The forum is pretty secure when it comes to someone editing anothers post. Considering someone added modules and edited tabs, it seems someone else is using an admin account. THis is a problem beyond the forum module. The only security flaws of this magnitude are actually exposed via third party modules as far as I know. TO let you know, DNN did undergo a security audit from an outside company who specializes in this and received a pretty nice grade. The audit also included all flaws which have been corrected since then.