VxVirusConstructorbased

Hunt Hypothesis

This detection identifies potential malware construction activities by leveraging a YARA signature that targets specific virus constructor patterns often used in early-stage infection campaigns. SOC teams should proactively hunt for this behavior within Azure Sentinel to uncover stealthy threats that may evade standard heuristic scans and establish a baseline of legitimate file creation activity before an outbreak occurs.

YARA Rule

rule VxVirusConstructorbased
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BB [2] B9 [2] 2E [4] 43 43 [2] 8B EC CC 8B [2] 81 [3] 06 1E B8 [2] CD 21 3D [4] 8C D8 48 8E D8 }
	$a1 = { E8 [2] 5D 81 [3] 06 1E E8 [2] E8 [4] 2E [6] B4 4A BB FF FF CD 21 83 [2] B4 4A CD 21 }

condition:
		$a0 at pe.entry_point 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

Here are 4 specific false positive scenarios for the VxVirusConstructorbased detection rule, including suggested filters and exclusions tailored for an enterprise environment:

• Antivirus Engine Self-Modification

  • Scenario: The primary endpoint protection suite (e.g., CrowdStrike Falcon, Microsoft Defender for Endpoint, or Symantec Endpoint Protection) performs routine updates to its own virus definition database or engine binaries. These processes often spawn child processes that construct new executable artifacts in memory, mimicking the behavior of a malicious constructor.
  • Filter/Exclusion: Create an exclusion rule based on the parent process name and path. Exclude any detection where ProcessName is one of C:\Program Files\CrowdStrike\fs_qrte.exe, C:\Windows\System32\mpcmdrun.exe (Defender), or C:\Program Files\Symantec Endpoint Protection\Smc.exe.

• Scheduled Deployment via Configuration Management

  • Scenario: Automated deployment tools such as Microsoft SCCM (Configuration Manager), Ansible, or Chocolatey execute scheduled jobs during maintenance windows. These tools frequently download, extract, and construct new application binaries on the endpoint to push software updates, triggering the constructor logic in the YARA rule.
  • Filter/Exclusion: Filter alerts where the parent process is a known deployment agent (e.g., ccmsetup.exe, choco.exe, or ansible-runner) AND the event occurs within defined maintenance windows (e.g., 02:00 – 06:00 UTC). Alternatively, exclude paths containing \AppData\Local\Temp when the parent is a deployment tool.

• Software Installation and Patching Services

  • Scenario: The Windows Update Standalone Installer (wusa.exe) or third-party patch management