pm.php (aka the PM system) in DeluxeBB 1.08, and possibly earlier, allows remote attackers to bypass authentication by providing an arbitrary username in the membercookie cookie parameter.
https://exchange.xforce.ibmcloud.com/vulnerabilities/28270
http://www.vupen.com/english/advisories/2006/3188
http://www.securityfocus.com/bid/19418
http://www.securityfocus.com/archive/1/442464/100/0/threaded