HTTP error 403.16 - client certificate trust issue

Question

I am trying to implement client certificate authentication on IIS 8. I have deployed my configuration on a development machine and verified it working as expected there. However after setting up on the server, whenever I navigate to the site and am prompted for the client cert, I select it and immediately get the 403.16 error. The failed requests log gives the error code 2148204809 and message "A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider."

I have a valid client cert and also a valid CA cert. The CA cert is installed in Trusted Root Authorities on the computer account on both the server and the client machine, and the client cert is installed in the Personal area of the Current User account on the client machine.

The client cert is signed directly by the root CA and as I said, both are valid. There are no other certs in the chain and there are no intermediate certs in the Trusted Root Authorities area.

The IIS configuration has sslFlags = SslNegotiateCert and iisClientCertificateMappingAuthentication is enabled.

The server is not configured to send a CTL and we have SendTrustedIssuerList = 0.

I cannot see why the client cert should not be trusted.

Answer 1

Windows 2012 introduced stricter certificate store validations. According to KB 2795828: Lync Server 2013 Front-End service cannot start in Windows Server 2012, the Trusted Root Certification Authorities (i.e. Root) store can only have certificates that are self-signed. If that store contains non-self-signed certificates, client certificate authentication under IIS returns with a 403.16 error code.

To solve the problem, you have to remove all non-self-signed certificates from the root store. This PowerShell command will identify non-self-signed certificates:

Get-Childitem cert:\LocalMachine\root -Recurse | 
    Where-Object {$_.Issuer -ne $_.Subject}

In my situation, we moved these non-self-signed certificates into the Intermediate Certification Authorities (i.e. CA) store:

Get-Childitem cert:\LocalMachine\root -Recurse | 
    Where-Object {$_.Issuer -ne $_.Subject} | 
    Move-Item -Destination Cert:\LocalMachine\CA

According to KB 2801679: SSL/TLS communication problems after you install KB 931125, you might also have too many trusted certificates.

[T]he maximum size of the trusted certificate authorities list that the Schannel security package supports is 16 kilobytes (KB). Having a large amount of Third-party Root Certication Authorities will go over the 16k limit, and you will experience TLS/SSL communication problems.

The solution in this situation is to remove any certification authority certificates you don't trust, or to stop sending the list of trusted certifiation authorities by setting the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\SecurityProviders\SCHANNEL\@SendTrustedIssuerList registry entry to 0 (the default, if not present, is 1).