Why you need to register authentication middleware even if your ASP.NET core web API does not handle authentication.

Sometimes ago, I was confused about the role of the Authentication middleware in an ASP.NET core web API that does not authenticate an user. It makes sense to me that you need to use the Authentication middleware if your web application handles the authentication. Specifically, I did not understand why you need to use Authentication middleware if your app is a web API that does not handle authentication. For instance, my web API performs token validation but it does not authenticate a user. Authentication handling is part of the client application which implements OpenID implicit flow to authenticate the user and obtains authorization to access the web API. I believed I only needed the Authorization middleware so that I can annotate the endpoints I want to protect with the [Authorized] attribute. The document states

The UseAuthentication method adds a single authentication middleware component, which is responsible for automatic authentication and the handling of remote authentication requests. 

Authentication Middleware and services

So if my web API does not handle authentication, why do I still need to call UseAuthentication to add the middleware?

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Implement OAuth2 Client-Credentials flow with Azure AD and Microsoft Identity Platform.

OAuth2 Client Credentials flow is a protocol to allow secure communication between two web APIs. Specifically, the protocol specifies the flow of obtaining authorization for a client to access protected endpoints of a resource server with no user interaction involved. With Microsoft Identity Platform, Azure portal, Microsoft Authentication Library (MSAL), and .NET core security middleware, you can implement the OAuth2 client credentials flow without much difficulty. In this post, I go over how to leverage those technologies to protect your ASP.NET core web APIs.

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Pass user’s identity and authorization from a client application to a web API to another web API using OAuth 2.0 On-Behalf-Of flow.

A few months ago, I gave an overview of the libraries I use to implement OpenID Connect implicit flow in an angular app, and On-Behalf-Of (OBO) flow in ASP.NET core backend APIs. You can checkout this post for more info. In that post, I talk about the security flow from the angular app to the downstream APIs. The angular app communicates only with a single backend API which acts as a gateway that forwards the requests from to other downstream APIs.

In this post, I go over the details of obtaining an access token via the OBO flow to call protected endpoints from a web API (which I refer to as the gateway in this post) to another web API .

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Using oidc-client-js to obtain tokens from Azure AD (v1.0) or Microsoft identity platform (v2.0) .

In my previous post, I mention using MSAL for angular to implement implicit flow in angular application. However, MSAL is still in preview and I could not get it to work in IE 11. In addition, I could not find a way to obtain both access and id tokens in a single call. I have switched to oidc-client-js. Besides adding the polyfills for IE, I did not have to do much for oidc-client-js to run in IE11. The library also allows me to configure response_type parameter of a request to the authorization endpoint to obtain both id and access tokens in one call. Overall, I have found the library to be more stable than MSAL for angular. In this post, I share how I configure oidc-client-js in an angular application to obtain tokens from Azure Active Directory (v1.0 endpoint) as well as some of the lessons I have learned.

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OIDC implicit flow in angular with MSAL for angular, Microsoft Identity Platform (v2.0) and Azure AD.

In this post, I share my experience about doing OpenID Connect (OIDC) implicit flow using Microsoft Authentication library (MSAL) for Angular, Microsoft Identity Platform (v2.0), and Azure AD. This post is part of the blog post series in which I cover implementing OIDC flows to protect as system that consists of an angular front-end application and asp.net core web apis. In the previous post, I give a high level overview of the technologies involved in protecting such a system.

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