Why Is Windows AllowNtAuthPolicyBypass Blocking Network? Fix Persistent CVE-2025-26647 Errors!

What Causes Ongoing Kerberos Authentication Failures After the CVE-2025-26647 Patch? Stop Annoying Certificate Problems Today!

Navigating the world of Windows security updates can sometimes feel like a complex puzzle. You apply a patch to fix one problem, only to find another issue has appeared. This is a situation many system administrators are facing with a security update for a vulnerability known as CVE-2025-26647. They are trying to do the right thing by securing their systems, but the process is causing unexpected trouble.

In April 2025, Microsoft released an update designed to strengthen Kerberos authentication, a core process that manages how users and computers are verified on a Windows network. This update introduced a new setting, a registry key called AllowNtAuthPolicyBypass, to help manage the rollout of the fix. However, reports have emerged from administrators, like a reader named Andy, who found that enabling the strongest protection level led to authentication failures for some of their client machines.

This article serves as your guide to this issue. It will walk you through what the vulnerability is, how the fix is supposed to work, and what might be going wrong. You will gain a clear understanding of the steps you can take to diagnose and resolve these problems in your own environment, ensuring your network is both secure and fully functional.

The Foundation: What Is CVE-2025-26647?

To understand the fix, you first need to understand the problem. Think of your computer network as a secure building. To get inside and access resources, you need to prove who you are. Kerberos is the security guard, or Key Distribution Center (KDC), that checks everyone’s identification.

The Vulnerability

The security flaw, identified as CVE-2025-26647, was a weakness in how this Kerberos “security guard” checked certain types of digital ID cards, known as certificates. It created a loophole where an attacker could potentially bypass the standard authentication process. This is a serious risk because it could allow unauthorized access to network resources.

The Microsoft Response

To close this loophole, Microsoft issued a security update. The update changes the rules for the Kerberos security guard, making its checks more strict. It now requires that all client certificates used for authentication must be linked to a trusted source that is listed in a special directory called the NTAuth store.

This change is essential for security. However, it also means that if any of your client machines are using certificates that don’t meet this new, stricter requirement, their attempts to log in will fail once the new rules are fully enforced.

The Control Switch: Explaining the AllowNtAuthPolicyBypass Registry Key

Microsoft understood that switching to this stricter security model overnight could break things. To give administrators control and time to adapt, they introduced a manual setting within the Windows Registry. This setting, or “key,” acts like a three-position switch, allowing you to control how strictly the Kerberos security guard enforces the new rules.

You can find this setting on your Domain Controllers at the following location:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Kdc

Inside this location, you must manually create a REG_DWORD value named AllowNtAuthPolicyBypass. This value can be set to 0, 1, or 2. Each setting has a very different effect on your network.

Value = 0 (Deactivated Mode)

This setting effectively tells the Kerberos guard to ignore the new security rule. It is like leaving the old, vulnerable system in place. No new checks are performed, and your network remains unprotected from the CVE-2025-26647 vulnerability. This mode is not recommended.

Value = 1 (Audit Mode)

This is the default setting if the key does not exist or is set to 1. Think of this as a “friendly warning” mode. The Kerberos guard will perform the new, stricter check on certificates. If it finds a certificate that doesn’t comply, it will still allow the login to succeed. However, it will make a note of the event by logging a warning in the System event log on the Domain Controller. This warning has an Event ID of 45. The purpose of this mode is to help you find which clients or services are using non-compliant certificates so you can fix them before you start blocking logins.

Value = 2 (Enforcement Mode)

This is the final, secure state. In this mode, the Kerberos guard acts as a strict gatekeeper. It performs the check, and if a certificate does not comply with the new rules, the authentication request is denied. The login fails. When this happens, an error is logged in the System event log on the Domain Controller with an Event ID of 21. This mode fully protects your network from the vulnerability.

Microsoft’s Phased Rollout Plan

To guide administrators through this transition, Microsoft laid out a three-phase plan for implementing the fix throughout 2025. Understanding these phases is crucial to knowing what to expect and when.

Phase 1: Initial Deployment (Started April 8, 2025)

The security update was released. By default, all Domain Controllers began operating in Audit Mode (AllowNtAuthPolicyBypass = 1). The goal during this phase was for administrators to monitor their event logs for Event ID 45. These warnings would point to the clients that needed attention before the enforcement phase began.

Phase 2: Enforcement by Default (Started July 8, 2025)

With the July updates, the default behavior was intended to shift. Systems would now be in Enforcement Mode unless an administrator manually created the AllowNtAuthPolicyBypass key and set it to 1. This phase raised the stakes, as insecure authentications would start being blocked by default. However, it still provided an escape hatch to revert to Audit Mode if major problems were discovered.

Phase 3: Full Enforcement (Starts October 14, 2025)

This is the final deadline. On this date, the ability to use the registry key to bypass the enforcement will be permanently removed. All systems will operate in Enforcement Mode (AllowNtAuthPolicyBypass = 2). Any remaining authentication issues related to non-compliant certificates will result in login failures with no option to revert.

A Real-World Case: When Enforcement Mode Causes Failures

The phased plan seems straightforward, but real-world environments are complex. A blog reader, Andy, shared his experience, which highlights a significant problem. His environment consisted of about 25 Windows Server 2016 Domain Controllers and roughly 1,500 Windows clients of various versions.

Andy and his team did exactly what was recommended. They monitored their Domain Controllers after the April update. They checked for Event ID 45 warnings and found none. Believing their environment was ready, they decided to proactively move to the secure state. They manually set the AllowNtAuthPolicyBypass registry key to 2 on their Domain Controllers.

Within minutes, problems began.

Domain Controller Errors

The system logs on the DCs started filling up with Event ID 21. The message indicated that a client certificate was not valid for smartcard logon because the “certification chain processed correctly, but one of the CA certificates is not trusted by the policy provider.”

Client-Side Errors

On the affected client machines (about 40 out of 1,500 initially), two new events appeared. Event ID 8 from Security-Kerberos noted a rejected client certificate. More damagingly, Event ID 1055 from GroupPolicy indicated that Group Policy processing had failed because the computer name could not be resolved.

Practical Consequences

Users on affected machines could no longer run gpupdate /force to refresh their policies. Attempts to request new certificates from their internal Certificate Authority (CA) would fail with an RPC server error. While users weren’t actively reporting that they couldn’t log in, critical background processes were clearly broken.

When Andy’s team set the AllowNtAuthPolicyBypass key back to 1 (Audit Mode), all the problems disappeared, and the affected machines started working correctly again. This confirmed that the issue was directly tied to the enforcement of the new Kerberos security policy.

Diagnosing the Root Cause

Andy’s situation points to a conflict between what is expected and what is happening. The official documentation from Microsoft, specifically the Windows Release Health information, acknowledges a similar issue. It states that updates released after April 8, 2025, could incorrectly log these events, particularly in environments using features like Windows Hello for Business (WHfB) Key Trust or Device Public Key Authentication. These features often rely on self-signed certificates linked to a device’s Active Directory object via the msds-KeyCredentialLink attribute.

The problem arises because these self-signed certificates, by their nature, do not chain up to a root CA that is listed in the NTAuth store. They are trusted through a different mechanism. The documentation states that the June 10, 2025, updates (like KB5061010) were intended to resolve this issue, making the KDC smart enough to handle these specific scenarios correctly even in Enforcement Mode.

However, in Andy’s case, his Domain Controllers were patched to the June 2025 level, yet the problem persisted. This suggests one of a few possibilities:

• The June 2025 fix is incomplete or does not cover all scenarios, possibly creating a bug.
• The issue is specific to certain operating system versions, like the Windows Server 2016 DCs in use.
• There is a subtle, underlying misconfiguration in the environment that was not exposed in Audit Mode but causes a hard failure in Enforcement Mode.

Your Action Plan: A Step-by-Step Guide for Administrators

If you are facing similar issues or are preparing to enable Enforcement Mode, follow this methodical approach to ensure a smooth and secure transition.

Revert to Audit Mode Temporarily

If you are experiencing active failures, your first priority is to restore service. Set the AllowNtAuthPolicyBypass registry value to 1 on all your Domain Controllers. This will stop the authentication failures and give you time to investigate properly.

Conduct a Thorough Log Investigation

Do not assume that “no Event ID 45” means you are safe. Go back and meticulously search the System event logs on all of your Domain Controllers for any instance of Event ID 45 since the April 2025 updates were installed. A single client connecting to a single DC and logging this event is a clue that must be investigated.

Verify Your NTAuth Store

The NTAuth store is the list of trusted certificate issuers for Active Directory. You must ensure that the Root and Intermediate CAs for your enterprise are correctly published here. You can check this on a Domain Controller by running the following command in Command Prompt:

certutil -viewstore NTAuth

This command will list the certificates currently in the store. If your issuing CAs are missing, you must add them. This is a common source of certificate validation failures.

Confirm Patch Levels Everywhere

A single, under-patched Domain Controller can cause intermittent and confusing issues. Verify that every single DC in your domain has the latest cumulative update installed. The issue was supposedly fixed in the June 2025 updates, but you should install the most recent ones available, such as the August 2025 updates, as they may contain additional fixes.

Establish a Controlled Test Environment

Do not test this in your full production environment. Set up a test lab that mimics your production setup. This should include at least one test DC and a few test client machines that represent the different configurations in your fleet. In this isolated environment, set the AllowNtAuthPolicyBypass key to 2 and perform extensive testing. Try to force Group Policy updates, request certificates, and perform other network-dependent actions.

Focus on Self-Signed Certificate Scenarios

Pay special attention to clients that use certificate-based authentication without a traditional PKI chain. This includes machines using Windows Hello for Business with Key Trust or other third-party solutions that leverage the msds-KeyCredentialLink attribute. These are the most likely candidates to experience this issue.

Consider Opening a Microsoft Support Case

If you have followed all these steps and can replicate the failure in a controlled environment, you may have found a genuine bug. Open a support case with Microsoft. Provide them with detailed information, including the exact OS versions and patch levels of your DCs and clients, the event logs, and a description of the failed operations.

Conclusion: Proceed with Caution and Diligence

The security fix for CVE-2025-26647 is not optional; it is a necessary step to protect your network from a known Kerberos vulnerability. The final enforcement deadline in October 2025 is approaching, and all administrators must prepare for it.

However, as experiences like Andy’s show, the path to full enforcement is not always a straight line. The AllowNtAuthPolicyBypass key, when set to Enforcement Mode, acts as a powerful diagnostic tool. It does not create new problems but rather exposes existing, often hidden, issues with certificate trust and configuration within your Active Directory environment.

Your strategy should be one of careful preparation. Use the time between now and the October deadline to operate in Audit Mode. Treat every Event ID 45 warning as a critical alert that must be resolved. Test Enforcement Mode thoroughly in a lab before deploying it. By taking a methodical and investigative approach, you can successfully navigate this challenge, achieving a network that is not only secure from this vulnerability but also more robust and correctly configured for the future.