Jelsoft vBulletin 3.5.4 allows remote attackers to register multiple arbitrary users and cause a denial of service (resource consumption) via a large number of requests to register.php.  NOTE: the vendor has disputed this vulnerability, stating "If you have the CAPTCHA enabled then the registrations wont even go through. ...  if you are talking about the flood being allowed in the first place then surely this is something that should be handled at the server level.