Joomla! Core is prone to a privilege escalation vulnerability. Exploiting this issue may allow attackers to bypass the expected capabilities check and perform otherwise restricted actions; subsequently, they can modify existing user accounts (reset username, password, and user group assignments). Joomla! Core versions ranging from 1.6.0 and up to and including 3.6.4 are vulnerable.
Update to Joomla! Core version 3.6.5 or latest