Cross-site scripting (XSS) vulnerability in MyBB before 1.6.5 allows remote attackers to inject arbitrary web script or HTML via vectors related to "usernames via AJAX."
https://exchange.xforce.ibmcloud.com/vulnerabilities/71461
http://www.securityfocus.com/bid/50816
http://secunia.com/advisories/46951
http://blog.mybb.com/2011/11/25/mybb-1-6-5-released-feature-update-security-maintenance-release/