Are you serious?
There were many old versions of phpBB 2 that had major security holes, however people did not update their forum software, causing large amounts of damage on many forums around the net.
Ever since then, phpBB has ensured to keep very tight code. When phpBB 3 was in beta, a professional third party code auditing team was brought in to audit the code to ensure they did a good/secure job.
I would say phpBB 3 is one of the most secure piece of PHP based online forum software. |