How to restrict a user to THEIR resources using JWT when there is no identity in the Access Token? - jwt

Question

We have a REST resource like this:
/customer/{customerId}/bill
We want to use the JWT tokens returned from AWS Cognito to secure access to this resource.
The {customerId} here is not the Cognito user id, it's a domain specific id. We have added this domain specific id to the Cognito user as a custom attribute. It comes in the ID token that Cognito returns like this:
{
"sub": "xxxxxxxx-852f-474d-aa9e-a50fd832bcb8",
"aud": "xxxxxxxxsijed6uf54dh0uhi",
"custom:customerId": "4044",
"event_id": "xxxxxx-fc0c-4ffc-affa-f8987714fb2b",
"token_use": "id",
....
}
If we use this ID Token in Authorization: Bearer <ID Token> we can write code (custom authoriser or in-app code) that ensures the customerId in /customer/{customerId}/bill is equal to the value of custom:customerId in the supplied token, and we have secured our API.
But then we read that you should not use ID tokens to secure APIs . The key point being:
"The audience (the aud claim) of the (ID) token is set to the application's identifier, which means that only this specific application should consume this token."
So it seems we need to send an Access Token to secure the API. With Cognito, there is no way we can add any concept of who the user is into the Access Token. We can't add a custom scope like user:4044 for example.
What folks suggest as an approach here is to call the /userinfo endpoint of Cognito on the server-side with the supplied Access Token to learn who the user is. This would enable us to write code (custom authoriser or in-app code) that calls this endpoint and asserts permission. But it's an endpoint call for every request, which seems crazy.
One thought that crossed our minds was to use the Access Token to secure access to the API itself, but also require the ID token, either as a query parameter or a header to allow us do the fine grained access control. But that too starts to feel wrong.
Surely this is a solved problem? What is the right thing to do here?

Answer 1

Sorry, this question is a year old, so my answer is probably irrelevant. But for future wanderer, I would say that, given the limitations of cognito in allowing custom claims in the access token, a call to the /userinfo route is definitely the best way.
The API GATEWAY lets you cache authorizer response for a given user, so you won't be calling the endpoint on every request. Note that some implementations recommend it as a way to make sure that the token haven't been revoked.