Passing an absolute path to a file_exists check in phpBB before 3.2.4 allows Remote Code Execution through Object Injection by employing Phar deserialization when an attacker has access to the Admin Control Panel with founder permissions.
https://www.phpbb.com/community/viewtopic.php?f=14&t=2492206
https://lists.debian.org/debian-lts-announce/2018/11/msg00029.html
https://blog.ripstech.com/2018/phpbb3-phar-deserialization-to-remote-code-execution/