PolyCryptorbySMTVersionv3v4

Hunt Hypothesis

This rule detects the presence of PolyCryptor ransomware variants (v3/v4) by identifying specific code patterns associated with its encryption routines. Proactively hunting for these signatures allows the SOC team to identify compromised hosts early in the infection lifecycle, enabling rapid isolation and remediation before the ransomware can fully encrypt critical data assets.

YARA Rule

rule PolyCryptorbySMTVersionv3v4
{
      meta:
		author="malware-lu"
strings:
		$a0 = { EB ?? 28 50 6F 6C 79 53 63 72 79 70 74 20 [3] 20 62 79 20 53 4D 54 29 }

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

• Scenario: A developer or system administrator manually compiles a legacy C/C++ application using an older version of Microsoft Visual Studio (e.g., VS 2010 or VS 2012) that defaults to the v3.0 or v4.0 SMT (Software Transactional Memory) runtime, resulting in a binary that matches the YARA signature for PolyCryptorbySMTVersionv3v4.

  • Filter/Exclusion: Exclude files located in known development directories (e.g., C:\Users\*\AppData\Local\Temp\, C:\Projects\, C:\Builds\) or exclude processes spawned by cl.exe, link.exe, or msbuild.exe where the output file is less than 24 hours old.

• Scenario: An enterprise application server running a custom Java or .NET application that utilizes a specific third-party library or native DLL compiled with an older SMT version for backward compatibility, which is frequently loaded during standard application startup or health check tasks.

  • Filter/Exclusion: Exclude files with extensions .dll or .exe that are signed by a trusted internal code-signing certificate (e.g., CN=InternalCorpCodeSign) and are located in the application’s install directory (e.g., C:\Program Files\CorpApp\bin\).

• Scenario: A scheduled maintenance job (e.g., a PowerShell script or batch file) that copies a backup of a legacy configuration module or a small utility tool from a network share to a local temp directory for processing, where the source file was originally built with an older compiler version.

  • Filter/Exclusion: Exclude files in C:\Windows\Temp\ or C:\Users\*\AppData\Local\Temp\ that were created within the last 15 minutes and