Cis GCP Foundations 5.2

5.2 Ensure That Cloud Storage Buckets Have Uniform Bucket-Level Access Enabled (Automated)

Profile Applicability

• Level 2

Description

It is recommended that uniform bucket-level access is enabled on Cloud Storage buckets.

Rationale

It is recommended to use uniform bucket-level access to unify and simplify how you grant access to your Cloud Storage resources.

Cloud Storage offers two systems for granting users permission to access your buckets and objects: Cloud Identity and Access Management (Cloud IAM) and Access Control Lists (ACLs). These systems act in parallel - in order for a user to access a Cloud Storage resource, only one of the systems needs to grant the user permission. Cloud IAM is used throughout Google Cloud and allows you to grant a variety of permissions at the bucket and project levels. ACLs are used only by Cloud Storage and have limited permission options, but they allow you to grant permissions on a per-object basis.

In order to support a uniform permissioning system, Cloud Storage has uniform bucket-level access. Using this feature disables ACLs for all Cloud Storage resources: access to Cloud Storage resources then is granted exclusively through Cloud IAM. Enabling uniform bucket-level access guarantees that if a Storage bucket is not publicly accessible, no object in the bucket is publicly accessible either.

Impact

If you enable uniform bucket-level access, you revoke access from users who gain their access solely through object ACLs.

Certain Google Cloud services, such as Stackdriver, Cloud Audit Logs, and Datastore, cannot export to Cloud Storage buckets that have uniform bucket-level access enabled.

Audit

From Google Cloud Console

1. Open the Cloud Storage browser in the Google Cloud Console by visiting: https://console.cloud.google.com/storage/browser
2. For each bucket, make sure that Access control column has the value Uniform.

From Google Cloud CLI

1. List all buckets in a project

gsutil ls

2. For each bucket, verify that uniform bucket-level access is enabled.

gsutil uniformbucketlevelaccess get gs://BUCKET_NAME/

If uniform bucket-level access is enabled, the response looks like:

Uniform bucket-level access setting for gs://BUCKET_NAME/:
    Enabled: True
    LockedTime: LOCK_DATE

Remediation

From Google Cloud Console

1. Open the Cloud Storage browser in the Google Cloud Console by visiting: https://console.cloud.google.com/storage/browser
2. In the list of buckets, click on the name of the desired bucket.
3. Select the Permissions tab near the top of the page.
4. In the text box that starts with This bucket uses fine-grained access control..., click Edit.
5. In the pop-up menu that appears, select Uniform.
6. Click Save.

From Google Cloud CLI

Use the on option in a uniformbucketlevelaccess set command:

gsutil uniformbucketlevelaccess set on gs://BUCKET_NAME/

Prevention

You can set up an Organization Policy to enforce that any new bucket has uniform bucket level access enabled. Learn more at: https://cloud.google.com/storage/docs/setting-org-policies#uniform-bucket

Default Value

By default, Cloud Storage buckets do not have uniform bucket-level access enabled.

References

1. https://cloud.google.com/storage/docs/uniform-bucket-level-access
2. https://cloud.google.com/storage/docs/using-uniform-bucket-level-access
3. https://cloud.google.com/storage/docs/setting-org-policies#uniform-bucket

Additional Information

Uniform bucket-level access can no longer be disabled if it has been active on a bucket for 90 consecutive days.

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