Table of Contents
- Can You Prevent Devastating Windows Hello & Kerberos Issues After April 2025 Updates? Essential Fixes for IT Pros
- What Happened?
- Who Is Affected?
- Symptoms and Event IDs
- Event ID 45
- Event ID 21
- Technical Background
- Workarounds and Recommendations
- If user logons are failing (Event ID 21)
- If only warnings are logged (Event ID 45)
- General advice
Can You Prevent Devastating Windows Hello & Kerberos Issues After April 2025 Updates? Essential Fixes for IT Pros
The April 2025 security updates for Windows Server introduced significant authentication issues, particularly affecting organizations using Active Directory Domain Controllers (DCs). These problems primarily impact enterprise environments and are unlikely to affect home users.
What Happened?
After installing the April 8, 2025 security update (KB5055523 and related patches), many Windows Server versions (2016, 2019, 2022, 2025) began experiencing failures with Kerberos authentication and Windows Hello logins in Key Trust mode. The issue is tied to changes in how DCs validate certificates for Kerberos authentication, specifically requiring certificates to chain to a root in the NTAuth store.
Who Is Affected?
Enterprise environments using:
- Windows Hello for Business (WHfB) Key Trust mode
- Device Public Key Authentication (Machine PKINIT)
- Smart card authentication products
- Third-party single sign-on (SSO) solutions
- Identity management systems
Home users are not impacted, as these features are rarely used outside enterprise settings.
Symptoms and Event IDs
Event ID 45
Occurs when the registry value AllowNtAuthPolicyBypass is set to “1”. The DC logs repeated warnings:
“The Key Distribution Center (KDC) encountered a client certificate that was valid but did not chain to a root in the NTAuth store.”
Despite the warnings, logons still succeed.
Event ID 21
Occurs when AllowNtAuthPolicyBypass is set to “2”. User logons fail, and the event log states:
“The client certificate for the user is not valid and resulted in a failed smartcard logon.”
This directly blocks user authentication.
Technical Background
The update enforces stricter validation for certificates used in Kerberos authentication, as part of protections for CVE-2025-26647 (Kerberos Authentication). The change is controlled by the registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Kdc\AllowNtAuthPolicyBypass
If the key is missing, the default behavior is as if set to “1”.
Workarounds and Recommendations
If user logons are failing (Event ID 21)
Set AllowNtAuthPolicyBypass to “1” instead of “2” to temporarily restore logon functionality.
If only warnings are logged (Event ID 45)
Logons are successful, but administrators should monitor for excessive event logging.
General advice
- Regularly install the latest updates, as Microsoft is actively working on a permanent fix.
- Closely monitor authentication logs and test security compliance after updates.
- Review certificate chains to ensure they are correctly anchored in the NTAuth store.
By understanding the root causes, symptoms, and workarounds, IT professionals can minimize disruption and maintain strong security postures during this critical update cycle.