Recently I wrote about denying access to Windows 10 Always On VPN users or computers. In that post I provided specific guidance for denying access to computers configured with the device tunnel. To summarize, the process involved exporting the device certificate from the issuing Certification Authority (CA) server and placing it in the Untrusted Certificates certificate store on each VPN server. In theory, simply revoking the device certificate should be all that’s required to prevent device tunnel connections.
Revocation Check Failure
As it turns out, a bug in Windows Server Routing and Remote Access prevents this from working as expected. Windows Server 2012 R2, 2016, and 2019 all fail to check the Certificate Revocation List (CRL) for IKEv2 VPN connections using machine certificate authentication (for example an Always On VPN device tunnel).
Update for Windows Server
Microsoft recently made a fix for this issue available for Windows Server 2016. It is included in the June 18, 2019 update KB4503294 (build 14393.3053). A fix for Windows Server 2019 is forthcoming. Windows Server 2012 R2 will not be updated. It is recommended that you upgrade to a later version of the Windows Server operating system to address this issue.
Note: This fix is now available for Windows Server 1903 (semi-annual channel). It is included in the June 27, 2019 update KB4501375 (build 18362.207).
Enable Revocation Check
Additional configuration is required to enable support for CRL checking. Microsoft published guidance for configuring CRL revocation checks for IKEv2 VPN connections using machine certificate authentication here. Specifically, administrators must enable the RootCertificateNameToAccept parameter and set a registry key to enable this functionality.
Open an elevated PowerShell window and run the following commands to enable CRL checking for IKEv2 VPN connections using machine certificate authentication.
$Thumbprint = ‘Root CA Certificate Thumbprint’
$RootCACert = (Get-ChildItem -Path cert:\LocalMachine\root | Where-Object {$_.Thumbprint -eq $Thumbprint})
Set-VpnAuthProtocol -RootCertificateNameToAccept $RootCACert -PassThru
New-ItemProperty -Path ‘HKLM:\SYSTEM\CurrentControlSet\Services\RemoteAccess\Parameters\Ikev2\’ -Name CertAuthFlags -PropertyTYpe DWORD -Value ‘4’ -Force
Restart-Service RemoteAccess -PassThru
A PowerShell script to update the RootCertificateNameToAccept parameter on multiple VPN servers can be found here.
Revoking Certificates
To prevent a Windows 10 Always On VPN device tunnel connection, the administrator must first revoke the certificate on the issuing CA. Next, open an elevated command window an enter the following commands. Repeat these steps on each VPN server in the enterprise.
certutil -urlcache * delete
certutil -setreg chain\ChainCacheResyncFiletime @now
Additional Information
Denying Access to Windows 10 Always On VPN Users or Computers
Blocking VPN Clients that use Revoked Certificates
PowerShell Script to Configure RootCertificateNameToAccept on GitHub
The Windows Server Routing and Remote Access Service (RRAS) includes support for the Secure Sockets Tunneling Protocol (SSTP), which is a Microsoft proprietary VPN protocol that uses SSL/TLS for security and privacy of VPN connections. The advantage of using SSTP for Always On VPN is that it is firewall friendly and ensures consistent remote connectivity even behind highly restrictive firewalls.
When deploying Windows 10 Always On VPN using Protected Extensible Authentication Protocol (PEAP) authentication with client certificates, administrators may find the VPN connection does not establish automatically. In this specific scenario the client is prompted to select a certificate to use to authenticate to the VPN server.
When deploying Windows 10 Always On VPN using Protected Extensible Authentication Protocol (PEAP) with client authentication certificates, the administrator may encounter a scenario in which the user can establish a VPN connection without issue, but when accessing internal resources they are prompted for credentials and receive the following error message.
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Important! Updated July 15, 2019 to support all versions of Windows Server including Windows Server 2012 and 2012 R2. Also added functionality to renew self-signed certificates individually.
Configuring Multifactor Authentication (MFA) is an excellent way to ensure the highest level of assurance for Always On VPN users. 
Once Windows 10 Always On VPN has been deployed in production, it may be necessary at some point for administrators to deny access to individual users or computers. Commonly this occurs when an employee is terminated or leaves the company, or if a device is lost, stolen, or otherwise compromised. Typically, this means that user accounts and computer accounts in Active Directory are disabled, and any issued certificates are revoked. However, additional steps may be required to disconnect current VPN sessions or prevent future remote connections.
A longstanding issue with Windows 10 Always On VPN is that of VPN tunnel connectivity reliability and device tunnel/user tunnel interoperability. Many administrators have reported that Always On VPN connections fail to establish automatically at times, that only one tunnel comes up at a time (user tunnel or device tunnel, but not both), or that VPN tunnels fail to establish when coming out of sleep or hibernate modes. Have a look at the comments on this post and you’ll get a good understanding of the issues with Always On VPN.
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