Researcher Denis Sinegubko says he has found a valid file hiding system called at.php, which executes a query to a remote server for 10,000 domains ordered alphabetically. The scan tool then connects to these domains and finds 14 files with the same names as Adminer's script or plugin.
Sinegubko said that once the detection page is running Adminer, it will save the page and the URL is working into a file with a simple name 'c'. After scanning, it will continue to work with other domains.
We can only estimate that the attacker either uses one of the Adminer vulnerabilities to gain access to the database management interface or to use the wrong test method to break into the instance. Adminer has a default or easy-to-guess password.
Adminer as well as phpMyAdmin, SQL Buddy and similar tools, does not have a protection system against the wrong type of attack. Webmasters who use a web-based GUI to manage databases need to consider switching to the CLI interface or installing WAF. If you can't use the free versions, you can use free tools like ModSecurity or NinjaFirewall.