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Avoid non-default ClusterRoleBindings and RoleBindings with the group system:unauthenticated, except the ClusterRoleBinding system:public-info- viewer.
Kubernetes assigns the group system:unauthenticated to API server requests that have no authentication information provided. Binding a role to this group gives any unauthenticated user the permissions granted by that role and is strongly discouraged.

Impact

Unauthenticated users will have privileges and permissions associated with roles associated with the configured bindings. Care should be taken before removing any non-default clusterrolebindings or rolebindings from the environment to ensure they were not required for operation of the cluster. Leverage 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 non-default ClusterRoleBindings to group system:unauthenticated:
$ kubectl get clusterrolebindings -o json | jq -r '["Name"], ["-----"], (.items[] | select((.subjects | length) > 0) | select(any(.subjects[]; .name == "system:unauthenticated")) | [.metadata.namespace, .metadata.name]) | @tsv'
Only the following default ClusterRoleBinding should be displayed: 
Name ----- system:public-info-viewer
If any non-default 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:unauthenticated group:
$ kubectl get rolebindings -A -o json | jq -r '["Namespace", "Name"], ["----- ----", "-----"], (.items[] | select((.subjects | length) > 0) | select(any(.subjects[]; .name == "system:unauthenticated")) | [.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 non-default clusterrolebindings and rolebindings to the group system:unauthenticated. 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 non-default, 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 non-default, unsafe bindings to the group system:unauthenticated, 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]

Default value

ClusterRoleBindings with group system:unauthenticated:
  • system:public-info-viewer
No RoleBindings with the group system:unauthenticated.