CodeCryptv0164

Hunt Hypothesis

This detection identifies potential code obfuscation or encryption activities within executable files that may indicate an adversary attempting to conceal malicious logic from static analysis tools. Proactively hunting for this behavior in Azure Sentinel is essential to uncover stealthy threats that evade traditional signature-based defenses by masking their underlying code structure before execution.

YARA Rule

rule CodeCryptv0164
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E9 2E 03 00 00 EB 02 83 3D 58 EB 02 FF 1D 5B EB 02 0F C7 5F EB 03 FF 1D 34 }

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 5 specific false positive scenarios for the CodeCryptv0164 detection rule, including suggested filters and exclusions:

• Scenario: Automated Deployment of Signed Artifacts via CI/CD Pipelines

  • Context: The rule triggers when a build server (e.g., Jenkins, GitLab Runner, or Azure DevOps) extracts a signed .nupkg or .jar file to the deployment agent’s working directory. The extraction process often involves temporary unpacking of encrypted or obfuscated payloads that match the YARA signature before the final installation occurs.
  • Filter/Exclusion: Exclude processes running under specific service accounts (e.g., NT SERVICE\jenkins, gitlab-runner) and restrict the rule to exclude file paths within standard build agent directories like C:\ProgramData\Jenkins\workspace or /var/lib/gitlab-runner.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: The detection fires during the nightly update cycle of enterprise endpoint protection suites (e.g., CrowdStrike Falcon, SentinelOne, or Microsoft Defender for Endpoint). These tools frequently download and unpack encrypted definition databases that contain obfuscated code structures similar to those targeted by CodeCryptv0164.
  • Filter/Exclusion: Create a process exclusion for the primary security engine binaries (e.g., C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe or MsMpEng.exe) and exclude file extensions associated with definition updates, such as .cab, .dat, and .mpst.

• Scenario: Execution of Obfuscated PowerShell Scripts for Patch Management

  • Context: IT administrators utilize scheduled tasks to run complex patching scripts (e.g., via SCCM/MECM or PDQ Deploy) that invoke `