ShellModify01pll621

Hunt Hypothesis

This detection identifies potential shell script modifications that may indicate an adversary establishing persistence or executing malicious payloads within the environment. Proactive hunting for this behavior in Azure Sentinel is essential to uncover subtle post-compromise activities that might evade standard alerting thresholds due to their low severity classification.

YARA Rule

rule ShellModify01pll621
{
      meta:
		author="malware-lu"
strings:
		$a0 = { 55 8B EC 6A FF 68 98 66 41 00 68 3C 3D 41 00 64 A1 00 00 00 00 }

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 ShellModify01pll621 detection rule, including suggested filters and exclusions:

• Scenario: Scheduled PowerShell Script Execution by System Account

  • Context: A nightly maintenance job runs a custom .ps1 script via Task Scheduler under the NT SYSTEM account to update inventory logs. This triggers the rule as it modifies shell objects or executes code in memory.
  • Filter/Exclusion: Create an exclusion based on the process image path and user context:
    • Process Image Path: C:\Windows\System32\Tasks\* (or specific task name)
    • User Account: NT AUTHORITY\SYSTEM
    • Action: Suppress alerts where the parent process is svchost.exe or TaskSchedulerService running as SYSTEM.

• Scenario: Legitimate Admin Deployment via SCCM/Intune

  • Context: The Configuration Manager (SCCM) or Microsoft Intune agent pushes a policy update that executes a PowerShell script to modify registry keys or install patches on endpoints. This often involves ccmexec.exe spawning powershell.exe.
  • Filter/Exclusion: Exclude known deployment tools and their child processes:
    • Process Image Name: ccmexec.exe, IntuneManagementExtension.exe
    • Parent Process Image Name: msiexec.exe, ccmsetup.exe
    • Action: Whitelist the hash of the specific deployment agent or exclude any PowerShell execution where the parent process name matches the known deployment suite.

• Scenario: Automated Backup and Reporting Tools

  • Context: Enterprise backup solutions like Veeam or Rubrik utilize embedded PowerShell scripts to generate daily reports or manage snapshot states, often invoking `