Prepare for Microsoft's Full Enforcement of Certificate-Based Authentication

With all the cybersecurity risks creeping in today's digital landscape, Microsoft is making moves to tighten authentication security in domain controller setups. If you're in the world of Active Directory and Windows Server, get ready because Full Enforcement mode related to certificate-based authentication is rolling out soon—and it isn’t messing around.
This article digs into KB5014754, Microsoft's detailed guidance on significant changes affecting certificate validation behaviors on Windows domain controllers. The deadline to prepare is quickly approaching, with February 11, 2025 marking the shift to Enforcement mode. Let's dive in and make sure you're armed with everything you need to handle this smoothly.

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What's the Fuss About?​

Microsoft has addressed several critical vulnerabilities (CVE-2022-34691, CVE-2022-26931, and CVE-2022-26923) to tighten the screws on certificate-based authentication. Pre-May 10, 2022 updates left scenarios where certificates could get spoofed if administrators weren’t keeping an eye on a dollar sign ($) in a machine name or resolving conflicts between User Principal Names (UPN) and sAMAccountName.
But now? The Key Distribution Center (KDC) can no longer let weak mappings slide. Microsoft wants organizations to pivot to smarter certificate mapping and get ready to embrace Full Enforcement mode.

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The Phases of Change: Patch Your Servers!​

This authentication overhaul is rolling out in phases. Here's your roadmap:

• Compatibility Mode: This arrived with the May 10, 2022 Security Update. If authentication works but triggers warnings due to weak certificate mappings, your KDC is operating in this mode. Administrators need to painfully comb through event logs for any warning signs pointing to issues with certificate mappings.
• Weak mappings are still accepted (but logged with warnings).
• Strong mappings get the green light, no problem.
• Enforcement Mode (Defaulting February 2025):
• If a certificate cannot latch onto a strong mapping, authentication will be outright denied.
• You can return to Compatibility mode using the StrongCertificateBindingEnforcement registry key—until September 2025.
• Full Enforcement Mode (September 2025 Deadline):
• This is the raise-your-shields defense strategy. Weak mappings will no longer survive, registry key settings will no longer work, and it'll be strong mappings or bust from this date forward.

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What Exactly is Strong Mapping?​

A strong mapping proves that the authenticated certificate and its associated user are one and the same—no loopholes, no exploits. Microsoft has defined mapping types that fall into two categories:

Weak (Insecure) Mappings:​

• X509IssuerSubject: Based on certificate issuer and subject names.
• X509SubjectOnly: Only considers the certificate’s subject.
• X509RFC822 (Email Address): Maps certificates based on email.
  These cannot be trusted because identifiers, like email addresses, are easy to duplicate or manipulate.

Strong (Secure) Mappings:​

• X509IssuerSerialNumber: Relies on the issuer and serial number of the certificate. Highly recommended.
• X509SKI: Leverages Subject Key Identifiers (SKI).
• X509SHA1PublicKey: Utilizes the SHA1 hash of the certificate’s public key. Another reliable option.

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How to Prepare for Full Enforcement​

Step 1: Audit and Verify Your Configurations​

Install the May 10, 2022 update and monitor event logs for warnings. Event IDs to look for:

• Event ID 39: No strong mapping available. Replace or map certificates explicitly.
• Event ID 40: User certificates predate the associated account (uh-oh!).
• Event ID 41: SID mismatches between certificate and Active Directory.

Step 2: Map Certificates Correctly​

Manually adjust the altSecurityIdentities attribute in Active Directory for certificates without built-in strong mappings. Use PowerShell if you're tweaking configurations in bulk.
Example for strong mapping through X509IssuerSerialNumber:

Set-ADUser 'UserAccount' -Replace @{
altSecurityIdentities = "X509:<I>DC=com,DC=contoso,CN=CONTOSO-CA<SR>12345678"
}

Remember to reverse the byte order of the certificate's serial number. Serial A1B2C3 becomes C3B2A1, not 3C2B1A.

Step 3: Update Windows Servers​

Administrators must ensure the following Windows Server OS versions are compliant:

• Windows Server 2016, 2019, 2022
• Legacy versions with Extended Security Updates (ESU), such as Server 2008 R2 and 2012 R2, also require configuration updates.

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Big Registry Key Players​

The future is automated, but today, you're still deep in the trenches with registry edits. Here are critical registry settings to know:

1. Key Distribution Center (KDC) Registry Key​

Enables different certificate mapping modes:

• 1: Allows weak mapping as a fallback (Compatibility Mode).
• 2: Full Enforcement—only strong mappings are accepted.

Registry Path:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Kdc

2. Certificate Backdating Compensation​

Addresses cases where certificates are issued before a user account was created—allowing backward date adjustments temporarily.

• Max Value = 50 years (0x5E0C89C0).

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Anticipating Common Issues​

• Authentication Failure?
  Check Active Directory timestamps against certificates or verify the certificate Secure Identifier (SID) extension matches the user account SID.
• TLS Certificate Mapping:
  Post-enforcement, services like IIS rely on Kerberos S4U2Self requests for authentication. Monitor these on the corresponding application server logs, not just domain controllers.

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Strong Mapping Default Changes​

A noteworthy change lands February 13, 2024: Certificate mapping defaults to strong types like X509IssuerSerialNumber.
Organizations relying on insecure mappings need to adapt ahead of this or risk legacy compatibility issues.

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What NOT to Do​

Microsoft explicitly discourages:

• Using Disabled Mode (removed in April 2023).
• Setting StrongCertificateBindingEnforcement to 0.
• Disabling SID extensions, as it impairs Kerberos protections.

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Frequently Asked Questions​

• Do I need to renew all certificates?
• No, not unless you're switching to certificates containing the ObjectSID extension.
• What happens on February 11, 2025?
• Devices move automatically to Audit or Full Enforcement mode, depending on the registry key setup.

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In Conclusion​

Microsoft’s Full Enforcement mode deadline is fast approaching. By September 2025, the leeway for weak certificate mappings will vanish. The time to act is now! Update your Windows servers, configure robust mappings, and start scouting your audit logs to avoid authentication chaos.
The future of authentication isn’t just secure—Microsoft is making sure it’s bulletproof. Ready for February 2025? Let us know how you're preparing and share your doubts or challenges in the forum!

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Source: Microsoft Announcements 60-day reminder: Full Enforcement mode for Certificate-based authentication changes on Windows DCs