Example Auth System for Mirror/Classic UNET

ExampleAuthSystem.cs

// This is an example way of making an auth system for Mirror/Classic UNET.
// This is a "kickstart" script that may or may not work for you. NO WARRANTY.
// Code written by SoftwareGuy/Coburn64 (@coburn64 on Twitter). Please don't claim as your own.

// START SNIPPET //
using UnityEngine;
using Mirror; // Change this to "using UnityEngine.Networking" if using classic UNET

public class CustomNetworkManager : NetworkManager {
  private int EXAMPLE_AUTH_PACKET_ID = 1000;
  
  public override void OnStartServer() {
    base.OnStartServer();
    
    // Register a handler for the auth packet ID.
    // The client will send this to the server.
    NetworkServer.RegisterHandler(EXAMPLE_AUTH_PACKET_ID, HandleAuthPacket);
  }
  
  public override void OnStartClient(NetworkClient client){
    base.OnStartClient(client);
    
    // Send an auth connection packet
    client.Send(EXAMPLE_AUTH_PACKET_ID, new AuthPacket(){ authToken = "Your_Clients_Auth_Token" });
  }
  
  public override void HandleAuthPacket(NetworkMessage netMsg) {
    AuthPacket ap = netMsg.ReadMessage<AuthPacket>();
    
    if(ap != null) {
      // You can now access the auth packet's contents.
      // An example is below:
      Debug.LogWarning("Auth Packet get from " + netMsg.connection.address);
      Debug.LogWarning("Auth Packet Token: " + ap.authToken);
      // You would then do something with that. Send it up stream to accounts server, send a message back to the client saying they're nongenuine, etc.
      // For example, to cut the connection:
      // netMsg.connection.Close();
    }
  }
}

public class AuthPacket : MessageBase {
  public string authToken = string.Empty;
}
// END SNIPPET //

If you have any issues you can either contact me on Discord or Twitter. No, I'm not going to write your netcode for you.