PECompactv110b7

Hunt Hypothesis

This hunt detects the execution of potentially obfuscated or packed Portable Executable (PE) files that match the specific signature defined by the PECompactv110b7 YARA rule, which often indicates an adversary attempting to evade static analysis through compression or packing techniques. A SOC team should proactively hunt for these artifacts in Azure Sentinel because low-severity detections of packed binaries can serve as early indicators of sophisticated threats that may bypass initial signature-based defenses while hiding malicious payloads within legitimate-looking processes.

YARA Rule

rule PECompactv110b7
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 06 68 [4] C3 9C 60 E8 02 [3] 33 C0 8B C4 83 C0 04 93 8B E3 8B 5B FC 81 EB 0F 60 40 ?? 87 DD 8B 85 9A 60 40 ?? 01 85 03 60 40 ?? 66 C7 85 ?? 60 40 ?? 90 90 01 85 92 60 40 ?? BB 14 }

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 PECompactv110b7 detection rule, including suggested filters and exclusions:

• Scenario: Microsoft Office Click-to-Run Updates

  • Context: The Office Click-to-Run service (OfficeClickToRun.exe) frequently downloads and installs updated components that utilize PE Compact v1.10b7 compression to reduce footprint during background updates. This often triggers the rule when new patches are applied silently.
  • Filter/Exclusion: Create a whitelist exclusion for the process C:\Program Files\Microsoft Office\root\Office16\OfficeClickToRun.exe and its child processes running within the Microsoft Update service context.

• Scenario: Scheduled Antivirus Definition Updates

  • Context: Enterprise Endpoint Detection and Response (EDR) agents, such as CrowdStrike Falcon or SentinelOne, periodically download compressed definition packs using PE Compact v1.10b7 during off-hours scheduled jobs. The extraction of these archives mimics the signature of a new executable deployment.
  • Filter/Exclusion: Exclude alerts where the parent process is C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe (or equivalent for SentinelOne) and the event occurs during the defined maintenance window (e.g., 02:00–04:00 local time).

• Scenario: Deployment of Internal Line-of-Business (LOB) Tools

  • Context: IT Operations teams often deploy custom internal utilities (e.g., a “Printer Config Tool” or “Asset Inventory Agent”) packaged with PE Compact v1.10b7 to ensure fast installation across thousands of endpoints via Microsoft Endpoint Configuration Manager (MECM/SCCM). These legitimate deployments are frequently flagged as anomalies.
  • **Filter