Cross-site scripting (XSS) vulnerability in the editavatar page in vBulletin 3.5.1 allows remote attackers to inject arbitrary web script or HTML via a URL in the remote avatar url field, in which the URL generates a parsing error, and possibly requiring a trailing extension such as .jpg.
http://www.vbulletin.com/forum/showthread.php?t=166391
http://www.securityfocus.com/bid/16128
http://pridels0.blogspot.com/2005/11/vbulletin-351-xss-vuln.html