y0dasCrypterv10

Hunt Hypothesis

This rule detects the presence of the y0dasCrypter v10 tool, a lightweight executable packer often used by threat actors to obfuscate malicious payloads and evade static analysis. Proactively hunting for this indicator in Azure Sentinel allows the SOC to identify potentially compromised endpoints or staging areas where attackers are preparing to deploy cryptic malware before execution.

YARA Rule

rule y0dasCrypterv10
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED E7 1A 40 00 E8 A1 00 00 00 E8 D1 00 00 00 E8 85 01 00 00 F7 85 }

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

• Legacy Application Deployment via Group Policy: When IT administrators deploy older, unmanaged line-of-business applications (e.g., specific versions of Adobe Reader, Java Runtime Environment, or legacy ERP clients) via Group Policy Software Installation, the installer or the application binary itself may match the y0dasCrypterv10 signature due to shared code patterns or generic encryption libraries.
  • Filter/Exclusion: Exclude files located in standard application directories (e.g., C:\Program Files\, C:\Program Files (x86)\) that are owned by known service accounts or have a valid digital signature from trusted vendors (e.g., Microsoft, Adobe, Oracle).
• Scheduled Backup and Archiving Jobs: Enterprise backup solutions (such as Veeam, Commvault, or Windows Server Backup) often create temporary archive files or encrypted backup sets during scheduled nightly or weekly jobs. If the backup agent uses a compression or encryption algorithm that shares characteristics with the YARA rule, the resulting .vbk, .bck, or .zip files in the backup repository may trigger the detection.
  • Filter/Exclusion: Exclude files with common backup extensions (.vbk, .bck, .bkf, .zip, .7z) located in designated backup share paths (e.g., \\backupserver\backups\) or created by known backup service accounts (e.g., svc_backup, vcenter).
• Development and Build Pipeline Artifacts: In CI/CD pipelines (e.g., Jenkins, Azure DevOps, GitHub Actions), build agents frequently compile, package, or encrypt artifacts (JARs, WARs, NUPKGS, or encrypted container images) in temporary working directories. These intermediate files may match the rule if they contain specific bytecode patterns or encrypted metadata.
  • Filter/Exclusion: Exclude files in temporary build directories (e