP SP1 ends in October and there are huge security benefits to XP SP2 so hopefully your're 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. http://support.microsoft.com/?kbid=923762 Workaround: Disable HTTP1.1 functionality as outlined by Microsoft in their knowledge base article: http://support.microsoft.com/?kbid=923762. Please review the caveats of doing this as outlined by Microsoft. Credit: Derek Soeder (eEye) Links: [1] - MS06-042 Bulletin - http://www.microsoft.com/technet/security/Bulletin/MS06-042.mspx [2] - SANS - http://isc.sans.org [3] - Microsoft KB Article - http://support.microsoft.com/?kbid=923762 [4] - SANS Thread - http://isc.sans.org/diary.php?storyid=1588 [5] - http://blogs.technet.com/msrc/archive/2006/08/16/447023.aspx Copyright (c) 1998-2006 eEye Digital Security Permission is hereby granted for the redistribution of this alert electronically. It is not to be edited in any way without express consent of eEye. If you wish to reprint the whole or any part of this alert in any other medium excluding electronic medium, please email alert (at) eEye (dot) com [email concealed] for permission. Disclaimer The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are no warranties, implied or express, with regard to this information. In no event shall the author be liable for any direct or indirect damages whatsoever arising out of or in connection with the use or spread of this information. Any use of this information is at the user's own risk.