Cross-service confused deputy prevention

The confused deputy problem is a security issue where an entity that doesn’t have permission to perform an action can coerce a more-privileged entity to perform the action. In AWS, cross-service impersonation can result in the confused deputy problem. Cross-service impersonation can occur when one service (the calling service) calls another service (the called service). The calling service can be manipulated to use its permissions to act on another customer’s resources in a way it shouldn't otherwise have permission to access. To prevent this, AWS provides tools that help you protect your data for all services with service principals that have been given access to resources in your account.

We recommend using the aws:SourceArn and aws:SourceAccount global condition context keys in resource policies to limit the permissions that Identity and Access Management (IAM) gives Amazon Nimble Studio to access your resources. If you use both global condition context keys, the aws:SourceAccount value and the account in the aws:SourceArn value must use the same account id when used in the same policy statement.

The value of aws:SourceArn must be the studio’s ARN and aws:SourceAccount must be your account id. You won’t know what the studio id is until the studio is created because it's generated by Nimble Studio. Once your studio is created, you can update the trust policy with the final studio id set as the aws:SourceArn.

The most effective way to protect against the confused deputy problem is to use the aws:SourceArn global condition context key with the full ARN of the resource. If you don’t know the full ARN of the resource or if you're specifying multiple resources, use the aws:SourceArn global context condition key with wildcards (*) for the unknown portions of the ARN. For example, arn:aws:nimble::123456789012:*.

Your end users assume your studio role when they sign in to the Nimble Studio portal. When you create your studio, AWS configures the role and evaluates the policy. AWS evaluates the policy every subsequent time one of your users logs in to the Nimble Studio portal. When you create a studio, you can’t modify the aws:SourceArn. After you finish creating your studio, you can use your studioArn for the aws:SourceArn.

The following example is an assume role policy that shows how you can use the aws:SourceArn and aws:SourceAccount global condition context keys in Nimble Studio to prevent the confused deputy problem.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "identity.nimble.amazonaws.com"
      },
      "Action": [
        "sts:AssumeRole",
        "sts:TagSession"
      ],
      "Condition": {
        "StringEquals": {
          "aws:SourceAccount": "123456789012"
        },
        "StringLike": {
          "aws:SourceArn": "arn:aws:nimble:us-west-2:123456789012:studio/*"
        }
      }
    }
  ]
}