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Vulnerabilities in CaptchaSecurityImages
From: "MustLive" <mustlive () websecurity com ua>
Date: Mon, 22 Mar 2010 21:53:20 +0200

Hello Full-Disclosure!

I want to warn you about security vulnerabilities in CaptchaSecurityImages.
It's captcha script which is using at many web sites and engines.

-----------------------------
Advisory: Vulnerabilities in CaptchaSecurityImages
-----------------------------
URL: http://websecurity.com.ua/4043/
-----------------------------
Timeline:
06.10.2007 - found Insufficient Anti-automation vulnerability, during
conducting of my project Month of Bugs in Captchas
(http://websecurity.com.ua/category/mobic/).
17.09.2009 - found Denial of Service vulnerability.
17.03.2010 - disclosed at my site.
18.03.2010 - informed developers.
-----------------------------
Details:

These are Insufficient Anti-automation and Denial of Service
vulnerabilities.

Insufficient Anti-automation:

Parameters characters, width and height fall under manipulation in the
captcha. They can be set in such way, that will allow easy bypass of the
captcha via half-automated or automated (with using of OCR) methods. And in
some systems (http://websecurity.com.ua/4046/) it's also possible to use
session reusing with constant captcha bypass method.

http://site/CaptchaSecurityImages.php?width=150&height=100&characters=2

In that way it's possible to set two characters and increase the size of the
captcha.

DoS:

http://site/CaptchaSecurityImages.php?width=1000&height=9000

With setting of large values of width and height it's possible to create
large load at the server.

Best wishes & regards,
MustLive
Administrator of Websecurity web site
http://websecurity.com.ua 

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