ASPRStripperv2xunpacked

Hunt Hypothesis

This hypothesis targets the presence of the ASPRStripperv2x unpacker, a tool frequently used by adversaries to remove anti-VM and anti-debugging checks from malware binaries to facilitate execution in virtualized environments. Proactively hunting for this indicator in Azure Sentinel allows the SOC to identify potential staging or execution phases of sophisticated attacks before they trigger more complex behavioral detections.

YARA Rule

rule ASPRStripperv2xunpacked
{
      meta:
		author="malware-lu"
strings:
		$a0 = { BB [4] E9 [4] 60 9C FC BF [4] B9 [4] F3 AA 9D 61 C3 55 8B EC }

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 Maintenance: An IT administrator runs a custom PowerShell script or batch file to strip debug symbols or PDB references from a legacy .NET assembly (e.g., MyLegacyApp.dll) before deploying it to a production server. This process often involves invoking a tool like ilasm or a specific stripper utility that matches the YARA signature.
  • Filter/Exclusion: Exclude processes where the parent process is powershell.exe or cmd.exe and the target file path contains \bin\ or \deploy\ directories, or specifically exclude files named *.pdb or *.dll in known application build folders.
• Software Packaging and Distribution: A DevOps engineer uses a tool like 7-Zip or Inno Setup to package an application installer. During the compression or stripping phase, the installer builder may temporarily unpack and re-compress binaries, triggering the “unpacked” signature if the YARA rule scans intermediate files in the build workspace.
  • Filter/Exclusion: Exclude files located in temporary build directories such as C:\Users\<user>\AppData\Local\Temp\ or C:\Builds\ where the parent process is 7z.exe, innosetup.exe, or msbuild.exe.
• Antivirus/EDR Self-Scanning or Quarantine: The enterprise EDR agent (e.g., CrowdStrike, SentinelOne) performs a deep scan or quarantines a file, temporarily unpacking it to a sandbox or memory-mapped view for analysis. If the YARA rule is applied to files in the quarantine or scan cache directory, it may flag the unpacked state.
  • Filter/Exclusion: Exclude files located in EDR-specific quarantine or cache paths (e.g., C:\ProgramData\CrowdStrike\quarantine\ or