HTTP 403 Access Denied: Amazon S3 Block Public Access

Amazon S3 Block Public Access (BPA) provides a centralized control to prevent the accidental exposure of data by overriding bucket policies and Access Control Lists (ACLs). This feature operates at both the account level and the individual bucket level, utilizing four distinct settings: BlockPublicAcls, IgnorePublicAcls, BlockPublicPolicy, and RestrictPublicBuckets.

When BPA is active, any request that attempts to apply a public ACL or a public bucket policy is rejected with an HTTP 403 response, ensuring the resource remains private regardless of the caller's permissions to modify policies.

Architectural Decision: Account-Level vs. Bucket-Level Enforcement

While account-level BPA offers the strongest security posture for all current and future buckets, it creates a conflict for workloads requiring legitimate public delivery, such as static website assets. The alternative is to disable account-level blocks and manage exceptions on a per-bucket basis, though this increases the risk of configuration drift.

For environments requiring both strict security and public delivery, the recommended pattern involves enabling full BPA and utilizing Amazon CloudFront with Origin Access Control (OAC) to serve content.

• Does enforcing BPA at the account level completely preclude the use of public buckets for static assets, or is there a documented mechanism for granular exceptions?
• When IgnorePublicAcls is enabled, how does S3 prioritize the evaluation between an existing public ACL and a restrictive bucket policy?