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Avoid non-default ClusterRoleBindings and RoleBindings with the group system:authenticated, except the ClusterRoleBindings system:basic-user, system:discovery, and system:public-info-viewer.
Google's approach to authentication is to make authenticating to Google Cloud and GKE as simple and secure as possible without adding complex configuration steps. The group system:authenticated includes all users with a Google account, which includes all Gmail accounts. Consider your authorization controls with this extended group scope when granting permissions. Thus, group system:authenticated is not recommended for non-default use.
GKE assigns the group system:authenticated to API server requests made by any user who is signed in with a Google Account, including all Gmail accounts. In practice, this isn't meaningfully different from system:unauthenticated because anyone can create a Google Account.
Binding a role to the group system:authenticated gives any user with a Google Account, including all Gmail accounts, the permissions granted by that role and is strongly discouraged.
Default value:
ClusterRoleBindings with group system:unauthenticated: 
system:public-info-viewer
No RoleBindings with the group system:unauthenticated.

Impact

Authenticated users in group system:authenticated should be treated similarly to users in system:unauthenticated, having 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

Use the following command to confirm there are no non-default ClusterRoleBindings to system:authenticated:
$ 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 ClusterRoleBindings should be displayed:
Name 
----- 
               system:public-info-viewer
               system:discovery 
               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:authenticated 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:authenticated. 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:authenticated, 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]