Simple Machines Forum (SMF) 1-0-5 and earlier supports the use of URLs for avatar images, which allows remote attackers to monitor sensitive information of forum visitors such as IP address and user agent, as demonstrated using a PHP script on a malicious server.
https://exchange.xforce.ibmcloud.com/vulnerabilities/22093
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-2818
http://securitytracker.com/id?1014828
http://secunia.com/advisories/16646