S3 buckets should restrict public bucket settings to prevent accidental or unauthorized public exposure of objects. This also ensures bucket-level public access controls are enforced.
In CloudFormation, AWS::S3::Bucket resources must define Properties.PublicAccessBlockConfiguration.RestrictPublicBuckets and set it to true. Resources missing PublicAccessBlockConfiguration, missing RestrictPublicBuckets, or with RestrictPublicBuckets: false will be flagged.