Ensure IAM users are not assigned the Service Account User or Service Account Token Creator roles at project level

It is recommended to assign the Service Account User (iam.serviceAccountUser) and Service Account Token Creator (iam.serviceAccountTokenCreator) roles to a user for a specific service account rather than assigning the role to a user at project level.

Rationale

A service account is a special Google account that belongs to an application or a virtual machine (VM), instead of to an individual end-user. The application/VM-Instance uses the service account to call the service's Google API so that users aren't directly involved. In addition to being an identity, a service account is a resource that has IAM policies attached to it. These policies determine who can use the service account.

Users with IAM roles to update the App Engine and Compute Engine instances (such as App Engine Deployer or Compute Instance Admin) can effectively run code as the service accounts used to run these instances, and indirectly gain access to all the resources for which the service accounts have access. Similarly, SSH access to a Compute Engine instance may also provide the ability to execute code as that instance/Service account.

Based on business needs, there could be multiple user-managed service accounts configured for a project. Granting the iam.serviceAccountUser or iam.serviceAccountTokenCreator roles to a user for a project gives the user access to all service accounts in the project, including service accounts that may be created in the future. This can result in elevation of privileges by using service accounts and corresponding Compute Engine instances.

In order to implement least privileges best practices, IAM users should not be assigned the Service Account User or Service Account Token Creator roles at the project level. Instead, these roles should be assigned to a user for a specific service account, giving that user access to the service account. The Service Account User allows a user to bind a service account to a long-running job service, whereas the Service Account Token Creator role allows a user to directly impersonate (or assert) the identity of a service account.

Impact

After revoking Service Account User or Service Account Token Creator roles at the project level from all impacted user account(s), these roles should be assigned to a user(s) for specific service account(s) according to business needs.