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 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.


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.


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.


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.


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


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.


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 = 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.


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

Microsoft Cloud PKI for Intune SCEP URL

Earlier this year, Microsoft announced Cloud PKI for Intune, a cloud service for issuing and managing digital certificates for Intune-managed endpoints. With Cloud PKI for Intune, administrators no longer need to deploy on-premises infrastructure to use certificates for user and device-based authentication for workloads such as Wi-Fi and VPN. Cloud PKI for Intune can be used standalone (cloud native) or integrated with an existing on-premises Active Directory Certificate Services (AD CS) enterprise PKI to extend an existing on-premises certificate services infrastructure.


Cloud PKI for Intune utilizes Simple Certificate Enrollment Protocol (SCEP) to enroll certificates for users and devices. To deploy Intune Cloud PKI certificates, administrators must create and deploy a SCEP Certificate device configuration policy in Intune.


When creating the SCEP certificate device configuration policy in Intune, administrators are asked to supply the SCEP server URL. Administrators will find this information by opening the Intune management console, navigating to Tenant Administration > Cloud PKI, clicking on the issuing certification authority, and then clicking Properties.

Administrators may notice the URL is unreachable if they try to connect to it using their web browser or PowerShell. Specifically, the FQDN is not shown in the URI; instead, it is represented as the variable {{CloudPKIFQDN}}, as highlighted above.

Policy Configuration

You can safely ignore this as it is not an error or misconfiguration. Simply copy and paste the entire URL into your SCEP certificate device configuration profile as is. Intune in the background will convert this to a fully formed URL with a proper FQDN accessible from the public Internet. This variable is used because it allows Microsoft to use different resources dynamically according to geography and availability.

Additional Information

RFC 8894 – Simple Certificate Enrollment Protocol

Microsoft Cloud PKI for Intune

Microsoft Cloud PKI for Intune and Active Directory

Microsoft Cloud PKI for Intune and Certificate Templates

Troubleshooting Intune Failed PKCS Request

Always On VPN administrators deploying on-premises enterprise PKI certificates using Microsoft Intune with PKCS may encounter a scenario where a certificate fails to be issued to a user or device. In this post, I’ll share some things to investigate when troubleshooting this issue.

Event 1001

To begin, open the Event Log and navigate to Applications and Services > Microsoft > Intune > CertificateConnectors > Admin. You will likely find an event ID 1001 from the CertificateConnectors source with the following error message.

Failed to process PKCS request.


Validate the following prerequisites have been met on the issuing Certification Authority (CA) server.

Certificate Template

Ensure the certificate template used for PKCS has the correct permissions and is published on an issuing CA server. Open the Certificate Templates management console (certtmpl.msc), right-click the certificate template, choose Properties, and then click on the Security tab. The certificate template must grant the Intune Certificate connector server’s computer account (or the PKCS connector’s service account if running as a service and not SYSTEM) the Read and Enroll permissions on the template.

CA Permissions

In addition to the permissions on the certificate template, ensure the correct permissions have been configured on the issuing CA itself. Right-click on the CA in the Certification Authority management console (certsrv.msc) and choose Security. Ensure the Intune Certificate connector server’s computer account (or the PKCS connector’s service account, if running as a service and not SYSTEM) is granted The Issue and Manage Certificates and Request Certificates permissions.

Intune Policy

Ensure the Intune device configuration policy is configured correctly. These three fields are critical and can result in failed PKCS certificate deployment if misconfigured.

Certification Authority

Enter the fully qualified domain name (FQDN) of the on-premises issuing CA server in this field.

Certification Authority Name

Enter the common name of the issuing CA in this field. You will find this information by running the following command on any domain-joined Windows system.

certutil.exe -dump

Certificate Template Name

Enter the name of the certificate template in Active Directory. Be aware that the template name and template display name are two different things. The template name is usually the template display name without spaces. However, that’s not a guarantee. On the General tab of the certificate template, look at the template name field on the certificate template to confirm.


This article is not a comprehensive troubleshooting guide for problems associated with failed PKCS certificate deployment using the Microsoft Intune Certificate connector and PKCS. However, it covers some of the more common problems administrators will likely encounter. If you cannot provision PKCS certificates correctly, drop me a note and I’ll provide further guidance.

Additional Information

Troubleshooting Failed Intune Certificate Connector Configuration – Part 1

Troubleshooting Failed Intune Certificate Connector Configuration – Part 2

Intune Certificate Connector Service Account and PKCS

Microsoft Intune Cloud PKI

Microsoft Intune Cloud PKI and Certificate Templates

Microsoft Intune Cloud PKI and Active Directory