Today, a customer called me and reported, on the first sight, a pretty weired error: Only Windows clients were unable to login into a WPA2-Enterprise wireless network. The setup itself was pretty simple: Cisco Meraki WiFi access points, a Windows Network Protection Server (NPS) on a Windows Server 2016 Domain Controller, and a Sophos SG 125 was acting as DHCP for different WiFi networks.
Windows clients failed to authenticate, but Apple iOS, Android, and even Windows 10 Tablets had no problem.
The following error was logged into the Windows Security event log.
Connection Request Policy Name: Use Windows authentication for all users
Network Policy Name: Wireless Users
Authentication Provider: Windows
Authentication Server: domaincontroller.domain.tld
Authentication Type: PEAP
EAP Type: -
Account Session Identifier: -
Logging Results: Accounting information was written to the local log file.
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.
The credentials were definitely correct, the customer and I tried different user and password combinations.
I also checked the NPS network policy. When choosing PEAP as authentication type, the NPS needs a valid server certificate. This is necessary, because the EAP session is protected by a TLS tunnel. A valid certificate was given, in this case a wildcard certificate. A second certificate was also in place, this was a certificate for the domain controller from the internal enterprise CA.
It was an educated guess, but I disabled the server certificate check for the WPA2-Enterprise conntection, and the client was able to login into the WiFi. This clearly showed, that the certificate was the problem. But it was valid, all necessary CA certificates were in place and there was no reason, why the certificate was the cause.
The customer told me, that they installed updates on friday (today is monday), and a reboot of the domain controller was issued. This also restarted the NPS service, and with this restart, the Wildcard certificate was used for client connections.
I switched to the domain controller certificate, restarted the NPS, and all Windows clients were again able to connect to the WiFi.
Try to avoid Wildcard certificates, or at least check the certificate that is used by the NPS, if you get authentication error with reason code 16.