The cache handler in MyBB (aka MyBulletinBoard) before 1.8.4 does not properly check the encoding of input to the var_export function, which allows attackers to have an unspecified impact via unknown vectors.
http://www.securitytracker.com/id/1031953
http://www.securityfocus.com/bid/73257
http://blog.mybb.com/2015/02/15/mybb-1-8-4-released-feature-update-security-maintenance-release/