The register method in the UsersModelRegistration class in controllers/user.php in the Users component in Joomla! before 3.6.4 allows remote attackers to gain privileges by leveraging incorrect use of unfiltered data when registering on a site.
Joomla! < 3.6.3 - Unauthenticated User Account Creation via UsersModelRegistration
The register method in the UsersModelRegistration class in controllers/user.php in the Users component in Joomla! before 3.6.4, when registration has been disabled, allows remote attackers to create user accounts by leveraging failure to check the Allow User Registration configuration setting.