UPackAltStubDwing

Hunt Hypothesis

This rule detects the presence of the UPack alternative stub, a common packer used to compress and obfuscate executable files, which adversaries often employ to reduce file size and hinder static analysis. Proactively hunting for this indicator in Azure Sentinel allows the SOC team to identify potentially packed binaries in memory or on disk that may be hiding malicious payloads or evading signature-based detection.

YARA Rule

rule UPackAltStubDwing
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 E8 09 00 00 00 C3 F6 00 00 E9 06 02 00 00 33 C9 5E 87 0E E3 F4 2B F1 8B DE AD 2B D8 AD }

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

• Legacy Application Deployment via MSI/EXE Wrappers: Many enterprise applications (e.g., older versions of Adobe Creative Suite, Oracle Java, or specific ERP clients) use custom installers that embed UPack stubs to handle dependency checks or silent installation logic.

  • Filter/Exclusion: Exclude files located in standard installation directories (e.g., C:\Program Files\, C:\Program Files (x86)\) where the parent process is a known installer service (e.g., msiexec.exe, setup.exe, install.exe) and the file extension is .exe or .msi.

• Scheduled Maintenance Jobs for Third-Party Utilities: Some vendor-provided maintenance tools (e.g., Symantec Endpoint Protection, McAfee Agent, or specific database backup utilities like Veeam or Commvault) may use packed executables for their scheduled cleanup or update tasks to reduce binary size or protect proprietary code.

  • Filter/Exclusion: Exclude processes spawned by known vendor service executables (e.g., smc.exe, mfemgr.exe, vscan.exe) or those running from vendor-specific service directories (e.g., C:\Program Files\Symantec\, C:\Program Files\McAfee\) with a valid digital signature.

• Development and QA Environments: Developers and QA engineers often run packed debuggers, profilers, or custom-built test harnesses (e.g., using UPack to compress large debug symbols or test binaries) during local testing or CI/CD pipeline execution on build agents.

  • Filter/Exclusion: Exclude files located in development-specific paths (e.g., C:\Users\<user>\Projects\, C:\builds\, C:\ci\) or those owned by user accounts with the Developer or QA security group membership