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Multi-vendor AV gateway image inspection bypass vulnerability



Multi-vendor AV gateway image inspection bypass vulnerability
January 10, 2005

A vulnerability has been discovered which allows a remote attacker to bypass anti-virus (as well other security technologies such as IDS and IPS) inspection of HTTP image content.

By leveraging techniques described in RFC 2397 for base64 encoding image content within the URL scheme. A remote attacker may encode a malicious image within the body of an HTML
formatted document to circumvent content inspection.

For example:

http://www.k-otik.com/exploits/09222004.ms04-28-cmd.c.php

The source code at the URL above will by default create a JPEG image that will attempt (and fail without tweaking) to exploit the Microsoft MS04-028 GDI+ vulnerability. The image itself is detected by all AV gateway engines tested (Trend, Sophos and McAfee), however, when the same image is base64 encoded using the technique described in RFC 2397 (documented below), inspection
is not performed and is delivered and rendered by the client.

While Microsoft Internet Explorer does not support the RFC 2397 URL scheme; Firefox, Safari, Mozilla and Opera do and will render the data and thus successfully execute the payload if the necessary
OS and/or application patches have not been applied.


## BEGIN HTML ##

<html>
<body>
<img src="data:image/gif;base64,/9j/4AAQSkZJRgABAQEAYABgAAD// gAARXhpZgAASUkqAAgAHPD9f0FBQUGWAgAAGgAAABzw
/X9BQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFBQUFB
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</body>
</html>

## END HTML ##


Solution:

While AV vendor patches are not yet available, fixes for all currently known image vulnerabilities are and have been for several months. If you have not yet applied them, you have your own
negligence to blame.


Contributions:

Thanks to Scott Roeder and Jacinto Rodriquez for their assistance in platform testing.


Thank you,

Darren Bounds
Intrusense, LLC.
http://www.intrusense.com

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