Microsoft Windows ANI Header Stack Buffer Overflow

Microsoft Windows 2000, XP, Server 2003, and Vista are affected. Applications that provide attack vectors include

• Microsoft Internet Explorer
• Microsoft Outlook
• Microsoft Outlook Express
• Microsoft Windows Mail
• Microsoft Windows Explorer

A buffer overflow vulnerability in the way Microsoft Windows handles animated cursor files is actively being exploited.

For updated solution information, see US-CERT Technical Cyber Security Alert TA07-093A and Microsoft Security Bulletin MS07-017.

A stack buffer overflow exists in the code that Microsoft Windows uses to process animated cursor files. Specifically, Windows fails to properly validate the size of an animated cursor file header supplied in animated cursor files.

Animated cursor files can be included with HTML files. For instance, a web site can use an animated cursor file to specify the icon that the mouse pointer should use when hovering over a hyperlink. Because of this, malicious web pages and HTML email messages can be used to exploit this vulnerability. In addition, animated cursor files are automatically parsed by Windows Explorer when the containing folder is opened or the file is used as a cursor. Consequently, opening a folder that contains a specially crafted animated cursor file will also trigger this vulnerability.

Note that Windows Explorer will process animated cursor files with several different file extensions, such as .ani, .cur, or .ico. Furthermore, Windows will automatically render animated cursor files referenced by HTML documents regardless of the animated cursor file extension.

This vulnerability is actively being exploited.

More information is available in Vulnerability Note VU#191609.

Impact

A remote, unauthenticated attacker may be able to execute arbitrary code. Exploitation may occur when a user clicks a malicious link, reads or forwards a specially crafted HTML email, or accesses a folder containing a malicious animated cursor file.

Solution

Install updates from Microsoft

Microsoft has released updates for this and other image processing vulnerabilities in Microsoft Security Bulletin MS07-017.

Workarounds

Until updates can be installed, refer to the Solution section of Vulnerability Note VU#191609 for the latest workarounds.

References

• US-CERT Technical Cyber Security Alert TA07-093A - http://www.us-cert.gov/cas/techalerts/TA07-093A.html
• Vulnerability Note VU#191609 - http://www.kb.cert.org/vuls/id/191609
• Microsoft Security Bulletin MS07-017 - http://www.microsoft.com/technet/security/Bulletin/ms07-017.mspx
• Microsoft Security Advisory (935423) - http://www.microsoft.com/technet/security/advisory/935423.mspx
• Microsoft Security Response Center Blog - http://blogs.technet.com/msrc/search.aspx?q=935423
• Unpatched Drive-By Exploit Found On The Web - http://www.avertlabs.com/research/blog/?p=230
• TROJ_ANICHMOO.AX - Description and Solution - http://www.trendmicro.com/vinfo/virusencyclo/default5.asp?VName=TROJ%5FANICMOO%2EAX

Revision History

• March 30, 2007: Initial release
  April 03, 2007: Updated with release of MS07-017. Title changed, previous title was "Microsoft Windows ANI header stack buffer overflow."