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.
https://github.com/joomla/joomla-cms/commit/bae1d43938c878480cfd73671e4945211538fdcf
https://developer.joomla.org/security-centre/660-20161002-core-elevated-privileges.html