Troubleshooting Always On VPN Error 853

Troubleshooting Always On VPN Error 691 and 812 – Part 2

Using Windows Server Network Policy Server (NPS) servers is a common choice for authenticating Microsoft Windows 10 Always On VPN user tunnel connections. The NPS server is joined to the domain and configured with a Network Policy that defines the authentication scheme used by clients for authentication when establishing an Always On VPN connection. Protected Extensible Authentication Protocol (PEAP) using client authentication certificates recommended for most Always On VPN deployment scenarios.

Can’t Connect

Users establishing an Always On VPN user tunnel connection using PEAP and client authentication certificates may encounter a scenario in which a VPN connection attempt fails with the following error message.

“The remote access connection completed, but authentication failed because the certificate that authenticates the client to the server is not valid. Ensure that the certificate used for authentication is valid.”

Error 853

In addition, the Application event log records an event ID 20227 from the RasClient source that includes the following error message.

“The user <username> dialed a connection named <connection name> which has failed. The error code is 853.”

Missing NTAuth Certificate

Error code 853 is commonly caused by a missing issuing Certification Authority (CA) certificate in the NTAuth store on the NPS server. The NPS server must have the issuing CA certificate included in this store to perform authentication using client certificates. You can see the contents of the NTAuth certificate store by opening an elevated command window on the NPS server and running the following command.

certutil.exe -enterprise -viewstore NTAuth

Install Certificate

To install the issuing CA server’s certificate into the NTAuth store, copy the CA certificate to the NPS server, open an elevated command window, then run the following command.

certutil.exe -enterprise -addstore NTAuth <issuing CA certificate>

Once complete, view the store again, and you’ll see the issuing CA certificate listed in the NTAuth certificate store.

Additional Information

Troubleshooting Always On VPN Error Code 858

Troubleshooting Always On VPN Error Code 864

Always On VPN and Windows Server 2019 NPS Bug

Always On VPN Network Policy Server (NPS) Load Balancing

Microsoft Network Policy Server (NPS) Reason Codes

Always On VPN SSTP Security Configuration

Always On VPN SSTP Security Configuration

When using Windows Server Routing and Remote Access Service (RRAS) to terminate Always On VPN client connections, administrators can leverage the Secure Socket Tunneling Protocol (SSTP) VPN protocol for client-based VPN connections. SSTP is a Microsoft proprietary VPN protocol that uses Transport Layer Security (TLS) to secure connections between the client and the VPN gateway. SSTP provides some crucial advantages over IKEv2 in terms of operational reliability. It uses the TCP port 443, the standard HTTPS port, which is universally available and ensures Always On VPN connectivity even behind highly restrictive firewalls.

TLS Certificate

When configuring SSTP, the first thing to consider is the certificate installed on the server. A certificate with an RSA key is most common, but for SSTP, provisioning a certificate with an ECDSA key is recommended for optimal security and performance. See the following two articles regarding SSTP certificate requirements and ECDSA Certificate Signing Request (CSR) creation.

Always On VPN SSL Certificate Requirements for SSTP

Always On VPN ECDSA SSL Certificate Request for SSTP

TLS Configuration

Much like IKEv2, the default TLS security settings for SSTP are less than optimal. However, SSTP can provide excellent security with some additional configuration.

TLS Protocols

There are several deprecated TLS protocols enabled by default in Windows Server. These include SSLv3.0, TLS 1.0, and TLS 1.1. They should be disabled to improve security for TLS. To do this, open an elevated PowerShell window on the VPN server and run the following commands.

New-Item -Path ‘HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\SSL 3.0\Server\’ -Force

New-ItemProperty -Path ‘HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\SSL 3.0\Server\’ -Name Enabled -PropertyType DWORD -Value ‘0’

New-Item -Path ‘HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.0\Server\’ -Force

New-ItemProperty -Path ‘HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.0\Server\’ -Name Enabled -PropertyType DWORD -Value ‘0’

New-Item -Path ‘HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Server\’ -Force

New-ItemProperty -Path ‘HKLM:\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\Protocols\TLS 1.1\Server\’ -Name Enabled -PropertyType DWORD -Value ‘0’

Cipher Suites

Many weak TLS cipher suites and enabled by default in Windows Server. To further enhance security and performance, they can be optimized using a tool such as IIS Crypto. For example, consider prioritizing cipher suites that use ECDHE and GCM with ECDSA to improve security. Also, remove ciphers that use AES-256 to enhance scalability and performance.

Note: AES-256 does not provide any additional practical security over AES-128. Details here.

PowerShell Script

I have published a PowerShell script on GitHub that performs security hardening and TLS cipher suite optimization to streamline the configuration TLS on Windows Server RRAS servers. You can download the script here.

Validation Testing

After running the script and restarting the server, visit the SSL Labs Server Test site to validate the configuration. You should receive an “A” rating, as shown here.

Note: An “A” rating is not achievable on Windows Server 2012 or Windows Server 2012 R2 when using an RSA TLS certificate. A TLS certificate using ECDSA is required to receive an “A” rating on these platforms.

Additional Information

Always On VPN SSL/TLS Certificate Requirements for SSTP

Always On VPN ECDSA SSL Certificate Request for SSTP

Qualys SSL Labs Server Test Site

Always On VPN Protocol Recommendations for Windows Server RRAS

Microsoft SSTP Specification on MSDN

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