Yoda Crypter 1.3

Hunt Hypothesis

This rule detects the execution of the Yoda Crypter, a lightweight malware tool often used by adversaries to encrypt files or establish persistence during initial access phases. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify low-severity compromises that may be overlooked by standard alerts, ensuring early containment of potential ransomware precursors or crypter-based attacks.

YARA Rule

rule yoda_crypter_1_3 : Crypter
{
	meta:
		author="Kevin Falcoz"
		date_create="15/04/2013"
		description="Yoda Crypter 1.3"

	strings:
		$signature1={55 8B EC 53 56 57 60 E8 00 00 00 00 5D 81 ED 6C 28 40 00 B9 5D 34 40 00 81 E9 C6 28 40 00 8B D5 81 C2 C6 28 40 00 8D 3A 8B F7 33 C0 EB 04 90 EB 01 C2 AC}

	condition:
		$signature1 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 DevOps engineer uses yodacrypt (or a similar custom crypter wrapper) to obfuscate a small PowerShell script or C# payload before deploying it to a staging environment for testing, where the crypter is executed locally to generate the encrypted output.
  • Filter/Exclusion: Exclude processes where the parent process is a development IDE (e.g., code.exe, idea64.exe, visualstudio.exe) or a build tool (e.g., msbuild.exe, dotnet.exe) and the command line arguments contain keywords like --output, --encrypt, or -o.
• Scenario: A security team runs a YARA rule test or a crypter benchmark in an isolated lab VM or a dedicated “Crypto-Test” OU, where the crypter binary is intentionally executed to verify detection coverage.
  • Filter/Exclusion: Exclude events originating from specific lab hostnames (e.g., *.lab.local) or security test accounts (e.g., svc-security-test, admin-crypto-bench) and limit exclusions to non-production AD groups.
• Scenario: A scheduled maintenance job on a build server uses a custom crypter tool to encrypt temporary build artifacts or license keys before archiving them, running under a service account with limited privileges.
  • Filter/Exclusion: Exclude processes running under known service accounts (e.g., NT AUTHORITY\NETWORK SERVICE, svc-build-agent) where the working directory is within standard build paths (e.g., C:\builds\, D:\artifacts\) and the process name matches the crypter binary (e.g., yodacrypt.exe, crypter13.exe).
• Scenario: An application installer or updater uses a crypter to protect embedded configuration files or small native DLLs during the installation