CPAV

Hunt Hypothesis

This rule detects the presence of CPAV, a YARA signature often associated with specific malware families or suspicious code patterns, indicating potential compromise or staging activity. Proactively hunting for this signature allows the SOC team to identify low-severity threats that may evade standard behavioral detections, ensuring early containment of emerging or niche malware variants within the Azure environment.

YARA Rule

rule CPAV: Packer PEiD
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [2] 4D 5A B1 01 93 01 00 00 02 }

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

• Antivirus/EDR On-Demand Scans: Legitimate security software like Microsoft Defender for Endpoint or CrowdStrike Falcon often spawns child processes (e.g., MsMpEng.exe, FalconSensor.exe) that execute scanning modules or update components. These processes may briefly exhibit memory patterns or file access behaviors that match the CPAV YARA signature if the rule targets generic process injection or specific API calls common in scanning engines.

  • Filter/Exclusion: Exclude processes where the parent process name is in the list of known AV/EDR daemons (e.g., MsMpEng.exe, FalconSensor.exe, Defender.exe) or where the image path resides in standard AV installation directories (e.g., C:\Program Files\Microsoft Security Client\, C:\Program Files\CrowdStrike\).

• Software Update Services: Enterprise update agents such as WSUS (Windows Server Update Services) clients or SCCM (System Center Configuration Manager) agents (ccmexec.exe, Wuaexcl.exe) frequently download and execute update packages or run integrity checks. These operations can involve loading temporary DLLs or modifying system files in a way that triggers YARA rules looking for code injection or unusual process behavior.

  • Filter/Exclusion: Exclude events where the process name is ccmexec.exe, Wuaexcl.exe, or wuauclt.exe, or where the command line contains parameters like /scan, /install, or /update.

• Backup and Recovery Tasks: Scheduled backup jobs using tools like Veeam Backup & Replication or Windows Server Backup (wbadmin.exe) often perform full-system or file-level backups. These processes may lock files, create shadow copies, or temporarily mount volumes, which can result in file system or memory states that falsely match patterns related