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
