SimpleUPXCryptorV3042005MANtiCORE

Hunt Hypothesis

This detection identifies potential cryptoworm or file encryption activity by matching process behavior against the specific signature of the SimpleUPXCryptorV3042005MANtiCORE YARA rule. SOC teams should proactively hunt for this low-severity indicator in Azure Sentinel to uncover early-stage lateral movement or data exfiltration attempts that may precede a larger ransomware incident.

YARA Rule

rule SimpleUPXCryptorV3042005MANtiCORE
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 60 B8 [4] B9 [8] E2 FA 61 68 [4] C3 }

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 SimpleUPXCryptorV3042005MANtiCORE detection rule, along with suggested filters and exclusions:

• Scenario: Microsoft Office Document Conversion via PowerShell

  • Context: Admins frequently use PowerShell scripts to batch-convert or process .docx/.xlsx files (which are essentially ZIP archives) using the System.IO.Compression namespace. The YARA rule may detect the internal UPX packing signature within these Office documents as a cryptor activity, mistaking standard document processing for a malicious cryptor execution.
  • Suggested Filter: Exclude processes where the parent process is powershell.exe or pwsh.exe and the file path matches the pattern C:\Program Files\Microsoft Office*\*. Additionally, filter out events where the command line contains arguments like -Command "Get-ChildItem ... | ForEach-Object { ... }".

• Scenario: Scheduled Antivirus Definition Updates (CrowdStrike/Carbon Black)

  • Context: During nightly scheduled jobs, endpoint protection agents often download and install new definition databases. These updates are frequently delivered as UPX-packed executables to reduce payload size. The detection logic may trigger on the extraction or execution of these update modules, flagging them as a cryptor rather than a routine maintenance task.
  • Suggested Filter: Create an exclusion for processes with the image path C:\ProgramData\CrowdStrike\*\csagent.exe (or equivalent for Carbon Black/SentinelOne) where the process command line contains keywords like “update”, “install”, or “definition”. Time-based filtering can also be applied to only alert during non-business hours if updates are strictly scheduled.

• Scenario: Deployment of Internal Line-of-Business (LOB) Applications via SCCM/Intune