Azure Authentication from client app to another API - azure-active-directory

Question

I'm trying to get azure AD authentication working between a Blazor WASM app, and another API that I have running locally but on a different port. I need both applications to use the Azure login, but I only want the user to have to log in once on the Blazor app which should then pass those credentials through to the API.
I've set up app registrations for both apps in the portal, created the redirect url, exposed the API with a scope and I can successfully log into the blazor app and see my name using #context.User.Identity.Name.
When it then tries to call the API though, I get a 401 error back and it doesn't hit any breakpoints in the API (presumably because there is no authentication being passed across in the http request).
My code in the Blazor app sets up a http client with the base address set to the API:
public static async Task Main(string[] args)
{
var builder = WebAssemblyHostBuilder.CreateDefault(args);
builder.RootComponents.Add<App>("#app");
builder.Services.AddHttpClient("APIClient", client => client.BaseAddress = new Uri("https://localhost:11001"))
.AddHttpMessageHandler<BaseAddressAuthorizationMessageHandler>();
builder.Services.AddScoped(sp => sp.GetRequiredService<IHttpClientFactory>().CreateClient("APIClient"));
builder.Services.AddMsalAuthentication<RemoteAuthenticationState, CustomUserAccount>(options =>
{
builder.Configuration.Bind("AzureAd", options.ProviderOptions.Authentication);
options.ProviderOptions.DefaultAccessTokenScopes.Add("api://d3152e51-9f5e-4ff7-85f2-8df5df5e2b2e/MyAPI");
//options.UserOptions.RoleClaim = "appRole";
});
await builder.Build().RunAsync();
}
In my API, I just have the Authorise attribute set on the class, and eventually will need roles in there too:
[Authorize]
[Route("api/[controller]")]
[ApiController]
public class CarController
Then, in my Blazor component, I then inject the http factory and try to make a request:
#inject IHttpClientFactory _factory
...
private async Task RetrieveCars()
{
var httpClient = _factory.CreateClient("APIClient");
HttpResponseMessage response = await httpClient.GetAsync("https://localhost:11001/api/cars");
var resp = await response.Content.ReadAsStringAsync();
cars = JsonSerializer.Deserialize<List<Car>>(resp);
}
but this returns the 401 error. I've also tried a few different variations like just injecting a http client (#inject HttpClient Http) but nothing seems to be adding my authorisation into the API calls. The options.UserOptions.RoleClaim is also commented out in the AddMsalAuthentication section as I wasn't sure if it was needed, but it doesn't work with or without it in there.
Can anyone explain what I'm doing wrong and what code I should be using?

Answer 1

Common causes.
Most cases ,we tend to forget to grant consent after giving API
permissions in the app registration portal,after exposing the api
which may lead to unauthorized error.
Other thing is when Audience doesn’t match the “aud” claim when we
track the token in jwt.io .Make sure ,Audience=clientId is configured
in the code in authentication scheme or Token validation parameters
by giving ValidAudiences.And also try with and without api:// prefix
in client id parameter.
Sometimes aud claim doesn’t match as we mistakenly send ID token
instead of Access tokens as access tokens are meant to call APIs .So
make sure you check mark both ID Token and access token in portal
while app registration.
While Enabling the authentication by injecting the [Authorize]
attribute to the Razor pages.Also add reference
Microsoft.AspNetCore.Authorization as(#using
Microsoft.AspNetCore.Authorization)
Please see the note in MS docs and some common-errors
If above are not the cases, please provide with additional error details and startup configurations or any link that you are following to investigate further.