PEPACK099

Hunt Hypothesis

This detection identifies potential file packing or obfuscation activities via the PEPACK099 YARA signature, which often indicates adversaries attempting to conceal malicious payloads within legitimate-looking executables. A proactive hunt is essential in Azure Sentinel to uncover these stealthy modifications early, as packed files frequently evade standard signature-based scanning and may signal the initial stages of a sophisticated intrusion campaign.

YARA Rule

rule PEPACK099
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 00 00 00 00 5D 83 ED 06 80 BD E0 04 00 00 01 0F 84 F2 }

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 PEPACK099 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Antivirus Definition Updates via Microsoft Defender

  • Context: The scheduled daily update task for Microsoft Defender for Endpoint (or Windows Update) downloads new YARA signature packs or definition files. These updates often contain embedded scripts or compressed archives that match the PEPACK099 logic, mimicking a potential malicious payload injection.
  • Suggested Filter/Exclusion: Create an exclusion based on the process name MpCmdRun.exe (Microsoft Defender) and the specific file path pattern C:\ProgramData\Microsoft\Windows Defender\Platform\*. Additionally, exclude events occurring during the configured maintenance window (e.g., 02:00–04:00 AM).

• Scenario: Enterprise Software Deployment via Microsoft SCCM/Intune

  • Context: When System Center Configuration Manager (SCCM) or Microsoft Intune pushes a new application package to endpoints, the deployment agent extracts and installs .msi or .exe packages. The extraction process often triggers PEPACK099 as it unpacks nested resources that resemble the rule’s detection criteria.
  • Suggested Filter/Exclusion: Exclude alerts where the parent process is ccmsetup.exe, TaskHost.exe, or IntuneManagementExtension.exe. Furthermore, filter out events where the file hash matches a known “Golden Image” hash stored in your asset management database for approved enterprise applications.

• Scenario: Automated Backup and Archiving with Veeam or Commvault

  • Context: Scheduled backup jobs (e.g., Veeam Agent or Commvault) frequently compress large datasets into .zip or proprietary archive formats before transmission. The YARA rule may