New – Simplify Access Management for Data Stored in Amazon S3

Update (4/27/2023): Amazon S3 now automatically enables S3 Block Public Access and disables S3 access control lists (ACLs) for all new S3 buckets in all AWS Regions.

Today, we are introducing a couple new features that simplify access management for data stored in Amazon Simple Storage Service (Amazon S3). First, we are introducing a new Amazon S3 Object Ownership setting that lets you disable access control lists (ACLs) to simplify access management for data stored in Amazon S3. Second, the Amazon S3 console policy editor now reports security warnings, errors, and suggestions powered by IAM Access Analyzer as you author your S3 policies.

Since launching 15 years ago, Amazon S3 buckets have been private by default. At first, the only way to grant access to objects was using ACLs. In 2011, AWS Identity and Access Management (IAM) was announced, which allowed the use of policies to define permissions and control access to buckets and objects in Amazon S3. Nowadays, you have several ways to control access to your data in Amazon S3, including IAM policies, S3 bucket policies, S3 Access Points policies, S3 Block Public Access, and ACLs.

ACLs are an access control mechanism in which each bucket and object has an ACL attached to it. ACLs define which AWS accounts or groups are granted access as well as the type of access. When an object is created, the ownership of it belongs to the creator.  This ownership information is embedded in the object ACL. When you upload an object to a bucket owned by another AWS account, and you want the bucket owner to access the object, then permissions need to be granted in the ACL. In many cases, ACLs and other kinds of policies are used within the same bucket.

The new Amazon S3 Object Ownership setting, Bucket owner enforced, lets you disable all of the ACLs associated with a bucket and the objects in it. When you apply this bucket-level setting, all of the objects in the bucket become owned by the AWS account that created the bucket, and ACLs are no longer used to grant access. Once applied, ownership changes automatically, and applications that write data to the bucket no longer need to specify any ACL. As a result, access to your data is based on policies. This simplifies access management for data stored in Amazon S3.

With this launch, when creating a new bucket in the Amazon S3 console, you can choose whether ACLs are enabled or disabled. In the Amazon S3 console, when you create a bucket, the default selection is that ACLs are disabled. If you wish to keep ACLs enabled, you can choose other configurations for Object Ownership, specifically:

• Bucket owner preferred: All new objects written to this bucket with the bucket-owner-full-control canned ACL will be owned by the bucket owner. ACLs are still used for access control.
• Object writer: The object writer remains the object owner. ACLs are still used for access control.

For existing buckets, you can view and manage this setting in the Permissions tab.

Before enabling the Bucket owner enforced setting for Object Ownership on an existing bucket, you must migrate access granted to other AWS accounts from the bucket ACL to the bucket policy. Otherwise, you will receive an error when enabling the setting. This helps you ensure applications writing data to your bucket are uninterrupted. Make sure to test your applications after you migrate the access.

Policy validation in the Amazon S3 console
We are also introducing policy validation in the Amazon S3 console to help you out when writing resource-based policies for Amazon S3. This simplifies authoring access control policies for Amazon S3 buckets and access points with over 100 actionable policy checks powered by IAM Access Analyzer.

To access policy validation in the Amazon S3 console, first go to the detail page for a bucket. Then, go to the Permissions tab and edit the bucket policy.

When you start writing your policy, you see that, as you type, different findings appear at the bottom of the screen. Policy checks from IAM Access Analyzer are designed to validate your policies and report security warnings, errors, and suggestions as findings based on their impact to help you make your policy more secure.

You can also perform these checks and validations using the IAM Access Analyzer’s ValidatePolicy API.

Availability
Amazon S3 Object Ownership is available at no additional cost in all AWS Regions, excluding the AWS China Regions and AWS GovCloud Regions. IAM Access Analyzer policy validation in the Amazon S3 console is available at no additional cost in all AWS Regions, including the AWS China Regions and AWS GovCloud Regions.

Get started with Amazon S3 Object Ownership through the Amazon S3 console, AWS Command Line Interface (AWS CLI), Amazon S3 REST API, AWS SDKs, or AWS CloudFormation. Learn more about this feature on the documentation page.

And to learn more and get started with policy validation in the Amazon S3 console, see the Access Analyzer policy validation documentation.

— Marcia