Demystifying S3 Authorization Process

all aws amazon s3 aws security aws storage s3 security Feb 06, 2024


Welcome to the world of AWS S3, where understanding how permissions work can feel like untangling a giant bowl of spaghetti. Fear not! This post aims to guide us through the maze of S3 authorization, breaking down the process into bite-sized, digestible pieces. Whether you're a developer, a system administrator, or just curious, you'll come away with a clear understanding of how S3 decides who gets to do what with our precious data.

Understanding User Context

At the heart of S3 authorization is the user context. This revolves around a simple question: is the person or service trying to access S3 resources authorized to do so? The journey to answer this begins with the IAM principal—a fancy term for the user or role attempting access. AWS checks if the IAM principal has been granted permission by the account to which it belongs. This permission comes in two flavours:

  • Identity-Based Policies: These are attached directly to an IAM user or role, specifying what actions they can or cannot perform on resources.
  • Resource-Based Policies: Unlike identity-based policies, these are attached to the resource itself (in this case, an S3 bucket or object) and define who can access this resource and how.

If the account that owns the IAM principal also owns the bucket or object in question, AWS then looks at bucket policies, Access Control Lists (ACLs), and object ACLs to decide on access.

Bucket Context: Who's the Boss?

Moving down to the bucket context, AWS scrutinizes the policies of the account that owns the bucket. Here, it specifically checks for any Explicit Deny statements. These are the big red stop signs of access policies—if an explicit deny is found, the game is over for the IAM principal trying to access the bucket, no matter what other permissions might say.

Object Context: Getting Granular

When it comes to individual objects within a bucket, the rules get more specific. The requester must have permission from the object's owner to access it. However, things simplify if the bucket owner and the object owner are the same; access is then governed by the bucket policy. If they're not the same, the object owner's ACL takes precedence.

A pro tip for keeping things simple is enabling the Bucket Owner Enforced Setting for Object Ownership. This ensures that all objects in our bucket are treated as if they're owned by the bucket owner, making bucket policies and IAM-based policies the sole gatekeepers of access. No more ACL headaches!

A Practical Example: Joe's Adventure with S3

Let's bring this to life with a story about Joe, who is part of three different AWS accounts. Joe wants to perform an operation on an object in S3. Here's how AWS decides if he can:

  1. Parental Permission: First, AWS checks if Joe's parent account has given her the thumbs up for the operation.
  2. Bucket Policy Check: Next, AWS looks at the bucket policy to see if there's an explicit deny against Joe's access.
  3. Object ACL Evaluation: Finally, if Joe has made it past the bucket policy, his access to the object is verified through the object's ACL.

Bucket Operations vs. Object Operations: Knowing the Difference

Understanding S3 authorization also means knowing the difference between bucket and object operations. For example, the ListBucket permission applies at the bucket level (think of it as asking, "What's in this bucket?"). On the other hand, permissions like GetObject, PutObject, and DeleteObject apply at the object level, specifying actions on individual items within a bucket.

  • For operations that apply at the bucket level, you use the ARN of the bucket without a trailing asterisk: arn:aws:s3:::example-bucket
  • When specifying permissions for actions that apply to objects within a bucket, you add a trailing asterisk (*) after a slash to indicate "all objects" in the bucket: arn:aws:s3:::example-bucket/*


By now, we should have a clearer picture of how S3 authorization works. Remember, it all starts with the IAM principal and cascades down through the layers of bucket and object ownership, with various policies and ACLs guiding the way. With this knowledge in hand, we're well-equipped to start managing access to our S3 resources effectively and securely. Whether we're setting up our first bucket or fine-tuning access for complex applications, keeping these principles in mind will help ensure that our data is both accessible to those who need it and secure from those who don't. 

Stay connected with news and updates!

Join our mailing list to receive the latest news and updates from our team.
Don't worry, your information will not be shared.

We hate SPAM. We will never sell your information, for any reason.