y0dasCrypterv12

Hunt Hypothesis

This rule detects the presence of the y0dasCrypter v12 malware, a tool often used by adversaries to obfuscate payloads and evade static analysis during initial access or lateral movement. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify compromised endpoints or staging environments before the crypter is deployed to execute more complex post-exploitation activities.

YARA Rule

rule y0dasCrypterv12
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED F3 1D 40 00 B9 7B 09 00 00 8D BD 3B 1E 40 00 8B F7 AC [48] AA E2 CC }

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 development team uses Visual Studio Code or JetBrains Rider to compile a C++ project that includes the y0dasCrypter library for obfuscating a specific module. The compiler generates a binary containing the specific string patterns or structural artifacts targeted by the rule.

  • Filter/Exclusion: Exclude files located in standard build directories (e.g., C:\Users\<User>\AppData\Local\Temp\, C:\Projects\<ProjectName>\build\, or C:\Projects\<ProjectName>\obj\) and exclude processes originating from known compiler executables such as cl.exe, msbuild.exe, or dotnet.exe.

• Scenario: An IT administrator runs a PowerShell script to deploy a custom internal utility that leverages the y0dasCrypter API for encrypting configuration files. The script executes a small helper binary (e.g., config_encryptor.exe) that matches the YARA signature.

  • Filter/Exclusion: Exclude processes with a specific command-line argument pattern (e.g., *config_encryptor.exe* /encrypt*) or exclude binaries located in the internal tooling directory (e.g., C:\CorpTools\Utilities\) that are signed by the internal corporate code-signing certificate.

• Scenario: A scheduled task runs a nightly backup verification job that uses a custom Perl or Python script to hash and verify encrypted backup archives. The script invokes a lightweight C-based verification tool that shares the same obfuscation characteristics as the crypter.

  • Filter/Exclusion: Exclude processes spawned by schtasks.exe or Task Scheduler that have a parent process of svchost.exe (if run as a service) or explorer.exe (if run interactively), and exclude files with extensions like .py,