As a GraphQL API gives you a lot of extra power and possibilities, it also introduces some new attack vectors. Nothing prevents the user of your (web) application to open the developer console and start creating and sending other queries to your GraphQL backend. By using the authentication token already available, he/she can call your API. So without further mitigations a user can create and run any query he/she can think of.
Luckily there are multiple ways to control this attack vector. I already talked about
- Assigning a complexity budget.
- Limiting the query depth.
Today I will focus on authorization.
Remark: I expect that you already have a kind of authentication built into your GraphQL API. So I'll ignore that part.
Doing an authorization check for a REST API is easy. As every REST call maps to a specific invocation requesting specific data, you have all the information you need to check if a user is allowed to fetch a specific set of data.
In ASP.NET Core this translates to the action level of a specific controller (or a specific endpoint if you are using minimal api's):
When using GraphQL you have a lot more flexibility at the query level so you don't know exactly what data will be fetched. Especially when you have created a rich graph, a lot of possible paths exist to get to a specific set of data.
So there is no single place where you can validate the authorization for the query the user wants to execute.
Authorization at the resolver level
A first option is to define authorization at the resolver level. In that case we use individual field resolvers to check user roles and make decisions as to what to return for each user.
HotChocolate builds further on the ASP.NET Core authorization middleware. It introduces an @authorize directive that can be applied to fields and types to denote that they require authorization.
If we do not specify any arguments to the @authorize directive, it will only enforce that the requestor is authenticated. But it is also possible to specify one or more roles or an an ASP.NET Core authorization policy:
Authorization at the business logic layer level
Doing authorization at the resolver level is fine for simple use cases. Thanks to the support for ASP.NET Core Authorization policies we can avoid duplication and decouple the authorization rules from the resolvers.
However you can go one step further and handle authorization at the business layer level. This helps to keep your resolvers thin, and put all of your business logic (including authorization) in one place.