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gcp

VM instance has 'Enable Connecting to Serial Ports' disabled

compute_instance_serial_ports_in_use
Service: computeSeverity: medium
OfficialBuilt-inVerified
Categories:identity-access
Resource Type:compute.googleapis.com/Instance

Compute Engine VM instance with the interactive serial console enabled via metadata serial-port-enable (1/true). Instances with this flag disabled do not allow interactive serial console connections.

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Risk

Enabling the serial console creates out-of-band access that can bypass network controls. Abuse can grant low-level OS interaction, expose sensitive boot logs, alter configuration, or disrupt services, degrading confidentiality, integrity, and availability.

Run this check with Prowler

Run in Prowler Cloud
Run
prowler gcp --checks compute_instance_serial_ports_in_use

Recommendation

Disable the interactive serial console on production VMs (serial-port-enable=false). Use it only for break-glass cases. Enforce least privilege for console roles, prefer controlled access paths (IAP/SSH or session tools), and monitor access. Apply defense in depth to reduce alternate entry points.

Remediation

CLI

gcloud compute instances add-metadata <INSTANCE_NAME> --zone=<ZONE> --metadata=serial-port-enable=false

Terraform

resource "google_compute_instance" "<example_resource_name>" {
  name         = "<example_resource_name>"
  machine_type = "e2-micro"
  zone         = "us-central1-a"

  boot_disk {
    initialize_params { image = "debian-cloud/debian-12" }
  }

  network_interface { network = "default" }

  metadata = {
    serial-port-enable = "false" # Critical: disables connecting to serial ports to pass the check
  }
}

Other

1In Google Cloud Console, go to Compute Engine > VM instances
2Click the target VM name, then click Edit
3Uncheck "Enable connecting to serial ports"
4Click Save

Check Code

from prowler.lib.check.models import Check, Check_Report_GCP
from prowler.providers.gcp.services.compute.compute_client import compute_client


class compute_instance_serial_ports_in_use(Check):
    def execute(self) -> Check_Report_GCP:
        findings = []
        for instance in compute_client.instances:
            report = Check_Report_GCP(metadata=self.metadata(), resource=instance)
            report.status = "PASS"
            report.status_extended = f"VM Instance {instance.name} has Enable Connecting to Serial Ports off."
            if instance.metadata.get("items"):
                for item in instance.metadata["items"]:
                    if item["key"] == "serial-port-enable" and item["value"] in [
                        "1",
                        "true",
                    ]:
                        report.status = "FAIL"
                        report.status_extended = f"VM Instance {instance.name} has Enable Connecting to Serial Ports set to on."
                        break
            findings.append(report)

        return findings

References