Public Vulnerability Reports

Microsoft WordPad Word97 Converter Integer Overflow Vulnerability

08.12.09

BACKGROUND

WordPad is the default text editing application included with nearly all Windows versions since Windows 95. The Word97 converter is used to convert Word documents into the format used by WordPad, and is present in all current versions of WordPad except Vista and Server 2008.

DESCRIPTION

Remote exploitation of an integer 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 DocumentSummaryInformation stream inside of a DOC file. This stream is used to provide information about the author of the document, date of creation, and similar data. Part of the data in this stream is a sequence of property name and value pairs. When reading in the names of these properties, the code performs a calculation using a 32bit integer from the file that represents the number of names present. The value is used without any check on its bounds, which can lead to an integer overflow. This results in an undersized heap buffer being allocated to store the property data. It is then possible to overflow this buffer 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 would not be associated with the .DOC file extension, and would not open it when the file is double clicked. However, by renaming the .doc file to a .wri extension (associated with WordPad), it is possible to make WordPad open the file simply by double clicking it.

DETECTION

iDefense has confirmed the existence of this vulnerability in WordPad version 5.1 for Windows XP SP3 and SP2. Other versions of Windows may also be affected. However, 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/MS09-073.mspx

CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2009-2506 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

12/18/2008 Initial Vendor Notification
12/19/2008 Initial Vendor Reply
12/08/2009 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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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.