Question

Difficulty: Very hardConfiguring Storage Access Controls and Uniform Bucket-Level Access

A security engineer is tasked with migrating a legacy production Cloud Storage bucket containing financial records from fine-grained Access Control Lists (ACLs) to Uniform Bucket-Level Access (UBLA) in accordance with company security posture mandates. Arrange the migration and enforcement steps in the correct sequential order to prevent service disruption while ensuring strict security policy enforcement.

  1. 1Analyze Cloud Audit Logs to identify principals accessing objects via legacy ACLs and assign them equivalent Cloud IAM predefined Storage roles at the bucket level.
  2. 2Execute the command `gcloud storage buckets update gs://[BUCKET_NAME] --uniform-bucket-level-access` to activate bucket-level access control.
  3. 3Monitor Cloud Monitoring metrics and Cloud Audit Logs for access denials to confirm all client applications are successfully operating under IAM authorization.
  4. 4Apply the `constraints/storage.uniformBucketLevelAccess` organization policy to the project hierarchy to enforce bucket-level access across all storage resources.

Answer

The correct procedural order is: 1) Analyze Cloud Audit Logs and grant equivalent Cloud IAM predefined roles; 2) Enable Uniform Bucket-Level Access on the Cloud Storage bucket; 3) Monitor metrics and audit logs for access denials during the grace period; 4) Apply the organization policy constraint for uniform bucket-level access.
Migrating to Uniform Bucket-Level Access requires a controlled sequence: first auditing and replacing object ACLs with equivalent IAM roles to preserve access; second enabling UBLA on the bucket to enforce IAM-only access; third monitoring audit logs and metrics during the 90-day window to catch unmapped edge cases; and finally locking security posture at the project/organization level using Organization Policy constraints.

Step-by-Step Solution

1
Audit current ACL utilization and map legacy object ACLs to IAM predefined roles.
Principals relying on object-level ACLs are granted bucket-level IAM roles (such as Storage Object Viewer or Storage Object Admin).
Once Uniform Bucket-Level Access is enabled, GCP ignores object ACLs entirely. IAM permissions must be present beforehand to avoid immediate access revocation.
2
Enable Uniform Bucket-Level Access on the target Cloud Storage bucket.
The bucket shifts permission evaluation strictly to Cloud IAM and starts a 90-day grace period.
Activating UBLA enforces IAM-only evaluation while retaining the technical safety net of reverting within 90 days if production workloads fail.
3
Perform runtime monitoring of audit logs and authorization telemetry.
Verification that zero 403 Forbidden errors occur due to unmapped object ACL dependencies.
Monitoring validates full operational stability under IAM before taking immutable or organization-wide enforcement steps.
4
Enforce the uniformBucketLevelAccess constraint via Organization Policy.
Fine-grained ACLs are completely disabled across the project scope.
Organization policies provide overarching governance to prevent policy regression or manual enabling of ACLs.

Key Concept

Uniform Bucket-Level Access Migration Workflow
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