This issue was first reported by lacton via the security tracker.
In certain conditions, phpBB is exposing the full path of cron.php. ie apache access log shows requests:
"GET /var/www/jadephpbb/httpdocs/cron.php?cron_type=tidy_search HTTP/1.1" 404 304 "https://forums.jadeworld.com/viewtopic.php?f=9&t=1206&start=0"
Support topic: http://www.phpbb.com/community/viewtopic.php?f=46&t=2121664