In case you are a Joomla! website owner do check your website as soon as possible. Joomla! is a well know CMS, second in popularity only to WordPress and as any other CMS does requires regular upgrades and updates.
In this case, Joomla! official website has released Joomla! 3.6.4 with two high priority Security Fixes. Both of these fixes prevent unwanted i.e. hacked users from registering on the website and thus exploiting it.
So even if the registration process has been disabled hackers can still create users that control the site, while the elevated privileges bug could enable users to perform advanced functions that ordinary users are not authorized to do.
What we’ve noticed with these releases is that as soon as they become publicly known, hackers start crawling the internet for sites with those exact vulnerabilities in hope owners did not know or are late to update their website.