Overview

In April 2025, Microsoft released security updates aimed at addressing vulnerabilities within Windows Server environments. However, these updates inadvertently introduced significant authentication issues, particularly affecting Kerberos-based logins and delegations. This article delves into the root causes, technical specifics, and the broader implications of these issues, providing guidance for IT professionals navigating this complex scenario.

Background on Kerberos Authentication

Kerberos is a network authentication protocol designed to provide strong authentication for client/server applications. It uses secret-key cryptography to enable secure communication over non-secure networks. Within Windows Server environments, Kerberos has been the default authentication protocol since Windows 2000, replacing NTLM due to its enhanced security features.

The April 2025 Security Updates and CVE-2025-26647

The April 2025 security updates were primarily released to mitigate a high-severity vulnerability identified as CVE-2025-26647. This vulnerability allowed authenticated attackers to escalate privileges remotely by exploiting improper input validation within the Windows Kerberos Key Distribution Center (KDC). Specifically, an attacker could obtain a certificate containing a target Subject Key Identifier (SKI) from a Certificate Authority (CA) and use it to acquire a Ticket Granting Ticket (TGT) for the target user from the KDC. This flaw posed a significant risk, potentially granting attackers elevated rights beyond their intended permissions.

Impact on Windows Server Environments

Post-update, organizations began reporting authentication failures across various Windows Server versions, including 2016, 2019, 2022, and 2025. The issues were particularly prevalent in environments utilizing:

  • Windows Hello for Business (WHfB) Key Trust: A passwordless authentication method that relies on public key infrastructure.
  • Device Public Key Authentication (Machine PKINIT): Facilitates device-based authentication without traditional user credentials.

These authentication failures also affected third-party solutions dependent on these protocols, such as single sign-on (SSO) platforms, identity management systems, and smart card authentication products.

Technical Details and Registry Modifications

The root cause of these authentication issues lies in the changes introduced to how Domain Controllers (DCs) validate certificates used for Kerberos authentication. With the April updates, DCs began verifying whether certificates chain to a root in the NTAuth store. This behavior is controlled by the INLINECODE0 registry key located at:

CODEBLOCK0

The possible values for this key are:

  • 0: Disables the NTAuth store check entirely.
  • 1: Enables the NTAuth store check and logs warning events (Audit mode). This is the default behavior post-update.
  • 2: Enforces the NTAuth store check, denying logons if the certificate does not chain to a root in the NTAuth store (Enforced mode).

If the INLINECODE1 key is not present, the system defaults to a value of INLINECODE2 .

Symptoms and Event Logs

Administrators may observe the following symptoms:

  • Event ID 45: Logged when the registry value is set to INLINECODE3 . The event indicates that the KDC encountered a client certificate that is valid but does not chain to a root in the NTAuth store. Despite the warning, authentication proceeds.
  • Event ID 21: Logged when the registry value is set to INLINECODE4 . This event signifies a failed smart card logon due to an invalid client certificate, resulting in authentication failures.

Mitigation Strategies

To address these issues, Microsoft recommends the following steps:

  1. Modify the Registry Key: Temporarily set the INLINECODE5 value to INLINECODE6 to prevent authentication failures. This adjustment places the system in Audit mode, allowing logons to succeed while logging warnings.
  2. Update the NTAuth Store: Ensure that all certificate authorities issuing logon certificates are included in the NTAuth store. This involves:
  • Identifying affected CAs through event logs.
  • Exporting the CA certificates.
  • Importing them into the NTAuth store using the command:

``INLINECODE7 `INLINECODE8 AllowNtAuthPolicyBypassINLINECODE9 2`.

Broader Implications

This incident underscores the delicate balance between implementing security patches and maintaining system stability. While addressing vulnerabilities is crucial, it's imperative for organizations to thoroughly test updates in controlled environments before widespread deployment. Additionally, maintaining an up-to-date and well-configured Public Key Infrastructure (PKI) is essential to prevent such authentication disruptions.

Conclusion

The April 2025 Windows Server updates highlight the complexities inherent in securing enterprise environments. By understanding the technical changes introduced, monitoring system behavior, and proactively managing certificate authorities, organizations can navigate these challenges effectively, ensuring both security and operational continuity.