Well, technically no forum software out there will be hack free, its just a matter of if hackers find these hacks or not. What do hackers want to do, hack of course. So they will exploit larger board softwares out there so they can hack mroe. If you go with a free, and small board software out there, chances are very high that there will be no hacks for it. Therefore, free softwares can be safe in that sense. If I were you, I would go with
PowerBB. I have personally tested it for exploits and it seems pretty safe.
EDIT-Also, do not think boards like IPB and vB are safe just cause you have to pay for them. There are many exploits out there too. But since it is a paid software, there will be more updates to keep it safer.