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.
www.rapid7.com/db/modules/auxiliary/admin/http/joomla_registration_privesc
www.securityfocus.com/bid/93883
www.securitytracker.com/id/1037108
blog.sucuri.net/2016/10/details-on-the-privilege-escalation-vulnerability-in-joomla.html
developer.joomla.org/security-centre/660-20161002-core-elevated-privileges.html
github.com/joomla/joomla-cms/commit/bae1d43938c878480cfd73671e4945211538fdcf
medium.com/%40showthread/joomla-3-6-4-account-creation-elevated-privileges-write-up-and-exploit-965d8fb46fa2#.rq4qh1v4r
www.exploit-db.com/exploits/40637/