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A malicious worm, Retadup, affected 850k Windows machines throughout Latin America. The objective of the Retadup worm is to obtain persistence on victims’ computers to spread itself far and wide and to install additional malware payloads on infected machines.

Source: Avast.io

The Avast antivirus team started closely monitoring activities of the Retadup worm in March 2019. Jan Vojtěšek, a malware analyst at Avast who led research into Retadup said, “The general functionality of this payload is pretty much what we have come to expect from common malicious stealthy miners.”  “In the vast majority of cases, the installed payload is a piece of malware mining cryptocurrency on the malware authors’ behalf. However, in some cases, we have also observed Retadup distributing the Stop ransomware and the Arkei password stealer,” Vojtěšek writes.

A few days ago, Vojtěšek shared a report informing users that Avast researchers, the French National Gendarmerie and FBI have together disinfected the Retadup virus, by making the threat to self-destruct.

When the Avast team analyzed the Retadup worm closely they identified a design flaw in the (Command-and-Control) C&C protocol that “would have allowed us to remove the malware from its victims’ computers had we taken over its C&C server,” Vojtěšek writes.

As Retadup’s C&C infrastructure was mostly located in France, Vojtěšek’s team decided to contact the  Cybercrime Fighting Center (C3N) of the French National Gendarmerie (one of two national police forces of France) at the end of March. The team shared their findings with the Gendarmerie proposing a disinfection scenario that involved taking over a C&C server and abusing the C&C design flaw in order to neutralize Retadup.

In July 2019, the Gendarmerie received the green light to legally proceed with the disinfection. To do this, they replaced the malicious C&C server with a prepared disinfection server that made connected instances of Retadup self-destruct. “In the very first second of its activity, several thousand bots connected to it in order to fetch commands from the server. The disinfection server responded to them and disinfected them, abusing the C&C protocol design flaw,” the report states.

The Gendarmerie also alerted the FBI of this worm as some parts of the C&C infrastructure were also located in the US. The FBI took them down successfully and on July 8, the malware authors no longer had any control over the malware bots, Vojtěšek said.

“Since it was the C&C server’s responsibility to give mining jobs to the bots, none of the bots received any new mining jobs to execute after this takedown. This meant that they could no longer drain the computing power of their victims and that the malware authors no longer received any monetary gain from mining,” the report explained.

Avast report highlights, “Over 85% of Retadup’s victims also had no third-party antivirus software installed. Some also had it disabled, which left them completely vulnerable to the worm and allowed them to unwittingly spread the infection further.”

Retadup has many different variants of its core, which is written in either AutoIt or AutoHotkey. Both cases contain two files, the clean scripting language interpreter and the malicious script. “In AutoHotkey variants of Retadup, the malicious script is distributed as source code, while in AutoIt variants, the script is first compiled and then distributed. Fortunately, since the compiled AutoIt bytecode is very high-level, it is not that hard to decompile it into a more readable form,” the report states.

Users and researchers are congratulating both the Avast team and the Gendarmerie to successfully disinfect the Retadup.

To know more about Retadup in detail, read Avast’s complete report.

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