Description

To follow the principle of least privilege and to prevent potential privilege escalation, assign instances to a service account other than the default Compute Engine service account. These accounts have a scope option of Allow full access to all Cloud APIs, which grants Editor rights on the project.

Rationale

When an instance is assigned the default compute engine and the non-default scope Allow full access to all Cloud APIs is selected, the instance has full Editor access on the Google Cloud project. This may allow users to perform malicious cloud operations and API calls leading to successful privilege escalation.

To defend against privilege escalation if your VM is compromised and prevent an attacker from gaining administrative rights to your project, it is recommended that you not use the default Compute Engine service account with an unrestricted scope. Instead, create a new service account and assign only the permissions needed by your instance.

The default Compute Engine service account is named [PROJECT_NUMBER]-compute@developer.gserviceaccount.com.

Exception

VMs created by GKE are excluded from this rule. These VMs have names that start with gke- and are labeled goog-gke-node.

Impact

To change a service account or scope for an instance, the instance must be stopped.

Remediation

From the console

1. Go to the VM instances page by visiting: https://console.cloud.google.com/compute/instances.
2. Click on the impacted VM instance.
3. If the instance is not stopped, click the Stop button. Wait for the instance to stop.
4. Click the Edit button.
5. Scroll down to the Service Account section.
6. Select a different service account or ensure that Allow full access to all Cloud APIs is not selected.
7. Click the Save button to save your changes and then click START.

From the command line

1. Stop the instance:

   gcloud compute instances stop <INSTANCE_NAME>
   

2. Update the instance:

   gcloud compute instances set-service-account <INSTANCE_NAME> --service-account=<SERVICE_ACCOUNT> --scopes [SCOPE1, SCOPE2...]
   

3. Restart the instance:

   gcloud compute instances start <INSTANCE_NAME>
   

Default value

By default, Compute instances are configured to use the default Compute Engine service account, but with a limited access scope that has read-only access to data in the project.

References

1. https://cloud.google.com/compute/docs/access/service-accounts#default_service_account
2. https://cloud.google.com/compute/docs/access/service-accounts#accesscopesiam
3. [https://cloud.google.com/compute/docs/access/service-accounts][4]
4. [https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances][5]
5. [https://cloud.google.com/sdk/gcloud/reference/compute/instances/set-service-account][6]

CIS Controls

Version 8 - 4.7: Manage Default Accounts on Enterprise Assets and Software

• Manage default accounts on enterprise assets and software, such as root, administrator, and other pre-configured vendor accounts. Example implementations can include: disabling default accounts or making them unusable.

Version 7 - 4.7 Limit Access to Script Tools

• Limit access to scripting tools (such as Microsoft PowerShell and Python) to only administrative or development users with the need to access those capabilities.