MZ Crypt10byBrainSt0rm

Hunt Hypothesis

This rule targets the execution of the MZ Crypt10byBrainSt0rm malware, a known crypter often used to obfuscate payloads and evade static analysis during initial access or lateral movement. Proactively hunting for this signature in Azure Sentinel allows the SOC to identify compromised endpoints early, as the presence of this specific crypter frequently indicates an adversary attempting to establish a foothold or deploy additional tooling within the environment.

YARA Rule

rule MZ_Crypt10byBrainSt0rm
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED 25 14 40 00 8B BD 77 14 40 00 8B 8D 7F 14 40 00 EB 28 83 7F 1C 07 75 1E 8B 77 0C 03 B5 7B 14 40 00 33 C0 EB 0C 50 8A A5 83 14 40 00 30 26 58 40 46 3B 47 10 76 EF 83 C7 28 49 0B C9 75 D4 8B 85 73 14 40 00 89 44 24 1C 61 FF E0 }

condition:
		$a0
}

Deployment Notes

This YARA rule can be deployed in the following contexts:

• Microsoft Defender for Endpoint — Custom indicators / advanced hunting
• Email Gateway — Attachment scanning
• File Share Monitoring — Periodic scanning of shared drives
• YARA CLI — Manual threat hunting on endpoints

This rule contains 1 string patterns in its detection logic.

• Source Rule

False Positive Guidance

• Antivirus/EDR Self-Scanning or Update Packages: When security tools like CrowdStrike Falcon, Microsoft Defender for Endpoint, or SentinelOne perform full-system scans or apply updates, they often load temporary executable files or memory-mapped images that may contain specific encryption headers or obfuscated payloads matching the MZ_Crypt10byBrainSt0rm signature.

  • Filter: Exclude processes originating from known security tool paths (e.g., C:\Program Files\CrowdStrike\, C:\Program Files\Microsoft Defender\) or filter by process name (e.g., FalconSensor.exe, MsMpEng.exe).

• Legacy .NET Framework Compilation or JIT Compilation: During the execution of large .NET applications or when the Just-In-Time (JIT) compiler generates native code, temporary assembly files or in-memory images may exhibit characteristics that YARA rules targeting packed or encrypted MZ headers can misinterpret as crypters. This is common in development environments or CI/CD pipelines running build agents.

  • Filter: Exclude processes related to .NET runtime or build tools (e.g., dotnet.exe, msbuild.exe, csc.exe) or filter by parent process being a build agent (e.g., Agent.Worker.exe for Azure DevOps, jenkins.exe for Jenkins).

• Software Update Installers (NSIS/Inno Setup): Major enterprise software updates (e.g., Adobe Creative Cloud, Oracle Java, or SAP components) often use installers that extract and execute temporary encrypted or compressed payloads. These temporary executables, created in Temp or ProgramData, can match generic MZ cryptor signatures if the installer uses specific compression or encryption algorithms.

  • Filter: Exclude processes with names containing “Setup”, “Install”, or “Uninstall” (e.g., Setup.exe, `