FishPEShield101HellFish

Hunt Hypothesis

This rule identifies the presence of the FishPEShield101HellFish YARA signature, which typically indicates the use of a specific PE shield or packing tool often employed by adversaries to obscure malicious binaries and evade static analysis. Proactively hunting for this indicator in Azure Sentinel allows the SOC to detect low-severity, potentially stealthy file modifications or packed executables that may serve as precursors to more advanced post-exploitation activities.

YARA Rule

rule FishPEShield101HellFish
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 83 C4 D0 53 56 57 8B 45 10 83 C0 0C 8B 00 89 45 DC 83 7D DC 00 75 08 E8 AD FF FF FF 89 45 DC E8 C1 FE FF FF 8B 10 03 55 DC 89 55 E4 83 C0 04 8B 10 89 55 FC 83 C0 04 8B 10 89 55 F4 83 C0 04 8B 10 89 55 F8 83 C0 04 8B 10 89 55 F0 83 C0 04 8B 10 89 55 EC 83 C0 04 8B 00 89 45 E8 8B 45 E4 8B 58 04 03 5D E4 8B FB 8B 45 E4 8B 30 4E 85 F6 72 2B 46 C7 45 E0 00 00 00 00 83 7B 04 00 74 14 }
	$a1 = { 60 E8 12 FE FF FF C3 90 09 00 00 00 2C 00 00 00 [4] C4 03 00 00 BC A0 00 00 00 40 01 00 [4] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 99 00 00 00 00 8A 00 00 00 10 00 00 28 88 00 00 40 ?? 4B 00 00 00 02 00 00 00 A0 00 00 18 01 00 00 40 ?? 4C 00 00 00 0C 00 00 00 B0 00 00 38 0A 00 00 40 ?? 4E 00 00 00 00 00 00 00 C0 00 00 40 39 00 00 40 ?? 4E 00 00 00 08 00 00 00 00 01 00 C8 06 00 00 40 }

condition:
		$a0 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

• Scenario: A developer or DevOps engineer runs a custom Python script that uses the pefile library to parse and analyze the headers of a newly built .NET executable or C++ binary for debugging purposes. The script loads the PE file into memory, and the YARA rule matches the specific structural patterns or string artifacts defined in FishPEShield101HellFish within the loaded process memory or the file on disk.

  • Filter/Exclusion: Exclude processes where the parent process is python.exe, python3.exe, or conda.exe and the command line contains arguments referencing pefile, inspect, or analyze. Alternatively, exclude file paths under development directories like C:\Projects\, D:\Dev\, or C:\Users\<User>\AppData\Local\Temp\ if the rule is file-based.

• Scenario: An IT administrator performs a manual integrity check on a critical application binary (e.g., app_server.exe) using a specialized PE header inspection tool like PE-bear or a custom PowerShell script that reads the DOS and PE headers to verify the checksum or section table before deploying a patch. The tool loads the binary into memory, triggering the YARA match on the specific “HellFish” signature pattern.

  • Filter/Exclusion: Exclude processes named PE-bear.exe, dumpbin.exe, or powershell.exe when the command line contains keywords such as header, check, verify, or inspect. Ensure the exclusion applies only to administrative service accounts or known IT management groups.

• Scenario: A scheduled maintenance job runs a custom backup verification script that uses a lightweight PE parser (e.g., a Go or Rust binary) to validate the structural integrity of backup files stored in a local staging area before uploading them to the cloud. The parser reads the PE structure of the