ExeSmashervxx

Hunt Hypothesis

This rule detects the execution of the ExeSmasher binary, a tool frequently used by adversaries to compress or obfuscate executables to evade static analysis and reduce file size during lateral movement or payload staging. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify potential post-compromise activity or staging operations that may not trigger high-severity alerts, thereby uncovering low-and-slow threats before they progress to more impactful stages of the kill chain.

YARA Rule

rule ExeSmashervxx
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 9C FE 03 ?? 60 BE [2] 41 ?? 8D BE ?? 10 FF FF 57 83 CD FF EB 10 }

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 IT engineer uses 7-Zip or WinRAR to extract a large software installer (e.g., Adobe Creative Cloud, Visual Studio) or a backup archive containing multiple .exe files into a user’s Downloads or Desktop folder. The YARA rule likely matches the specific binary structure or resource strings of these common archivers or the extracted executables themselves.

  • Filter/Exclusion: Exclude processes where the parent process is 7z.exe, WinRAR.exe, or explorer.exe (if triggered by double-click extraction) and the file path contains \Downloads\ or \Desktop\. Additionally, exclude known hash values of the standard 7-Zip or WinRAR binaries.

• Scenario: An administrator runs Sysinternals Process Explorer or Autoruns to troubleshoot startup items or investigate a hung process. These tools often load or interact with various system DLLs and EXEs, potentially triggering signature-based detections if the rule inspects loaded modules or specific PE headers associated with debugging utilities.

  • Filter/Exclusion: Exclude processes where the parent process is procexp.exe, autoruns.exe, or procmon.exe. Ensure the exclusion applies to the specific user accounts with the Domain Admins or IT Support security group membership.

• Scenario: A scheduled task or service runs Microsoft Defender Antivirus or CrowdStrike Falcon during a full system scan, which involves reading and analyzing numerous executable files on disk. If the YARA rule is sensitive to certain PE header fields or resource sections that are common in system binaries or updated by security tools, it may flag these legitimate scans.

  • Filter/Exclusion: Exclude processes where the parent process is MsMpEng.exe (Defender) or FalconSensor.exe (CrowdStrike).