WinUpackv039finalByDwingc2005h1

Hunt Hypothesis

This detection identifies the execution of a specific unpacking utility (WinUpack) that adversaries often leverage to extract and stage malicious payloads within the Windows environment. Proactively hunting for this behavior in Azure Sentinel allows the SOC team to uncover early-stage file extraction activities that may precede more complex attack chains, ensuring visibility into potentially benign-looking tools being repurposed by attackers.

YARA Rule

rule WinUpackv039finalByDwingc2005h1
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BE B0 11 [2] AD 50 FF 76 34 EB 7C 48 01 [2] 0B 01 4C 6F 61 64 4C 69 62 72 61 72 79 41 00 00 18 10 00 00 10 00 00 00 00 [3] 00 00 [2] 00 10 00 00 00 02 00 00 04 00 00 00 00 00 39 00 04 00 00 00 00 00 00 00 00 [3] 00 02 00 00 00 00 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 WinUpackv039finalByDwingc2005h1 YARA rule, which typically targets unpacking behaviors or specific PE header structures often associated with legitimate enterprise software:

• Scenario: Legitimate deployment of Microsoft Office updates via the Office Deployment Tool (ODT).

  • Context: When the ODT runs a scheduled task to download and install cumulative updates, it extracts compressed .appx or .msi packages that match the unpacking signature defined in this rule.
  • Filter/Exclusion: Exclude processes where ImageName contains setup.exe AND CommandLine contains /download or /update, specifically targeting paths under C:\Program Files\Microsoft Office\root\Office16.

• Scenario: Execution of the System Center Configuration Manager (SCCM) Client Agent.

  • Context: The SCCM client (ccmexec.exe) frequently unpacks application payloads and software updates in the background, triggering the YARA rule due to its internal resource extraction mechanisms.
  • Filter/Exclusion: Create an exclusion for ImageName equal to ccmexec.exe running under the context of the “System” or “Network Service” account, located within the C:\Windows\CCM directory.

• Scenario: Automated antivirus definition updates by CrowdStrike Falcon or Symantec Endpoint Protection.

  • Context: These EDR solutions periodically download and unpack new threat intelligence definitions (often in compressed archives) which mimic the structural characteristics detected by this rule.
  • Filter/Exclusion: Filter out events where ImageName matches C:\Program Files\CrowdStrike\FalconSensor\csfalcon.exe or Symantec Endpoint Protection Client\SmcService.exe, provided the parent