Always On VPN NPS and PEAP Vulnerabilities

The February 2023 security updates for Windows Server address multiple vulnerabilities that affect Microsoft Always On VPN administrators. This latest update addresses multiple critical and important vulnerabilities in the Network Policy Server (NPS), commonly used to perform RADIUS authentication for Always On VPN servers. Specifically, there are several Remote Code Execution (RCE) and Denial of Service (DoS) vulnerabilities with Protected Extensible Authentication Protocol (PEAP). PEAP with user authentication certificates is the authentication protocol of choice for Always On VPN user tunnel authentication.

Vulnerabilities

The following is a list of vulnerabilities in PEAP addressed in the February 2023 security update.

  • CVE-2023-21689Microsoft PEAP Remote Code Execution Vulnerability (critical)
  • CVE-2023-21690Microsoft PEAP Remote Code Execution Vulnerability (critical)
  • CVE-2023-21691Microsoft PEAP Information Disclosure vulnerability (important)
  • CVE-2023-21692Microsoft PEAP Remote Code Execution Vulnerability (critical)
  • CVE-2023-21695Microsoft PEAP Remote Code Execution Vulnerability (important)
  • CVE-2023-21701Microsoft PEAP Denial of Service Vulnerability (important)

Mitigation

Unauthenticated attackers can exploit the RCE vulnerabilities in PEAP on Microsoft Windows NPS servers. However, NPS servers should not be exposed directly to the Internet and would require an attacker to have access to the internal network already. However, administrators are advised to apply this update to their NPS servers as soon as possible. In addition, organizations that deploy the NPS role on enterprise domain controllers should update immediately.

Additional Information

February 2023 Update for Windows Server 2022 (KB5022842)

February 2023 Update for Windows Server 2019 (KB022840)

February 2023 Update for Windows Server 2016 (KB5022838)

Troubleshooting Always On VPN Error 853

Troubleshooting Always On VPN Error 691 and 812 – Part 2

Using Windows Server Network Policy Server (NPS) servers is a common choice for authenticating Microsoft Windows 10 Always On VPN user tunnel connections. The NPS server is joined to the domain and configured with a Network Policy that defines the authentication scheme used by clients for authentication when establishing an Always On VPN connection. Protected Extensible Authentication Protocol (PEAP) using client authentication certificates recommended for most Always On VPN deployment scenarios.

Experiencing error 853 on Windows 11? Click here for more information.

Can’t Connect

Users establishing an Always On VPN user tunnel connection using PEAP and client authentication certificates may encounter a scenario in which a VPN connection attempt fails with the following error message.

“The remote access connection completed, but authentication failed because the certificate that authenticates the client to the server is not valid. Ensure that the certificate used for authentication is valid.”

Error 853

In addition, the Application event log records an event ID 20227 from the RasClient source that includes the following error message.

“The user <username> dialed a connection named <connection name> which has failed. The error code is 853.”

Missing NTAuth Certificate

Error code 853 is commonly caused by a missing issuing Certification Authority (CA) certificate in the NTAuth store on the NPS server. The NPS server must have the issuing CA certificate included in this store to perform authentication using client certificates. You can see the contents of the NTAuth certificate store by opening an elevated command window on the NPS server and running the following command.

certutil.exe -enterprise -viewstore NTAuth

Install Certificate

To install the issuing CA server’s certificate into the NTAuth store, copy the CA certificate to the NPS server, open an elevated command window, then run the following command.

certutil.exe -enterprise -addstore NTAuth <issuing CA certificate>

Once complete, view the store again, and you’ll see the issuing CA certificate listed in the NTAuth certificate store.

Additional Information

Always On VPN Error 853 on Windows 11

Troubleshooting Always On VPN Error Code 858

Troubleshooting Always On VPN Error Code 864

Always On VPN and Windows Server 2019 NPS Bug

Always On VPN Network Policy Server (NPS) Load Balancing

Microsoft Network Policy Server (NPS) Reason Codes

Always On VPN Continue Connecting Prompt

Using the Extensible Authentication Protocol (EAP) with client certificates is the recommended best practice for authentication for Windows 10 Always On VPN deployments. EAP, and especially Protected EAP (PEAP), has a lot of settings to configure and it is not uncommon to encounter issues related to some parameters being defined incorrectly. This post covers one of the more common issues related to EAP/PEAP misconfiguration.

Action Needed?

When establishing an Always On VPN user tunnel connection, users may find the connection does not complete automatically, and they are informed that additional action is needed.

Clicking on the VPN connection and then clicking Connect prompts the user with the following message.

“Action needed. Continue connecting? We don’t have enough info to validate the server. You can still connect if you trust this server.”

Common Causes

This message can occur when (EAP) is used and is configured to perform server validation with a restricted set of NPS servers, as shown here.

NPS Server Certificate

The NPS server performing authentication for the connection request must have a certificate that includes a subject name that matches one of the names of the NPS servers defined in the EAP configuration. The certificate must be issued by the organizations private certification authority (CA).

EAP Configuration

Alternatively, the client-side EAP configuration may be incorrect. Although the NPS server may have the correct hostname configured on its certificate, it may not be entered correctly on the client. Ensure the hostname listed in the “Connect to these servers” field matches the subject name or SAN of the NPS server certificate defined in the network policy used for the Always On VPN user tunnel. Look carefully at the syntax when defining multiple NPS servers. Multiple servers are separated by a semi-colon and there are no additional spaces. Missing either one of these critical details will result in connection prompts. Also, ensure that all NPS servers used for authentication (those defined on the VPN server) are included in this list.

Note: Administrators must ensure that all VPN clients have updated their EAP configuration before adding additional NPS servers to the environment. Failure to do so will result in connection prompts.

Security Best Practice

To be clear, the behavior above is not ideal from a security perspective. Validating the NPS server before authenticating is crucial to ensuring the highest level of security and assurance, preventing credential theft from a man-in-the-middle attack. For this reason, it is recommended that users not be given the choice to authorize an NPS server. Authorized NPS servers should be defined by administrators exclusively. This is accomplished by selecting the option “Don’t ask user to authorize new servers or trusted CAs” in the Notifications before connecting drop-down list, and by selecting the option “Don’t prompt user to authorize new servers or trusted certification authorities“.

Additional Information

Always On VPN Network Policy Server (NPS) Load Balancing

Always On VPN and Windows Server 2019 NPS Bug

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