Recent cybersecurity research has uncovered AvosLocker ransomware samples abusing the Avast Anti-Rootkit Driver file to disable anti-virus, which allows adversaries to evade detection and block defense. AvosLocker is known to represent a relatively novel ransomware family that appeared in the cyber threat arena to replace the infamous REvil, which was one of the most active ransomware variants in 2021 until the official shutdown of its operators.
In this latest cyber-attack, AvosLocker ransomware is also observed to weaponize a set of endpoints for Log4Shell, a notorious zero-day vulnerability in Apache Log4j Java logging library that has compromised hundreds of millions of devices across the globe. The ransomware enabled scanning for Log4Shell leveraging the malicious Nmap NSE script.
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First observed in July 2021 and acting as a Ransomware-as-a-Service (RaaS) model, AvosLocker ransomware targets food and beverage sectors, tech and finance industries, telecom and government entities, with India, Canada, and the U.S. being spotted as the top affected countries based on the malicious activity spanning half a year from July 2021 through February 2022. According to the joint Cybersecurity Advisory issued by FBI and FinCEN, AvosLocker ransomware has also hit critical U.S. infrastructure, including financial services and government entities.
Based on the new research by Trend Micro security analysts, a new variant of AvosLocker ransomware began sweeping across the globe, standing out from other strains of this ransomware family as the first one to disable anti-virus solutions on the infected devices.
The most probable initial point of access is the Zoho ManageEngine ADSelfService Plus (ADSS) exploit. Upon the successful penetration, adversaries launch mshta.exe to remotely execute an HTML application (HTA) file from their C&C server. The HTA ran an obfuscated PowerShell script with a shellcode that allowed it to connect to the server and run arbitrary commands on a host operating system. Besides, PowerShell downloads and launches the remote desktop tool AnyDeskMSI, used to distribute ransomware payload and tools leveraged for further system compromise.
Apart from scanning for an infamous Log4Shell vulnerability, tracked as CVE-2021-44228, AvosLocker ransomware targets other unpatched vulnerabilities to penetrate a targeted network. This new variant of AvosLocker ransomware samples misuses a driver file (Avast Anti-Rootkit Driver) to disable anti-virus software to establish its stealthy presence. After disabling defense, the AvosLocker operators transfer other tools, including Mimikatz and Impacket.
Adversaries use PDQ, which is a software deployment tool to deliver a malicious batch script on a targeted system. The batch script has a broad range of features, including the ability to kill the processes of several Windows products, such as Windows Error Recovery or Windows Update, as well as prohibit security software safe boot execution, create a new admin account, and run the malicious code to spread infection.
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