S3 buckets should block public ACLs to prevent objects from being granted public access via ACLs. Public ACLs can lead to unintended data exposure. For AWS::S3::Bucket resources, Properties.PublicAccessBlockConfiguration.BlockPublicAcls must be set to true. Resources missing PublicAccessBlockConfiguration, or with BlockPublicAcls: false, will be flagged. Consider also enabling other PublicAccessBlockConfiguration flags (for example, IgnorePublicAcls, BlockPublicPolicy, and RestrictPublicBuckets) for broader protection.