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

Configure DirectAccess with OTP Authentication

Updated 6/10/2015: This post was revised to include instructions for enabling OTP support for Windows 7 clients and for configuring OTP on the DirectAccess server using the Remote Access Management console.

Introduction

DirectAccess in Windows Server 2012 R2 provides significantly improved authentication over traditional client-based VPN solutions. When configured to use certificate authentication (a recommended best practice) the DirectAccess client is authenticated using its machine certificate and its Active Directory computer account. Once the client machine has been authenticated, the user is also authenticated via Kerberos against a live domain controller over the existing DirectAccess connection. These multiple authentication steps provide a high level of assurance for DirectAccess-connected clients. If that’s not enough to meet your needs, additional strong user authentication is supported using dynamic One-Time Passwords (OTP).

Drawbacks for DirectAccess with OTP

While OTP provides an additional level of assurance, it does come with a few drawbacks. OTP adds additional complexity and makes troubleshooting more difficult. OTP cannot be configured with force tunneling; the two security features are mutually exclusive. DirectAccess OTP does not support RADIUS challenge-response. For Windows 7 clients, the DirectAccess Connectivity Assistant (DCA) v2.0 must be deployed. In addition, enabling OTP with DirectAccess disables the use of null cipher suites for IP-HTTPS. This can potentially have a negative effect on performance and scalability (more details here). Also, OTP fundamentally breaks the seamless and transparent nature of DirectAccess.

Configuring DirectAccess OTP

OTP for DirectAccess makes use of short-lived certificates for user authentication. Thus, enabling OTP for DirectAccess requires making changes to the internal Public Key Infrastructure (PKI). DirectAccess in Windows Server 2012 R2 can be configured to use the same Certificate Authority (CA) that is used to issue computer certificates to the DirectAccess clients and servers. This differs from DirectAccess with Forefront Unified Access Gateway (UAG) 2010, where a separate, dedicated CA was required.

To configure DirectAccess OTP, follow the instructions below.

OTP Certificate Request Signing Template

Open the Certification Authority management console, right-click Certificate Templates, and then choose Manage. Alternatively you can enter certtmpl.msc in the Start/Run box or search from the Windows Start menu. Right-click the Computer template and choose Duplicate Template. On a Windows Server 2008 or 2008 R2 CA, select Windows Server 2008 Enterprise when prompted for the duplicate certificate template version.

Configure DirectAccess with OTP Authentication

On a Windows Server 2012 or 2012 R2 CA, select Compatibility tab and then select Windows Server 2008 R2 for the Certification Authority and Windows 7/Windows Server 2008 R2 for the Certificate recipient.

Configure DirectAccess with OTP Authentication

Select the General tab and provide a descriptive name for the Template Display Name. Specify a validity period of 2 days and a renewal period of 1 day.

Configure DirectAccess with OTP Authentication

Select the Security tab and click Add. Click Object Types and then select Computers and click Ok. Enter the names of each DirectAccess server separated by semicolons and click Check Names. Click Ok when finished. For each DirectAccess server, grant Read, Enroll, and Autoenroll permissions. Select Authenticated Users and remove any permissions other than Read. Select Domain Computers and remove the Enroll permission. Select Domain Admins and grant Full Control permission. Do the same for Enterprise Admins.

Configure DirectAccess with OTP Authentication

Select the Subject Name tab and choose the option to Build from this Active Directory information. Select DNS name in the Subject name format drop-down list and confirm that DNS name is checked under Include this information in alternate subject name.

Configure DirectAccess with OTP Authentication

Select the Extensions tab, highlight Application Policies and click Edit.

Configure DirectAccess with OTP Authentication

Remove all existing application policies and then click Add and then New. Provide a descriptive name for the new application policy and enter 1.3.6.1.4.1.311.81.1.1 for the Object Identifier. Click Ok for all remaining dialog boxes.

Configure DirectAccess with OTP Authentication

OTP Certificate Template

In the Certificate Templates Console, right-click the Smartcard Logon certificate template and choose Duplicate Template. On a Windows Server 2008 or 2008 R2 CA, select Windows Server 2008 Enterprise when prompted for the duplicate certificate template version.

Configure DirectAccess with OTP Authentication

On a Windows Server 2012 or 2012 R2 CA, select the Compatibility tab and then select Windows Server 2008 R2 for the Certification Authority and Windows 7/Windows Server 2008 R2 for the Certificate recipient.

Configure DirectAccess with OTP Authentication

Select the General tab and provide a descriptive name for the Template Display Name. Specify a validity period of 1 hour and a renewal period of 0 hours.

Configure DirectAccess with OTP Authentication

Note: It is not possible to set the validity period to hours on a Windows Server 2003 Certificate Authority (CA). As a workaround, use the Certificate Templates snap-in on another system running Windows 7/Windows Server 2008 R2 or later. Also, if the CA is running Windows Server 2008 R2, the template must be configured to use a Renewal Period of 1 or 2 hours and a Validity Period that is longer but no more than 4 hours.

Select the Security tab, then highlight Authenticated Users and grant Read and Enroll permissions. Select Domain Admins and grant Full Control permission. Do the same for Enterprise Admins.

Configure DirectAccess with OTP Authentication

Select the Subject Name tab and choose the option to Build from this Active Directory information. Select Fully distinguished name in the Subject name format drop-down list and confirm that User principal name (UPN) is checked under Include this information in alternate subject name.

Configure DirectAccess with OTP Authentication

Select the Server tab and choose the option Do not store certificates and requests in the CA database. Clear the checkbox next to Do not include revocation information issued in certificates.

Configure DirectAccess with OTP Authentication

Select the Issuance Requirements tab and set the value for This number of authorized signatures to 1. Confirm that Application Policy is selected from the Policy type required in signature drop-down list and choose the OTP certificate request signing template created previously.

Configure DirectAccess with OTP Authentication

Select the Extensions tab, highlight Application Policies and click Edit. Highlight Client Authentication and click Remove. Ensure that the only application policy listed is Smart Card Logon.

Configure DirectAccess with OTP Authentication

Certificate Authority Configuration

In the Certificate Authority management console, right-click Certificate Templates, choose New, and then Certificate Template to Issue. Highlight both of the certificate templates created previously and click Ok.

Configure DirectAccess with OTP Authentication

Open an elevated command prompt and enter the following command:

certutil.exe -setreg dbflags +DBFLAGS_ENABLEVOLATILEREQUESTS

Configure DirectAccess with OTP Authentication

Restart the Certificate Authority service by right-clicking the CA in the Certificate Authority management console and choosing All Tasks and then Stop Service. Once complete, repeat these steps and choose Start Service.

DirectAccess Server Configuration

In the Remote Access Management console, select DirectAccess and VPN under Configuration in the navigate pane and then click Edit on Step 2 – Remote Access Server. Select Authentication, choose Two-factor authentication (smart card or one-time password (OTP)), and then check the option to Use OTP.

Configure DirectAccess with OTP Authentication

Click Next and then add the RADIUS servers that will be used for OTP authentication. Provide the hostname, FQDN, or IP address of the server, the shared secret, and specify the service port.

Configure DirectAccess with OTP Authentication

Click Next, select the CA server that will be used to issue certificates to DirectAccess clients for OTP authentication, and then click Add.

Click Next, select the CA server that will be used to issue certificates to DirectAccess clients for OTP authentication, and then click Add.

Note: When performing this step you may receive the following error.

No CA servers can be detected, and OTP cannot be configured. Ensure that
servers added to the list are available on each domain controller in the
corporate network.

Configure DirectAccess with OTP Authentication

If this occurs, close out of the Remote Access Management console and install this hotfix.

Click Next and select the certificate templates to be used for the enrollment of certificates that are issued for OTP authentication. Also select a certificate template used to enroll the certificate used by the DirectAccess server to sign OTP certificate enrollment requests.

Configure DirectAccess with OTP Authentication

Click Next and specify whether selected DirectAccess users can authenticate with a user name and password when OTP authentication is disabled. If some users need to be exempted from using OTP, specify the security group as required and click Finish.

Configure DirectAccess with OTP Authentication

Click Edit on Step 3 – Infrastructure Servers. Select Management and add the CA server used for OTP authentication to the list of management servers.

Configure DirectAccess with OTP Authentication

Click Ok and then Finish. Click Finish once more and then apply the changes.

DirectAccess OTP Client Experience

When a DirectAccess client is outside of the corporate network and has established DirectAccess connectivity, users can log on to their machine and access their desktop, but they will not be able to access corporate resources without first providing their OTP.

For Windows 8 clients, swipe in from the right side of the screen or press Window Key + I and click on the active network connection. The DirectAccess Workplace Connection will indicate that action is needed. Clicking on the Workplace Connection will indicate that credentials are needed. Clicking Continue will prompt the user to press Ctrl+Alt+Delete and provide their OTP.

Configure DirectAccess with OTP Authentication

For Windows 7 clients, an alert from the DirectAccess Connectivity Assistant (DCA) in the system tray will indicate that Windows needs your smart card credentials. Clicking on the notification Window will prompt the user to provide their OTP.

Configure DirectAccess with OTP Authentication

Alternatively the user can click on the DCA icon in the system tray and then click Lock and unlock your computer with a smartcard or a one-time password. The user will then press CTRL+ALT+DELETE, choose Other Credentials, select One-time password (OTP), and then provide their OTP.

Configure DirectAccess with OTP Authentication

Summary

Using dynamic, one-time passwords is an effective way to provide the highest level of assurance for remote DirectAccess clients. It does come with some potential drawbacks, so be sure to consider those before implementing OTP.

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