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

Details.
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:

http://www.senseofsecurity.com.au/advisories/SOS-11-007.zip
Mirror: https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/17309.zip (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
vulnerability.

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

Discovered by.
Phil Taylor from Sense of Security Labs.

About us.
Sense of Security is a leading provider of information security and 
risk management solutions. Our team has expert skills in assessment
and assurance, strategy and architecture, and deployment through to
ongoing management. We are Australia's premier application penetration
testing firm and trusted IT security advisor to many of the country's
largest organisations.

Sense of Security Pty Ltd 
Level 8, 66 King St
Sydney NSW 2000
AUSTRALIA

T: +61 (0)2 9290 4444
F: +61 (0)2 9290 4455
W: http://www.senseofsecurity.com.au
E: info@senseofsecurity.com.au
Twitter: @ITsecurityAU

The latest version of this advisory can be found at:
http://www.senseofsecurity.com.au/advisories/SOS-11-007.pdf 

Other Sense of Security advisories can be found at:
http://www.senseofsecurity.com.au/research/it-security-advisories.php