Always On VPN and VpnStrategy

NetMotion Mobility for DirectAccess Administrators – Split vs. Force Tunneling

Always On VPN supports a variety of VPN protocols for the user tunnel. Internet Key Exchange version 2 (IKEv2) and Secure Socket Tunneling Protocol (SSTP) are the most common. I wrote about the advantages and disadvantages of each in this post. To summarize, IKEv2 provides the highest security options but suffers from operational limitations. SSTP offers excellent security and is generally more reliable.

VpnStrategy

Always On VPN administrators must choose between IKEv2 and SSTP when configuring the Always On VPN user tunnel. Some administrators may prefer to use IKEv2 when available but then fall back to SSTP if it is not. To accomplish this requires editing the rasphone.pbk file and setting the value of VpnStrategy to 8, as described here.

Challenges

Unfortunately, setting the VpnStrategy value to 8 poses some challenges. Updating rasphone.pbk requires editing a text file on each endpoint post-deployment. Updating rasphone.pbk can be automated using the Update-Rasphone.ps1 script or Microsoft Intune proactive remediation.

Limitations

By default, Windows will overwrite the VpnStrategy setting in rasphone.pbk when fallback occurs. For example, setting VpnStrategy to prefer IKEv2 over SSTP will be reset to use SSTP first if a connection with IKEv2 fails. There’s a registry setting available that’s supposed to prevent this, but it doesn’t always work as expected.

Windows 11

There’s good news for administrators deploying Always On VPN on Windows 11. Microsoft recently introduced support for additional NativeProtocol types in XML. Specifically, VPN protocol preference can now be defined using the ProtocolList native protocol type. When using the ProtocolList native protocol type, each supported VPN protocol is listed in order of preference using the syntax shown below.

In addition, the RetryTimeInHours value defines the time Windows will try the last successful connection protocol. Setting this value to 0 overrides this and ensures the preferred protocol (the first protocol in the list) will always be attempted first.

SSTP Only

Previously the VPNv2CSP only supported IKEv2 or Automatic as values for the native protocol type. Windows 11 now supports SSTP as a native protocol type. Administrators configuring Always On VPN user tunnel connections using SSTP exclusively can now use this option.

Caveats

While the settings above are supported in both Windows 11 21H2 and 22H2, there are some known issues when enabling these settings. Specifically, when administrators define the ProtocolList value for the native protocol type, IKEv2 is always shown as the active protocol, even when an SSTP connection is established.

Also, if ProtocolList is used, the VPN connection cannot be managed using PowerShell. The VPN profile will not be displayed when running Get-VpnConnection at the time of this writing. Hopefully Microsoft will fix this soon.

Additional Information

Always On VPN CSP Updates

Always On VPN IKEv2 and SSTP Fallback

Always On VPN and Intune Proactive Remediation

Always On VPN Protocol Recommendations for Windows Server RRAS

Always On VPN IKEv2 Features and Limitations

Always On VPN May 2023 Security Updates

Hey, Always On VPN administrators! It’s the second Tuesday of the month, which means security updates for Windows have been released. This month’s batch includes an update to address a critical vulnerability likely to affect many Always On VPN implementations using Windows Server.

SSTP Vulnerability

CVE-2023-24903 documents a vulnerability on Windows Servers with the Routing and Remote Access Service (RRAS) configured to support Secure Socket Tunneling Protocol (SSTP) for VPN connections. This is a remote code execution (RCE) vulnerability that can be exploited when an attacker sends a specifically crafted malicious packet to the server. Administrators are encouraged to update as soon as possible.

Mitigation

SSTP is commonly used for Always On VPN user tunnels. However, if administrators have configured user tunnels using IKEv2, or are using the device tunnel only, consider blocking inbound TCP 443 at the edge firewall to prevent attacks from the Internet. In addition, if SSTP is not in use, consider disabling support for SSTP by opening an elevated PowerShell command window and running the following commands.

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

Restart-Service RemoteAccess -PassThru

Alternatively, SSTP can be disabled in the RRAS management console by following the steps below.

  1. Open the RRAS management console (rrasmgmt.msc).
  2. Expand the server.
  3. Right-click Ports.
  4. Choose Properties.
  5. Highlight WAN Miniport (SSTP).
  6. Click Configure.
  7. Uncheck Remote access connections (inbound only).
  8. Uncheck Demand-dial routing connections (inbound and outbound).
  9. Enter 0 in the Maximum ports field.
  10. Click Ok.

Additional Information

Windows SSTP Remote Code Execution Vulnerability (CVE-2023-24903)

May 2023 Security Updates for Windows Server 2016 (KB5026363)

May 2023 Security Updates for Windows Server 2019 (KB5026362)

May 2023 Security Updates for Windows Server 2022 (KB5026370)

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