robzilla - 9:07 am on Jul 5, 2012 (gmt 0)
As far as open-source forum software goes, phpBB is about as secure and reliable as you can get, and my experience with it has been positive. It has a relatively solid reputation, and I believe phpBB3 has in recent years been subjected to an external security audit. Of course, you need to do your part in keeping it (and any software it relies on) up to date, and exert caution when installing third-party plug-ins. If you are paranoid or dealing with truly sensitive data on the server, and your budgets allows for it, you could always consider getting a seperate server or hosting package for the forum and host phpBB on a subdomain.