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By Damien Guard
This tutorial demonstrates how to add authorization to an ASP.NET OWIN API using the standard JWT middleware.We recommend that you log in to follow this quickstart with examples configured for your account.
New to Auth0? Learn how Auth0 works and read about implementing API authentication and authorization using the OAuth 2.0 framework.

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Configure Auth0 APIs

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Create an API

In the APIs section of the Auth0 dashboard, click Create API. Provide a name and an identifier for your API, for example, https://quickstarts/api. You will use the identifier as an audience later, when you are configuring the Access Token verification. Leave the Signing Algorithm as RS256.
Create API
By default, your API uses RS256 as the algorithm for signing tokens. Since RS256 uses a private/public keypair, it verifies the tokens against the public key for your Auth0 account. The public key is in the JSON Web Key Set (JWKS) format, and can be accessed here.

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Define permissions

Permissions let you define how resources can be accessed on behalf of the user with a given access token. For example, you might choose to grant read access to the messages resource if users have the manager access level, and a write access to that resource if they have the administrator access level. You can define allowed permissions in the Permissions view of the Auth0 Dashboard’s APIs section.
Configure Permissions
This example uses the read:messages scope.
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, using the JSON Web Key Set (JWKS) for your Auth0 account. To learn more about validating Access Tokens, see Validate Access Tokens.

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Configure the Sample Project

The sample code has an appsettings section in Web.config which configures it to use the correct Auth0 Domain and API Identifier for your API. If you download the code from this page it will be automatically filled. If you use the example from Github, you will need to fill it yourself.

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Validate Access Tokens

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Install dependencies

To use Auth0 Access Tokens with ASP.NET you will use the OWIN JWT Middleware which is available in the Microsoft.Owin.Security.Jwt NuGet package. 
Install-Package Microsoft.Owin.Security.Jwt

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Verifying the token signature

As the OWIN JWT middleware doesn’t use Open ID Connect Discovery by default, you will need to provide a custom IssuerSigningKeyResolver. To do this, add the following to the Support/OpenIdConnectSigningKeyResolver.cs file:
Such a custom resolver was previously published as part of the Auth0.OpenIdConnectSigningKeyResolver package through Nuget. As this package is not available anymore, you will need to provide this yourself.
public class OpenIdConnectSigningKeyResolver
{
    private readonly OpenIdConnectConfiguration openIdConfig;

    public OpenIdConnectSigningKeyResolver(string authority)
    {
        var cm = new ConfigurationManager<OpenIdConnectConfiguration>($"{authority.TrimEnd('/')}/.well-known/openid-configuration", new OpenIdConnectConfigurationRetriever());
        openIdConfig = AsyncHelper.RunSync(async () => await cm.GetConfigurationAsync());
    }

    public SecurityKey[] GetSigningKey(string kid)
    {
        return new[] { openIdConfig.JsonWebKeySet.GetSigningKeys().FirstOrDefault(t => t.KeyId == kid) };
    }
}
The OpenIdConnectSigningKeyResolver will automatically download the JSON Web Key Set used to sign the RS256 tokens from the OpenID Connect Configuration endpoint (at /.well-known/openid-configuration). You can then use it subsequently to resolve the Issuer Signing Key, as will be demonstrated in the JWT registration code below.

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Configuration

Go to the Configuration method of your Startup class and add a call to UseJwtBearerAuthentication passing in the configured JwtBearerAuthenticationOptions. The JwtBearerAuthenticationOptions needs to specify your Auth0 API Identifier in the ValidAudience property, and the full path to your Auth0 domain as the ValidIssuer. You will need to configure the IssuerSigningKeyResolver to use the instance of OpenIdConnectSigningKeyResolver to resolve the signing key: 
// Startup.cs

public void Configuration(IAppBuilder app)
{
    var domain = $"https://{ConfigurationManager.AppSettings["Auth0Domain"]}/";
    var apiIdentifier = ConfigurationManager.AppSettings["Auth0ApiIdentifier"];
    var keyResolver = new OpenIdConnectSigningKeyResolver(domain);

    app.UseJwtBearerAuthentication(
        new JwtBearerAuthenticationOptions
        {
            AuthenticationMode = AuthenticationMode.Active,
            TokenValidationParameters = new TokenValidationParameters()
            {
                ValidAudience = apiIdentifier,
                ValidIssuer = domain,
                IssuerSigningKeyResolver = (token, securityToken, kid, parameters) => keyResolver.GetSigningKey(kid)
            }
        });

    // Configure Web API
    WebApiConfig.Configure(app);
}

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Do not forget the trailing backslash

Please ensure that the URL specified for ValidIssuer contains a trailing backslash as this needs to match exactly with the issuer claim of the JWT. This is a common misconfiguration error which will cause your API calls to not be authenticated correctly.

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Validate scopes

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. Create a class called ScopeAuthorizeAttribute which inherits from System.Web.Http.AuthorizeAttribute. This Authorization Attribute 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. 
// Controllers/ScopeAuthorizeAttribute.cs

public class ScopeAuthorizeAttribute : AuthorizeAttribute
{
    private readonly string scope;

    public ScopeAuthorizeAttribute(string scope)
    {
        this.scope = scope;
    }

    public override void OnAuthorization(HttpActionContext actionContext)
    {
        base.OnAuthorization(actionContext);

        // Get the Auth0 domain, in order to validate the issuer
        var domain = $"https://{ConfigurationManager.AppSettings["Auth0Domain"]}/";

        // Get the claim principal
        ClaimsPrincipal principal = actionContext.ControllerContext.RequestContext.Principal as ClaimsPrincipal;

        // Get the scope claim. Ensure that the issuer is for the correct Auth0 domain
        var scopeClaim = principal?.Claims.FirstOrDefault(c => c.Type == "scope" && c.Issuer == domain);
        if (scopeClaim != null)
        {
            // Split scopes
            var scopes = scopeClaim.Value.Split(' ');

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

        HandleUnauthorizedRequest(actionContext);
    }
}

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Protect API Endpoints

The routes shown below are available for the following requests:
  • GET /api/public: available for non-authenticated requests
  • GET /api/private: available for authenticated requests containing an access token with no additional scopes
  • GET /api/private-scoped: available for authenticated requests containing an access token with the read:messages scope granted
The JWT middleware integrates with the standard ASP.NET Authentication and Authorization mechanisms, so you only need to decorate your controller action with the [Authorize] attribute to secure an endpoint. 
// Controllers/ApiController.cs

[RoutePrefix("api")]
public class ApiController : ApiController
{
    [HttpGet]
    [Route("private")]
    [Authorize]
    public IHttpActionResult Private()
    {
        return Json(new
        {
            Message = "Hello from a private endpoint! You need to be authenticated to see this."
        });
    }
}
To ensure that a scope is present in order to call a particular API endpoint, you simply need to decorate the action with the ScopeAuthorize attribute and pass the name of the required scope in the scope parameter. 
// Controllers/ApiController.cs

[RoutePrefix("api")]
public class ApiController : ApiController
{
    [HttpGet]
    [Route("private-scoped")]
    [ScopeAuthorize("read:messages")]
    public IHttpActionResult 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."
        });
    }
}
What can you do next?
Configure other identity providersEnable multifactor authentication
Learn about attack protectionLearn about rules
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