PECryptv102

Hunt Hypothesis

This rule detects the presence of the PECrypt v102 malware family, which is commonly used to encrypt and obfuscate malicious payloads to evade static analysis. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify compromised endpoints or staging servers before the encrypted payload is decrypted and executed, reducing the risk of lateral movement or data exfiltration.

YARA Rule

rule PECryptv102
{
      meta:
		author="malware-lu"
strings:
		$a0 = { E8 [4] 5B 83 EB 05 EB 04 52 4E 44 }

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

• Legacy .NET Application Deployment: The rule may trigger on older versions of .NET Framework assemblies (specifically those compiled with certain versions of csc.exe or vbc.exe prior to .NET 4.5) that utilize specific encryption headers or obfuscation patterns common in legacy business logic libraries.

  • Filter/Exclusion: Exclude files located in standard .NET GAC (Global Assembly Cache) paths (e.g., C:\Windows\Microsoft.NET\assembly\GAC_*) or specific legacy application directories (e.g., C:\Program Files\LegacyERP\bin\) where the file extension is .dll or .exe and the size is less than 2MB.

• Obfuscated PowerShell Scripts in Scheduled Tasks: Enterprise IT teams often use tools like PSObfuscator or ISEverything to protect sensitive administrative scripts (e.g., password rotation, patch management) stored in Windows Task Scheduler. These scripts may be compiled into executables or stored as base64-encoded strings that match the PECryptv102 pattern.

  • Filter/Exclusion: Exclude processes or files where the parent process is schtasks.exe or taskschd.msc, or where the file path contains \ScheduledTasks\ or \Scripts\ and the file extension is .ps1, .psm1, or .exe with a known hash from the internal obfuscation toolchain.

• Third-Party Anti-Malware or EDR Agent Components: Some commercial EDR or anti-malware solutions (e.g., CrowdStrike Falcon, Carbon Black, or SentinelOne) use self-encrypting or obfuscated memory regions for their agent binaries to resist tampering. The PECryptv102 YARA rule may match the internal structure of these agent components during memory scanning or file