SimplePack1XMethod2bagie

Hunt Hypothesis

This hypothesis targets the execution of SimplePack1, a lightweight Windows packer frequently used by threat actors to obfuscate malicious payloads and evade static analysis. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to identify potentially compromised hosts or staged binaries before they are unpacked and executed, reducing the window of opportunity for adversaries to establish persistence.

YARA Rule

rule SimplePack1XMethod2bagie
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 4D 5A 90 EB 01 00 52 E9 ?? 01 00 00 50 45 00 00 4C 01 02 00 00 00 00 00 00 00 00 00 00 00 00 00 E0 00 0F 03 0B 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0C 00 00 00 00 [3] 00 10 00 00 00 02 00 00 01 00 00 00 00 00 00 00 04 00 00 00 00 00 00 00 }

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

• Scenario: A developer or DevOps engineer uses SimplePack (a lightweight .NET packer) to obfuscate a small utility script or internal tool before deploying it to a test environment or staging server.

  • Filter/Exclusion: Exclude files located in specific development directories (e.g., C:\Dev\, C:\Projects\, C:\Temp\) or exclude processes where the parent process is a known IDE or build tool (e.g., dotnet.exe, msbuild.exe, code.exe).

• Scenario: An automated backup or archival job uses a custom script that packs configuration files or small data blobs using SimplePack to reduce size or protect sensitive keys before transferring them to a secure storage location.

  • Filter/Exclusion: Exclude files with specific extensions (e.g., .cfg, .json, .xml) that are being written by known service accounts or scheduled tasks (e.g., Task Scheduler jobs named Backup_Config, Archive_Secrets).

• Scenario: A security team or red team uses SimplePack to create a benign test payload for validating detection coverage in a non-production environment (e.g., a CI/CD pipeline or a dedicated test VM).

  • Filter/Exclusion: Exclude hosts tagged as “Test”, “Dev”, or “CI-CD” in your asset management system, or exclude processes running under user accounts with names like svc-test, ci-runner, or build-agent.

• Scenario: A third-party application installer or updater uses SimplePack internally to unpack or pack components during installation, particularly in older or niche enterprise applications that rely on custom packaging.

  • Filter/Exclusion: Exclude files located in vendor-specific installation directories (e.g., C:\Program Files\VendorApp\, `C:\Program Files