Always On VPN SSL Certificate Requirements for SSTP

Always On VPN Certificate Requirements for SSTPThe Windows Server 2016 Routing and Remote Access Service (RRAS) is commonly deployed as a VPN server for Windows 10 Always On VPN deployments. Using RRAS, Always On VPN administrators can take advantage of Microsoft’s proprietary Secure Socket Tunneling Protocol (SSTP) VPN protocol. SSTP is a Transport Layer Security (TLS) based VPN protocol that uses HTTPS over the standard TCP port 443 to encapsulate and encrypt communication between the Always On VPN client and the RRAS VPN server. SSTP is a firewall-friendly protocol that ensures ubiquitous remote network connectivity. Although IKEv2 is the protocol of choice when the highest level of security is required for VPN connections, SSTP can still provide very good security when implementation best practices are followed.

SSTP Certificate

Since SSTP uses HTTPS for transport, a common SSL certificate must be installed in the Local Computer/Personal/Certificates store on the RRAS VPN server. The certificate must include the Server Authentication Enhanced Key Usage (EKU) at a minimum. Often SSL certificates include both the Server Authentication and Client Authentication EKUs, but the Client Authentication EKU is not strictly required. The subject name on the certificate, or at least one of the Subject Alternative Name entries, must match the public hostname used by VPN clients to connect to the VPN server. Multi-SAN (sometimes referred to as UC certificates) and wildcard certificates are supported.

Always On VPN Certificate Requirements for SSTP

Certification Authority

It is recommended that the SSL certificate used for SSTP be issued by a public Certification Authority (CA). Public CAs typically have their Certificate Revocation Lists (CRLs) hosted on robust, highly available infrastructure. This reduces the chance of failed VPN connection attempts caused by the CRL being offline or unreachable.

Using an SSL certificate issued by an internal, private CA is supported if the CRL for the internal PKI is publicly available.

Key Type

RSA is the most common key type used for SSL certificates. However, Elliptic Curve Cryptography (ECC) keys offer better security and performance, so it is recommended that the SSTP SSL certificate be created using an ECC key instead.

Always On VPN Certificate Requirements for SSTP

To use an ECC key, be sure to specify the use of a Cryptographic Next Generation (CNG) key and select the ECDSA_P256 Microsoft Software Key Storage Provider (CSP) (or greater) when creating the Certificate Signing Request (CSR) for the SSTP SSL certificate.

Always On VPN Certificate Requirements for SSTP

Most public CAs will support certificate signing using ECC and Elliptic Curve Digital Signature Algorithm (ECDSA). If yours does not, find a better CA. 😉

Forward Secrecy

Forward secrecy (sometimes referred to as perfect forward secrecy, or PFS) ensures that session keys can’t be compromised even if the server’s private key is compromised. Using forward secrecy for SSTP is crucial to ensuring the highest levels of security for VPN connections.

To enforce the use of forward secrecy, the TLS configuration on the VPN server should be prioritized to prefer cipher suites with Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) key exchange.

Authenticated Encryption

Authenticated encryption (AE) and authenticated encryption with associated data (AEAD) is a form of encryption that provides better data protection and integrity compared to older block or stream ciphers such as CBC or RC4.

To enforce the use of authenticated encryption, the TLS configuration on the VPN server should be prioritized to prefer cipher suites that support Galois/Counter Mode (GCM) block ciphers.

Important Note: In Windows Server 2016, GCM ciphers can be used with both RSA and ECC certificates. However, in Windows Server 2012 R2 GCM ciphers can only be used when an ECC certificate is used.

SSL Offload

Offloading SSL to a load balancer or application delivery controller (ADC) can be enabled to improve scalability and performance for SSTP VPN connections. I will cover SSL offload for SSTP in detail in a future post.

Summary

SSTP can provide good security for VPN connections when implementation and security best practices are followed. For optimum security, use an SSL certificate with an EC key and optimize the TLS configuration to use forward secrecy and authenticated cipher suites.

Additional Information

Always On VPN ECDSA SSL Certificate Request for SSTP

Always On VPN and Windows Server Routing and Remote Access Service (RRAS)

Always On VPN Protocol Recommendations for Windows Server RRAS

Always On VPN Certificate Requirements for IKEv2

3 Important Advantages of Always On VPN over DirectAccess

Microsoft SSTP Specification on MSDN

Always On VPN Protocol Recommendations for Windows Server Routing and Remote Access Service (RRAS)

Always On VPN Protocol Recommendations for Windows Server Routing and Remote Access Service (RRAS)Windows 10 Always On VPN is infrastructure independent and can be implemented using third-party VPN devices. It is not necessary to deploy any Windows servers at all to support an Always On VPN solution. However, in a recent blog post I outlined some compelling reasons to consider using Windows Server 2016’s Routing and Remote Access Service (RRAS) feature to terminate VPN connections. RRAS supports both modern and legacy VPN protocols, each with their own advantages and disadvantages. The choice of which protocols to support will be determined by many factors, but it is important to understand the capabilities of each to make an informed decision.

RRAS VPN Protocols

Windows RRAS supports the following VPN protocols.

  • Internet Key Exchange version 2 (IKEv2) – RFC7296
  • Secure Sockets Tunneling Protocol (SSTP) – Microsoft
  • Layer Two Tunneling Protocol over IPsec (L2TP/IPsec) – RFC2661
  • Point-to-Point Tunneling Protocol (PPTP) – RFC2637

There are pros and cons associated with each of these VPN protocols. Here’s a breakdown of each.

IKEv2

This IPsec-based VPN protocol is the preferred choice for most deployments. IKEv2 provides the best security and performance, with native features that enhance mobility. This latest version of IKE (v2) features streamlined messaging during connection establishment and enhanced session management that reduce protocol overhead and improve performance.

Advantages: Best security and performance.
Disadvantages: Firewalls may block required UDP ports.

SSTP

SSTP is an excellent alternative to IKEv2. It uses industry standard Transport Layer Security (TLS), making it widely accessible from most locations. It provides good security out of the box, but can be improved upon with additional configuration. SSTP lends itself well to load balancing, making it much easier to scale out than IKEv2. Optionally, TLS can be offloaded to an Application Delivery Controller (ADC) to reduce resource utilization on the RRAS server and further improve performance.

Advantages: Easy to configure with firewall friendly access.
Disadvantages: Not as secure IKEv2.

L2TP

While technically supported for Always On VPN, L2TP is a legacy VPN protocol that offers no real advantages over IKEv2. Its use is unnecessary and should be avoided.

Advantages: None.
Disadvantages: Firewalls may block required UDP ports.

PPTP

PPTP is considered an obsolete VPN protocol with many known security vulnerabilities. Its use should be avoided at all costs.

Advantages: None.
Disadvantages: Insecure.

Summary

Implementation best practices dictate that IKEv2 and SSTP be enabled to support Windows 10 Always On VPN connections when using Windows Server 2016 RRAS. The use of L2TP/IPsec and PPTP should be avoided. The combination of IKEv2 and SSTP will provide the best security and availability for remote workers. Clients that can establish IKEv2 VPN connections can take advantages of the security and performance benefits it provides. SSTP can be enabled as a fallback for clients that are unable to establish an IKEv2 connection due to restricted firewall access.

Always On VPN Hands-On Training

Interested in learning more about Windows 10 Always On VPN? Hands-on training classes are now forming. More details here.

Additional Resources

Frequently Asked Questions about Microsoft’s PPTP Implementation

Always On VPN and Windows Server Routing and Remote Access Services (RRAS)

Windows 10 Always On VPN and the Future of DirectAccess 

5 Things DirectAccess Administrators Should Know about Always On VPN 

3 Important Advantages of Windows 10 Always On VPN over DirectAccess 

Windows 10 Always On VPN Hands-On Training Classes

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