PEcryptbyarchphase

Hunt Hypothesis

This detection identifies potential adversaries attempting to evade analysis by encrypting Portable Executable (PE) files during specific architectural phases of their execution lifecycle. A proactive hunt is essential in Azure Sentinel to uncover stealthy malware that utilizes encryption to obscure its behavior and bypass traditional signature-based defenses.

YARA Rule

rule PEcryptbyarchphase
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 83 C4 E0 53 56 33 C0 89 45 E4 89 45 E0 89 45 EC [4] 64 82 40 00 E8 7C C7 FF FF 33 C0 55 68 BE 84 40 00 64 FF 30 64 89 20 68 CC 84 40 00 [4] 00 A1 10 A7 40 00 50 E8 1D C8 FF FF 8B D8 85 DB 75 39 E8 3A C8 FF FF 6A 00 6A 00 68 A0 A9 40 00 68 00 04 00 00 50 6A 00 68 00 13 00 00 E8 FF C7 FF FF 6A 00 68 E0 84 40 00 A1 A0 A9 40 00 50 6A 00 E8 [4] E9 7D 01 00 00 53 A1 10 A7 40 00 50 E8 42 C8 FF FF 8B F0 85 F6 75 18 6A 00 68 E0 84 40 00 68 E4 84 40 00 6A 00 E8 71 C8 FF FF E9 53 01 00 00 53 6A 00 E8 2C C8 FF FF A3 [4] 83 3D 48 A8 40 00 00 75 18 6A 00 68 E0 84 40 00 68 F8 84 40 00 6A 00 E8 43 C8 FF FF E9 25 01 00 00 56 E8 F8 C7 FF FF A3 4C A8 40 00 A1 48 A8 40 00 E8 91 A1 FF FF 8B D8 8B 15 48 A8 40 00 85 D2 7C 16 42 33 C0 8B 0D 4C A8 40 00 03 C8 8A 09 8D 34 18 88 0E 40 4A 75 ED 8B 15 48 A8 40 00 85 D2 7C 32 42 33 C0 8D 34 18 8A 0E 80 F9 01 75 05 C6 06 FF EB 1C 8D 0C 18 8A 09 84 [5] 00 EB 0E 8B 0D 4C A8 40 00 03 C8 0F B6 09 49 88 0E 40 4A 75 D1 8D [4] E8 A5 A3 FF FF 8B 45 E8 8D 55 EC E8 56 D5 FF FF 8D 45 EC BA 18 85 40 00 E8 79 BA FF FF 8B 45 EC E8 39 BB FF FF 8B D0 B8 54 A8 40 00 E8 31 A6 FF FF BA 01 00 00 00 B8 54 A8 40 00 E8 12 A9 FF FF E8 DD A1 FF FF 68 50 A8 40 00 8B D3 8B 0D 48 A8 40 00 B8 54 A8 40 00 E8 56 A7 FF FF E8 C1 A1 FF FF }

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 PEcryptbyarchphase detection rule, along with suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Automated Antivirus Definition Updates

  • Context: Enterprise Endpoint Protection suites (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint) frequently download new signature databases. These updates often involve downloading a compressed PE archive, extracting it in memory, and executing the installer which may trigger the rule due to the “archival” phase of the update package.
  • Filter/Exclusion: Exclude processes where the parent process is the specific AV service (e.g., csfalcon.exe, MsMpEng.exe) AND the file path contains known update directories like \Program Files\Microsoft Defender\Updates or \CrowdStrike\Falcon.

• Scenario: Scheduled Backup and Archiving Jobs

  • Context: System administrators often run nightly backup tasks using tools like Veeam, Commvault, or Windows Task Scheduler. These jobs compress large volumes of PE executables into .7z or .zip archives. The extraction phase creates temporary executable files that match the rule’s criteria for “archival” behavior.
  • Filter/Exclusion: Create a whitelist for scheduled tasks running under specific service accounts (e.g., NT SERVICE\VEEAM, BackupAdmin). Filter out events where the process command line contains keywords like -archive, -compress, or paths within designated backup storage folders (e.g., D:\Backups\Staging).

• Scenario: Software Deployment via Configuration Management

  • Context: IT teams use tools like Microsoft SCCM, Ansible, or Chocolatey to deploy applications. During the deployment phase, these tools often unpack application installers (MSI/EXE) from a