Earlier findings highlighted how Russian-linked groups exploited Microsoft Hyper-V to install covert Linux virtual machines on targeted systems. However, because enterprise environments typically restrict or closely monitor Hyper-V, attackers have shifted to less scrutinized alternatives.
Security firm Sophos reports active misuse of QEMU, which enables attackers to operate a full Linux system within a Windows host. Activities carried out inside these virtual machines are largely undetectable by endpoint protection tools such as Windows Defender.
“Rather than deploying a pre-built toolkit, the attackers manually install and compile their full attack suite within the VM, including Impacket, KrbRelayx, Coercer, BloodHound.py, NetExec, Kerbrute, Metasploit, and supporting libraries for Python, Rust, Ruby, and C++,” Sophos said in a report detailing active exploitation campaigns.
Attackers frequently rely on Alpine Linux, particularly version 3.22.0, due to its minimal size and low resource consumption. This allows the malicious VM to operate with almost no visible impact on the host system.
Once their objectives are achieved, attackers can simply shut down the VM, erase its image, and disappear without leaving significant traces.
“Attackers are drawn to QEMU and more common hypervisor-based virtualization tools like Hyper-V, VirtualBox, and VMware,” Sophos researchers said.
“Malicious activity within a virtual machine (VM) is essentially invisible to endpoint security controls and leaves little forensic evidence on the host itself.”
One group leveraging this technique is linked to the PayoutsKing ransomware campaign and tracked as STAC4713. In observed cases, attackers used QEMU to establish covert reverse SSH backdoors, enabling them to deploy additional malicious payloads.
Even though a basic QEMU setup can run without administrative privileges, attackers often escalate access by launching VMs under a SYSTEM account via scheduled tasks. They disguise virtual disk files as innocuous items like “vault.db” and later shift to obscure DLL filenames such as “birsv.dll.”
Through these hidden VMs, attackers create reverse SSH tunnels to remote servers, granting full control over compromised systems. They also exploit built-in Windows applications like Paint, Notepad, and Edge to explore network shares and access files.
Another threat actor, identified as STAC3725, deployed a QEMU-based VM in February to conduct credential harvesting and system reconnaissance. This setup enabled activities such as Kerberos enumeration, Active Directory mapping, and even running FTP servers for staging malware or exfiltrating data.
“The abuse of QEMU represents a growing evasion trend where threat actors leverage legitimate virtualization software to conceal malicious actions from endpoint protection agents and audit logs,” Sophos warns.
“A hidden VM with a pre-loaded or compiled attack toolkit can enable a threat actor to have long-term access to a network, providing the ability to deploy malware, harvest credentials, and move laterally without leaving evidence on the host itself.”
To mitigate such risks, researchers advise IT teams to regularly audit systems for unexpected QEMU installations and suspicious scheduled tasks, especially those running under SYSTEM-level privileges. Indicators of compromise may include unusual SSH port forwarding (particularly port 22), outbound SSH connections from uncommon ports, and virtual disk files with atypical extensions such as .db, .dll, or .qcow2.