Always On VPN Authentication Failure with Azure Conditional Access

Always On VPN Clients Prompted for Authentication when Accessing Internal Resources

Integrating Microsoft Azure Conditional Access with Windows 10 Always On VPN has several important benefits. The most important is that it allows administrators to improve their security posture by enforcing access polices that can be dynamically applied. For example, requiring multifactor authentication (MFA) for privileged users (e.g., administrators) or sign-ins that appear to be risky, the type of device they are connecting with, the health of the endpoint, and much more.

Authentication Failure

When configuring Always On VPN to support Azure Conditional Access, administrators may expeirence a failed authentication during preliminary testing. Specifically, an event ID 20227 from the RasClient source may be encountered with the following error message.

“The user <username> dialed a connection named <connection name> which has failed. The error code returned on failure is 812.”

Looking at the event logs on the Network Policy Server (NPS) server reveals an event ID 6273 from the Microsoft Windows security auditing source with Reason Code 258 and the following Reason.

“The revocation function was unable to check revocation for the certificate.”

Root Cause

When Azure Conditional Access is configured for Always On VPN, a short-lived certificate (1 hour lifetime) is provisioned by Azure. This certificate does not include revocation information because, by design, a short-lived certificate does not need to be revoked. However, by default NPS always checks revocation when client authentication certificates are used for authentication. Since the certificate does not include this information, certificate revocation fails.

Resolution

The way to resolve this issue is to disable certificate revocation checking for Protected Extensible Authentication Protocol (PEAP) authentication requests. To do this, open an elevated PowerShell window on the NPS server and run the following command.

New-ItemProperty -Path ‘HKLM:\SYSTEM\CurrentControlSet\Services\RasMan\PPP\EAP\13\’ -Name IgnoreNoRevocationCheck -PropertyType DWORD -Value 1 -Force

Once complete, restart the NPS server for the changes to take effect.

Additional Information

Windows 10 Always On VPN Network Policy Server (NPS) Load Balancing

Windows 10 Always On VPN Network Policy Server (NPS) Server 2019 Bug

Always On VPN and Autopilot Hybrid Azure AD Join

Always On VPN and Autopilot Hybrid Azure AD Join

Windows Autopilot is a cloud-based technology that administrators can use to configure new devices wherever they may be, whether on-premises or in the field. Devices provisioned with Autopilot are Azure AD joined by default and managed using Microsoft Endpoint Manager. Optionally, an administrator can enable hybrid Azure AD join by also joining the device to an on-premises Active Directory domain using a domain join configuration profile in conjunction with the offline domain-join connector. Although enabling hybrid Azure AD join might sound appealing, there are specific deployment scenarios that present some rather unique and challenging problems when using this option.

Offline Hybrid Azure AD Join

For field-based devices, the device must have connectivity to a domain controller to support the initial login when the user has no local cached credentials. The Always On VPN device tunnel can be deployed in this scenario to provide connectivity and allow the user to log in to a new device the first time without being on-premises. The Always On VPN device tunnel is easily deployed using a Microsoft Endpoint Manager configuration profile. Certificates required to support the device tunnel can be deployed with Microsoft Endpoint Manager and one of the certificate connectors for Microsoft Endpoint Manager.

Windows 10 Professional

If a Windows 10 Professional device is configured using Autopilot, and hybrid Azure AD joined is enabled, the Always On VPN device tunnel can still be provisioned, but it won’t start automatically because it requires Enterprise Edition to be fully functional. This prevents the user from being able to logon the first time. The device must be upgraded to Enterprise Edition before the first user logon. There are multiple ways to accomplish this depending on the deployment scenario and activation requirements.

Multiple Activation Key

The easiest way to upgrade Windows 10 Professional to Enterprise Edition is to obtain a Multiple Activation Key (MAK) and deploy that to clients using a Microsoft Endpoint Manager configuration profile. Follow the steps below to create a configuration profile to perform this upgrade.

  1. Open the Microsoft Endpoint Manager console and click on Devices > Configuration Profiles.
  2. Click Create profile.
  3. Select Windows 10 and later in the Platform drop-down list.
  4. Select Templates in the Profile type drop-down list.
  5. Select Edition upgrade and mode switch from the list of templates.
  6. Click Create.

Use the following steps to configure the settings for the configuration profile.

  1. Enter a descriptive name for the configuration profile in the Name field.
  2. Enter a description for the profile in the Description field (optional).
  3. Click Next.
  4. Expand the Edition Upgrade section and select Windows 10 Enterprise from the Edition to upgrade to drop-down list.
  5. Enter your multiple activation product key in the Product Key field.

    Always On VPN and Autopilot Hybrid Azure AD Join

Once complete, assign the configuration profile to the appropriate groups and click Create.

KMS Activation

If Key Management Service (KMS) activation is required, follow the steps listed previously for MAK. Enter the KMS client setup key for Windows 10 Enterprise which is NPPR9-FWDCX-D2C8J-H872K-2YT43. The device will complete KMS activation when it can connect to the on-premises KMS host.

Subscription Activation

Windows 10 Enterprise Edition licensing is included in some Microsoft 365 subscriptions. This poses a unique challenge for hybrid Azure AD join scenarios, however. Specifically, subscription activation is a “step-up” process that requires Windows 10 Professional to have been successfully activated previously. Also, this occurs after the user logs on, but the user cannot log on unless the device tunnel is active. Catch 22!

Workaround

A multi-step process is required to address the limitations imposed by subscription activation. To begin, the device must be upgraded to Enterprise Edition, so the device tunnel is available for the initial user logon. This is a temporary, one-time upgrade to Enterprise Edition solely for the purpose of getting the device tunnel to connect and allow the user to authenticate.

To begin, download this PowerShell script and follow the steps below to deploy it to Windows 10 devices using Microsoft Endpoint Manager.

  1. Open the Microsoft Endpoint Manager console and click on Devices > Scripts.
  2. Click Add and select Windows 10.
  3. Enter a descriptive name for the configuration profile in the Name field.
  4. Enter a description for the profile in the Description field (optional).
  5. Click Next.
  6. Enter the location of the PowerShell script in the Script location field.
  7. Click Next, then assign the script to the appropriate device group(s) and click Add.

The PowerShell script will automatically install the KMS client setup key for Windows 10 Enterprise Edition, then restart the network interfaces to ensure the device tunnel starts. This will immediately upgrade the client device to Windows 10 Enterprise Edition and allow the user to authenticate.

Subscription activation with a step-up upgrade to Enterprise Edition still requires that Windows 10 Professional be activated first. To accomplish this, the embedded Windows 10 Professional key must be re-installed on the client. To do this, download this PowerShell script and follow the same steps listed previously to deploy a PowerShell script with Microsoft Endpoint Manager. However, this script should be assigned to users, not devices.

Once this script is run on the client it will be downgraded (temporarily) to Windows 10 Professional edition. After activation is successful, subscription activation will once again upgrade the client to Windows 10 Enterprise Edition.

Considerations

As you can see, the process of getting a Windows 10 Professional edition client onboarded in a hybrid Azure AD joined scenario is somewhat complex. My advice is to avoid this scenario whenever possible. Access to on-premises resources with the Always On VPN user tunnel with full single sign-on support is still available for users on Windows 10 devices that are Azure AD joined only. Unless there is a specific requirement to manage client devices using on-premises Active Directory and group policy, consider choosing native Azure AD join with Autopilot and manage devices using Microsoft Endpoint Manager exclusively.

Special Thanks

I would like to extend a special thank you to everyone in the Microsoft Endpoint Manager community who provided valuable input and feedback for me on this topic, especially John Marcum, Michael Niehaus, and Sandy Zeng. Follow the #MEMCM hashtag on Twitter to keep up on all things Microsoft Endpoint Manager.

Additional Information

Overview of Windows Autopilot

Windows 10 Subscription Activation

Windows 10 Always On VPN Class-Based Default Route and Microsoft Endpoint Manager

Windows 10 Always On VPN Device Tunnel and Custom Cryptography in Microsoft Endpoint Manager

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