Starting in April of 2023 we will be making two changes to Amazon Simple Storage Service (Amazon S3) to put our latest best practices for bucket security into effect automatically. The changes will begin to go into effect in April and will be rolled out to all AWS Regions within weeks.
Once the changes are in effect for a target Region, all newly created buckets in the Region will by default have S3 Block Public Access enabled and access control lists (ACLs) disabled. Both of these options are already console defaults and have long been recommended as best practices. The options will become the default for buckets that are created using the S3 API, S3 CLI, the AWS SDKs, or AWS CloudFormation templates.
As a bit of history, S3 buckets and objects have always been private by default. We added Block Public Access in 2018 and the ability to disable ACLs in 2021 in order to give you more control, and have long been recommending the use of AWS Identity and Access Management (IAM) policies as a modern and more flexible alternative.
In light of this change, we recommend a deliberate and thoughtful approach to the creation of new buckets that rely on public buckets or ACLs, and believe that most applications do not need either one. If your application turns out to be one that does, then you will need to make the changes that I outline below (be sure to review your code, scripts, AWS CloudFormation templates, and any other automation).
What’s Changing
Let’s take a closer look at the changes that we are making:
S3 Block Public Access – All four of the bucket-level settings described in this post will be enabled for newly created buckets:
A subsequent attempt to set a bucket policy or an access point policy that grants public access will be rejected with a 403 Access Denied error. If you need public access for a new bucket you can create it as usual and then delete the public access block by calling DeletePublicAccessBlock (you will need s3:PutBucketPublicAccessBlock permission in order to call this function; read Block Public Access to learn more about the functions and the permissions).
ACLs Disabled – The Bucket owner enforced setting will be enabled for newly created buckets, making bucket ACLs and object ACLs ineffective, and ensuring that the bucket owner is the object owner no matter who uploads the object. If you want to enable ACLs for a bucket, you can set the ObjectOwnership parameter to ObjectWriter in your CreateBucket request or you can call DeleteBucketOwnershipControls after you create the bucket. You will need s3:PutBucketOwnershipControls permission in order to use the parameter or to call the function; read Controlling Ownership of Objects and Creating a Bucket to learn more.