Always On VPN April 2023 Security Updates

Heads up, Always On VPN administrators! This month’s patch Tuesday includes fixes for critical security vulnerabilities affecting Windows Server Routing and Remote Access Service (RRAS). Crucially there are remote code execution (RCE) vulnerabilities in the Point-to-Point Tunneling Protocol (PPTP) (CVE-2023-28232), the Layer Two Tunneling Protocol (L2TP) (CVE-2023-28219, CVE-2023-28220), the Point-to-Point over Ethernet (PPPoE) protocol (CVE-2023-28224), and the Internet Key Exchange (IKE) protocol (CVE-2023-28238). The vulnerabilities in PPTP and L2TP are especially urgent as they allow an unauthenticated attacker to exploit them. There is also a denial-of-service (DoS) vulnerability (CVE-2023-28234) in the Secure Socket Tunneling Protocol (SSTP) protocol.

Exposure and Risk

The RCEs in PPTP, L2TP, and PPPoE should present limited risk as these protocols aren’t commonly used for Always On VPN (PPPoE and PPTP aren’t supported for Always On VPN, in fact). However, organizations may be using these protocols for other purposes. In addition, improperly configured edge firewalls could allow these connections even though administrators may not be actively using them. An attacker could also exploit these vulnerabilities with access to the RRAS server from the internal network.

Attack Surface Reduction

Always On VPN administrators are advised to ensure that only protocols and ports for VPN protocols in use are allowed through the edge firewall. Also, administrators should disable any unused protocols and services in RRAS to reduce the attack surface on their RRAS servers. To do this, open an elevated PowerShell command window on the RRAS server and run the following commands to disable support for the PPTP, L2TP, and PPPoE protocols.

netsh.exe ras set wanports device = “WAN Miniport (L2TP)” rasinonly = disabled ddinout = disabled ddoutonly = disabled maxports = 0

netsh.exe ras set wanports device = “WAN Miniport (PPTP)” rasinonly = disabled ddinout = disabled ddoutonly = disabled maxports = 1

netsh.exe ras set wanports device = “WAN Miniport (PPPOE)” ddoutonly = disabled

Restart-Service RemoteAccess -PassThru

Additional Vulnerabilities

This month’s update also includes fixes for other vulnerabilities that may impact Always On VPN deployments. Specifically, there are RCEs in Windows Network Address Translation (NAT) (CVE-2023-28217) and Windows Network Load Balancing (NLB) (CVE-2023-28240), and a DoS vulnerability in Windows Transport Layer Security (TLS) (CVE-2023-28234).

Update Now

Administrators should patch their RRAS servers as soon as possible to avoid potential compromise of the RRAS server in their environments.

Additional Information

Always On VPN SSTP Security Configuration

Always On VPN Load Balancing for RRAS in Azure

Always On VPN Load Balancing for RRAS in AzurePreviously I wrote about Always On VPN options for Microsoft Azure deployments. In that post I indicated that running Windows Server with the Routing and Remote Access Service (RRAS) role for VPN was an option to be considered, even though it is not a formally supported workload. Despite the lack of support by Microsoft, deploying RRAS in Azure works well and is quite popular. In fact, I recently published some configuration guidance for RRAS in Azure.

Load Balancing Options for RRAS

Multiple RRAS servers can be deployed in Azure to provide failover/redundancy or to increase capacity. While Windows Network Load Balancing (NLB) can be used on-premises for RRAS load balancing, NLB is not supported and doesn’t work in Azure. With that, there are several options for load balancing RRAS in Azure. They include DNS round robin, Azure Traffic Manager, the native Azure load balancer, Azure Application Gateway, or a dedicated load balancing virtual appliance.

DNS Round Robin

The easiest way to provide load balancing for RRAS in Azure is to use round robin DNS. However, using this method has some serious limitations. Simple DNS round robin can lead to connection attempts to a server that is offline. In addition, this method doesn’t accurately balance the load and often results in uneven distribution of client connections.

Azure Traffic Manager

Using Azure Traffic Manager is another alternative for load balancing RRAS in Azure. In this scenario each VPN server will have its own public IP address and FQDN for which Azure Traffic Manager will intelligently distribute traffic. Details on configuring Azure Traffic Manager for Always On VPN can be found here.

Azure Load Balancer

The native Azure load balancer can be configured to provide load balancing for RRAS in Azure. However, it has some serious limitations. Consider the following.

  • Supports Secure Socket Tunneling Protocol (SSTP) only.
  • Basic health check functionality (port probe only).
  • Limited visibility.
  • Does not work with IKEv2.
  • Does not support TLS offload for SSTP.

More information about the Azure Load Balancer can be found here.

Azure Application Gateway

The Azure Application Gateway can be used for load balancing RRAS SSTP VPN connections where advanced capabilities such as enhanced health checks and TLS offload are required. More information about the Azure Application Gateway can be found here.

Load Balancing Appliance

Using a dedicated Application Delivery Controller (ADC), or load balancer is a very effective way to eliminate single points of failure for Always On VPN deployments hosted in Azure. ADCs provide many advanced features and capabilities to ensure full support for all RRAS VPN protocols. In addition, ADCs offer much better visibility and granular control over VPN connections. There are many solutions available as virtual appliances in the Azure marketplace that can be deployed to provide RRAS load balancing in Azure.

Summary

Deploying Windows Server RRAS in Azure for Always On VPN can be a cost-effective solution for many organizations. Although not a formally supported workload, I’ve deployed it numerous times and it works quite well. Consider using a dedicated ADC to increase scalability or provide failover and redundancy for RRAS in Azure whenever possible.

Additional Information

Windows 10 Always On VPN Options for Azure Deployments

Windows 10 Always On VPN and RRAS in Microsoft Azure

Windows 10 Always On VPN with Microsoft Azure Gateway

DirectAccess Manage Out with ISATAP and NLB Clustering

DirectAccess Manage Out with ISATAP and NLB ClusteringDirectAccess connections are bidirectional, allowing administrators to remotely connect to clients and manage them when they are out of the office. DirectAccess clients use IPv6 exclusively, so any communication initiated from the internal network to remote DirectAccess clients must also use IPv6. If IPv6 is not deployed natively on the internal network, the Intrasite Automatic Tunnel Addressing Protocol (ISATAP) IPv6 transition technology can be used to enable manage out.

ISATAP Supportability

According to Microsoft’s support guidelines for DirectAccess, using ISATAP for manage out is only supported for single server deployments. ISATAP is not supported when deployed in a multisite or load-balanced environment.

Not supported” is not the same as “doesn’t work” though. For example, ISATAP can easily be deployed in single site DirectAccess deployments where load balancing is provided using Network Load Balancing (NLB).

ISATAP Configuration

To do this, you must first create DNS A resource records for the internal IPv4 address for each DirectAccess server as well as the internal virtual IP address (VIP) assigned to the cluster.

DirectAccess Manage Out with ISATAP and NLB Clustering

Note: Do NOT use the name ISATAP. This name is included in the DNS query block list on most DNS servers and will not resolve unless it is removed. Removing it is not recommended either, as it will result in ALL IPv6-enabled hosts on the network configuring an ISATAP tunnel adapter.

Once the DNS records have been added, you can configure a single computer for manage out by opening an elevated PowerShell command window and running the following command:

Set-NetIsatapConfiguration -State Enabled -Router [ISATAP FQDN] -PassThru

DirectAccess Manage Out with ISATAP and NLB Clustering

Once complete, an ISATAP tunnel adapter network interface with a unicast IPv6 address will appear in the output of ipconfig.exe, as shown here.

DirectAccess Manage Out with ISATAP and NLB Clustering

Running the Get-NetRoute -AddressFamily IPv6 PowerShell command will show routes to the client IPv6 prefixes assigned to each DirectAccess server.

DirectAccess Manage Out with ISATAP and NLB Clustering

Finally, verify network connectivity from the manage out host to the remote DirectAccess client.

Note: There is a known issue with some versions of Windows 10 and Windows Server 2016 that may prevent manage out using ISATAP from working correctly. There’s a simple workaround, however. More details can be found here.

Group Policy Deployment

If you have more than a few systems on which to enable ISATAP manage out, using Active Directory Group Policy Objects (GPOs) to distribute these settings is a much better idea. You can find guidance for creating GPOs for ISATAP manage out here.

DirectAccess Client Firewall Configuration

Simply enabling ISATAP on a server or workstation isn’t all that’s required to perform remote management on DirectAccess clients. The Windows firewall running on the DirectAccess client computer must also be configured to securely allow remote administration traffic from the internal network. Guidance for configuring the Windows firewall on DirectAccess clients for ISATAP manage out can be found here.

ISATAP Manage Out for Multisite and ELB

The configuration guidance in this post will not work if DirectAccess multisite is enabled or external load balancers (ELB) are used. However, ISATAP can still be used. For more information about enabling ISATAP manage out with external load balancers and/or multisite deployments, fill out the form below and I’ll provide you with more details.

Summary

Once ISATAP is enabled for manage out, administrators on the internal network can remotely manage DirectAccess clients wherever they happen to be. Native Windows remote administration tools such as Remote Desktop, Windows Remote Assistance, and the Computer Management MMC can be used to manage remote DirectAccess clients. In addition, enterprise administration tools such as PowerShell remoting and System Center Configuration Manger (SCCM) Remote Control can also be used. Further, third-party remote administration tools such as VNC, TeamViewer, LogMeIn, GoToMyPC, Bomgar, and many others will also work with DirectAccess ISATAP manage out.

Additional Information

ISATAP Recommendations for DirectAccess Deployments

DirectAccess Manage Out with ISATAP Fails on Windows 10 and Windows Server 2016 

DirectAccess Client Firewall Rule Configuration for ISATAP Manage Out

DirectAccess Manage Out and System Center Configuration Manager (SCCM)

Contact Me

Interested in learning more about ISATAP manage out for multisite and external load balancer deployments? Fill out the form below and I’ll get in touch with you.

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