Always On VPN Security Updates August 2024

Today is the second Tuesday of the month, so Windows Server administrators everywhere know what that means – it’s Update Tuesday! For Always On VPN administrators in particular there are a few security updates that affect Windows Server Routing and Remote Access (RRAS), which is a popular VPN server used to support Always On VPN implementations. While many of these updates address Remote Code Execution vulnerabilities, non are considered critical.

RRAS Updates

This month there are six vulnerabilities disclosed affecting Windows Server RRAS.

CVE-2024-38120 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38121 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38128 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38130 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38154 – Windows RRAS Remote Code Execution Vulnerability (Important)

CVE-2024-38214 – Windows RRAS Remote Code Execution Vulnerability (Important)

Additional Updates

In addition to the updates addressing vulnerabilities in Windows Server RRAS, there are also updates available for the Windows Network Address Translation (NAT), Windows Transport Layer Security (TLS), and Windows IP Routing Management snapin that could potentially impact Always On VPN deployments.

Recommendations

None of the security vulnerabilities disclosed this month are critical. Although the RRAS vulnerabilities are remote code execution, exploitation is unlikely. However, administrators are encouraged to update their systems as soon as possible.

Additional Information

Microsoft August 2024 Security Updates

Always On VPN Device Tunnel Fails to Connect Automatically

After the April 2024 Microsoft security updates were released, many Always On VPN administrators noticed that the device tunnel suddenly stopped connecting automatically for many, if not all, their endpoints.

Note: There were additional problems with the April 2024 security update that affected the Always On VPN device tunnel. Details here.

Troubleshooting

When this problem occurs, administrators can establish the device tunnel connection successfully if it is initiated manually. This indicates that there are no issues with the IKEv2 VPN connection or security configuration.

Subscription Activation

The root cause of this issue is related to a subscription activation issue broken in the April 2024 security updates. In this case, Windows 10/11 Enterprise Edition devices that were initially provisioned using Professional Edition and used a step-up upgrade (subscription activation) to Enterprise Edition are reverting to Professional Edition. The Always On VPN device tunnel requires Enterprise Edition to work correctly. Although you can deploy a device tunnel to Windows Professional, it will not connect automatically. It will, however, connect manually.

KB5040527

On July 25, 2024, Microsoft released a preview of updates (KB5040527), including a fix for this subscription activation issue. Administrators experiencing problems with Always On VPN device tunnels where their devices revert to Professional Edition can install this update to resolve this issue.

Additional Information

Always On VPN Device Tunnel Issue with the Microsoft April 2024 Security Update

Always On VPN Device Tunnel Status Indicator

Always On VPN Devcie Tunnel Only Deployment Considerations

Always On VPN and Blast-RADIUS

Microsoft released an update for the Windows Server Network Policy Server (NPS) to address recently disclosed vulnerabilities in the Remote Access Dial-In User Service (RADIUS) protocol in the July 2024 security updates. RADIUS is an industry-standard authentication protocol widely used for remote access, including Always On VPN. The RADIUS protocol was first introduced in the early 1990s and, unfortunately, still relies on the deprecated MD5 cryptographic hash function. The good news is that this vulnerability does not affect Always On VPN. Read on to learn more.

Blast-RADIUS

Blast-RADIUS is an attack on the RADIUS protocol that allows an attacker to alter network authentication packets to gain access to a service relying on RADIUS for authentication by exploiting the weakness of MD5 integrity checks in RADIUS. In the absence other controls, an attacker could alter an authentication response and change the reply from Access-Reject to Access-Accept.

Considerations

It’s important to note that leveraging this attack is not trivial. It requires local network access, so the attacker must have a presence on the target network to carry out this attack. However, cloud-hosted RADIUS services are inherently more vulnerable. In addition, the attack is mostly academic today because the default timeout for authentication requests is typically short, usually between 5 and 30 seconds. This is not enough time (today) for an attacker to mount the attack. However, this attack could become more feasible if authentication timeouts are increased (sometimes required to support MFA) or if an attacker has access to vast computing resources.

Affected Protocols

Although Blast-RADIUS is a vulnerability in the RADIUS protocol itself, not all authentication protocols are affected. Specifically, this vulnerability affects services leveraging PAP, CHAP, MS-CHAP, and MS-CHAPv2. Extensible Authentication Protocol (EAP) and Protected Extensible Authentication Protocol (PEAP) are not vulnerable to this attack. Since Always On VPN requires EAP authentication, it is not susceptible to this attack.

Mitigation

Microsoft has published guidance in KB5040268 for mitigating Blast-RADIUS attacks on Windows NPS servers. Specifically, administrators are encouraged to enable the Message-Authenticator attribute in Access-Request packets sent by the network access server and to ensure the NPS server requires the Message-Authenticator attribute in any Access-Request messages it receives.

Note: The following changes are not required for Always On VPN or any other workload using EAP-TLS or Protected EAP, as these protocols use TLS natively to protect the authentication exchange.

NPS

To configure this setting in the UI, open the NPS management console (nps.msc) and perform the following steps.

  1. Expand RADIUS Clients and Servers.
  2. Highlight RADIUS Clients.
  3. Right-click the RADIUS client to configure and choose Properties.
  4. Select the Advanced tab.
  5. Check the box next to Access-Request messages must contain the Message-Authenticator attribute.

PowerShell

To configure this setting using PowerShell, open an elevated PowerShell command window and run the following command.

Set-NpsRadiusClient -Name <RADIUS client name> -AuthAttributeRequired $True

Additional NPS Settings

Administrators should also run the following commands on their NPS servers to further protect their infrastructure from Blast-RADIUS attacks.

netsh.exe nps set limitproxystate all = enable

netsh.exe nps set requiremsgauth all = enable

RRAS

When using Windows Server Routing and Remote Access (RRAS) without EAP, ensure the RADIUS server configuration always includes the Message-Authenticator. To configure this setting, open the Routing and Remote Access console (rrasmgmt.msc) on the RRAS server and perform the following steps.

  1. Right-click the VPN server and choose Properties.
  2. Select the Security tab.
  3. Click the Configure button next to the Authentication provider drop-down list.
  4. Highlight the RADIUS server and choose Edit.
  5. Check the box next to Always use message authenticator.

Repeat these steps for any additional configured RADIUS servers.

CLI

Administrators can implement this change at the command line by opening an elevated command window and entering the following command.

netsh.exe ras aaaa set authserver name = <name of RADIUS server> signature = enabled

For example:

netsh.exe ras aaaa set authserver name = nps.lab.richardhicks.net signature = enabled

New NPS Events

After installing the KB5040268 update on NPS servers, the NPS server will record event ID 4421 from the NPS source after a service start if the RequireMsgAuth or LimitProxyState settings are not configured.

“RequireMsgAuth and/or limitProxyState configuration is in Disable mode. These settings should be configured in Enable mode for security purposes.”

Optional Mitigation

If administrators cannot configure the above settings, consider using IPsec to secure network traffic at the transport layer. IPsec will protect all RADIUS traffic at the network layer to mitigate Blast-RADIUS attacks. Unfortunately, Windows Server NPS does not support TLS or DTLS, so IPsec is your only option.

Summary

Always On VPN is not vulnerable to the Blast-RADIUS attack. However, NPS is commonly a shared service in many organizations, and other workloads may use older, vulnerable protocols. Consider implementing the changes detailed in KB5040268 as outlined in above to ensure the integrity of your environment and mitigate these potential attacks.

More Information

Microsoft KB5040268: how to manage Access-Request packets attack vulnerability associated with CVE-2024-3596

RADIUS Protocol Vulnerability Exposes Networks to MitM Attacks

New Blast-RADIUS attack breaks 30-year-old protocol used in networks everywhere

Overview of Microsoft Protected Extensible Authentication Protocol (PEAP)

Always On VPN NPS Auditing and Logging