apiVersion: templates.gatekeeper.sh/v1
kind: ConstraintTemplate
metadata:
  name: k8sdisallowanonymous
  annotations:
    metadata.gatekeeper.sh/title: "Disallow Anonymous Access"
    metadata.gatekeeper.sh/version: 1.1.0
    description: Disallows associating ClusterRole and Role resources to the system:anonymous user and system:unauthenticated group.
spec:
  crd:
    spec:
      names:
        kind: K8sDisallowAnonymous
      validation:
        # Schema for the `parameters` field
        openAPIV3Schema:
          type: object
          properties:
            allowedRoles:
              description: >-
                The list of ClusterRoles and Roles that may be associated
                with the `system:unauthenticated` group and `system:anonymous`
                user.
              type: array
              items:
                type: string
            disallowAuthenticated:
              description: >-
                A boolean indicating whether `system:authenticated` should also
                be disallowed by this policy.
              type: boolean
              default: false
  targets:
    - target: admission.k8s.gatekeeper.sh
      rego: |
        package k8sdisallowanonymous

        violation[{"msg": msg}] {
          not is_allowed(input.review.object.roleRef, object.get(input, ["parameters", "allowedRoles"], []))

          group := ["system:unauthenticated", "system:anonymous"][_]
          subject_is(input.review.object.subjects[_], group)

          msg := message(group)
        }

        violation[{"msg": msg}] {
          not is_allowed(input.review.object.roleRef, object.get(input, ["parameters", "allowedRoles"], []))

          object.get(input, ["parameters", "disallowAuthenticated"], false)

          group := "system:authenticated"
          subject_is(input.review.object.subjects[_], group)

          msg := message(group)
        }

        is_allowed(role, allowedRoles) {
          role.name == allowedRoles[_]
        }

        subject_is(subject, expected) {
          subject.name == expected
        }

        message(name) := val {
          val := sprintf("%v is not allowed as a subject name in %v %v", [name, input.review.object.kind, input.review.object.metadata.name])
        }