In modern cloud environments, managing access across multiple AWS accounts is a common challenge. Whether you’re operating within a multi-account AWS organization or collaborating with external partners, AWS provides robust mechanisms for secure cross-account access. This blog post will break down how cross-account access works and the best practices for implementing it.

What is Cross-Account Access in AWS?

Cross-account access in AWS allows users, roles, or services in one AWS account to access resources in another account. This eliminates the need to share long-term credentials while maintaining security and control over permissions.

Methods for Implementing Cross-Account Access

AWS provides multiple ways to enable secure access between accounts. The most commonly used methods include:

1. Resource-Based Policies

Resource-based policies are attached directly to AWS resources, specifying which external AWS accounts or principals (users, roles) can access them.

Use Case:

  • Ideal when you want to grant access to specific resources without modifying the principal’s permissions.

Example:

  • An Amazon S3 bucket policy that grants another AWS account permission to read from the bucket.

2. IAM Roles with Trust Relationships

IAM roles are a powerful way to delegate permissions across AWS accounts securely. A trust relationship specifies which external AWS accounts can assume the role.

Use Case:

  • Suitable when the resource does not support resource-based policies or when centralizing permission management is preferred.

Example:

  • An IAM role in Account A allows a user from Account B to assume the role and access resources in Account A.

Steps to Implement Cross-Account Access Using IAM Roles

To configure secure access using IAM roles, follow these steps:

Step 1: Create a Role in the Resource Account

  • Define an IAM role in the AWS account where the resource is located.
  • Attach a permissions policy that specifies allowed actions on the resource.

Step 2: Configure the Trust Relationship

  • Set up a trust policy that defines which AWS accounts (or specific IAM users/roles) are allowed to assume the role.

Step 3: Assume the Role from the External Account

  • The external AWS account can assume the role using AWS Security Token Service (STS).
  • The AWS user or service in the external account receives temporary credentials to access the specified resources.

Real-World Example: Sharing S3 Access Across Accounts

Consider an organization with two AWS accounts:

  • Account A (Production Environment)
  • Account B (Development Environment)

Developers in Account B need to access an S3 bucket in Account A.

Solution:

  • In Account A:
    • Create an IAM role granting the necessary S3 permissions.
    • Set a trust policy allowing the role to be assumed by users from Account B.
  • In Account B:
    • Developers assume the IAM role from Account A using STS.
    • Temporary credentials are used to access the S3 bucket securely.

Best Practices for Cross-Account Access

To ensure security and compliance, follow these best practices:

  • Use IAM roles instead of IAM users to avoid long-term credentials.
  • Implement least privilege access by granting only necessary permissions.
  • Monitor access logs using AWS CloudTrail for auditing and compliance.
  • Regularly review permissions to remove unnecessary access.

Conclusion

Cross-account access in AWS provides a secure and efficient way to manage permissions across multiple AWS accounts. By leveraging resource-based policies and IAM roles with trust relationships, organizations can ensure secure collaboration without compromising security.

For a detailed walkthrough, refer to the AWS IAM documentation on delegating access across AWS accounts.