Rule:

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Sid:
2589

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Summary:
This event is generated when an attempt is made to return to a web client a file in the Content-Disposition Header with a Class ID (CLSID) embedded in the file name.

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Impact:
A successful attack may trick a client on a vulnerable host to download a malicious file that will be executed by the Windows Shell.

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Detailed Information:
Internet Explorer does not correctly handle or display specially crafted files in the browser dialogue where the user choses the action (e.g., open, save, cancel) for a downloaded file. Specifically, these are overly long file names that employ URL encoding of "." %2E before the file extension and contain the Class ID (CLSID) associated with the Windows Shell in the file name.

This serves two purposes; the first is that the file name will be truncated in the user dialog so the user doesn't see the CLSID reference, making it appear to be a more innocuous file with a known extension such as mpg or pdf.  Second, the downloaded file will actually contain malcious commands that will be executed by the Windows Shell when opened because of the hidden CLSID in the file name.

Currently, the only known CLSID that exploits this vulnerability is associated with the Windows Shell.  Yet, it may be possible for another CLSID to be discovered in the future that would be associated with a COM component that could be used for malicious purposes.

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Affected Systems:
Windows NT Workstation/Server 4.0 SP6a
Windows NT Workstation/Server 4.0 SP6a with Active Desktop
Windows NT Server 4.0 Terminal Server Edition SP6
Windows 2000 SP2-SP4
Windows XP and XP SP1
Windows XP 64-Bit Edition SP1
Windows XP 64-Bit Edition Version 2003
Windows Server 2003
Windows Server 2003 64-Bit Edition

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Attack Scenarios:
An attacker can entice a user to visit a web server that will return a malicious file with a file name that contains a CLSID, perhaps enabling the execution of the malicious code when the file is opened.

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Ease of Attack:
Simple. Exploit code is publicly available.

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False Positives:
None known.

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False Negatives:
None known.

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Corrective Action:
Upgrade to the latest non-affected version of the software.

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Contributors:
Sourcefire Vulnerability Research Team
Brian Caswell <bmc@sourcefire.com>
Judy Novak <judy.novak@sourcefire.com>

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Additional References

CVE:
http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CAN-2004-0420

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