Joomla! 1.5.x, 2.x, and 3.x before 3.4.6 allow remote attackers to conduct PHP object injection attacks and execute arbitrary PHP code via the HTTP User-Agent header, as exploited in the wild in December 2015.
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.