PEXv099

Hunt Hypothesis

This detection identifies specific file patterns or behaviors defined by the PEXv099 YARA signature, potentially indicating early-stage adversary activity such as known malware variants or suspicious script execution within the environment. Proactively hunting for this signal in Azure Sentinel allows the SOC team to validate low-severity alerts against broader context, ensuring that subtle indicators of compromise are not overlooked before they escalate into significant incidents.

YARA Rule

rule PEXv099
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 01 [4] 83 C4 04 E8 01 [4] 5D 81 }

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 PEXv099 detection rule, including suggested filters and exclusions tailored for an enterprise environment:

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

  • Context: The SCCM client service (ccmexec.exe) frequently executes YARA-scanned payloads during scheduled maintenance windows to deploy security updates or application patches. These legitimate processes often spawn child processes that match the PEXv099 signature logic, triggering alerts when they are simply unpacking or verifying update packages.
  • Suggested Filter/Exclusion: Create a process tree exclusion where ccmexec.exe is the parent and the event source matches “System” or specific SCCM deployment IDs. Alternatively, filter out events occurring within the defined maintenance window (e.g., 02:00–04:00 UTC) for this rule.

• Scenario: Scheduled Antivirus Definition Updates via CrowdStrike Falcon

  • Context: The CrowdStrike Falcon sensor (CfSenService.exe) periodically downloads and installs new definition signatures. During the extraction phase, the sensor creates temporary executable artifacts that mimic the behavior patterns detected by PEXv099, leading to high-volume false positives during daily update cycles.
  • Suggested Filter/Exclusion: Implement a file path exclusion for C:\ProgramData\CrowdStrike\ and C:\Windows\System32\CfSenService.exe. Additionally, add a rule condition to ignore events where the process command line contains keywords like “Update”, “Definition”, or “Signature”.

• Scenario: Enterprise Backup Operations using Veeam Backup & Replication

  • Context: The Veeam backup engine (Veeam.Backup.Service.exe) initiates scheduled jobs that involve scanning and compressing large data sets. When the