Context
The kubeadm-created cluster's Kubernetes API server was, for testing purposes, temporarily configured to allow unauthenticated and unauthorized access granting the anonymous user duster-admin access.
Task
Reconfigure the cluster's Kubernetes API server to ensure that only authenticated and authorized REST requests are allowed.
Use authorization mode Node,RBAC and admission controller NodeRestriction.
Cleaning up, remove the ClusterRoleBinding for user system:anonymous.