Certificate-Based Authentication Changes and Always On VPN

Microsoft introduced important changes affecting certificate-based authentication on Windows domain controllers as part of the May 10, 2022 update KB5014754 that may affect Always On VPN deployments. The update addresses privilege escalation vulnerabilities when a domain controller is processing a certificate-based authentication request. The recommendation from Microsoft is that the update be applied to all Windows domain controllers and Active Directory Certificate Services (AD CS) servers as soon as possible.

Updated 5/20/2022: An out-of-band update to address authentication issues reported with this update is now available. Updates are available for Windows Server 2022, Windows Server 20H2, Windows Server 2019, Windows Server 2016, Windows Server 2012 R2, Windows Server 2012, Windows Server 2008 R2 SP1, and Windows Server 2008 SP2.

Certificate Services

After applying the update to certification authority (CA) servers, a non-critical extension with Object Identifier (OID) 1.3.6.1.4.1.311.25.2 is added to all issued certificates with the user or device security identifier (SID) included. Domain controllers with the update installed will use this information to validate the certificate used for authentication and ensure that it matches the information in Active Directory.

Domain Controllers

The update operates in Compatibility Mode, by default, when applied to domain controllers. Windows monitors authentication requests and records audit events for certificates presented for authentication under the following conditions.

No strong mapping (event ID 39) – The certificate has not been mapped explicitly to a domain account, and the certificate did not include the new SID extension.

Certificate predates account (event ID 40) – A certificate was issued before the user existed in Active Directory, and no explicit mapping could be found.

User’s SID does not match certificate (event ID 41) – A certificate contains the new SID extension, but it does not match the SID of the corresponding user account.

Certificate Mapping

Administrators can map certificates explicitly to accounts in Active Directory, but this results in a significant administrative burden in most environments. A better option is to reissue user and device authentication certificates after applying the KB5014754 update to all issuing CA servers.

Reenroll Certificates

Administrators should reissue user and device authentication certificates after applying the KB5014754 update. Open the Certificate Templates management console (certtmpl.msc), identify the user or device authentication certificate template, then right-click on the template and choose Reenroll All Certificate Holders.

Enforcement Mode

After applying update KB5014754, administrators should monitor domain controller event logs for event IDs 39, 40, and 41. Once all certificates have been updated, and none of these events have been recorded for 30 days, administrators can switch to Full Enforcement Mode by enabling it in the registry on all domain controllers.

Key: HKLM\SYSTEM\CurrentControlSet\Services\KDC
Value: StrongCertificateBindingEnforcement
Type: DWORD
Data: 2

Note: Microsoft will automatically switch to Full Enforcement Mode beginning May 9, 2023.

Known Issues

There have been some reports of authentication issues after installing the KB5014754 update. Early indications are that device authentication certificates missing a Subject Alternative Name (SAN) entry are to blame. Administrators are encouraged to update their device certificates to include the SAN entry. Optionally, but not recommended, administrators can place the update in disabled mode by editing the registry.

Note: An out-of-band update for these authentication issues is now available. See the reference links at the top of this article for more information.

Caveat

It’s important to understand that this new OID is added only to online templates. Online templates are those that build the subject information from Active Directory. Unfortunately, this new OID is NOT applied to offline templates (templates where the subject name is supplied in the request), such as those used for delivering certificates with Microsoft Endpoint Manager/Intune using PKCS or SCEP. It is impossible to move to enforcement mode when issuing user or device authentication certificates with Microsoft Endpoint Manager or Intune today. Microsoft is aware of this limitation and is working to address this issue as we speak. I expect a fix to be available sometime before the May 2023 deadline when Microsoft permanently switches on enforcement mode.

Additional Information

KB5014754 – Certificate-based authentication changes on Windows domain controllers

Microsoft Windows Always On VPN Users Prompted for Certificate

Microsoft Windows Always On VPN Clients Prompted for Authentication when Accessing Internal Resources

Always On VPN Updates for RRAS and IKEv2

Always On VPN Updates for RRAS and IKEv2

Many users have reported connection stability issues using Windows Server 2019 Routing and Remote Access Service (RRAS) and the IKEv2 VPN protocol. Specifically, there have been reports of random disconnects for which the connection cannot be re-established for an extended period. At the same time, other VPN connections may work without issue.

KB5003703

Microsoft has identified an issue in RRAS where the RemoteAccess service enters DoS protection mode, limiting incoming IKEv2 connection attempts. They released an update on June 15 (OS Build 17763.2028) that addresses this issue. Previously, the only workaround was to restart the IKEEXT service, which was highly disruptive if performed during peak hours.

No More Files

In addition, this update includes another Always On VPN-related fix for Windows 10 1809 clients. An Always On VPN user tunnel connection may fail, with an error message stating, “There are no more files.” The problem can occur after an existing user’s certificate is automatically renewed.

Additional Information

Microsoft Update June 15, 2021 KB5003703 (OS Build 17763.2028)

Always On VPN Updates for Windows 10 2004

Always On VPN Updates for Windows 10 2004Microsoft recently made available an update for Windows 10 2004 that includes many important fixes for outstanding issues with Windows 10 Always On VPN. KB4571744 (build 19041.488) addresses many challenges faced by Always On VPN administrators today, including the following.

TPM

This update addresses an issue that prevents hash signing from working correctly using the Microsoft Platform Crypto Provider for Trusted Platform Module (TPM). This issue can occur when administrators configure Always On VPN to use Protected Extensible Authentication Protocol (PEAP) with client certificate authentication using a FortiGate security device.

Sleep/Hibernate

This update also addresses issues with Windows 10 Always On VPN failing to automatically reconnect when resuming from sleep or hibernate. I’ve written about issues with Always On VPN and sleep/hibernate in the past. This is an issue that has plagued Always On VPN since its introduction, so let’s hope this finally provides some meaningful relief from this persistent problem.

Certificate Authentication

When both the Always On VPN device tunnel and user tunnel are provisioned to a Windows 10 clients, user tunnel connections may be authenticated using the machine certificate and not EAP/PEAP. This can result in connections that are not validated as intended, and allowing a user to bypass configured NPS policies, MFA requirements, or conditional access rules. This update includes a fix for this issue, restoring proper authentication for the user tunnel when the device tunnel is also provisioned.

Device and User Tunnel Coexistence

A bug that first appeared when Windows 10 2004 was introduced prevented a device tunnel and user tunnel Always On VPN connection from being established to the same VPN server if the user tunnel used Internet Key Exchange Version 2 (IKEv2). This update restores full functionality under those conditions.

Update KB4571744

To resolve these issues with Windows 10 Always On VPN as well as others, download and install update KB4571744 today. If you are experiencing any of these issues with releases of Windows 10 prior to 2004, look for updates for those build to come later this year.

Additional Information

September 3, 2020 – KB4571744 (OS Build 19041.488) Preview

Windows 10 Always On VPN Connection Issues after Sleep or Hibernate

Windows 10 Always On VPN Bug in Windows 10 2004

%d bloggers like this: