This Quickstart will guide you through the various tasks related to using Auth0-issued Access Tokens to secure your ASP.NET Core Web API.

If you would like to follow along with this Quickstart you can download the seed project. The seed project is just a basic ASP.NET Web API with a simple controller and some of the NuGet packages which will be needed included. It also contains an appSettings.json file where you can configure the various Auth0-related settings for your application.

The final project after each of the steps is also available in the Quickstart folder of the Samples repository. You can find the final result for each step in the relevant folder inside the repository.

Also, update the appsettings.json file in your project with the correct Domain and API Identifier for your API, such as

{
  "Auth0": {
    "Domain": "YOUR_AUTH0_DOMAIN",
    "ApiIdentifier": "{YOUR_API_IDENTIFIER}"
  }
}

This example demonstrates:

• How to check for a JSON Web Token (JWT) in the Authorization header of an incoming HTTP request
• How to check if the token is valid with the standard ASP.NET Core JWT middleware

To use Auth0 Access Tokens with ASP.NET Core you will use the JWT Middleware. Add the Microsoft.AspNetCore.Authentication.JwtBearer package to your application.

Install-Package Microsoft.AspNetCore.Authentication.JwtBearer

The ASP.NET Core JWT middleware will handle downloading the JSON Web Key Set (JWKS) file containing the public key for you, and will use that to verify the access_token signature.

To add the JWT middleware to your application's middleware pipeline, go to the Configure method of your Startup class and add a call to UseJwtBearerAuthentication passing in the configured JwtBearerOptions. The JwtBearerOptions needs to specify your Auth0 API Identifier as the Audience, and the full path to your Auth0 domain as the Authority:

// Startup.cs

public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    loggerFactory.AddConsole(Configuration.GetSection("Logging"));
    loggerFactory.AddDebug();

    var options = new JwtBearerOptions
    {
        Audience = Configuration["Auth0:ApiIdentifier"],
        Authority = $"https://{Configuration["Auth0:Domain"]}/"
    };
    app.UseJwtBearerAuthentication(options);

    app.UseMvc();
}

The JWT middleware integrates with the standard ASP.NET Core Authentication and Authorization mechanisms. To secure an endpoint you only need to decorate your controller action with the [Authorize] attribute:

// Controllers/ApiController.cs

[Route("api")]
public class ApiController : Controller
{
    [HttpGet]
    [Route("private")]
    [Authorize]
    public IActionResult Private()
    {
        return Json(new
        {
            Message = "Hello from a private endpoint! You need to be authenticated to see this."
        });
    }
}

The JWT middleware above verifies that the access_token included in the request is valid; however, it doesn't yet include any mechanism for checking that the token has the sufficient scope to access the requested resources.

To make sure that an Access Token contains the correct scope, use the Policy-Based Authorization in ASP.NET Core.

Create a new Authorization Requirement called HasScopeRequirement. This requirement will check that the scope claim issued by your Auth0 tenant is present, and if so it will ensure that the scope claim contains the requested scope. If it does then the Authorization Requirement is met.

// HasScopeRequirement.cs

public class HasScopeRequirement : AuthorizationHandler<HasScopeRequirement>, IAuthorizationRequirement
{
    private readonly string issuer;
    private readonly string scope;

    public HasScopeRequirement(string scope, string issuer)
    {
        this.scope = scope;
        this.issuer = issuer;
    }

    protected override Task HandleRequirementAsync(AuthorizationHandlerContext context, HasScopeRequirement requirement)
    {
        // If user does not have the scope claim, get out of here
        if (!context.User.HasClaim(c => c.Type == "scope" && c.Issuer == issuer))
            return Task.CompletedTask;

        // Split the scopes string into an array
        var scopes = context.User.FindFirst(c => c.Type == "scope" && c.Issuer == issuer).Value.Split(' ');

        // Succeed if the scope array contains the required scope
        if (scopes.Any(s => s == scope))
            context.Succeed(requirement);

        return Task.CompletedTask;
    }
}

Next, you can define a policy for each of the scopes in your application in the ConfigureServices method of your Startup class:

// Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddMvc();

    string domain = $"https://{Configuration["Auth0:Domain"]}/";
    services.AddAuthorization(options =>
    {
        options.AddPolicy("read:messages",
            policy => policy.Requirements.Add(new HasScopeRequirement("read:messages", domain)));
    });
}

To secure the API endpoint, we need to make sure that the correct scope is present in the access_token. To do that, add the Authorize attribute to the Scoped action, passing read:messages as the policy parameter.

// Controllers/ApiController.cs

[Route("api")]
public class ApiController : Controller
{
    [HttpGet]
    [Route("private-scoped")]
    [Authorize("read:messages")]
    public IActionResult Scoped()
    {
        return Json(new
        {
            Message = "Hello from a private endpoint! You need to be authenticated and have a scope of read:messages to see this."
        });
    }
}

• To learn how you can call your API from applications, please refer to the Using your API section.

• If your experience problems with your API, please refer to the Troubleshooting section.