Cereal Malware Toolset Linked to the Turla APT Group

Summary

ESET has uncovered a new modular .NET malware toolset called Cereal, attributed to the Turla APT group. The framework is specifically built to target Microsoft Exchange servers for long-term cyberespionage. It uses advanced techniques such as hijacking email transport pipelines and abusing the .NET native image cache to maintain stealth and evade detection.

Investigation

The investigation began after researchers detected suspicious activity on a government Exchange server in Kazakhstan in November 2025. Further analysis exposed a modular toolset containing components such as CerealLoader and CerealBox. Additional telemetry later revealed similar activity targeting a Romanian government organization in December 2025.

Mitigation

Organizations should monitor for abnormal .NET execution behavior, particularly unmanaged entry points and Vtable fixups. Hardening Microsoft Exchange environments and detecting unauthorized changes to the EdgeTransport.exe process or existing transport agents are critical. Auditing the .NET native image cache for unexpected DLLs can also help uncover persistence mechanisms.

Response

When malicious activity is detected, responders should isolate the affected Exchange server and perform memory forensic analysis to identify injected modules. Review web traffic for unusual URL paths associated with CerealNeuron communication patterns. Conduct a comprehensive audit of installed Exchange transport agents and system-level .NET assemblies for unauthorized modifications.

Simulation Execution

• Attack Narrative & Commands: An adversary has successfully compromised an environment and is attempting to deploy the Turla Cereal toolset to establish command-and-control (C2) capabilities. To avoid detection by basic AV, they attempt to run the CerealLoader component. The adversary’s goal is to execute the loader which will then side-load malicious DLLs into a legitimate process. The simulation will mimic this by executing a dummy process with the exact command-line argument CerealLoader to verify if the SOC receives the alert.

• Regression Test Script:

  # Simulation script to trigger the Turla Cereal detection rule.
  # This mimics the execution of the CerealLoader component.
  
  $TargetString = "CerealLoader"
  Write-Host "[+] Starting simulation: Executing process with command line containing $TargetString"
  
  # We use cmd.exe to execute a dummy command that includes the target string in the arguments.
  Start-Process "cmd.exe" -ArgumentList "/c echo Simulation of $TargetString execution" -WindowStyle Hidden
  
  Write-Host "[+] Simulation command sent. Check SIEM for alert."

• Cleanup Commands:

  # No permanent files were created during this simulation.
  # If files were dropped, they would be removed here.
  Write-Host "[+] Cleanup complete. No artifacts to remove."