The domecode function in inc/functions_post.php in MyBulletinBoard (MyBB) 1.1.2, and possibly other versions, allows remote attackers to execute arbitrary PHP code via the username field, which is used in a preg_replace function call with a /e (executable) modifier.
https://exchange.xforce.ibmcloud.com/vulnerabilities/27046
http://www.vupen.com/english/advisories/2006/2288
http://www.securityfocus.com/bid/18396
http://www.securityfocus.com/archive/1/437509/100/100/threaded
http://www.securityfocus.com/archive/1/436767/100/0/threaded
http://www.514.es/download/mybibi.pl
http://securitytracker.com/id?1016270
http://securityreason.com/securityalert/1086