S3 buckets can be configured to allow the global principal to access the bucket via the bucket policy or through ACL. This policy should be restricted only to known users or accounts. Setting the bucket policy to public access would make all the contents in the bucket public.
AWS allows users to set visibility and access permissions for a bucket and the objects within at a granular level using bucket policies. One of the most dangerous configurations is to have a bucket with public Internet wide access with all objects within becoming public. An S3 bucket with full access to any Internet user has the All Users group permission, represented by http://acs.amazonaws.com/groups/global/AllUsers assigned to it. An attacker can simply browse to the HTTP address of the S3 bucket with a browser and download objects from within.
The most common cause of data breaches in AWS, the exposure of buckets and their contains will result in a data incident. Based on the quantity and type of data uncovered, it may be possible for additional attacks within the AWS target environment.
Steps to enable public access block for Bucket
Log in to your AWS account and navigate to S3 dashboard.
In the left navigation panel, choose Buckets to access the S3 buckets list
Select the bucket, click on Permissions tab and in the section Block public access (bucket settings) click on Edit
Check the checkbox Block all public access and click on Save changes