Description
Joomla! Core is prone to multiple vulnerabilities, including security bypass and privilege escalation vulnerabilities. Exploiting these issues could allow an attacker to perform otherwise restricted actions and subsequently register on a site even when registration has been disabled, or to bypass the expected capabilities check and register with elevated privileges. Joomla! Core versions 3.x.x ranging from 3.4.4 and up to and including 3.6.3 are vulnerable.
Remediation
Update to Joomla! Core version 3.6.4 or latest
References
https://gist.github.com/melvinsh/4b28f78ace12ec0c02b183bbadc76da8
https://blog.fortinet.com/2016/10/27/joomla-from-no-one-to-the-highest-privilege
https://blog.sucuri.net/2016/10/details-on-the-privilege-escalation-vulnerability-in-joomla.html
https://github.com/XiphosResearch/exploits/tree/master/Joomraa
http://blog.knownsec.com/2016/10/joomla-register-users/
https://developer.joomla.org/security-centre/659-20161001-core-account-creation.html
https://developer.joomla.org/security-centre/660-20161002-core-elevated-privileges.html
https://developer.joomla.org/security-centre/661-20161003-core-account-modifications.html