y0dasCrypterv1xModified

Hunt Hypothesis

This detection identifies potential file integrity modifications by the Y0DAS Crypter v1.x malware, signaling an adversary’s attempt to alter system artifacts as part of a persistence or evasion strategy. Proactively hunting for this behavior in Azure Sentinel is essential to uncover low-severity anomalies that may indicate early-stage compromise before they escalate into significant data encryption events.

YARA Rule

rule y0dasCrypterv1xModified
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 81 ED [4] B9 [2] 00 00 8D BD [4] 8B F7 AC }

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

Here are 4 specific false positive scenarios for the y0dasCrypterv1xModified detection rule, including suggested filters and exclusions:

• Scenario: Scheduled Antivirus Definition Updates

  • Context: The enterprise endpoint protection suite (e.g., CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint) performs nightly definition updates. During this process, the update engine modifies its local database files and executable components within the C:\ProgramData\Antivirus directory to apply new signatures.
  • Suggested Filter: Exclude file modification events where the source process is the specific AV service (e.g., falcon.sys, s1agent.exe) AND the modified file path contains keywords like “definitions”, “signatures”, or “updates”.

• Scenario: System Backup and Archiving Jobs

  • Context: A scheduled task running via Windows Task Scheduler triggers a backup utility (e.g., Veeam Agent, Acronis Cyber Protect, or Robocopy) to archive logs or compress user data. These tools often modify existing log files, configuration manifests, or temporary cache folders during the archiving process, which may match the YARA signature for modified cryptographic artifacts.
  • Suggested Filter: Create an exclusion rule based on the parent process name (e.g., VeeamService.exe, AcronisBackup.exe) and restrict the scope to specific backup directories such as D:\Backups or C:\ProgramData\Archive.

• Scenario: Software Deployment via Configuration Management

  • Context: The IT operations team uses Microsoft Endpoint Configuration Manager (MECM/SCCM) or Ansible to push a patch or application update. During the deployment, the installer modifies registry keys and updates local installation binaries in `C:\Program Files\AppName