Stealthy Shadow Copy Exploitation: Bypassing EDR with GUI-Based VSS Access

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Introduction

Attackers are increasingly leveraging Windows’ built-in “Previous Versions” feature to access Volume Shadow Copy Service (VSS) snapshots without triggering traditional command-line detection mechanisms. This technique allows adversaries to extract critical files like `ntds.dit` (Active Directory database) while evading Endpoint Detection and Response (EDR) solutions. Below, we explore detection gaps, mitigation strategies, and verified commands to identify and counter such attacks.

Learning Objectives

  • Understand how attackers exploit GUI-based VSS access to bypass EDR monitoring.
  • Learn detection methods for stealthy shadow copy manipulation.
  • Implement hardening measures to prevent unauthorized VSS snapshot access.

You Should Know

  1. How Attackers Access VSS Snapshots via UNC Path

Verified Command/Path:

\localhost\C$\@GMT-2025.06.21-10.53.43\Windows\NTDS\ntds.dit

Step-by-Step Explanation:

  1. Attackers gain initial access (e.g., via phishing or RDP).
  2. Instead of running vssadmin list shadows, they manually browse shadow copies via File Explorer > Properties > Previous Versions.
  3. The snapshot path follows the format @GMT-YYYY.MM.DD-HH.MM.SS, allowing direct UNC access.
  4. Critical files (e.g., ntds.dit, SAM) are copied without executing suspicious commands.

Detection:

  • Monitor for unusual file access patterns in `\Windows\NTDS\` or \System32\config\.
  • Enable Sysmon Event ID 11 (FileCreate) for tracking shadow copy interactions.

2. Detecting Malicious VSS Access with PowerShell

Verified PowerShell Script:

Get-WinEvent -LogName "Microsoft-Windows-Sysmon/Operational" | 
Where-Object { $<em>.Id -eq 11 -and $</em>.Message -like "@GMT" }

Step-by-Step Explanation:

  1. Sysmon logs file creation events (Event ID 11).
  2. The script filters events containing @GMT, indicating shadow copy access.

3. Investigate any matches for unauthorized activity.

Mitigation:

  • Restrict shadow copy access via Group Policy: 
    Computer Configuration > Policies > Windows Settings > Security Settings > File System > Deny access to \.\GLOBALROOT\Device\HarddiskVolumeShadowCopy

3. Hardening VSS Against Exploitation

Verified Command (Disable VSSAdmin for Non-Admins):

reg add "HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\SystemRestore" /v "DisableSR" /t REG_DWORD /d 1 /f

Step-by-Step Explanation:

  1. This registry key disables System Restore (and VSS) for non-administrative users.
  2. Prevents attackers from creating or listing shadow copies without elevated privileges.

Alternative:

  • Use Windows Defender Application Control (WDAC) to block `vssadmin.exe` execution.

4. Monitoring EDR Bypass Techniques

Verified Sigma Rule (Detection):

title: Suspicious Shadow Copy Access via UNC 
description: Detects access to VSS snapshots via '@GMT' UNC paths 
logsource: 
product: windows 
service: sysmon 
detection: 
selection: 
EventID: 11 
TargetFilename: "@GMT" 
condition: selection

Step-by-Step Explanation:

  1. Deploy this Sigma rule in SIEM (e.g., Splunk, Elastic) to alert on shadow copy access.
  2. Correlate with unusual RDP/logon events to identify compromise chains.

5. Mitigating Credential Theft via ntds.dit

Verified Command (Backup ntds.dit Safely):

ntdsutil "ac i ntds" "ifm" "create full C:\SafeBackup" q q

Step-by-Step Explanation:

  1. Legitimate admins can use `ntdsutil` for AD backups.
  2. Monitor for unauthorized use of this command (often precedes DC exploitation).

Detection Tip:

  • Alert on ntdsutil.exe execution outside of backup windows.

What Undercode Say

  • Key Takeaway 1: GUI-based VSS exploitation is a growing threat, bypassing traditional command-line monitoring.
  • Key Takeaway 2: Defenders must combine filesystem auditing, Sysmon, and least-privilege controls to close detection gaps.

Analysis:

This technique highlights the need for behavioral detection over signature-based rules. While EDR tools focus on vssadmin.exe, attackers pivot to GUI/UNC methods. Future attacks may abuse WMI or PowerShell for stealthier VSS interaction, requiring defenders to monitor Win32_ShadowCopy calls and shadow copy storage depletion (a sign of mass exfiltration).

Prediction

As EDR solutions improve command-line monitoring, attackers will shift to GUI-based or API-driven data theft. Microsoft may eventually restrict shadow copy access by default, but until then, proactive hardening and anomaly detection are critical. Expect more Living-off-the-Land (LOTL) tactics leveraging built-in Windows features for stealth.

Actionable Next Steps:

1. Audit your environment for `@GMT` path access.

  1. Test the Sigma rule and PowerShell detection script.

3. Restrict VSS via Group Policy for non-admins.

By closing this gap, organizations can mitigate a low-noise, high-impact attack vector. 🚀

IT/Security Reporter URL:

Reported By: Stephan Berger – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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