phpBB 2.0.20 does not verify user-specified input variable types before being passed to type-dependent functions, which allows remote attackers to obtain sensitive information, as demonstrated by the (1) mode parameter to memberlist.php and the (2) highlight parameter to viewtopic.php that are used as an argument to the htmlspecialchars or urlencode functions, which displays the installation path in the resulting error message.
phpBB 2.0.19 and earlier allows remote attackers to cause a denial of service (application crash) by (1) registering many users through profile.php or (2) using search.php to search in a certain way that confuses the database.