Overview

In April 2025, Microsoft released a security update aimed at addressing a critical vulnerability in the Kerberos authentication protocol, identified as CVE-2025-26647. This update, however, has led to significant authentication issues within Windows Server environments, particularly affecting Active Directory Domain Controllers (DCs) and systems utilizing certificate-based authentication methods.

Background on CVE-2025-26647

CVE-2025-26647 is an elevation-of-privilege vulnerability within the Windows Kerberos authentication system. An authenticated attacker could exploit this flaw by obtaining a certificate containing a target Subject Key Identifier (SKI) from a Certificate Authority (CA). This certificate could then be used to acquire a Ticket Granting Ticket (TGT) for the target user from the Key Distribution Center (KDC), potentially granting unauthorized access to sensitive resources. (support.microsoft.com)

Impact of the April 2025 Security Update

Following the deployment of the April 8, 2025, security update (KB5055523), organizations reported widespread authentication failures. These issues predominantly affected environments utilizing:

  • Windows Hello for Business (WHfB) Key Trust: A passwordless authentication method for enterprise Windows devices.
  • Device Public Key Authentication (Machine PKINIT): Enables device-based authentication without traditional user credentials.

The update introduced stricter validation checks, requiring that certificates used for authentication chain to a root in the NTAuth store. Certificates failing this validation resulted in authentication errors, disrupting operations reliant on these authentication methods. (learn.microsoft.com)

Technical Details

The security update altered the behavior of Domain Controllers by enforcing that certificates used in Kerberos authentication must chain to a trusted root in the NTAuth store. This change was controlled via the INLINECODE0 registry key located at INLINECODE1 . The key could be set to:

  • 0: Disables the new behavior entirely.
  • 1: Enables the NTAuth check and logs warning events for certificates not chaining to the NTAuth store (Audit mode).
  • 2: Enforces the NTAuth check, denying logon requests for certificates failing the validation (Enforced mode).

By default, the update set this value to INLINECODE2 , initiating Audit mode. Administrators who manually set the value to INLINECODE3 experienced immediate authentication failures for certificates not meeting the new validation criteria. (support.microsoft.com)

Mitigation Steps

To address the authentication issues, Microsoft recommended the following steps:

  1. Update Domain Controllers: Ensure all DCs are updated with the latest Windows update released on or after April 8, 2025.
  2. Monitor Event Logs: Review domain controller event logs for Event ID 45, indicating certificates that do not chain to the NTAuth store.
  3. Adjust Registry Settings: Temporarily set the INLINECODE4 registry value to INLINECODE5 to allow authentication while addressing certificate issues.
  4. Validate Certificates: Ensure all client certificates are valid and chain to a trusted Issuing CA in the NTAuth store.
  5. Enable Enforcement Mode: Once all certificates are compliant, set the registry value to INLINECODE6 to enforce the new security measures.

Administrators are advised to exercise caution when modifying registry settings and to thoroughly test changes in a controlled environment before widespread deployment. (support.microsoft.com)

Implications for IT Administration

The unintended consequences of the April 2025 security update underscore the delicate balance between implementing security measures and maintaining system stability. IT administrators must remain vigilant, ensuring that security patches do not inadvertently disrupt critical authentication processes. This incident highlights the importance of:

  • Comprehensive Testing: Before deploying updates, conduct thorough testing in a controlled environment to identify potential issues.
  • Monitoring and Logging: Regularly monitor system logs to detect and address authentication failures promptly.
  • User Education: Inform users about potential authentication issues and provide guidance on reporting problems.

By adopting these practices, organizations can better navigate the complexities of maintaining secure and functional authentication systems.

Conclusion

While the April 2025 security update aimed to address a critical vulnerability in the Kerberos authentication protocol, it inadvertently introduced significant challenges for Windows Server environments. Through careful management, monitoring, and adherence to Microsoft's guidance, IT administrators can mitigate these issues and maintain both security and operational continuity.