iDefense Security Advisory 09.13.11
http://labs.idefense.com/intelligence/vulnerabilities/
Sep 13, 2011

I. BACKGROUND

Excel is the spreadsheet application included with Microsoft Corp.'s
Office productivity software suite. More information is available at the
following website:

http://office.microsoft.com/excel/

II. DESCRIPTION

Remote exploitation of an integer signedness vulnerability in Microsoft
Corp.'s Excel could allow an attacker to execute arbitrary code with the
privileges of the current user.

The vulnerability is an integer signedness issue that leads to an
invalid array indexing vulnerability. It is triggered by a certain
record with a negative 'iax' field.

It is possible to pass negative 16-bit values, which are later sign
extended to 32 bits. The sign extended value is later used as an index
into a heap-based array. Due to the incomplete validation of the 'iax'
field, it is possible to index outside of the bounds of the array, which
can lead to a controlled overwrite of arbitrary memory locations with
user data. This can lead to the execution of arbitrary code.

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. Attackers typically accomplish this by e-mailing a targeted user
the file or hosting the file on a Web page.

IV. DETECTION

Microsoft has reported the following products vulnerable:

    * Microsoft Excel 2003 SP 3
    * Microsoft Excel 2007 SP 2
    * Microsoft Office 2007 SP 2
    * Microsoft Excel 2010 (32-bit editions)
    * Microsoft Excel 2010 SP 1 (32-bit editions)
    * Microsoft Office 2010 and Microsoft Office 2010 SP 1 (32-bit editions)
    * Microsoft Excel 2010 (64-bit editions)
    * Microsoft Excel 2010 SP 1 (64-bit editions)
    * Microsoft Office 2010 and Microsoft Office 2010 SP 1 (64-bit editions)
    * Microsoft Office 2004 for Mac
    * Microsoft Office 2008 for Mac
    * Microsoft Office for Mac 2011
    * Open XML File Format Converter for Mac
    * Microsoft Excel Viewer SP 2
    * Microsoft Office Compatibility Pack for Word, Excel, and
PowerPoint 2007 File Formats SP 2
    * Excel Services
    * Microsoft Excel Web App 2010 and Microsoft Excel Web App 2010 SP 1

V. WORKAROUND

Microsoft suggested workarounds can be found under the Workaround
section within Microsoft Security Bulletin MS11-072.

http://technet.microsoft.com/en-us/security/bulletin/ms11-072

VI. VENDOR RESPONSE

Microsoft has released fixes which addresses this issue. Information
about downloadable vendor updates can be found by clicking on the URLs
shown.

http://technet.microsoft.com/en-us/security/bulletin/ms11-072

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2011-1987 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

02/25/2011  Initial Vendor Notification
02/25/2011  Vendor Reply
09/13/2011  Coordinated Public Disclosure

IX. CREDIT

This vulnerability was reported to iDefense by Sean Larsson, iDefense
Labs.

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X. LEGAL NOTICES

Copyright © 2011 Verisign

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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.