Always On VPN NPS Auditing and Logging

The Network Policy Server (NPS) event log is incredibly valuable for administrators when troubleshooting Always On VPN user tunnel connectivity issues. Administrators can find these pertinent events by opening the Event Viewer on the NPS server (eventvwr.msc) and navigating to Custom View > Server Roles > Network Policy and Access Services.

Event Logs

When configured correctly, event logs will record the disposition of all authentication requests, allowed or denied. The two most common recorded events are event IDs 6272 (access granted) and 6273 (access denied).

NPS Event ID 6272 – Access granted.

NPS Event ID 6273 – Access denied.

Auditing

In some cases, administrators may find none of these events recorded even though user authentication is working correctly. Here, the only events recorded are NPS informational events indicating which domain controller the NPS server is using to perform authentication.

The lack of 6272 and 6273 events in the event log indicates that auditing for NPS events is not enabled. Open an elevated PowerShell window and run the following command to view the current auditing setting for NPS events.

auditpol.exe /get /subcategory:”Network Policy Server”

Open an elevated PowerShell window and run the following command to enable auditing for NPS events.

auditpol.exe /set /subcategory:”Network Policy Server” /success:enable /failure:enable

Group Policy

Alternatively, consider using Active Directory group policy to enforce the NPS server auditing settings. Open the Group Policy Management Console (GPMC) and create a new GPO. Next, navigate to Computer Configuration > Policies > Windows Settings > Security Settings > Local Policies > Audit Policy > Audit account logon events and select the option to audit both success and failure attempts.

Once complete, link this GPO to the OU where the NPS servers reside.

Missing Events

If auditing is enabled and there are no recorded 6272 or 6273 events, the NPS server did not receive any authentication requests from the VPN server. Review the event logs on any other NPS servers if there is more than one configured. In addition, this may indicate that network communication between the VPN and NPS server is blocked. Ensure network connectivity and name resolution are working as expected.

Troubleshooting Guides

Are you interested in learning more about Always On VPN troubleshooting? My Always On VPN book contains an entire chapter dedicated to troubleshooting. Also, my Always On VPN video training course on Pluralsight includes a module on troubleshooting. The video training course is available to Pluralsight subscribers only. If you don’t have a Pluralsight subscription, you can sign up for a free trial here.

Additional Information

Troubleshooting Always On VPN Errors 691 and 812

Troubleshooting Always On VPN Errors 691 and 812 – Part 2

Troubleshooting Always On VPN Errors 691 and 812 – Part 3

Always On VPN NPS Load Balancing

Certificate-Based Authentication Changes and Always On VPN

Microsoft introduced important changes affecting certificate-based authentication on Windows domain controllers as part of the May 10, 2022 update KB5014754 that may affect Always On VPN deployments. The update addresses privilege escalation vulnerabilities when a domain controller is processing a certificate-based authentication request. The recommendation from Microsoft is that the update be applied to all Windows domain controllers and Active Directory Certificate Services (AD CS) servers as soon as possible.

Updated 5/20/2022: An out-of-band update to address authentication issues reported with this update is now available. Updates are available for Windows Server 2022, Windows Server 20H2, Windows Server 2019, Windows Server 2016, Windows Server 2012 R2, Windows Server 2012, Windows Server 2008 R2 SP1, and Windows Server 2008 SP2.

Certificate Services

After applying the update to certification authority (CA) servers, a non-critical extension with Object Identifier (OID) 1.3.6.1.4.1.311.25.2 is added to all issued certificates with the user or device security identifier (SID) included. Domain controllers with the update installed will use this information to validate the certificate used for authentication and ensure that it matches the information in Active Directory.

Domain Controllers

The update operates in Compatibility Mode, by default, when applied to domain controllers. Windows monitors authentication requests and records audit events for certificates presented for authentication under the following conditions.

No strong mapping (event ID 39) – The certificate has not been mapped explicitly to a domain account, and the certificate did not include the new SID extension.

Certificate predates account (event ID 40) – A certificate was issued before the user existed in Active Directory, and no explicit mapping could be found.

User’s SID does not match certificate (event ID 41) – A certificate contains the new SID extension, but it does not match the SID of the corresponding user account.

Certificate Mapping

Administrators can map certificates explicitly to accounts in Active Directory, but this results in a significant administrative burden in most environments. A better option is to reissue user and device authentication certificates after applying the KB5014754 update to all issuing CA servers.

Reenroll Certificates

Administrators should reissue user and device authentication certificates after applying the KB5014754 update. Open the Certificate Templates management console (certtmpl.msc), identify the user or device authentication certificate template, then right-click on the template and choose Reenroll All Certificate Holders.

Enforcement Mode

After applying update KB5014754, administrators should monitor domain controller event logs for event IDs 39, 40, and 41. Once all certificates have been updated, and none of these events have been recorded for 30 days, administrators can switch to Full Enforcement Mode by enabling it in the registry on all domain controllers.

Key: HKLM\SYSTEM\CurrentControlSet\Services\KDC
Value: StrongCertificateBindingEnforcement
Type: DWORD
Data: 2

Note: Microsoft will automatically switch to Full Enforcement Mode beginning May 9, 2023.

Known Issues

There have been some reports of authentication issues after installing the KB5014754 update. Early indications are that device authentication certificates missing a Subject Alternative Name (SAN) entry are to blame. Administrators are encouraged to update their device certificates to include the SAN entry. Optionally, but not recommended, administrators can place the update in disabled mode by editing the registry.

Note: An out-of-band update for these authentication issues is now available. See the reference links at the top of this article for more information.

Caveat

It’s important to understand that this new OID is added only to online templates. Online templates are those that build the subject information from Active Directory. Unfortunately, this new OID is NOT applied to offline templates (templates where the subject name is supplied in the request), such as those used for delivering certificates with Microsoft Endpoint Manager/Intune using PKCS or SCEP. It is impossible to move to enforcement mode when issuing user or device authentication certificates with Microsoft Endpoint Manager or Intune today. Microsoft is aware of this limitation and is working to address this issue as we speak. I expect a fix to be available sometime before the May 2023 deadline when Microsoft permanently switches on enforcement mode.

Additional Information

KB5014754 – Certificate-based authentication changes on Windows domain controllers

Microsoft Windows Always On VPN Users Prompted for Certificate

Microsoft Windows Always On VPN Clients Prompted for Authentication when Accessing Internal Resources

Always On VPN Book Available for Pre-Order

Great news! My new book, Implementing Always On VPN, is now available for pre-order on Amazon.com. This new book, scheduled for release in late 2021, is a comprehensive implementation guide for Windows 10 Always On VPN. Drawing on many years of experience deploying Always On VPN for organizations worldwide, it covers all aspects of an Always On VPN deployment, including planning and design, prerequisite gathering, infrastructure preparation, and client deployment.

In addition, it contains detailed, prescriptive guidance for advanced configuration options such as application and traffic filtering and proxy server configuration. Cloud deployments using Azure VPN gateway and Virtual WAN are covered, and it includes guidance for configuring Azure MFA and Conditional Access.

Also, the book includes thorough guidance for provisioning certificates using Microsoft Endpoint Manager/Intune using both PKCS and SCEP. It outlines options for high availability for VPN and authentication infrastructure and provides details for ongoing system maintenance and operational support.

Finally, the book has an entire chapter dedicated to troubleshooting and resolving common (and not so common!) issues encountered with Windows 10 Always On VPN.

Reserve your copy today. Pre-order Implementing Always On VPN now!

Chapter List

  1. Always On VPN Overview
  2. Plan an Always On VPN Deployment
  3. Prepare the Infrastructure
  4. Configure Windows Server for Always On VPN
  5. Provision Always On VPN clients
  6. Advanced Configuration
  7. Cloud Deployments
  8. Deploy Certificates with Intune
  9. Integrating Azure MFA
  10. High Availability
  11. Monitor and Report
  12. Troubleshooting
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