Forum branches 0.4, 0.5 and master have recently been patched to cover a security exploit.
Tags 0.4.1 and 0.5.1 contain the fixes as well as the master branch
https://github.com/silverstripe/silverstripe-forum/archive/0.4.1.zip
https://github.com/silverstripe/silverstripe-forum/archive/0.5.1.zip
It is recommended you upgrade or patch any existing installs.
Thanks.