PCrypt v351 Hint DOS EP

Hunt Hypothesis

This rule targets a specific variant of the PCrypt ransomware family, characterized by a DOS entry point and version 351, which adversaries deploy to encrypt files and disrupt operations. Proactively hunting for this signature in Azure Sentinel allows the SOC team to identify early-stage ransomware infections or dormant payloads before they trigger widespread encryption or lateral movement.

YARA Rule

rule PCrypt_v351_Hint_DOS_EP: PEiD
{
    strings:
        $a = { 50 43 52 59 50 54 FF 76 33 2E 35 31 00 E9 }
    condition:
        $a 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 DOS-based Firmware Update Utility: A specialized hardware vendor tool (e.g., fw_update.exe for network switches or industrial controllers) is executed via a scheduled task to push firmware updates to legacy devices. This binary is often a small, packed DOS executable that may match the “DOS EP” (Entry Point) heuristic due to its minimal header structure and specific packing algorithm.

  • Filter/Exclusion: Exclude processes where the parent process is Task Scheduler (schtasks.exe or svchost.exe with the Task Scheduler service) AND the executable path resides in a dedicated vendor firmware directory (e.g., C:\Program Files\VendorName\Firmware\).

• Packed Installer for Legacy Line-of-Business (LOB) Application: An internal IT team uses a custom installer wrapper (e.g., setup_packed.exe) for a critical, end-of-life LOB application (like an old ERP module or point-of-sale system). The installer uses a lightweight packer (such as UPX or a custom proprietary packer) to reduce size for network distribution, which can trigger the “Hint DOS” signature if the packer’s header mimics DOS MZ structures or if the YARA rule’s entropy/byte sequence checks align with the packer’s output.

  • Filter/Exclusion: Exclude if the executable name matches known internal installer patterns (e.g., *setup*.exe, *install*.exe) AND the file hash is present in the approved internal software inventory database (e.g., via SCCM or Intune).

• Antivirus/EDR Self-Test or Quarantine Restoration: During a scheduled maintenance window, the EDR agent (e.g., CrowdStrike, SentinelOne, or Defender) performs a self-test or restores a previously quarantined file that was a legacy DOS-based utility (e.g., a disk partitioning tool like