Shrinkerv34

Hunt Hypothesis

This rule targets the execution of Shrinkerv34, a known malware variant, indicating potential post-exploitation activity or persistence mechanisms within the environment. Proactively hunting for this signature allows the SOC team to identify compromised hosts early, even in the absence of correlated alerts, thereby reducing the dwell time of low-severity threats that may serve as precursors to more impactful attacks.

YARA Rule

rule Shrinkerv34
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 83 3D B4 [4] 55 8B EC 56 57 75 6B 68 00 01 00 00 E8 ?? 0B 00 00 83 C4 04 8B 75 08 A3 B4 [3] 85 F6 74 23 83 7D 0C 03 77 1D 68 FF }
	$a1 = { BB [2] BA [2] 81 C3 07 00 B8 40 B4 B1 04 D3 E8 03 C3 8C D9 49 8E C1 26 03 0E 03 00 2B }

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

• Scenario: A developer or DevOps engineer runs a local build script that invokes a custom, obfuscated PowerShell or Python wrapper (named shrinker_v34.exe or similar) to compress and package application artifacts before deployment to a staging environment.

  • Filter/Exclusion: Exclude processes where the parent process is a known build tool (e.g., msbuild.exe, dotnet.exe, npm.exe, or python.exe) and the file path resides within a designated build or artifact directory (e.g., C:\Builds\, D:\Artifacts\).

• Scenario: An IT administrator performs a manual cleanup of temporary files or logs using a third-party utility or a custom script named shrinkerv34.bat or shrinkerv34.ps1 to reduce disk usage on a specific server, often executed via a scheduled task or directly from the command line.

  • Filter/Exclusion: Exclude if the process is launched by cmd.exe or powershell.exe and the working directory is a standard temp or log location (e.g., C:\Temp\, C:\Logs\, C:\Windows\Temp\), or if the process command line contains keywords like clean, delete, or compress.

• Scenario: A security team or QA engineer executes a controlled test of a new internal tool or a known benign binary that has been renamed to shrinkerv34 to verify detection coverage in a non-production test environment.

  • Filter/Exclusion: Exclude if the process is running on a host tagged as TestEnvironment or Lab in your CMDB, or if the user account belongs to a service group like svc-test or qa-team.

• Scenario: A legacy application or plugin uses a helper binary named `shrink