Cis GCP Foundations 5.1

5.1 Ensure That Cloud Storage Bucket Is Not Anonymously or Publicly Accessible (Automated)

Profile Applicability

• Level 1

Description

It is recommended that IAM policy on Cloud Storage bucket does not allows anonymous or public access.

Rationale

Allowing anonymous or public access grants permissions to anyone to access bucket content. Such access might not be desired if you are storing any sensitive data. Hence, ensure that anonymous or public access to a bucket is not allowed.

Impact

No storage buckets would be publicly accessible. You would have to explicitly administer bucket access.

Audit

From Google Cloud Console

1. Go to Storage browser by visiting https://console.cloud.google.com/storage/browser.
2. Click on each bucket name to go to its Bucket details page.
3. Click on the Permissions tab.
4. Ensure that allUsers and allAuthenticatedUsers are not in the Members list.

From Google Cloud CLI

1. List all buckets in a project

gsutil ls

2. Check the IAM Policy for each bucket:

gsutil iam get gs://BUCKET_NAME

No role should contain allUsers and/or allAuthenticatedUsers as a member.

Using Rest API

1. List all buckets in a project

Get https://www.googleapis.com/storage/v1/b?project=<ProjectName>

2. Check the IAM Policy for each bucket

GET https://www.googleapis.com/storage/v1/b/<bucketName>/iam

No role should contain allUsers and/or allAuthenticatedUsers as a member.

Remediation

From Google Cloud Console

1. Go to Storage browser by visiting https://console.cloud.google.com/storage/browser.
2. Click on the bucket name to go to its Bucket details page.
3. Click on the Permissions tab.
4. Click Delete button in front of allUsers and allAuthenticatedUsers to remove that particular role assignment.

From Google Cloud CLI

Remove allUsers and allAuthenticatedUsers access.

gsutil iam ch -d allUsers gs://BUCKET_NAME
gsutil iam ch -d allAuthenticatedUsers gs://BUCKET_NAME

Prevention

You can prevent Storage buckets from becoming publicly accessible by setting up the Domain restricted sharing organization policy at: https://console.cloud.google.com/iam-admin/orgpolicies/iam-allowedPolicyMemberDomains

Default Value

By Default, Storage buckets are not publicly shared.

References

1. https://cloud.google.com/storage/docs/access-control/iam-reference
2. https://cloud.google.com/storage/docs/access-control/making-data-public
3. https://cloud.google.com/storage/docs/gsutil/commands/iam

Additional Information

To implement Access restrictions on buckets, configuring Bucket IAM is preferred way than configuring Bucket ACL. On GCP console, "Edit Permissions" for bucket exposes IAM configurations only. Bucket ACLs are configured automatically as per need in order to implement/support User enforced Bucket IAM policy. In-case administrator changes bucket ACL using command-line(gsutils)/API bucket IAM also gets updated automatically.

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