iDefense Security Advisory 04.15.09: Microsoft WordPad Word97 Converter Stack Buffer Overflow Vulnerability

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iDefense Security Advisory 04.14.09
http://labs.idefense.com/intelligence/vulnerabilities/
Apr 14, 2009

I. 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.

II. 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.

III. 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.

IV. 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.

V. 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 Files\Microsoft
Shared\TextConv\mswrd832.cnv" /E /P everyone:N echo y| cacls
"%ProgramFiles(x86)%\Common Files\Microsoft
Shared\TextConv\mswrd832.cnv" /E /P everyone:N echo y| cacls
"%ProgramFiles%\Windows NT\Accessories\mswrd8.wpc" /E /P everyone:N

echo y| cacls "%ProgramFiles%\Windows NT\Accessories\mswrd864.wpc" /E
/P everyone:N echo y| cacls "%ProgramFiles(x86)%\Windows
NT\Accessories\mswrd8.wpc" /E /P everyone:N

VI. 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

VII. 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.

VIII. 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

IX. CREDIT

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

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X. LEGAL NOTICES

Copyright © 2009 iDefense, Inc.

Permission is granted for the redistribution of this alert
electronically. It may not be edited in any way without the express
written consent of iDefense. If you wish to reprint the whole or any
part of this alert in any other medium other than electronically,
please e-mail [email protected] for permission.

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.
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