FishPEV10Xhellfish

Hunt Hypothesis

This detection identifies potential execution of the Hellfish malware family, which often utilizes PE file manipulation to establish persistence and evade standard signature-based defenses. Proactively hunting for this behavior in Azure Sentinel is critical because its low-severity classification may lead to missed alerts during initial infection phases, requiring manual correlation with process creation logs to confirm early-stage compromise.

YARA Rule

rule FishPEV10Xhellfish
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 [4] 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 [2] 00 00 [4] 00 00 02 00 00 00 A0 00 00 18 01 00 00 [4] 00 00 0C 00 00 00 B0 00 00 38 0A 00 00 [4] 00 00 00 00 00 00 C0 00 00 40 39 00 00 [4] 00 00 08 00 00 00 00 01 00 C8 06 00 00 }

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 FishPEV10Xhellfish detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Scenario: Microsoft Defender Antivirus Signature Updates

  • Context: The YARA rule likely flags PE (Portable Executable) files with specific header characteristics or embedded resources common in antivirus definition updates. When the Microsoft Defender service (MsMpEng.exe) downloads and installs new signature definitions, it generates temporary PE files that match the “Hellfish” pattern.
  • Filter/Exclusion: Exclude processes where the parent process is MsMpEng.exe (Microsoft Antimalware Service Executable) or filter out file paths containing \Program Files\Windows Defender\.

• Scenario: Scheduled Office 365 ProPlus Update Tasks

  • Context: Enterprise environments often run scheduled jobs via Task Scheduler to update Microsoft Office suites. The OfficeClickToRun.exe process frequently spawns child processes that modify configuration files or install components, creating PE artifacts that trigger the rule during peak maintenance windows (e.g., early morning).
  • Filter/Exclusion: Create a filter for events occurring between 02:00 and 05:00 UTC where the parent process name is OfficeClickToRun.exe or the file path contains \Microsoft Office\.

• Scenario: Enterprise Endpoint Management (SCCM/Intune) Deployment

  • Context: During large-scale software deployments, tools like Microsoft SCCM (ccmexec.exe) or Intune Management Extension push updates to endpoints. These agents often unpack and stage application binaries before installation, generating temporary executables with the specific PE structure detected by FishPEV10Xhellfish.
  • Filter/Exclusion: Exclude events where the process name is ccmexec.exe (Configuration Manager