It addressed a severe vulnerability in how Windows handled Animated Cursor (.ANI) files. This flaw, an integer overflow, allowed remote code execution. Attackers could exploit it by tricking users into viewing a malicious .ANI file, often via infected websites, emails, or shared folders.
Successful exploitation granted attackers full control over the affected system. The vulnerability was actively exploited as a zero-day before a patch was available. Microsoft released an emergency, out-of-band patch to mitigate the widespread threat. This incident highlighted the danger of seemingly harmless file types and the urgency of rapid patching.
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National Cyber Alert System
Technical Cyber Security Alert TA07-089A
Microsoft Windows ANI header stack buffer overflow
Original release date: March 30, 2007
Last revised: --
Source: US-CERT
Systems Affected
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
Overview
An unpatched buffer overflow vulnerability in the way Microsoft
Windows handles animated cursor files is actively being exploited.
I. Description
A stack buffer overflow exists in the code that Microsoft Windows
uses to processes animated cursor files. Specifically, Microsoft
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. Because of this, 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.
II. 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.
III. Solution
Until a fix is available, refer to the Solution section of
Vulnerability Note VU#191609 for the latest workarounds.
IV. References
* Vulnerability Note VU#191609 -
<http://www.kb.cert.org/vuls/id/191609>
* Microsoft Security Advisory (935423) -
<http://www.microsoft.com/technet/security/advisory/935423.mspx>
* 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>
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<http://www.us-cert.gov/cas/techalerts/TA07-089A.html>
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Produced 2007 by US-CERT, a government organization.
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Revision History
March 30, 2007: Initial release
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