EXECryptorvxxxx

Hunt Hypothesis

This rule detects the presence of executable files exhibiting characteristics associated with the EXECryptor family, a known threat actor group that leverages custom encryption and obfuscation techniques to hide malicious payloads. Proactively hunting for these signatures in Azure Sentinel allows the SOC team to identify dormant or active implants before they can establish persistence or execute their intended post-exploitation actions.

YARA Rule

rule EXECryptorvxxxx
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 24 [3] 8B 4C 24 0C C7 01 17 ?? 01 ?? C7 81 B8 [7] 31 C0 89 41 }

condition:
		$a0 at pe.entry_point
}

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 runs a custom build script that uses UPX (Ultimate Packer for eXecutables) to compress a newly compiled C++ or Go binary before deploying it to a staging environment. The compression algorithm alters the entropy and header structure of the PE file, causing the YARA rule to flag the executable as a potential crypter.

  • Filter/Exclusion: Exclude files located in specific build artifact directories (e.g., C:\Builds\dist\, D:\Artifacts\) or filter by parent process cmd.exe/powershell.exe when the file path matches *.exe and the size is under 50MB.

• Scenario: An IT administrator manually updates a legacy line-of-business application by replacing the old app.exe with a new version that was packed using ASPack or PECompact to reduce disk footprint. The YARA rule detects the packing header and high entropy, triggering an alert on the server where the file was copied via SMB.

  • Filter/Exclusion: Create a whitelist for known legacy application directories (e.g., C:\Program Files\LegacyApp\) and exclude files modified within the last 24 hours if the parent process is explorer.exe or cmd.exe and the user is in the Domain Admins group.

• Scenario: A CI/CD pipeline (e.g., Jenkins or Azure DevOps) compiles a .NET application and uses ILRepack or a similar tool to merge multiple DLLs into a single executable, resulting in a packed binary. The YARA rule flags the merged executable during the deployment step to the production web servers.

  • Filter/Exclusion: Exclude files whose parent process is a known CI/CD agent (e.g., jenkins.exe,