How Can a Critical Windows Flaw Let Any User Disable Group Policies for 25 Years?

Why Is Microsoft Ignoring This Alarming Windows Security Weakness?

A longstanding Windows design flaw allows any standard user to bypass user group policies, effectively nullifying administrator-imposed restrictions. This vulnerability has existed for over 25 years and remains unaddressed by Microsoft, despite multiple responsible disclosures from security researchers.

Nature of the Flaw

Windows stores user group policy settings in the registry under the user's profile, specifically in the HKCU (HKEY_CURRENT_USER) hive.

While these policy registry keys are protected by access control lists (DACLs), the entire user profile folder—including critical registry hive files—is owned and writable by the user.

How the Bypass Works

Attackers (or malware) with standard user rights can use legitimate Windows tools and DLLs (such as Offreg.dll) to export their current registry hive (excluding protected policy keys) and create a new registry hive file named ntuser.man in their profile directory.

Upon next login, Windows prioritizes loading ntuser.man over the standard ntuser.dat, effectively replacing the administrator-enforced policies with the user's customized settings.

This process does not require administrator privileges and can be executed entirely with normal user permissions.

Impact

Any group policy restrictions applied to the user—such as disabling the command prompt, registry editor, or Office macro hardening—can be silently removed.

This undermines security controls, especially in environments relying on group policies for compliance or attack surface reduction.

Office security hardening, which relies on registry-based policies, can be completely bypassed, re-enabling previously blocked features like macros in old file formats.

Microsoft's Position

Microsoft considers this "expected behavior," arguing that since the HKCU hive is owned by the user, modifying it does not breach a security boundary.

No plans exist to address or mitigate this vulnerability, despite acknowledgment from security researchers and prior reports from organizations like Tenable.

Step-by-Step Attack Summary

1. Use a tool (such as Offreg.dll or a custom script) to export the current user registry hive, excluding policy keys.
2. Save the exported hive as ntuser.man in the user profile directory.
3. Upon next login, Windows loads ntuser.man, effectively removing group policy restrictions.

Result: Group policies set by administrators are no longer enforced for that user.

Security Implications

This flaw represents a significant and persistent weakness in Windows security architecture, allowing users or malware to easily bypass critical administrative controls. Without changes from Microsoft, organizations cannot reliably enforce user-specific group policies against determined users.

Recommended Countermeasures

While Microsoft has not provided a fix, researchers suggest advanced NTFS permissions as a partial mitigation:

• Deny users the ability to create or modify files (like ntuser.man) in their profile directory.
• Restrict permissions on ntuser.dat to prevent unauthorized changes.

However, these workarounds are complex and may introduce usability issues. They are not suitable for most enterprise environments.

Solution 1: Harden NTFS Permissions on User Profile Directories

Restrict File Creation and Modification

Deny users the ability to create or modify files like ntuser.man in their own profile directory (%USERPROFILE%). This can be achieved by adding a Deny Access Control Entry (ACE) specifically targeting file creation and DACL (Discretionary Access Control List) modifications within the user profile folder.

Example approach:

Use the CACLS.exe or icacls.exe command to modify permissions, inserting a Deny ACE for file creation and DACL changes before existing permissions.

This prevents standard users from introducing a malicious ntuser.man file that Windows would prioritize over ntuser.dat during login.

Lock Down Registry Hive Files

Add an NTFS ACE to explicitly deny users the ability to modify the DACL or add extended attributes to ntuser.dat. This step helps prevent users from granting themselves or others unauthorized access to their registry hive.

Example command:

Use CACLS.exe ntuser.dat /S and adjust the output to insert a Deny ACE for write permissions.

Solution 2: Restrict Registry Permissions

Limit Write Access on Sensitive Registry Keys

Use tools like icacls or PowerShell to ensure only authorized users (e.g., SYSTEM and Administrators) have write access to critical registry keys, especially those under HKCU\Software\Policies and HKCU\Software\Microsoft\Windows\CurrentVersion\Policies.

Regularly review and adjust permissions on these keys to block unauthorized modifications that could disable group policies.

Enable Registry Auditing

Implement registry auditing using auditpol to monitor and log attempts to modify sensitive registry keys. This provides early detection of suspicious activity and can help in incident response.

Solution 3: Prevent Creation of ntuser.man via Policy or Script

Automated Monitoring and Removal

Deploy scripts or endpoint management tools that routinely scan user profile directories for unauthorized ntuser.man files and remove them if detected.

Alert administrators when such files are found, indicating a potential bypass attempt.

Solution 4: Leverage Group Policy for Additional Restrictions

Restrict Registry Editor Usage

Use Group Policy to block access to registry editing tools for standard users, reducing the risk of manual registry manipulation.

Set policies to disable command-line tools that could be used to export or manipulate registry hives.

Solution 5: Educate and Limit Local Admin Rights

Minimize Local Admin Access

Limit the number of users with local administrator privileges, as these users can easily bypass group policy and other hardening measures.

Regularly audit membership of privileged groups and enforce the principle of least privilege.

Solution 6: Monitor for Policy Bypass Indicators

Detect Suspicious Logins and Policy Changes

Monitor for unexpected changes to user policies, failed policy applications, or the presence of ntuser.man files.

Use endpoint detection and response (EDR) solutions to flag and investigate anomalies.

Solution 7: Consider Application Whitelisting

Restrict Execution of Non-Approved Tools

Implement application whitelisting to prevent unauthorized tools (such as custom scripts or registry editors) from running, further reducing the attack surface.

Important Note: These countermeasures are complex and may impact usability or require significant administrative overhead. They should be tested thoroughly in a controlled environment before deployment to production systems.

This Windows design flaw allows any standard user to override administrator-enforced group policies, nullifying critical security controls. Microsoft does not consider this a security boundary issue and has no plans to address it. Organizations relying on group policy for user-level security should be aware of this limitation and consider alternative or additional security measures