One flaw was patched in version 3.6.3 released on 25 October, four days after the Joomla! team was made aware of the bug, which was described thus: "Incorrect use of unfiltered data allows for users to register on a site with elevated privileges."
The team then released 3.6.4 on 27 October and issued an updated advisory.
In it they said: "As stated in the (previous) advisory, the incorrect use of unfiltered data in Joomla! 3.4.4 through 3.6.3 allowed a malicious user to register on a website with elevated privileges.
"Although the issue relies on an earlier exploit which has already been resolved in the 3.6.4 release, security advisory 20161003 was published for this issue."
Web security firm Sucuri said that both vulnerabilities were being actively exploited. The first attacks, according to Sucuri, began at 1pm UTC on 26 October (midnight the same day AEDT) less than 24 hours after the Joomla! team acknowledged the first exploit.
"Most of them were looking for the user.register tasks and trying to create users. They were especially targeting some of the most popular Joomla! sites," an entry on the Sucuri blog said, describing these as "initial pokes". They would register in web logs as POST /index.php?option=com_users&task=user.register HTTP/1.1.
About seven hours later, Sucuri wrote, "a couple of IPs from Romania started a mass attack against thousands of different Joomla! sites. In all of them, they tried to create a username called db_cfg with the password fsugmze3. They were going to the same URL with a payload that looks like:
188.8.131.52 - - [26/Oct/2016:18:09:24 -0400]
user[name] = db_cfg
user[username] = db_cfg
user[password1] = fsugmze3
user[password2] = fsugmze3
Sucuri said Joomla! users could look for the db_cfg username on their sites to see if their sites had been compromised. Else, the presence of the IP addresses 184.108.40.206, 220.127.116.11 and 18.104.22.168 in logs would indicate a hack.
But after these attempts, a host of others had joined in, and "some of them are even automating the upload of backdoors and using some unique techniques to bypass the media uploader (using .pht files)", Sucuri wrote. "That led to a massive increase in IP addresses trying to exploit this vulnerability using different patterns and techniques."