To my (quite extensive, heh) knowledge, Energizer is right; there is no secure forum.
But I also believe IPB is by far the most secure at the moment, Assuming you use proper precautions.
The security center in ACP helps with security a great deal though, and I don't know any exploits in IPB myself currently.
Long story short, a hacking isn't neccesarily an issue with the software, no, and IPB is the most secure out there at the moment in my opinion, yes ;)