It is all clear how to authorize Non Interactive Client (NIC) which is running on our backend server - use Client Credentials Grant.
But we have third party NIC running on end user’s backend. Since it is NIC, there is no user who can authenticate. So we can’t use Implicit Grant or any other similar grant.
We cannot use Client Credentials Grant because it does not contain any information about enduser (id_token) and we do not want to enable access to all data on our Resource Server.
So, how should we proper authenticate NIC on enduser side?