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Description

To support principle of least privileges and prevent potential privilege escalation it is recommended that instances are not assigned to default service account Compute Engine default service account with Scope Allow full access to all Cloud APIs.

Rationale​

Along with ability to optionally create, manage and use user managed custom service accounts, Google Compute Engine provides default service account Compute Engine default service account for an instances to access necessary cloud services. Project Editor role is assigned to Compute Engine default service account hence, This service account has almost all capabilities over all cloud services except billing. However, when Compute Engine default service account assigned to an instance it can operate in 3 scopes.

        1. Allow default access: Allows only minimum access required to run an Instance (Least Privileges) 
        2. Allow full access to all Cloud APIs: Allow full access to all the cloud APIs/Services (Too much access) 
        3. Set access for each API: Allows Instance administrator to choose only those APIs that are needed to perform specific business functionality expected by instance

When an instance is configured with Compute Engine default service account with Scope Allow full access to all Cloud APIs, based on IAM roles assigned to the user(s) accessing Instance, it may allow user to perform cloud operations/API calls that user is not supposed to perform leading to successful privilege escalation.

Impact​

In order to change service account or scope for an instance, it needs to be stopped.

Audit​

From Google Cloud Console​

  1. Go to the VM instances page by visiting: https://console.cloud.google.com/compute/instances.
  2. Click on each instance name to go to its VM instance details page.
  3. Under the API and identity management, ensure that Cloud API access scopes is not set to Allow full access to all Cloud APIs.

From Google Cloud CLI​

  1. List the instances in your project and get details on each instance:

         gcloud compute instances list --format=json | jq -r '. | "SA Scopes: \(.[].serviceAccounts[].scopes) Name: \(.[].name) Email: \(.[].serviceAccounts[].email)"'

  2. Ensure that the service account section has an email that does not match the pattern [PROJECT_NUMBER]-compute@developer.gserviceaccount.com.

Exception​

VMs created by GKE should be excluded. These VMs have names that start with gke- and are labeled goog-gke-node

Default Value​

While creating an VM instance, default service account is used with scope Allow default access.

References​

  1. https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances
  2. https://cloud.google.com/compute/docs/access/service-accounts

Additional Information​

• User IAM roles will override service account scope but configuring minimal scope ensures defense in depth

• Non-default service accounts do not offer selection of access scopes like default service account. IAM roles with non-default service accounts should be used to control VM access.