Always On VPN SSTP with Let’s Encrypt Certificates

Always On VPN SSTP Security Configuration

When configuring the Windows Server Routing and Remote Access Service (RRAS) to support Secure Socket Tunneling Protocol (SSTP) for Always On VPN user tunnel connections, administrators must install a Transport Layer Security (TLS) certificate on the VPN server. The best practice is to use a certificate issued by a public Certification Authority (CA). In addition, administrators should use a TLS certificate using Elliptic Curve Digital Signature Algorithm (ECDSA) for optimal security and performance.

Let’s Encrypt

Obtaining a public TLS certificate is not inherently difficult, nor is it expensive. However, Let’s Encrypt is a nonprofit public CA issues TLS certificates entirely for free. Always On VPN supports Let’s Encrypt TLS certificates, and installing a Let’s Encrypt certificate on the Always On VPN RRAS server is quite simple.

Pros and Cons

Using Let’s Encrypt certificates for Always On VPN has several significant advantages over traditional public CAs.

  • Cost – Let’s Encrypt certificates are free! No cost whatsoever.
  • Speed – Enrolling for a Let’s Encrypt certificate takes just a few minutes.
  • Trusted – Let’s Encrypt certificates are trusted by default in Windows 10 and Windows 11.

Let’s Encrypt is not without some drawbacks, however.

  • Lifetime – Let’s Encrypt certificates are only valid for 90 days.
  • Administration – Certificates must be redeployed frequently (every 90 days).
  • Security – PFX files (which include private keys) are left on disk by default.

It is possible to mitigate some of these drawbacks, though. For example, deleting PFX files after import can improve security. Alternatively, using a Certificate Signing Request (CSR) eliminates PFX files completely.

Also, it is possible to fully automate the Let’s Encrypt certificate enrollment and RRAS configuration process, which eases the administrative burden. And rotating certificates every 90 days could be considered an advantage from a security perspective! Enrolling new certificates (and specifically certificates with unique keys) is advantageous in that respect.

Certificate Enrollment

There are several different ways to enroll for Let’s Encrypt certificates. The preferred method is using PowerShell, as it works on both Windows Server with Desktop Experience (GUI) and Windows Server Core. Using PowerShell, administrators can also fully automate the enrollment and assignment of the certificate in RRAS.

PowerShell Module

To enroll for Let’s Encrypt TLS certificates on the VPN server, install the Posh-ACME PowerShell module. On the RRAS server, open an elevated PowerShell window and run the following command.

Install-Module Posh-ACME

Certificate Request

After installing the Posh-ACME PowerShell module, select a Let’s Encrypt environment by running the following command. Use LE_PROD for the production Let’s Encrypt server or LE_STAGE for the staging environment (used for testing).

Set-PAServer LE_PROD

Next, request a new certificate using the following command.

New-PACertificate -Domain vpn.example.net -Contact ‘[email protected]’ -CertKeyLength ec-256 -AcceptTOS -Install

The administrator is prompted to create a TXT record in public DNS to prove ownership of the domain. Using the example above, create a DNS record called _acme-challenge.vpn in the example.net DNS zone.

Once complete, the TLS certificate is automatically installed in the local computer certificate store on the VPN server and can be assigned in the RRAS management console, as shown here.

Note: R3 is a Let’s Encrypt issuing certification authority.

DNS Plugin

The Posh-ACME PowerShell module supports DNS plugins that allow administrators to automate the creation of the DNS TXT record used to authorize certificate enrollment. DNS plugins for many public DNS providers are available. Some of the more popular DNS providers are listed here.

  • Microsoft Azure
  • Amazon Route53
  • Cloudflare
  • Akamai
  • GoDaddy
  • Infoblox
  • Windows Server

A list of all supported DNS plugins for Posh-ACME can be found here.

Certificate Binding

Administrators can use the following PowerShell example code to automate the process of binding the new TLS certificate to the SSTP listener in RRAS.

$Thumbprint = <TLS certificate thumbprint>
$Cert = Get-ChildItem -Path Cert:\LocalMachine\My\$thumbprint
Set-RemoteAccess -SslCertificate $Cert
Restart-Service RemoteAccess -Passthru

Additional Information

Posh-ACME Tutorial

Windows 10 Always On VPN TLS Certificate Requirements for SSTP

Windows 10 Always On VPN SSTP Security Configuration

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

Always On VPN SSTP Certificate Binding Error

Always On VPN SSTP Certificate Binding ErrorWhen configuring a Windows Server with the Routing and Remote Access Service (RRAS) role to support Windows 10 Always On VPN connections, the administrator may encounter the following error message when installing or updating the TLS certificate used for Secure Socket Tunneling Protocol (SSTP) connections.

“The thumbprint (cert hash) of the certificate used for Secure Socket Tunneling Protocol (SSTP) is different than the certificate bound to the Web listener (HTTP.sys). Configure SSTP to use the default certificate or the certificate bound to SSL. You can configure web server applications to use the same certificate used by SSTP.”

Always On VPN SSTP Certificate Binding Error

IIS Binding

Most commonly this error can occur if an administrator mistakenly binds a TLS certificate directly in IIS. To resolve this problem, open the IIS management console (inetmgr.exe), navigate to the Default Web Site and click Bindings in the Actions section. Highlight the HTTPS binding and click Remove. Once complete, open an elevated command window and run the iisreset.exe command.

Always On VPN SSTP Certificate Binding Error

Netsh

In some instances, the administrator may find no certificate bindings in the IIS management console. However, a certificate binding may still be present. To confirm, open an elevated command window and run the following command.

netsh.exe http show sslcert

Always On VPN SSTP Certificate Binding Error

Remove existing certificate binding by running the following commands.

netsh.exe http delete sslcert ipport=0.0.0.0:443
netsh.exe http delete sslcert ipport=[::]:443

SSTP Configuration

When configuring SSTP in RRAS for Always On VPN, certificate assignment should always be performed using the Routing and Remote Access management console (rrasmgmt.msc). No changes are required to be made in the IIS management console for SSTP.

Additional Information

Windows 10 Always On VPN SSL Certificate Requirements for SSTP

Windows 10 Always On VPN SSTP Load Balancing with Citrix NetScaler ADC Load Balancer

Windows 10 Always On VPN SSTP Load Balancing with Kemp LoadMaster Load Balancer

Windows 10 Always On VPN SSTP Load Balancing with F5 BIG-IP Load Balancer

%d bloggers like this: