NSFree

Hunt Hypothesis

The NSFree rule detects potential adversary behavior involving the use of a free network service to exfiltrate data or establish command and control, which may indicate covert communication channels. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate early-stage threat activity that could evade traditional detection methods.

YARA Rule

rule NSFree : Family
{
    meta:
        description = "NSFree"
        author = "Seth Hardy"
        last_modified = "2014-06-24"
        
    condition:
        NSFreeCode or NSFreeStrings
}

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
• Source Rule

False Positive Guidance

• Scenario: Scheduled system maintenance or patching using WSUS (Windows Server Update Services)
  Filter/Exclusion: Check for EventID 19031 (WSUS client event) or filter by process name wuauclt.exe with known WSUS update schedules.

• Scenario: Regular backup jobs using Veeam Backup & Replication
  Filter/Exclusion: Filter by process name Veeam.Backup.exe or check for known backup job names in the task scheduler or Veeam console.

• Scenario: Logon/logoff events during normal user activity using Event Viewer
  Filter/Exclusion: Filter by EventID 4624 (successful logon) or EventID 4634 (logoff), and exclude events from known user accounts or during business hours.

• Scenario: Scheduled task execution for system monitoring or health checks using Task Scheduler
  Filter/Exclusion: Filter by task name or command line containing keywords like healthcheck, monitor, or diagtrack.exe (for Windows diagnostics).

• Scenario: Administrative tool usage such as Group Policy Management Console (GPMC) or PowerShell remoting for configuration changes
  Filter/Exclusion: Check for process names like gpmc.msc, gpupdate.exe, or powershell.exe with command lines containing gpregistry or gpupdate /force.