Public Vulnerability Reports

Microsoft Word CSS Processing Memory Corruption Vulnerability



Microsoft Word is a word processing application that is distributed with Microsoft Office. Cascading Style Sheets (CSS) is a stylesheet language used to describe the presentation of a document written in a markup language. For more information about Microsoft Word, visit the following URL.


Remote exploitation of a memory corruption vulnerability in Microsoft Corp.'s Word could allow attackers to execute arbitrary code with the privileges of the logged in user.

This vulnerability exists in the way Word handles CSS rules in an HTML document. When the number of CSS selectors is above some specific amount, an unspecified object will be corrupted causing Word to access a memory region that has already been freed.


Exploitation allows remote attackers to execute arbitrary code on the affected host in the context of the user. Exploitation requires that the user opens a specially crafted HTML document using Microsoft Word. The most likely exploitation vector involves convincing a user to open an HTML document, with a DOC extension, sent to them via e-mail or linked on a website.


iDefense has confirmed fully patched Microsoft Word 2003 SP2, Microsoft Word XP SP3, Microsoft Word 2000 SP3 are vulnerable. Microsoft Word 2003 SP3 and Microsoft Word 2007 do not appear to be affected.

Microsoft reports that all supported versions of Word, Word Viewer, and Outlook 2007 are vulnerable.


Users of Office 2003 and Office 2007 that have installed all security patches as of May 2007 can use the Microsoft Office File Block policy to prevent opening files of this type. To deploy this workaround, save and import the following registry file for the corresponding version of Office.

Office 2003:

 Windows Registry Editor Version 5.00 [HKEY_CURRENT_USERSoftwareMicrosoftOffice11.0WordSecurityFileOpenBlock] "HTMLFiles"=dword:00000001 

Office 2007:

 Windows Registry Editor Version 5.00 [HKEY_CURRENT_USERSoftwarePoliciesMicrosoftOffice12.0WordSecurityFileOpenBlock] "HTMLFiles"=dword:00000001 


Microsoft has officially addressed this vulnerability with Security Bulletin MS08-026. For more information, consult their bulletin at the following URL.


The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2008-1434 to this issue. This is a candidate for inclusion in the CVE list (, which standardizes names for security problems.


11/08/2007 Initial vendor notification
11/08/2007 Initial vendor response
05/13/2008 Coordinated public disclosure


This vulnerability was discovered by Jun Mao of iDefense Labs.

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