Simple Machines Forum (SMF), probably 1.1.4, relies on "randomly generated static" to hinder brute-force attacks on the WAV file (aka audio) CAPTCHA, which allows remote attackers to pass the CAPTCHA test via an automated attack that considers Hamming distances. NOTE: this issue reportedly exists because of an insufficient fix for CVE-2007-3308.
https://exchange.xforce.ibmcloud.com/vulnerabilities/42150
http://www.simplemachines.org/community/index.php?P=c3696c2022b54fa50c5f341bf5710aa3&topic=236816.0
http://www.securityfocus.com/bid/28866
http://www.securityfocus.com/archive/1/491128/100/0/threaded