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.
https://exchange.xforce.ibmcloud.com/vulnerabilities/26537
http://securityreason.com/securityalert/1470
http://secunia.com/advisories/20093
http://archives.neohapsis.com/archives/bugtraq/2006-05/0238.html