CodeCryptv015b

Hunt Hypothesis

This detection identifies potential code obfuscation or encryption activities within executable files that may indicate an adversary attempting to evade static analysis and hide malicious logic. The SOC team should proactively hunt for this behavior in Azure Sentinel to uncover stealthy threats that traditional signature-based tools might miss, ensuring early identification of sophisticated fileless or packed malware campaigns.

YARA Rule

rule CodeCryptv015b
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E9 31 03 00 00 EB 02 83 3D 58 EB 02 FF 1D 5B EB 02 0F C7 5F }

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 3-5 specific false positive scenarios for the CodeCryptv015b detection rule, including suggested filters and exclusions:

• Scenario: Antivirus Engine Real-Time Scanning of Large Archives

  • Context: Enterprise antivirus solutions like Microsoft Defender for Endpoint or CrowdStrike Falcon often perform real-time scanning on large .zip or .7z archives containing encrypted payloads. When the AV engine extracts and scans these files, its internal cryptographic modules may trigger the CodeCryptv015b signature due to the presence of embedded encryption libraries within the archive structure.
  • Filter/Exclusion: Add a file path exclusion for the antivirus quarantine or scan directories (e.g., C:\ProgramData\Microsoft\Windows Defender\Scans\History). Alternatively, filter alerts where the parent process is the specific AV engine service (e.g., MsMpEng.exe or FalconSensorService.exe) and the file extension matches .zip, .7z, or .cab.

• Scenario: Scheduled Backup Jobs Executing Encrypted Snapshots

  • Context: Automated backup tools such as Veeam Backup & Replication, Commvault, or Rubrik frequently run scheduled jobs during off-hours to create encrypted disk snapshots or transfer data to cloud storage. These agents utilize internal encryption routines that match the YARA signature logic, causing alerts when they write temporary encrypted chunks to the file system.
  • Filter/Exclusion: Create a time-based exclusion window (e.g., 02:00–06:00 local time) for processes known to be backup agents (e.g., VeeamBRService.exe, CommServeAgent.exe). Additionally, exclude file paths located within dedicated backup staging folders like D:\BackupStaging\ or `C:\Program Files