Clusters should use client certificates for authentication

Explanation

Client certificates are the most secure and recommended method of authentication.

Possible Impact

Less secure authentication method in use

Suggested Resolution

Use client certificates for authentication

Insecure Example

The following example will fail the google-gke-no-legacy-auth check.

resource "google_service_account" "default" {
  account_id   = "service-account-id"
  display_name = "Service Account"
}

resource "google_container_cluster" "good_example" {
  name     = "my-gke-cluster"
  location = "us-central1"

  # We can't create a cluster with no node pool defined, but we want to only use
  # separately managed node pools. So we create the smallest possible default
  # node pool and immediately delete it.
  remove_default_node_pool = true
  initial_node_count       = 1
  master_auth {
    client_certificate_config {
      issue_client_certificate = true
    }
  }
}

resource "google_container_node_pool" "primary_preemptible_nodes" {
  name       = "my-node-pool"
  location   = "us-central1"
  cluster    = google_container_cluster.primary.name
  node_count = 1

  node_config {
    preemptible  = true
    machine_type = "e2-medium"

    # Google recommends custom service accounts that have cloud-platform scope and permissions granted via IAM Roles.
    service_account = google_service_account.default.email
    oauth_scopes    = [
      "https://www.googleapis.com/auth/cloud-platform"
    ]
  }
}

Secure Example

The following example will pass the google-gke-no-legacy-auth check.

resource "google_service_account" "default" {
  account_id   = "service-account-id"
  display_name = "Service Account"
}

resource "google_container_cluster" "good_example" {
  name     = "my-gke-cluster"
  location = "us-central1"

  # We can't create a cluster with no node pool defined, but we want to only use
  # separately managed node pools. So we create the smallest possible default
  # node pool and immediately delete it.
  remove_default_node_pool = true
  initial_node_count       = 1
}

resource "google_container_node_pool" "primary_preemptible_nodes" {
  name       = "my-node-pool"
  location   = "us-central1"
  cluster    = google_container_cluster.primary.name
  node_count = 1

  node_config {
    preemptible  = true
    machine_type = "e2-medium"

    # Google recommends custom service accounts that have cloud-platform scope and permissions granted via IAM Roles.
    service_account = google_service_account.default.email
    oauth_scopes    = [
      "https://www.googleapis.com/auth/cloud-platform"
    ]
  }
}

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