From Non-Auth User to Local Admin: Exploiting Null Sessions and GPP Vulnerabilities

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Introduction

Null sessions and Group Policy Preference (GPP) vulnerabilities remain critical attack vectors in Windows environments. Attackers leverage these weaknesses to escalate privileges from an unauthenticated user to local administrator, often leading to full domain compromise. This article dissects the three-step attack chain and provides mitigation strategies.

Learning Objectives

  • Understand how null sessions expose sensitive system information.
  • Learn to exploit GPP cPasswords for privilege escalation.
  • Apply hardening techniques to prevent these attacks.

1. Exploiting Null Sessions for Initial Access

Command:

net use \dc\ipc$ "" /u:""

Step-by-Step Guide:

  1. Null Session Setup: The above command establishes a null session (anonymous connection) to the Domain Controller’s IPC$ share.
  2. Enumeration: Attackers use this to list shares, users, and groups without authentication.
  3. Impact: Exposes critical system details like SIDs, group memberships, and shared resources.

Mitigation:

  • Disable null sessions via `RestrictAnonymous` in Group Policy (Computer Configuration > Windows Settings > Security Settings > Local Policies > Security Options).

2. Hunting for GPP cPasswords

Command:

dir \dc\sysvol\contoso.corp\Policies -r -I .xml | Select-String cPassword >> dump.txt

Step-by-Step Guide:

  1. Search GPOs: The command recursively searches SYSVOL for XML files containing `cPassword` (encrypted credentials).
  2. Extract Data: Saved to `dump.txt` for offline analysis.

3. Decrypt: Use `gpp-decrypt` to reveal plaintext passwords:

gpp-decrypt <encrypted_string>

Mitigation:

  • Delete GPP files with cPasswords.
  • Use LAPS (Local Administrator Password Solution) for managed local admin passwords.

3. Privilege Escalation via Decrypted Credentials

Scenario:

  1. Decrypted credentials often belong to domain admins or local administrators.
  2. Use `runas` or RDP to gain elevated access:
    runas /user:DOMAIN\admin cmd.exe

Mitigation:

  • Regularly audit GPOs for sensitive data.
  • Enforce principle of least privilege.

4. Detecting Null Session Attacks

Command (Audit):

Get-WinEvent -LogName Security | Where-Object {$_.ID -eq 5145}

Purpose: Monitors share access attempts (Event ID 5145).

5. Hardening SYSVOL Permissions

Command:

icacls \dc\sysvol /deny "Authenticated Users":(R)

Purpose: Restricts read access to SYSVOL, preventing GPP leaks.

What Undercode Say

Key Takeaways:

  1. Legacy Threats Persist: Null sessions and GPP misconfigurations are still prevalent in enterprise networks.
  2. Automated Exploitation: Tools like Metasploit automate these attacks, making them accessible to low-skilled attackers.
  3. Proactive Defense: Regular audits and disabling legacy protocols are critical.

Analysis:

Despite being documented for over a decade, these vulnerabilities persist due to misconfigured legacy systems and lack of patch management. Organizations must prioritize hardening AD environments, monitoring SYSVOL, and replacing GPP with modern solutions like LAPS or Azure AD.

Prediction

As cloud adoption grows, attackers will shift focus to hybrid AD/Azure misconfigurations, but on-prem vulnerabilities like null sessions will remain a threat in legacy-heavy industries (e.g., manufacturing, healthcare). Continuous monitoring and zero-trust frameworks will become non-negotiable.

IT/Security Reporter URL:

Reported By: Zlatanh From – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅

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