Unspecified vulnerability in MyBB (aka MyBulletinBoard) before 1.8.4 has unknown attack vectors related to "Group join request notifications sent to wrong group leaders."
https://euvd.enisa.europa.eu/vulnerability/EUVD-2015-2875
http://www.securityfocus.com/bid/73394
http://blog.mybb.com/2015/02/15/mybb-1-8-4-released-feature-update-security-maintenance-release/