If you are using the Private Messaging plugin
by Joshua Hutchins / ardentfrost.rayd.org
or even Detective’s mod version of it, you need to take it offline immediately.
There are some very serious, multiple security problems with it.
If you are running it, your site can easily be hacked.
Please take this warning seriously, it’s not worth the headache.
Make sure you delete not only the plugin file
but the additional files it uses in the bbPress root.