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Terraform

Terraform Wildcard in S3 Bucket Principal Policy

By Ondřej DolanskĆ½ on 01/01/2025

Learn how to use wildcards in Terraform principals for your S3 buckets to grant permissions to specific users, roles, or services.

Terraform Wildcard in S3 Bucket Principal Policy

Table of Contents

Introduction

In AWS Identity and Access Management (IAM) policies, you cannot use wildcards within Amazon Resource Names (ARNs) specified in the Principal element. To grant access to all users, utilize "" for the entire Principal field. For instance, in a Terraform policy definition, setting "type" to "" and "identifiers" to ["*"] within the "principals" block will grant access to everyone.

Step-by-Step Guide

You cannot use wildcards within ARN specifications for the Principal element in AWS IAM policies.

To grant access to everyone, use "*" for the entire Principal field.

Example (Terraform):

data "aws_iam_policy_document" "example" {
  statement {
    effect = "Allow"
    actions = ["s3:GetObject"]
    resources = ["arn:aws:s3:::examplebucket/*"]

    principals {
      type = "*"
      identifiers = ["*"]
    }
  }
}

This policy allows anyone to perform the s3:GetObject action on objects within the examplebucket S3 bucket.

Code Example

The Terraform code defines an AWS IAM policy document that grants everyone permission to perform the "s3:GetObject" action on all objects within the "examplebucket" S3 bucket.

data "aws_iam_policy_document" "example" {
  statement {
    effect = "Allow"
    actions = ["s3:GetObject"]
    resources = ["arn:aws:s3:::examplebucket/*"]

    # Correct: Grant access to everyone
    principals {
      type = "*"
    }
  }
}

Explanation:

  • We removed the identifiers attribute entirely. When type is set to "*", the identifiers attribute is not used and should be omitted.
  • Using "*" for the entire Principal field (by specifying only the type as "*") signifies that the policy applies to everyone. This includes anonymous users and users from other AWS accounts.

Important Note: Granting access to everyone is generally discouraged due to security risks. It's crucial to restrict access to only the necessary users or services in a production environment.

Additional Notes

  • Alternatives to "*": While using "*" for the principal is possible, it's crucial to explore more secure alternatives for production environments. Consider these options:

    • Specific ARNs: Explicitly define the ARNs of the users, roles, or services that require access. This provides the most granular control.
    • AWS Account IDs: If you need to grant access to all users within a specific AWS account, use the account ID as the principal.
    • IAM Conditions: Utilize IAM conditions to further refine access based on factors like IP address, time of day, or specific request attributes.

  • Policy Evaluation Logic: Remember that IAM policies are evaluated based on "least privilege." If a more restrictive policy exists alongside a broader one, the more restrictive policy takes precedence.

  • Regularly Review Policies: Periodically review your IAM policies to ensure they haven't become overly permissive over time. Remove any unnecessary permissions and update them as your requirements evolve.

  • Use Tools for Analysis: Leverage AWS tools like IAM Access Analyzer to help identify and mitigate potential security risks associated with overly permissive policies.

  • Principle of Least Privilege: Always strive to adhere to the principle of least privilege. Grant only the minimum necessary permissions required for users and services to perform their intended tasks.

Summary

Topic Description
Wildcards in IAM Policy Principals Not Supported: You cannot use wildcards within ARN specifications for the Principal element in AWS IAM policies.
Granting Access to Everyone Use "*": To grant access to everyone, use a single asterisk ("*") for the entire Principal field.
Example (Terraform) The provided Terraform code demonstrates creating an IAM policy that allows anyone to perform the s3:GetObject action on objects within a specific S3 bucket. The key point is the use of "*" for both type and identifiers within the principals block.

Conclusion

When defining IAM policies in AWS, remember that wildcards within ARN specifications are not allowed for the Principal element. To grant access to all users, use "*" for the entire Principal field. However, granting access to everyone is generally discouraged in production environments due to security risks. Instead, prioritize more restrictive approaches like using specific ARNs, AWS account IDs, or IAM conditions to control access. Regularly review and refine your IAM policies, adhering to the principle of least privilege to ensure only necessary permissions are granted. Leverage AWS tools like IAM Access Analyzer to identify and mitigate potential security risks associated with overly permissive policies.

References

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