Joomla! 1.6.x/1.7.x/2.5.0-2.5.2 suffers from a privilege escalation vulnerability that allows users to be registered into any group not having 'core.admin' privileges.
Joomla! versions 1.0.x, 1.5.x, and 2.5.3+ are not vulnerable. No patch has been issued for 1.6.x or 1.7.x and users of these versions are strongly urged to upgrade to 2.5.3 immediately.