How to Fix 4625 Logon Failures with SSL Stream AuthenticateAsServer and AuthenticateAsClient

When you use SslStream.AuthenticateAsServer or AuthenticateAsClient to enable TLS communication in .NET applications, Windows may generate hundreds or thousands of Event ID 4625 – An Account Failed to Log On entries.

This often confuses developers into thinking it is a security breach, but in almost every case, the failure comes from certificate authentication mapping, not from an actual login attempt.

Why You See 4625 Errors With SSL Stream

Each time SslStream performs a certificate handshake, Windows (Schannel) tries to map the client certificate to a valid Windows or Active Directory user.

If the certificate:

• Does not match any domain account
• Has no valid UPN (User Principal Name)
• Has no certificate mapping configured
• Has an invalid or expired chain of trust

Windows attempts a logon using Kerberos/NTLM, fails, and records a 4625 entry.

So you’re not seeing intrusion — you’re seeing TLS handshake ↠ failed user mapping.

How to Fix the 4625 Logon Failure (Two Ways)

There are two correct approaches, depending on what you want:

Option 1 → If you want the certificate to map to a real Windows/AD account

(Recommended for enterprise authentication)

Step-by-Step Configuration

1. Open the client certificate

• Run certmgr.msc or use MMC Certificate Snap-in.
• Ensure the certificate contains:
  • Client Authentication in Enhanced Key Usage
  • A UPN entry inside Subject Alternative Name (SAN)

2. Match certificate UPN to an AD User

• Open Active Directory Users & Computers
• Right-click the intended user → Properties
• Set UPN (same as certificate UPN)
• Example: [email protected]

3. Configure Explicit Certificate Mapping (if needed)

For stricter security:

• Open AD user → Account tab → Map certificate
• Import the exact client certificate
• Save changes

4. Restart handshake test using SslStream

• Now Schannel recognizes the certificate
• Maps it to a real identity
• 4625 stops appearing.

Option 2 → If you only need TLS encryption, not Windows login

(Most common for microservices, API clients, internal apps)

Many developers only need SSL for secure packets, not for Kerberos identity. In that case, let the certificate authenticate inside your app, not as an OS logon.

How to implement cleanly:

1. Continue using AuthenticateAsServer & AuthenticateAsClient

Your TLS handshake remains secure.

2. Extract certificate for your own auth layer

var clientCert = sslStream.RemoteCertificate;

if (clientCert != null)
{
    // Validate inside your application
    var thumb = clientCert.GetCertHashString();
    if (thumb == "YOUR_CERT_THUMBPRINT")
    {
        // Authorize manually
    }
}

3. Result

• TLS remains secure
• No OS-level Kerberos login needed
• You filter/approve certificates yourself
• 4625 entries may still appear, but harmless
  (Optional log filtering can hide them)

If your client applications represent real users. Enable certificate-to-user mapping to stop 4625 completely.

If your application only needs secure traffic. Use certificate identity at code level and ignore OS logon mapping.

Both approaches work — choose based on architecture.

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