usercp_avatar.php in PHPBB 2.0.20, when avatar uploading is enabled, allows remote attackers to use the server as a web proxy by submitting a URL to the avatarurl parameter, which is then used in an HTTP GET request.
Cross-site scripting vulnerability in phpBB 1.4.4 and earlier allows remote attackers to execute arbitrary Javascript on web clients by embedding the script within an IMG image tag while editing a message.