Profile applicability: Level 2
Avoid ClusterRoleBindings nor RoleBindings with the user system:anonymous.
Kubernetes assigns user system:anonymous to API server requests that have no authentication information provided. Binding a role to user system:anonymous gives any unauthenticated user the permissions granted by that role and is strongly discouraged.
Note
Note
No clusterrolebindings nor rolebindings with user system:anonymous.

Impact

Unauthenticated users will have privileges and permissions associated with roles associated with the configured bindings.
Care should be taken before removing any clusterrolebindings or rolebindings from the environment to ensure they were not required for operation of the cluster. Use a more specific and authenticated user for cluster operations.

Audit

Both CusterRoleBindings and RoleBindings should be audited. Use the following command to confirm there are no ClusterRoleBindings to system:anonymous:
$ kubectl get clusterrolebindings -o json | jq -r '["Name"], ["-----"], 
(.items[] | select((.subjects | length) > 0) | select(any(.subjects[]; .name 
== "system:anonymous")) | [.metadata.namespace, .metadata.name]) | @tsv'
There should be no ClusterRoleBindings listed. If any bindings exist, review their permissions with the following command and reassess their privilege: 
$ kubectl get clusterrolebinding [CLUSTER_ROLE_BINDING_NAME] -o json \ 
    | jq ' .roleRef.name +" " + .roleRef.kind' \ 
    | sed -e 's/"//g' \ 
    | xargs -l bash -c 'kubectl get $1 $0 -o yaml'
Confirm that there are no RoleBindings including the system:anonymous user:
$ kubectl get rolebindings -A -o json \ 
     | jq -r '["Namespace", "Name"], ["---------", "-----"], (.items[] | 
select((.subjects | length) > 0) | select(any(.subjects[]; .name == 
"system:anonymous")) | [.metadata.namespace, .metadata.name]) | @tsv'
There should be no RoleBindings listed.
If any bindings exist, review their permissions with the following command and reassess their privilege.
$ kubectl get rolebinding [ROLE_BINDING_NAME] --namespace 
[ROLE_BINDING_NAMESPACE] -o json \ 
    | jq ' .roleRef.name +" " + .roleRef.kind' \ 
    | sed -e 's/"//g' \ 
    | xargs -l bash -c 'kubectl get $1 $0 -o yaml --namespace 
[ROLE_BINDING_NAMESPACE]'

Remediation

Identify all clusterrolebindings and rolebindings to the user system:anonymous.
Check if they are used and review the permissions associated with the binding using the commands in the Audit section above or refer to GKE documentation.
Strongly consider replacing unsafe bindings with an authenticated, user-defined group. Where possible, bind to non-default, user-defined groups with least-privilege roles. If there are any unsafe bindings to the user system:anonymous, proceed to delete them after consideration for cluster operations with only necessary, safer bindings.
kubectl delete clusterrolebinding 
[CLUSTER_ROLE_BINDING_NAME] 
kubectl delete rolebinding 
[ROLE_BINDING_NAME] 
--namespace 
[ROLE_BINDING_NAMESPACE]