PureBasic4xDLL

Hunt Hypothesis

This rule detects the presence of PureBasic 4x DLLs, which are often leveraged by adversaries to execute custom or obfuscated payloads within Windows environments. Proactively hunting for these artifacts helps identify potential living-off-the-land techniques or custom tooling that may bypass standard application whitelisting controls.

YARA Rule

rule PureBasic4xDLL : Neil Hodgson
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 83 7C 24 08 01 75 0E 8B 44 24 04 A3 ?? ?? ?? 10 E8 22 00 00 00 83 7C 24 08 02 75 00 83 7C 24 08 00 75 05 E8 ?? 00 00 00 83 7C 24 08 03 75 00 B8 01 00 00 00 C2 0C 00 68 00 00 00 00 68 00 10 00 00 68 00 00 00 00 E8 ?? 0F 00 00 A3 }

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: A third-party legacy line-of-business application (e.g., an older inventory management system or specialized CAD plugin) utilizes a PureBasic-compiled DLL for its core logic. When the application is updated or reinstalled via a standard MSI package, the new DLL is dropped into the C:\Program Files\ directory, triggering the YARA rule.
  • Filter/Exclusion: Exclude files located in specific vendor directories (e.g., C:\Program Files\LegacyApp\) or exclude DLLs with specific version strings that match the known legacy application build.
• Internal Tooling Deployment: The DevOps team deploys a custom internal utility written in PureBasic (e.g., a log parser or network diagnostic tool) to the C:\Tools\ or C:\Scripts\ directories on jump servers or build agents. This is a one-time deployment or a scheduled update via a PowerShell script.
  • Filter/Exclusion: Exclude paths under C:\Tools\ or C:\Scripts\ on designated jump servers, or exclude files owned by the devops-admin service account during the deployment window.
• Scheduled Backup/Archive Job: A scheduled task runs a custom PureBasic-based script to compress and move old log files to a backup share. The script’s DLL is loaded into memory from a temporary directory (%TEMP% or C:\Temp\) during execution, which is a common location for transient executables.
  • Filter/Exclusion: Exclude files in %TEMP% or C:\Temp\ if the parent process is a known scheduled task executable (e.g., schtasks.exe or powershell.exe running a specific script path) and the file is deleted shortly after execution.
• Driver or Hardware Utility Update: A hardware vendor (e.g., for specialized scanners or