PureBasicDLL

Hunt Hypothesis

This rule detects the presence of PureBasic DLLs, which adversaries may leverage to execute custom, low-profile payloads or perform in-memory manipulation to bypass standard application whitelisting controls. Proactively hunting for these artifacts in Azure Sentinel allows the SOC team to identify potential staging or execution components that could be used for lateral movement or privilege escalation before they are fully leveraged in an attack chain.

YARA Rule

rule PureBasicDLL : Neil Hodgson
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 83 7C 24 08 01 75 ?? 8B 44 24 04 A3 ?? ?? ?? 10 E8 }

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 Internal Tooling Maintenance: An IT support engineer is updating a custom-built internal inventory management application developed in-house using PureBasic years ago. The updated InventoryManager.dll is copied to the C:\Program Files\InternalTools\ directory during a routine patching cycle.
  • Filter/Exclusion: Exclude files located in specific application directories (e.g., C:\Program Files\InternalTools\) or exclude the specific hash of the known-good internal DLL if the YARA rule is hash-based.
• Game Development Asset Pipeline: A game developer’s CI/CD pipeline compiles a new version of a game engine module written in PureBasic. The resulting GameEngineCore.dll is deployed to a shared network drive (\\fileserver\dev\builds\) for QA testing.
  • Filter/Exclusion: Exclude files with the .dll extension located in designated development or build output directories (e.g., \\fileserver\dev\builds\ or C:\Users\Public\DevBuilds\).
• Scheduled Backup of Custom Scripts: A scheduled task runs nightly to back up custom automation scripts and their associated libraries. One of the libraries, AutoDeploy.dll (compiled from PureBasic source), is copied from the source server to the backup repository on the backup server.
  • Filter/Exclusion: Exclude files being written by specific backup service accounts (e.g., svc_backup) or exclude paths within the backup repository (e.g., D:\BackupRepo\Scripts\).
• Third-Party Plugin Installation: An administrator installs a third-party PDF annotation plugin for a document management system. The plugin includes a helper library PdfHelper.dll written in PureBasic for performance optimization, which is installed to C:\Program Files\DocManager\Plugins\.
  • *Filter/Ex