Cis Gke V170 4.1.10

4.1.10 Avoid non-default bindings to system:authenticated (Automated)

Profile Applicability

• Level 1

Description

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.

Rationale

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.

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:basic-user
        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]

Default Value

ClusterRoleBindings with group system:authenticated:

• system:basic-user
• system:discovery

No RoleBindings with group system:authenticated.

References

1. https://kubernetes.io/docs/reference/access-authn-authz/rbac/#discovery-roles

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