y0dasCrypterv11

Hunt Hypothesis

This rule detects the presence of the y0dasCrypter v1.1, a lightweight PE file crypter frequently used by threat actors to obfuscate malicious payloads and evade static analysis. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify compromised endpoints or staging environments where attackers are preparing to deploy encrypted malware, enabling early intervention before execution occurs.

YARA Rule

rule y0dasCrypterv11
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED 8A 1C 40 00 B9 9E 00 00 00 8D BD 4C 23 40 00 8B F7 33 }

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: Legitimate Software Packaging and Obfuscation

  • Context: Development teams or IT operations use tools like Inno Setup, NSIS, or Advanced Installer to create installers. Some custom packaging scripts or specific versions of 7-Zip with encryption/obfuscation flags may exhibit byte patterns that match the y0dasCrypterv11 signature, particularly if the rule targets generic XOR or RC4-like encryption headers often found in packed executables.
  • Filter/Exclusion: Exclude files located in standard build directories (e.g., C:\Builds\, C:\Temp\Installers\) or filter by parent process names such as iscc.exe (Inno Setup Compiler), makensis.exe (NSIS), or 7z.exe.

• Scenario: Enterprise Backup and Archiving Agents

  • Context: Backup solutions like Veeam Backup & Replication, Commvault, or Acronis True Image create encrypted backup archives (.vbk, .avb, .tib). If the YARA rule matches on encrypted container headers or specific magic bytes associated with encrypted archives, these large files on backup agents or backup servers may trigger the detection.
  • Filter/Exclusion: Exclude file extensions .vbk, .avb, .tib, .bak, and .zip when the parent process is a known backup agent (e.g., VeeamBackup.exe, commvaultagent.exe, acronisagent.exe). Additionally, exclude paths containing \Backup\ or \Archive\.

• Scenario: Virtual Machine Snapshot and Disk Image Files

  • Context: Hypervisors like VMware ESXi or Hyper-V generate virtual disk files (.vmdk, .vhdx, `.v