UPolyXv05

Hunt Hypothesis

This hypothesis detects adversaries utilizing the UPolyXv05 polymorphic obfuscation technique to evade static signature-based detection by dynamically altering malware characteristics during execution. A proactive hunt is essential in Azure Sentinel because this low-severity rule may miss sophisticated attacks that rely on runtime behavior analysis rather than fixed signatures, requiring deeper investigation of file integrity and process anomalies.

YARA Rule

rule UPolyXv05
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC ?? 00 BD 46 00 8B ?? B9 ?? 00 00 00 80 [2] 51 [4] 00 [26] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }
	$a1 = { 83 EC 04 89 14 24 59 BA ?? 00 00 00 52 [56] 00 [13] 00 }
	$a2 = { BB 00 BD 46 00 83 EC 04 89 1C 24 ?? B9 ?? 00 00 00 80 33 [6] 00 [13] 00 [12] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }
	$a3 = { E8 00 00 00 00 59 83 C1 07 51 C3 C3 ?? 00 BD 46 00 83 EC 04 89 ?? 24 B9 ?? 00 00 00 81 [4] 00 [21] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }
	$a4 = { E8 00 00 00 00 59 83 C1 07 51 C3 C3 ?? 00 BD 46 00 ?? B9 ?? 00 00 00 [27] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }
	$a5 = { EB 01 C3 ?? 00 BD 46 00 [46] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 }

condition:
		$a0 or $a1 or $a2 or $a3 or $a4 or $a5
}

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 6 string patterns in its detection logic.

• Source Rule

False Positive Guidance

Here are 3-5 specific false positive scenarios for the UPolyXv05 detection rule, tailored for a legitimate enterprise environment:

• Scenario: Antivirus Engine Signature Updates via Scheduled Task

  • Context: The YARA rule triggers when the Microsoft Defender Antivirus (or similar EDR agents like CrowdStrike) executes its daily scheduled update job. This process often involves unpacking compressed signature databases and executing a local updater binary that matches the polymorphic behavior patterns defined in UPolyXv05.
  • Suggested Filter/Exclusion: Add an exclusion based on the executable path C:\Program Files\Windows Defender\MsMpEng.exe (or specific vendor paths) combined with a parent process filter for svchost.exe running under the “System” account during the scheduled maintenance window.

• Scenario: Enterprise Software Deployment via SCCM/Intune

  • Context: During mass deployment cycles, tools like Microsoft Endpoint Configuration Manager (SCCM) or Microsoft Intune push application installers to endpoints. The installation agents often extract and execute temporary installer binaries that exhibit polymorphic characteristics (changing hashes upon extraction) which the UPolyXv05 rule flags as suspicious.
  • Suggested Filter/Exclusion: Create a whitelist for processes initiated by ccmsetup.exe or IntuneManagementExtension.exe. The filter should exclude alerts where the parent process is one of these deployment agents and the file extension is .msi, .exe, or .cab located within the C:\Windows\CCMCache directory.

• Scenario: Automated Backup and Archive Operations

  • Context: Enterprise backup solutions like Veeam Agent for Windows or Acronis Cyber Protect run nightly jobs that compress large datasets into temporary archives before uploading. The compression engine generates temporary