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Microsoft WordPad Word97 Converter Stack Buffer Overflow Vulnerability

04.14.09

BACKGROUND

WordPad is a word processing application included with Microsoft Windows. The Word97 converter is used to convert Word97 format documents into RTF format used by WordPad, and is present in all current versions of WordPad except Vista and Server 2008.

DESCRIPTION

Remote exploitation of a stack buffer overflow vulnerability in Microsoft Corp.'s WordPad could allow an attacker to execute arbitrary code with the privileges of the current user.

The vulnerability occurs when parsing the content of a Word97 format file. When reading in the data, the code uses a 32-bit integer from the file to check a buffer length while using the lower 16-bit value to do the actual copy. This results in a stack buffer overflow. This stack buffer is overwritten with data from the file.

ANALYSIS

Exploitation of this vulnerability results in the execution of arbitrary code with the privileges of the user opening the file. To exploit this vulnerability, an attacker needs to convince a user to open a malicious file. Usually, WordPad is associated with the .DOC file extension unless Microsoft Word is installed. However, by renaming the .doc file to a .wri extension, it is possible to make WordPad open the file simply by double clicking it regardless of Microsoft Word being installed or not.

DETECTION

iDefense has confirmed the existence of this vulnerability in Wordpad on Windows 2000 SP4. Windows XP SP3 is not affected. Vista and Server 2008 are not affected as they no longer contain the Word97 converter.

WORKAROUND

It is possible to disable access to the converter by changing the ACLs on the file. This can be accomplished as follows, depending on the version of Windows being used:

 echo y| cacls "%ProgramFiles%Common FilesMicrosoft SharedTextConvmswrd832.cnv" /E /P everyone:N echo y| cacls "%ProgramFiles(x86)%Common FilesMicrosoft SharedTextConvmswrd832.cnv" /E /P everyone:N echo y| cacls "%ProgramFiles%Windows NTAccessoriesmswrd8.wpc" /E /P everyone:N 
 echo y| cacls "%ProgramFiles%Windows NTAccessoriesmswrd864.wpc" /E /P everyone:N echo y| cacls "%ProgramFiles(x86)%Windows NTAccessoriesmswrd8.wpc" /E /P everyone:N 

VENDOR RESPONSE

Microsoft has released a patch which addresses this issue. This patch removes the vulnerability by changing the way that the WordPad Text Converters handle opening specially crafted Word 97 files. For more information, consult their advisory at the following URL:

http://www.microsoft.com/technet/security/bulletin/MS09-010.mspx

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2009-0235 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org/), which standardizes names for security problems.

DISCLOSURE TIMELINE

12/19/2008 - Initial Contact
12/31/2008 - PoC Request
01/06/2009 - PoC Sent
01/07/2009 - PoC Rcpt. ACK
02/19/2009 - Vendor Status Update
03/31/2009 - CVE Assigned
04/14/2009 - Coordinated Public Disclosure

CREDIT

This vulnerability was discovered by Jun Mao and Sean Larsson, iDefense Labs.

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LEGAL NOTICES

Copyright © 2009 Verisign, Inc.

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