You can use VPC Service Controls to improve the security of your Cloud Domains configuration.
VPC Service Controls provides additional security for Cloud Domains to help mitigate the risk of data exfiltration. By using VPC Service Controls, you can add projects to service perimeters that protect resources and services from requests that originate outside the perimeter.
To learn how to create a service perimeter, see Create a service perimeter in the VPC Service Controls documentation.