PHP Captcha / Securimage 2.0.2 - Authentication Bypass - SO-11-007 2011-05-20 16:15:03

Sense of Security - Security Advisory - SOS-11-007

Release Date. 20-May-2011
Last Update. -
Vendor Notification Date. 04-Apr-2011
Product. Securimage / PHPCaptcha
Platform. PHP
Affected versions. 1.0.4 - 2.0.2
Severity Rating. Medium
Impact. Authentication bypass
Attack Vector. Remote without authentication
Solution Status. Vendor workaround
(remove securimage_play.php)
CVE reference. Not yet assigned

PHPCaptcha, also known as Securimage, is a popular Open Source PHP
CAPTCHA library. It is also used in popular WordPress plugins such as
the 'Fast Secure Contact Form'. Insufficient distortion in the audio
version of the CAPTCHA allows an attacker to quickly decode the CAPTCHA
by performing basic binary analysis of the generated audio file. The
issue is compounded by the fact that even if the audio feature of the
CAPTCHA has been disabled, it can still be accessed by forceful
browsing to the /secure_play.php URI.

Proof of Concept.
Proof of concept code that works against the example_form.php page and
the MP3 file format provided with the standard PHPCaptcha package
available from www.phpcaptcha.org is available at:

Mirror: http://www.exploit-db.com/sploits/SOS-11-007.zip

Proof of concept code is only available for the MP3 version of the
audio, however the WAV audio format is also affected by the same

Remove the script securimage_play.php and disable the use of the Audio

Discovered by.
Phil Taylor from Sense of Security Labs.

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