Cross-account roles

What is a cross-account role in AWS, and why would you use it?

A cross-account role is an IAM role that allows you to delegate access to resources in one AWS account to a user or service in another AWS account. It is used to provide controlled access without having to create IAM users in each account, which helps in maintaining a principle of least privilege and reducing administrative overhead.

How do you secure cross-account role access?

To secure cross-account role access, you should implement a strong permission policy that defines what actions can be performed by the trusted entity. Additionally, use condition keys in the role’s trust policy to restrict access based on factors such as source IP or Multi-Factor Authentication (MFA), and regularly review and rotate access keys if they are used.

Can you describe the trust policy and permission policy in cross-account roles?

In cross-account roles, the trust policy determines who is allowed to assume the role (the trusted entities), such as another AWS account. Meanwhile, the permission policy specifies what actions the entity assuming the role has permissions to execute within the account that owns the role.

How can you monitor the usage of cross-account roles within your AWS environment?

You can monitor the usage of cross-account roles by enabling AWS CloudTrail logs, which capture all API calls for IAM including AssumeRole requests. You can then analyze these logs for any unusual activity or unauthorized access attempts.

What is the difference between resource-based policies and cross-account access with roles?

Resource-based policies are attached directly to AWS resources to define who has access to that resource and how they can interact with it, regardless of which account they belong to. Cross-account access with roles involves creating IAM roles that users or services from one account can assume to access resources in another account, with permissions defined in the role’s permission policies.

How do you automate the process of rotating keys for a cross-account role?

For cross-account roles, the primary security feature is the role itself, not the access keys. IAM roles do not have access keys by default; instead, temporary credentials are generated when the role is assumed. Rotation of these temporary credentials is handled automatically by AWS, and new credentials are provided each time the AssumeRole API is called.

Can you limit which AWS services are able to assume a cross-account role?

Yes, you can limit which AWS services are able to assume a cross-account role by specifying the service principal in the trust policy of the role. This explicitly allows only the specified AWS service to assume the role.

What steps would you take to troubleshoot an issue where a user is unable to assume a cross-account role?

To troubleshoot this issue, first verify the trust policy of the role to ensure that the user’s account has permission to assume the role. Next, check the permission policy of the assuming user to confirm they have the `sts:AssumeRole` permission for the particular role. Also, examine any condition keys used in the policies and ensure the user’s request meets those conditions. Finally, review CloudTrail logs for any denied access attempts that could give insights into the issue.

What are the implications of using a cross-account access with external ID and when should it be used?

The external ID is a unique identifier that you can use in a trust policy to ensure that only the entity that knows the external ID can assume the role. It is especially useful when you want to restrict access to a third party and prevent the confused deputy problem, where one AWS account could inadvertently act on behalf of another if permissions are too broadly defined.

Describe a scenario where a cross-account role would be necessary, instead of using an IAM user.

A cross-account role would be necessary in a scenario where you have a centralized logging account and you want to allow that account to access logs stored in S3 buckets across different AWS accounts. Instead of creating multiple IAM users in each account with access rights, you can create a cross-account role with the necessary permissions that the logging account can assume.

How do you set up a cross-account role to be assumed only by a specific IAM user in another account?

To set up a cross-account role to be assumed only by a specific IAM user, you would include a condition in the role’s trust policy that checks the `arn:aws:iam:::user/` of the IAM user. This ensures that only the specified IAM user from the trusted account can assume the role.

Explain the significance of ‘externalId’ when setting up a cross-account role trust relationship.

The ‘externalId’ is an optional condition that you can set in the trust policy of a cross-account role to enhance security when third-party services or accounts assume the role. It acts as a shared secret that prevents the “confused deputy” problem. The external service must provide the ‘externalId’ as a condition when making the `AssumeRole` call, ensuring that only the entity in possession of the ‘externalId’ can access the resources, even if the role ARN is somehow compromised.