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Securing your S3 data is paramount. AWS provides multiple layers of access control for S3, and understanding how they interact is critical for preventing data breaches. In this lesson, we cover IAM policies, bucket policies, Access Control Lists (ACLs), Block Public Access, and access points.
| Layer | Scope | Written In | Attached To |
|---|---|---|---|
| IAM policies | User/role-level | JSON | IAM users, groups, roles |
| Bucket policies | Bucket-level | JSON | The bucket itself |
| ACLs | Object or bucket level | XML | Bucket or individual objects |
| Block Public Access | Account or bucket level | Toggle settings | Account or bucket |
When a request arrives, S3 evaluates all applicable policies. The request is allowed only if at least one policy grants access and no policy explicitly denies it.
IAM policies control what AWS principals (users, groups, roles) can do with S3. These are identity-based policies.
Example: Allow a user to read from a specific bucket:
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