Question: Is phpBB safe to use?
Answer: Yes, generally speaking phpBB is safe. Over a decade ago phpBB had some security issues but since that time the code for phpBB has been improved and audited a few times.
phpBB hasn’t made headlines over the last few years for any major security exploits. There has been an improvement in the security procedures and practices for phpBB that has led it to become more secure.
Although the base package for phpBB may be secure with no known exploits it does not mean that your web hosting account or server may not be exploited. There could be other vectors and unknown exploits or weakness in your web hosting environment that can lead your web hosting account to be compromised.