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.
Cross-site scripting (XSS) vulnerability in private.php in MyBB 1.1.2 allows remote attackers to inject arbitrary web script or HTML via the do parameter.