Using default service account

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Metadata

Id: terraform-gcp-using-default-service-account

Provider: GCP

Platform: Terraform

Severity: Medium

Category: Insecure Defaults

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Description

Google Compute Engine instances should not be configured to use the default service account, which has broad permissions and full access to all Cloud APIs. If the attribute service_account—specifically the email sub-attribute—is missing, empty, or set to the default Google Compute Engine service account, it increases the risk of privilege escalation and unauthorized access to sensitive resources. Instead, instances should explicitly specify a custom service account with only the necessary permissions, such as in the following example:

service_account {
  email  = "custom-sa@project-id.iam.gserviceaccount.com"
  scopes = ["userinfo-email", "compute-ro", "storage-ro"]
}

Compliant Code Examples

#this code is a correct code for which the query should not find any result
resource "google_compute_instance" "negative1" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  // Local SSD disk
  scratch_disk {
    interface = "SCSI"
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  service_account {
    email = "email@email.com"
    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
  }
}

# service_account.email references a google_service_account created in the
# same plan (google_service_account.custom.email), so it's unresolved
# pre-apply and absent from planned_values.values in negative2.json (a real
# create plan for this exact configuration). The rule confirms via
# _dd_tfplan_meta.configuration_expressions that the reference targets a
# google_service_account resource -- which can never resolve to the reserved
# default Compute Engine service account -- so it correctly stays silent
# instead of treating this as an unconfigured/default service account.
resource "google_compute_instance" "negative2" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  service_account {
    email  = google_service_account.custom.email
    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
  }
}

resource "google_service_account" "custom" {
  account_id   = "custom-sa"
  display_name = "Custom Service Account"
}

Non-Compliant Code Examples

#this is a problematic code where the query should report a result(s)
resource "google_compute_instance" "positive1" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

}

resource "google_compute_instance" "positive2" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  service_account {
    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
  }
}

resource "google_compute_instance" "positive3" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  service_account {
    email = ""
    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
  }
}

resource "google_compute_instance" "positive4" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  service_account {
    email = "a"
    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
  }
}

resource "google_compute_instance" "positive5" {
  name         = "test"
  machine_type = "e2-medium"
  zone         = "us-central1-a"

  tags = ["foo", "bar"]

  boot_disk {
    initialize_params {
      image = "debian-cloud/debian-9"
    }
  }

  network_interface {
    network = "default"

    access_config {
      // Ephemeral IP
    }
  }

  service_account {
    email = "email@developer.gserviceaccount.com"
    scopes = ["userinfo-email", "compute-ro", "storage-ro"]
  }
}