The good guys won one. It took a commercial AV vendor teaming up with the French Gendarmerie to make it happen, but happen it did.
Let's start at the beginning. Retadup is a malicious worm affecting Windows machines located in Latin America. It tries to achieve persistence on its victims' computers, to spread itself far and wide and to install additional malware payloads on infected machines. Usually, it's malware mining cryptocurrency on the malware authors' behalf. But Stop ransomware and the Arkei password stealer has also been seen using the worm.
AV vendor Avast first looked at the worm in detail and started monitoring its activity closely in March 2019. How it was evading Avast's AV detection piqued the company's interest.
Avast described what it found in the blog that it posted about the effort. "After analyzing Retadup more closely," it said, "we found that while it is very prevalent, its C&C [command and control] communication protocol is quite simple. We identified a design flaw in the C&C protocol that would have allowed us to remove the malware from its victims' computers had we taken over its C&C server. This made it possible to put an end to Retadup and protect everyone from it, not just Avast users (note that while it is often possible to clean malware infections by taking over a C&C server and pushing a 'malware removal' script to the victims through the malware's established arbitrary code execution channel, the design flaw we found did not involve making the victims execute any extra code)."
Avast found a way to kill the C&C server and the malware with one action.
Retadup's C&C infrastructure was mostly located in France so Avast decided to contact the French National Gendarmerie at the end of March to share what it had found.
Les flics gave their own cooperation. They obtained a snapshot of the C&C server's disk from its hosting provider and shared parts of it (no PII!) with Avast so they could start to reverse engineer the contents of the C&C server.
In July 2019, the Gendarmerie received the green light to proceed with the disinfection. They replaced the malicious C&C server with a prepared by Avast disinfection server that made connected instances of Retadup self-destruct.
Some parts of the C&C infrastructure were found to be located in the US. The Gendarmerie alerted the FBI who took them down on July 8. Avast was able to determine post-mortem the most infected computers had either two or four cores (the average number of infected computer cores was 2.94) and that the majority of victims used Windows 7. Over 85% of Retadup's victims also had no third-party antivirus software installed.
The good guys got lucky this time. A flaw in the malware could be exploited to cause its downfall. The next time luck may not be a factor.
— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.