To fix the issue where authorization fails because the usernames do not exist in the authentication source, you can configure rules in HPE Aruba Networking ClearPass Policy Manager (CPPM) to strip the domain name from the username. When certificates are issued by a Windows CA, the username in the certificate often includes the domain (e.g., user@domain.com). ClearPass might not be able to find this format in the authentication source. By stripping the domain name, you ensure that ClearPass searches for just the username (e.g., user) in the authentication source, allowing successful authentication.