AvosLocker is a fairly recent ransomware-as-a service that has already been used to attack Windows and Linux systems in the Americas, Middle East and Asia-Pacific, according to Sophos.
AnyDesk is a remote desktop application for Windows, macOS, Linux, iOS and Android.
"Sophos discovered that the AvosLocker attackers installed AnyDesk so it works in Safe Mode, tried to disable the components of security solutions that run in Safe Mode, and then ran the ransomware in Safe Mode. This creates a scenario where the attackers have full remote control over every machine they've set up with AnyDesk, while the target organisation is likely locked out of remote access to those computers. Sophos has never seen some of these components used with ransomware, and certainly not together," said Sophos director of incident response Peter Mackenzie.
"The message for IT security teams facing such an attack is that even if the ransomware fails to run, until they clean every trace of the attackers' AnyDesk deployment from every impacted machine, they will remain exposed as the attackers have access to their organisation's network and can lock them out again at any time."
On Windows, AvosLocker is deployed via a batch file that disables Windows update services and Windows Defender, attempts to disable the components of commercial security software solutions that can run in Safe Mode, installs AnyDesk and sets it to run in Safe Mode, sets up a new account with auto login details, and then connects to a domain controller to remotely access and run the ransomware itself.
"The techniques used by AvosLocker are simple, but very clever. They ensure that the ransomware has the best chance of running in Safe Mode and allow the attackers to retain remote access to the machines throughout the attack," said Mackenzie.
"Sophos has reported on Snatch and BlackMatter implementing the technique, however, neither of these ransomware groups attempted to install a subsequent application, such as AnyDesk, for command and control of the machines while in Safe Mode. We believe we're seeing this for the first time."
Understandably, Sophos points out that its endpoint products – including Intercept X – protect users by detecting the actions and behaviours of ransomware and other attacks, such as those described in this Sophos research.
More details of Sophos's analysis are available here.