AWS S3 Bucket Policy Multiple Principals

Amazon S3 (Simple Storage Service) is a highly scalable, reliable, and cost - effective object storage service provided by Amazon Web Services (AWS). Bucket policies in S3 are a powerful way to manage access to your S3 buckets and the objects within them. A principal in an S3 bucket policy is an entity that can perform actions on the bucket or its objects, such as an AWS account, an IAM user, an IAM role, or a federated user. When dealing with real - world scenarios, you often need to grant access to multiple principals. This is where the concept of multiple principals in an S3 bucket policy comes into play. In this blog post, we will explore the core concepts, typical usage scenarios, common practices, and best practices related to AWS S3 bucket policy multiple principals.

Table of Contents#

  1. Core Concepts
    • What is an S3 Bucket Policy?
    • What is a Principal?
    • Multiple Principals in S3 Bucket Policies
  2. Typical Usage Scenarios
    • Team - Based Access
    • Cross - Account Access
    • Third - Party Service Access
  3. Common Practices
    • Defining Multiple Principals in a Policy
    • Using Wildcards for Principals
  4. Best Practices
    • Least Privilege Principle
    • Regular Policy Review
    • Separation of Duties
  5. Conclusion
  6. FAQ
  7. References

Article#

Core Concepts#

What is an S3 Bucket Policy?#

An S3 bucket policy is a JSON - formatted document that you can attach to an S3 bucket. It allows you to define permissions for the bucket and the objects within it. The policy can specify who (the principal) can access the bucket, what actions they can perform (e.g., s3:GetObject, s3:PutObject), and under what conditions the access is allowed.

What is a Principal?#

A principal in an S3 bucket policy is the entity that is being granted or denied access to the bucket or its objects. Principals can be of different types:

  • AWS Account: You can specify an entire AWS account as a principal. For example, if you want to grant access to all resources within an account.
  • IAM User: An individual user within an AWS account.
  • IAM Role: A role that can be assumed by users, applications, or services.
  • Federated User: A user who has been authenticated through an external identity provider and federated with AWS.

Multiple Principals in S3 Bucket Policies#

In an S3 bucket policy, you can specify multiple principals. This is useful when you need to grant the same set of permissions to different entities. You can list the principals as an array in the Principal element of the bucket policy JSON.

Typical Usage Scenarios#

Team - Based Access#

Suppose you have a development team and a testing team, and both teams need access to a specific S3 bucket. You can create an S3 bucket policy with multiple principals, where each principal represents an IAM group for the development and testing teams respectively. This way, all members of these teams can access the bucket with the defined permissions.

Cross - Account Access#

In a multi - account AWS environment, you may need to grant access to an S3 bucket in one account to users or roles in another account. You can list the AWS accounts, IAM users, or IAM roles from the other account as multiple principals in the bucket policy of the source account.

Third - Party Service Access#

If your application integrates with third - party services that need access to your S3 bucket, you can specify the service principals in the bucket policy. For example, if you are using a data analytics service that needs to read data from your S3 bucket, you can add the service's principal to the bucket policy along with other internal principals.

Common Practices#

Defining Multiple Principals in a Policy#

Here is an example of an S3 bucket policy with multiple principals:

{
    "Version": "2012 - 10 - 17",
    "Statement": [
        {
            "Sid": "AllowMultiplePrincipals",
            "Effect": "Allow",
            "Principal": {
                "AWS": [
                    "arn:aws:iam::123456789012:user/User1",
                    "arn:aws:iam::123456789012:role/Role1",
                    "arn:aws:iam::234567890123:root"
                ]
            },
            "Action": [
                "s3:GetObject",
                "s3:ListBucket"
            ],
            "Resource": [
                "arn:aws:s3:::my - bucket",
                "arn:aws:s3:::my - bucket/*"
            ]
        }
    ]
}

In this example, three different principals (an IAM user, an IAM role, and an entire AWS account) are granted the s3:GetObject and s3:ListBucket permissions on the specified bucket and its objects.

Using Wildcards for Principals#

You can use wildcards in the principal ARNs to simplify the policy when dealing with multiple related principals. For example, if you have multiple IAM users with names starting with dev -, you can use a wildcard like arn:aws:iam::123456789012:user/dev - *.

Best Practices#

Least Privilege Principle#

When using multiple principals in an S3 bucket policy, always follow the least privilege principle. Only grant the minimum set of permissions required for each principal to perform its tasks. For example, if a principal only needs to read objects from the bucket, do not grant write permissions.

Regular Policy Review#

Regularly review your S3 bucket policies, especially when there are changes in your organization's structure, team membership, or security requirements. Remove any unnecessary principals or permissions to reduce the risk of unauthorized access.

Separation of Duties#

Separate the management of different types of principals and their permissions. For example, have different teams or individuals responsible for managing IAM users, roles, and cross - account access. This helps in maintaining security and accountability.

Conclusion#

AWS S3 bucket policy multiple principals provide a flexible way to manage access to your S3 buckets. By understanding the core concepts, typical usage scenarios, common practices, and best practices, software engineers can effectively use multiple principals in S3 bucket policies to meet their organization's security and access requirements. This ensures that the right entities have the right level of access to the S3 resources, while minimizing the risk of security breaches.

FAQ#

Q: Can I mix different types of principals (e.g., IAM users and AWS accounts) in the same policy?#

A: Yes, you can mix different types of principals in the same S3 bucket policy. You just need to list them as an array in the Principal element of the policy JSON.

Q: What if I make a mistake in the principal ARN?#

A: If you make a mistake in the principal ARN, the principal will not be able to access the bucket as expected. You should double - check the ARNs and test the policy to ensure that the correct principals have the intended access.

Q: Can I use multiple principals in a deny statement?#

A: Yes, you can use multiple principals in a deny statement. A deny statement always takes precedence over an allow statement, so if a principal is listed in a deny statement, they will be denied access regardless of any allow statements.

References#

2025-10