iDefense Security Advisory 05.13.08
http://labs.idefense.com/intelligence/vulnerabilities/ 
May 13, 2008

I. BACKGROUND

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.

http://office.microsoft.com/en-us/word/default.aspx 

II. DESCRIPTION

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.

III. ANALYSIS

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.

IV. DETECTION

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.

V. WORKAROUND

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_USER\Software\Microsoft\Office\11.0\Word\Security\FileOpenBlock]
  "HTMLFiles"=dword:00000001

Office 2007:

  Windows Registry Editor Version 5.00
  
[HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\12.0\Word\Security\FileOpenBlock]
  "HTMLFiles"=dword:00000001

VI. VENDOR RESPONSE

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

http://www.microsoft.com/technet/security/bulletin/ms08-026.mspx 

VII. CVE INFORMATION

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 (http://cve.mitre.org/), which standardizes names for 
security problems.

VIII. DISCLOSURE TIMELINE

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

IX. CREDIT

This vulnerability was discovered by Jun Mao of iDefense Labs.

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X. LEGAL NOTICES

Copyright =A9 2008 iDefense, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
 There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct,
indirect, or consequential loss or damage arising from use of, or
reliance on, this information.