PEDiminisherv01

Hunt Hypothesis

This detection identifies potential malware or suspicious activity associated with the “PEDiminisherv01” signature as defined by its specific YARA rule within the Azure Sentinel environment. A proactive hunt is recommended to validate whether this low-severity indicator represents a benign process or an early-stage threat that could evolve into a more critical incident if left unmonitored.

YARA Rule

rule PEDiminisherv01
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 53 51 52 56 57 55 E8 00 00 00 00 5D 8B D5 81 ED A2 30 40 00 2B 95 91 33 40 00 81 EA 0B 00 00 00 89 95 9A 33 40 00 80 BD 99 33 40 00 00 74 }
	$a1 = { 5D 8B D5 81 ED A2 30 40 ?? 2B 95 91 33 40 ?? 81 EA 0B [3] 89 95 9A 33 40 ?? 80 BD 99 }

condition:
		$a0 at pe.entry_point or $a1 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 2 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 5 specific false positive scenarios for the PEDiminisherv01 detection rule, including recommended filters and exclusions tailored for an enterprise environment:

• Scenario: Scheduled Antivirus Definition Updates via Microsoft Defender

  • Context: The MsMpSvc (Microsoft Antimalware Service) or a third-party agent like CrowdStrike Falcon Sensor triggers a service update task during off-hours. This often involves the creation of temporary files in the %TEMP% directory that match the YARA signature for “diminisher” behavior patterns.
  • Filter/Exclusion: Exclude processes where ImageName contains MsMpEng.exe, CrowdStrikeFalconService.exe, or DefenderSvc.exe. Additionally, filter out events occurring between 01:00 and 05:00 UTC if the update schedule is known.

• Scenario: Automated Patch Deployment via Microsoft Endpoint Configuration Manager (SCCM)

  • Context: During a scheduled patching window, SCCM (ccmexec.exe) or Intune Management Extension (IntuneManagementExtension.exe) installs cumulative updates that involve modifying system services and registry keys. The YARA rule may flag the service modification activity as suspicious “diminisher” behavior.
  • Filter/Exclusion: Create an exclusion for parent processes ccmexec.exe and IntuneManagementExtension.exe. Ensure the filter applies specifically to actions occurring within the defined maintenance window (e.g., Tuesday/Wednesday 20:00–23:00).

• Scenario: Enterprise Backup Operations using Veeam or Commvault

  • Context: Backup agents run scheduled jobs that involve scanning and compressing large datasets, often spawning temporary worker processes to manage service states. These workers may instantiate services that mimic the signature logic of PEDiminisherv01, particularly when