On the 25th of October 2016, the Joomla team issued a patch for a high severity security flaw that could allow a remote unauthenticated attacker to create administrative accounts on the target system. AppCheck was updated on the same day to detect and safely exploit the vulnerability.

Our security researchers observed scanning for this flaw against our honeypot systems within 24 hours of the initial disclosure, a common trend with popular CMS systems.

It is recommended that all Joomla installations should be updated immediately to version 3.6.4 or later.

https://www.joomla.org/announcements/release-news/5678-joomla-3-6-4-released.html

AppCheck can be used now to scan your environment for the vulnerability. If you don’t yet have an AppCheck account, use the Free Trial option to request an evaluation license.