gcloud-org-shared-vpc-subnets: Shared VPC Subnetworks Not Restricted

id: gcloud-org-shared-vpc-subnets

info:
  name: Shared VPC Subnetworks Not Restricted
  author: princechaddha
  severity: medium
  description: |
    Ensure that the set of shared VPC subnetworks that eligible Google Cloud resources can use, are defined using the "Restrict Shared VPC Subnetworks" constraint policy. The allowed list of VPC subnetworks must be specified in the following form: projects/<project-id>/regions/<subnetwork-region>/subnetworks/<subnetwork-name>. You can also define the list of allowed subnetworks in a project, folder, or organization.
  impact: |
    By default, eligible Google Cloud resources can use any shared Virtual Private Cloud (VPC) subnetwork. Without restrictions, resources could be deployed in unauthorized or non-compliant network segments.
  remediation: |
    Configure the "Restrict Shared VPC Subnetworks" policy at the organization level to explicitly specify which VPC subnetworks can be used. Use the format projects/<project-id>/regions/<region>/subnetworks/<name> or under: prefix for broader scopes.
  reference:
    - https://www.trendmicro.com/cloudoneconformity/knowledge-base/gcp/ResourceManager/restrict-shared-vps-subnetworks.html
    - https://cloud.google.com/vpc/docs/shared-vpc
  tags: cloud,devops,gcp,gcloud,resourcemanager,security,networking,vpc,gcp-cloud-config

flow: |
  code(1)
  for(let orgId of iterate(template.orgIds)){
    set("orgId", orgId)
    code(2)
  }

self-contained: true

code:
  - engine:
      - sh
      - bash
    source: |
      gcloud organizations list --format="json(name)"

    extractors:
      - type: json
        name: orgIds
        internal: true
        json:
          - '.[].name'

  - engine:
      - sh
      - bash
    source: |
      gcloud alpha resource-manager org-policies describe compute.restrictSharedVpcSubnetworks --organization=$orgId --effective --format="json(listPolicy.allValues)"

    matchers:
      - type: word
        words:
          - "ALLOW"

    extractors:
      - type: dsl
        dsl:
          - '"Organization " + orgId + " has not restricted shared VPC subnetworks, allowing resources to use any shared subnet"'
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