PEncrypt20junkcode

Hunt Hypothesis

This rule detects the presence of PEncrypt20, a lightweight encryption utility often embedded in malicious scripts or payloads to obfuscate data and hinder forensic analysis. Proactively hunting for this artifact allows the SOC team to identify potential low-and-slow persistence or data staging activities that may otherwise go unnoticed due to the tool’s minimal footprint and low severity classification.

YARA Rule

rule PEncrypt20junkcode
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB 25 00 00 F7 BF 00 00 00 00 00 00 00 00 00 00 12 00 E8 00 56 69 72 74 75 61 6C 50 72 6F 74 65 63 74 00 00 00 00 00 E8 00 00 00 00 5D 81 ED 2C 10 40 00 8D B5 14 10 40 00 E8 33 00 00 00 89 85 10 10 40 00 BF 00 00 40 00 8B F7 03 7F 3C 8B 4F 54 51 56 8D 85 }

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

• Scenario: A developer or DevOps engineer manually compiles a custom C/C++ utility using gcc or clang with optimization flags (e.g., -O2 or -Os) that insert padding or alignment bytes, resulting in a binary containing sequences of 0x00 or 0xFF that match the “junk code” pattern.
  • Filter/Exclusion: Exclude files located in standard build directories (e.g., C:\build\, /opt/build/, ~/projects/) or files with extensions .a, .so, .dll, and .exe if they are owned by service accounts or developer groups.
• Scenario: An enterprise backup solution (e.g., Veeam, Commvault, or Windows Server Backup) creates temporary snapshot files or delta backups that contain large blocks of uninitialized memory or zero-filled sectors before compression is applied.
  • Filter/Exclusion: Exclude files in backup staging directories (e.g., C:\ProgramData\Veeam\Backup\, /var/lib/commvault/) and files with extensions .vbk, .vib, .bkp, or .tmp created by known backup service accounts.
• Scenario: A Java or .NET application creates temporary serialization files or memory-mapped files that are pre-allocated with zero values (0x00) before data is written, triggering the junk code detection on the file header or padding regions.
  • Filter/Exclusion: Exclude files in C:\Windows\Temp\, /tmp/, or application-specific temp directories (e.g., C:\Users\<user>\AppData\Local\Temp\) if the file size is less than 1MB and the creation time is recent (< 1 hour).
• Scenario: A disk defragmentation or cleanup utility (e.g., `