Microsoft recently introduced Entra Private Access, an identity-centric Zero Trust Network Access (ZTNA) solution to provide secure remote access to on-premises resources. With Entra Private Access, administrators can leverage Entra Conditional Access to enforce policy-based access control for network access. However, Entra Private Access isn’t for everyone. It does not provide full feature parity with Always On VPN, and there are also licensing considerations. However, for those organizations using Always On VPN, the good news is that you can integrate Entra Conditional Access with Always On VPN today to gain some of the security benefits it provides.
Conditional Access
Microsoft Entra Conditional Access is a security feature that enables administrators to create and enforce policies that specify how users can access resources. In the specific case of Always On VPN, conditional access is critical to ensuring legitimate access to authenticated users on authorized devices.
Signals
Conditional access policies use a wide variety of signals for policy enforcement, such as:
User Identity: Who is making this access request?
User Properties: Is this user a member of a specific group?
Location: Where is this access request originating?
Device Management: Is this device joined to Entra ID?
Device State: Is this device compliant with security policies?
Device Platform: Is this a Windows device?
Risk Level: Is this login considered risky?
Access Control
Based on these signals, administrators can design a conditional access policy to enforce granular access control, such as:
Enforce specific authentication types (e.g., phishing-resistant credentials)
Allow access only from specific device platforms (e.g., Windows only)
Require Entra hybrid-joined device
Block access when a device is not compliant with security policies
Always On VPN
Entra Conditional Access works with Always On VPN by issuing a special, short-lived user authentication certificate once the user has been authorized. The Always On VPN infrastructure can be configured to use this certificate to grant access to the VPN. Integrating conditional access with Always On VPN can significantly improve the security posture of organizations using this feature.
Are you ready? In just a few short weeks(!) Microsoft will release the February 2025 security updates. This is a critical update because Microsoft plans to enable full enforcement of strong certificate mapping on Active Directory Domain Controllers (DCs) with this release. Administrators unprepared for this may incur outages for workloads using certificate-based authentication such as Always On VPN, Wi-Fi, and others.
Microsoft introduced strong certificate mapping with the May 2022 update KB5014754 to address vulnerabilities identified with certificate-based authentication. The update makes changes to Active Directory Certificate Services (AD CS) certification authorities (CAs) to embed the principal’s Security Identifier (SID) on issued certificates with a new certificate extension. The update also changes domain controller behavior to monitor and optionally enforce strong certificate mapping for authentication.
Enforcement Mode
When first introduced, the update is configured in compatibility mode. If a certificate that isn’t strongly mapped is presented for authentication, an event is recorded in the event log indicating that. Microsoft has been planning for years to enable full enforcement. After many delays, that time is now upon us. Specifically, full enforcement for strong certificate mapping will be enabled by default on DCs after applying the February 2025 security updates.
Note: Administrators can switch back to compatibility mode for now. See below for more details.
Limitations
Initially, the strong certificate mapping update was applied only to online certificate templates. Specifically, those templates are configured to build the subject name from Active Directory information. However, offline templates, where the subject name is supplied in the request, do not include this information by default. Crucially, any certificate issued with Microsoft Intune with PKCS or SCEP uses offline templates and is not strongly mapped. The lack of strong certificate mapping options for Intune-issued certificates forced Microsoft to delay its full enforcement deadline until these limitations were resolved.
Updates
In October 2024, Microsoft Intune announced support for strong certificate mapping for PKCS and SCEP certificates. Administrators can now configure these certificates to include strong certificate mapping. However, administrators must take action to affect this change.
PKCS
To enable strong certificate mapping for PKCS certificates, administrators must ensure that the certificate connector is running at least version 6.2406.0.1001. In addition, the following registry key must be configured on the connector server.
The Intune Certificate Connector server must be restarted for this change to take effect. No changes are required on the PKCS certificate policy in Intune.
SCEP
To enable strong certificate mapping for SCEP certificates, administrators must add the following attribute/value pair to the Subject alternative name settings on their existing Intune SCEP certificate policy.
Attribute: URI Value: {{OnPremisesSecurityIdentifier}}
Preparation
Administrators using certificate-based authentication against on-premises Active Directory should ensure all user and device authentication certificates include embedded SID information. For certificates issued on-premises, with Intune using PKCS or certificates issued by Entra Conditional Access, the certificate should now have the extension 1.3.6.1.4.1.311.25.2, including the principal’s SID.
SCEP certificates issued using Intune will include the following information in the Subject Alternative Name field.
URL=tag:microsoft.com,2022-09-24:sid:<sid>
Note: This applies to certificates issued using Cloud PKI for Microsoft Intune as those certificates are deployed using a SCEP device configuration policy.
Opt-Out
With the February 2025 security update, all domain controllers will be switched to full enforcement mode. Authentication requests using certificates without strong mapping will be denied in this configuration.
If your organization is not prepared to move to full enforcement mode, the February 2025 update allows administrators to opt out and switch back to compatibility mode by enabling the following registry key on all domain controllers.
Administrators are strongly encouraged to update all user and device authentication certificates before September 2025. With the September 2025 security update, Microsoft will no longer honor the opt-out registry settings and strictly enforce strong certificate mapping for all certificate-based authentication requests.
Troubleshooting
Certificate authentication is commonly used for Always On VPN and Wi-Fi authentication. If full enforcement mode is enabled on domain controllers and a certificate is presented for authentication that is not strongly mapped, administrators may see the following event log information recorded on the Network Policy Server (NPS).
Network Policy Server denied access to a user.
The details of the event include the following.
Reason Code: 16 Reason: Authentication failed due to a user credentials mismatch. Either the user name provided does not map to an existing user account or the password was incorrect.
Obviously, the user does not enter their password when using certificates for authentication. However, the indication of a credential mismatch can be caused by missing strong certificate mapping information when the DC is in full enforcement mode.
Note: There are other causes for reason code 16 failures on NPS. Further investigation may be required to determine the root cause.
Last week Microsoft introduced new Security Service Edge (SSE) capabilities as part of the Microsoft Entra suite of technologies. Included in these announcements, Microsoft introduced the public preview of two new secure remote access technologies – Microsoft Entra Internet Access and Microsoft Entra Private Access. The latter of these will particularly interest Microsoft Always On VPN administrators in some deployment scenarios.
Microsoft Entra Internet Access
Microsoft Entra Internet Access is a new Secure Web Gateway (SWG) cloud service solution designed to protect users from threats on the public Internet. Features include web content filtering, malware inspection, TLS inspection, and more. In addition, Entra Internet Access can protect Microsoft 365 applications. Azure Conditional Access policies can be enforced for Internet traffic. Network conditions are now included with Azure Conditional Access, which can further protect against attacks by requiring access from specific trusted or compliant networks. Today, the public preview is available for Microsoft 365 scenarios only. Internet traffic and other SaaS applications will be available later this year.
Microsoft Entra Private Access
Microsoft Entra Private Access is a Zero Trust Network Access (ZTNA) cloud service solution that leverages the Azure Application Proxy access model. With Azure App Proxy, administrators can easily publish private, on-premises web applications by installing the connector on an on-premises server. Administrators can leverage Azure AD authentication and conditional access policies to ensure device compliance or enforce multifactor authentication (MFA), if required. Microsoft Entra Private Access extends the capabilities of the Azure Application Proxy to support TCP and UDP-based applications.
Private Access vs. Always On VPN
Microsoft Entra Private Access will be a compelling alternative to Always On VPN in the future. Specifically, organizations using native Azure AD join devices could benefit tremendously from this technology. Microsoft Entra Private Access is much simpler to implement than Always On VPN and requires no on-premises infrastructure other than the Azure Application Proxy connector. Using Microsoft Entra Private Access also means that no inbound access from the Internet is required, making the solution inherently more secure and reducing the public attack surface. For organizations using hybrid Azure AD join, Always On VPN continues to be the best Microsoft solution for these scenarios.
I’ve had the privilege of participating in the private preview for Microsoft Entra Internet Access and Private Access. If you’d like to learn more about these technologies and how they can help your organization, fill out the form below, and I’ll provide more information.
