Cross-Account Access with Resource-Based Policy vs. Role-Based Access: A Detailed Explanation

When it comes to granting access to AWS resources across multiple AWS accounts, there are two common approaches: using a resource-based policy and using a role-based approach. Both methods have their advantages, but cross-account access with a resource-based policy provides certain benefits, especially when it comes to maintaining user permissions.

Resource-Based Policy:

A resource-based policy is a policy attached directly to a resource, such as an Amazon S3 bucket, AWS KMS key, or Amazon ES domain. This policy specifies who (in the form of a list of AWS account ID numbers) can access that resource.

Role-Based Access:

In contrast, a role-based approach involves creating an IAM (Identity and Access Management) role in the trusting AWS account. Users or applications in the trusted AWS account assume this role to access resources in the trusting account. This method is typically used when you want to provide temporary or cross-account access to a specific resource, and it’s a common choice for granting access to AWS services like EC2 instances.

Advantages of Resource-Based Policy:

  1. User Permissions Preservation: One of the significant advantages of using a resource-based policy is that the user in the trusted account retains their user permissions. This means that while the user is granted access to the resource in the trusting account (the other AWS account), they don’t have to give up their permissions in their home or trusted account.
  2. Simultaneous Access: This approach allows the user to work within the trusted account and the trusting account simultaneously. In practical terms, it means that the user can perform actions in their own AWS account while also accessing resources in the other account. This is incredibly useful for tasks like copying information to or from the shared resource in the other account.

Use Cases:

Let’s break down some use cases where cross-account access with resource-based policies is beneficial:

  1. Data Sharing: Imagine a media company that hosts its infrastructure on AWS. They need to share specific data or files stored in an S3 bucket with a partner organization’s AWS account. Using a resource-based policy, the partner’s users can access the S3 bucket without losing their own account’s permissions. This setup allows the media company to maintain control over the shared resource.
  2. Encryption and Security: Cross-account access can be crucial for security and compliance. For example, a company may use a KMS key in one AWS account to encrypt data stored in an S3 bucket in another AWS account. Users can access the KMS key and the S3 bucket without losing their original account’s permissions.
  3. Monitoring and Auditing: Organizations may need to grant an auditing or monitoring service in another AWS account access to their logs stored in an Amazon Elasticsearch domain. Resource-based policies allow this access without affecting the organization’s own permissions.

In Summary:

Resource-based policies offer a flexible and user-friendly way to enable cross-account access to shared resources while ensuring that users in the trusted account retain their own permissions. This approach is particularly valuable when you need to collaborate, share data, and perform tasks that involve multiple AWS accounts while maintaining control over your resources and permissions.

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