Deciphering the Simple Machines Forum audio Captcha

2008-04-21T00:00:00
ID SECURITYVULNS:DOC:19705
Type securityvulns
Reporter Securityvulns
Modified 2008-04-21T00:00:00

Description

The Simple Machine’s Forum audio Captcha that has been hardened from attack. I have contacted SMF about this flaw and it has been verified.

I go into greater detail of how i am able to break this captcha here: http://www.rooksecurity.com/blog/?p=6

Exploit Code: http://www.rooksecurity.com/exploits/smf_captcha.zip

This captcha has been broken before and exploit code is available here: http://www.securityfocus.com/archive/1/471641 . The fix was to add a randomly generated static.

I was able to write code to decipher the audio file using a Fuzzy Logic comparison http://en.wikipedia.org/wiki/Fuzzy_logic . The comparison is the Hamming Distance http://en.wikipedia.org/wiki/Hamming_distance between the original audio file and the damaged one generated by the Captcha.

peace