Jason Watmore's Blog A Web Developer in Sydney
Published: December 14 2021

.NET 6.0 - JWT Authentication Tutorial with Example API

Tutorial built with .NET 6.0

Other versions available:

In this tutorial we'll go through a simple example of how to implement custom JWT (JSON Web Token) authentication in a .NET 6.0 API with C#.

For an extended example that includes refresh tokens see .NET 6.0 - JWT Authentication with Refresh Tokens Tutorial with Example API.

The example API has just two endpoints/routes to demonstrate authenticating with JWT and accessing a restricted route with JWT:

  • /users/authenticate - public route that accepts HTTP POST requests containing the username and password in the body. If the username and password are correct then a JWT authentication token and the user details are returned.
  • /users - secure route that accepts HTTP GET requests and returns a list of all the users in the application if the HTTP Authorization header contains a valid JWT token. If there is no auth token or the token is invalid then a 401 Unauthorized response is returned.

The tutorial project is available on GitHub at https://github.com/cornflourblue/dotnet-6-jwt-authentication-api.


Tutorial Contents


Tools required to run the .NET 6.0 JWT Example Locally

To develop and run .NET 6.0 applications locally, download and install the following:

  • .NET SDK - includes the .NET runtime and command line tools
  • Visual Studio Code - code editor that runs on Windows, Mac and Linux
  • C# extension for Visual Studio Code - adds support to VS Code for developing .NET applications


Run the .NET JWT Authentication API Locally

  1. Download or clone the tutorial project code from https://github.com/cornflourblue/dotnet-6-jwt-authentication-api
  2. Start the api by running dotnet run from the command line in the project root folder (where the WebApi.csproj file is located), you should see the message Now listening on: http://localhost:4000. Follow the instructions below to test with Postman or hook up with one of the example single page applications available (Angular, Blazor, React or Vue).

NOTE: You can also start the application in debug mode in VS Code by opening the project root folder in VS Code and pressing F5 or by selecting Debug -> Start Debugging from the top menu. Running in debug mode allows you to attach breakpoints to pause execution and step through the application code. For more info on debugging .NET in VS Code see VS Code + .NET - Debug a .NET Web App in Visual Studio Code.


Test the .NET 6.0 JWT Auth API with Postman

Postman is a great tool for testing APIs, you can download it at https://www.postman.com/downloads.

Below are instructions on how to use Postman to authenticate a user to get a JWT token from the api, and then make an authenticated request with the JWT token to retrieve a list of users from the api.

How to authenticate a user with Postman

To authenticate a user with the api and get a JWT token follow these steps:

  1. Open a new request tab by clicking the plus (+) button at the end of the tabs.
  2. Change the HTTP method to POST with the dropdown selector on the left of the URL input field.
  3. In the URL field enter the address to the authenticate route of your local API - http://localhost:4000/users/authenticate.
  4. Select the Body tab below the URL field, change the body type radio button to raw, and change the format dropdown selector to JSON.
  5. Enter a JSON object containing the test username and password in the Body textarea: 
    {
    "username": "test",
    "password": "test"
}
  6. Click the Send button, you should receive a "200 OK" response with the user details and a JWT token in the response body, make a copy of the token value because we'll be using it in the next step to make an authenticated request.

Here's a screenshot of Postman after the request is sent and the user has been authenticated:


How to make an authenticated request to retrieve all users

To make an authenticated request using the JWT token from the previous step, follow these steps:

  1. Open a new request tab by clicking the plus (+) button at the end of the tabs.
  2. Change the HTTP method to GET with the dropdown selector on the left of the URL input field.
  3. In the URL field enter the address to the users route of your local API - http://localhost:4000/users.
  4. Select the Authorization tab below the URL field, set the Type selector to Bearer Token, and paste the JWT token from the previous authenticate step into the Token field.
  5. Click the Send button, you should receive a "200 OK" response containing a JSON array with all the user records in the system (just the one test user in the example).

Here's a screenshot of Postman after making an authenticated request to get all users:

 


Run an Angular client app with the .NET JWT Auth API

For full details about the example Angular application see the post Angular 10 - JWT Authentication Example & Tutorial. But to get up and running quickly just follow the below steps.

  1. Install Node.js and npm from https://nodejs.org.
  2. Download or clone the Angular tutorial code from https://github.com/cornflourblue/angular-10-jwt-authentication-example
  3. Install all required npm packages by running npm install or npm i from the command line in the project root folder (where the package.json is located).
  4. Remove or comment out the line below the comment // provider used to create fake backend located in the /src/app/app.module.ts file.
  5. Start the application by running npm start from the command line in the project root folder, this will launch a browser displaying the Angular example application and it should be hooked up with the .NET 6.0 JWT Auth API that you already have running.


Run a Blazor WebAssembly (WASM) client app with the .NET JWT Auth API

For full details about the example Blazor application see the post Blazor WebAssembly - JWT Authentication Example & Tutorial. But to get up and running quickly just follow the below steps.

  1. Download or clone the tutorial project code from https://github.com/cornflourblue/blazor-webassembly-jwt-authentication-example
  2. Change the "fakeBackend" setting to "false" in the /wwwroot/appsettings.json file.
  3. Start the app by running dotnet run from the command line in the project root folder (where the BlazorApp.csproj file is located)
  4. Open a new browser tab and navigate to the URL http://localhost:5000, the Blazor app should be hooked up with the .NET 6.0 JWT Auth API that you already have running.

NOTE: To enable hot reloading during development so the app automatically restarts when a file is changed, start the app with the command dotnet watch run.


Run a React client app with the .NET JWT Auth API

For full details about the example React application see the post React + Recoil - JWT Authentication Tutorial & Example. But to get up and running quickly just follow the below steps.

  1. Install Node.js and npm from https://nodejs.org.
  2. Download or clone the React tutorial code from https://github.com/cornflourblue/react-recoil-jwt-authentication-example
  3. Install all required npm packages by running npm install from the command line in the project root folder (where the package.json is located).
  4. Remove or comment out the 2 lines below the comment // setup fake backend located in the /src/index.jsx file.
  5. Start the application by running npm start from the command line in the project root folder, this will launch a browser displaying the React example application and it should be hooked up with the .NET 6.0 JWT Auth API that you already have running.


Run a Vue.js client app with the .NET JWT Auth API

For full details about the example VueJS JWT application see the post Vue.js + Vuex - JWT Authentication Tutorial & Example. But to get up and running quickly just follow the below steps.

  1. Install Node.js and npm from https://nodejs.org.
  2. Download or clone the VueJS tutorial code from https://github.com/cornflourblue/vue-vuex-jwt-authentication-example
  3. Install all required npm packages by running npm install from the command line in the project root folder (where the package.json is located).
  4. Remove or comment out the 2 lines below the comment // setup fake backend located in the /src/index.js file.
  5. Start the application by running npm start from the command line in the project root folder, this will launch a browser displaying the VueJS example application and it should be hooked up with the .NET 6.0 JWT Auth API that you already have running.
 

.NET 6.0 JWT Authentication API Project Structure

The tutorial project is organised into the following folders:
Controllers - define the end points / routes for the web api, controllers are the entry point into the web api from client applications via http requests.
Models - represent request and response models for controller methods, request models define the parameters for incoming requests, and response models can be used to define what data is returned.
Services - contain business logic, validation and data access code.
Entities - represent the application data.
Helpers - anything that doesn't fit into the above folders.

Click any of the below links to jump down to a description of each file along with its code:

 

.NET JWT Users Controller

Path: /Controllers/UsersController.cs

The .NET users controller defines and handles all routes / endpoints for the api that relate to users, this includes authentication and standard CRUD operations. Within each route the controller calls the user service to perform the action required which keeps the controller 'lean' and completely separated from the business logic and data access code.

Routes restricted to authenticated users are decorated with the [Authorize] attribute. The auth logic is implemented in the custom authorize attribute.

namespace WebApi.Controllers;

using Microsoft.AspNetCore.Mvc;
using WebApi.Helpers;
using WebApi.Models;
using WebApi.Services;

[ApiController]
[Route("[controller]")]
public class UsersController : ControllerBase
{
    private IUserService _userService;

    public UsersController(IUserService userService)
    {
        _userService = userService;
    }

    [HttpPost("authenticate")]
    public IActionResult Authenticate(AuthenticateRequest model)
    {
        var response = _userService.Authenticate(model);

        if (response == null)
            return BadRequest(new { message = "Username or password is incorrect" });

        return Ok(response);
    }

    [Authorize]
    [HttpGet]
    public IActionResult GetAll()
    {
        var users = _userService.GetAll();
        return Ok(users);
    }
}
 

.NET JWT User Entity

Path: /Entities/User.cs

The user entity class represents the data for a user in the application. Entity classes are used to pass data between different parts of the application (e.g. between services and controllers) and can be used to return http response data from controller action methods. If multiple types of entities or other custom data is required to be returned from a controller method then a custom model class should be created in the Models folder for the response.

The [JsonIgnore] attribute prevents the password property from being serialized and returned in api responses.

namespace WebApi.Entities;

using System.Text.Json.Serialization;

public class User
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Username { get; set; }

    [JsonIgnore]
    public string Password { get; set; }
}
 

.NET JWT App Settings

Path: /Helpers/AppSettings.cs

The app settings class contains properties defined in the appsettings.json file and is used for accessing application settings via objects that are injected into classes using the .NET built in dependency injection (DI) system. For example the user service accesses app settings via an IOptions<AppSettings> appSettings object that is injected into the constructor.

Mapping of configuration sections to classes is done in the Program.cs file.

namespace WebApi.Helpers;

public class AppSettings
{
    public string Secret { get; set; }
}
 

.NET Custom Authorize Attribute

Path: /Helpers/AuthorizeAttribute.cs

The custom authorize attribute is added to controller action methods that require the user to be authenticated.

Authorization is performed by the OnAuthorization method which checks if there is an authenticated user attached to the current request (context.HttpContext.Items["User"]). An authenticated user is attached by the custom jwt middleware if the request contains a valid JWT access token.

On successful authorization no action is taken and the request is passed through to the controller action method, if authorization fails a 401 Unauthorized response is returned.

namespace WebApi.Helpers;

using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;
using WebApi.Entities;

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]
public class AuthorizeAttribute : Attribute, IAuthorizationFilter
{
    public void OnAuthorization(AuthorizationFilterContext context)
    {
        var user = (User)context.HttpContext.Items["User"];
        if (user == null)
        {
            // not logged in
            context.Result = new JsonResult(new { message = "Unauthorized" }) { StatusCode = StatusCodes.Status401Unauthorized };
        }
    }
}
 

.NET Custom JWT Middleware

Path: /Helpers/JwtMiddleware.cs

The custom JWT middleware checks if there is a token in the request Authorization header, and if so attempts to:

  1. Validate the token
  2. Extract the user id from the token
  3. Attach the authenticated user to the current HttpContext.Items collection to make it accessible within the scope of the current request

If there is no token in the request header or if any of the above steps fail then no user is attached to the http context and the request is only be able to access public routes. Authorization is performed by the custom authorize attribute which checks that a user is attached to the http context, if authorization fails a 401 Unauthorized response is returned.

namespace WebApi.Helpers;

using Microsoft.Extensions.Options;
using Microsoft.IdentityModel.Tokens;
using System.IdentityModel.Tokens.Jwt;
using System.Text;
using WebApi.Services;

public class JwtMiddleware
{
    private readonly RequestDelegate _next;
    private readonly AppSettings _appSettings;

    public JwtMiddleware(RequestDelegate next, IOptions<AppSettings> appSettings)
    {
        _next = next;
        _appSettings = appSettings.Value;
    }

    public async Task Invoke(HttpContext context, IUserService userService)
    {
        var token = context.Request.Headers["Authorization"].FirstOrDefault()?.Split(" ").Last();

        if (token != null)
            attachUserToContext(context, userService, token);

        await _next(context);
    }

    private void attachUserToContext(HttpContext context, IUserService userService, string token)
    {
        try
        {
            var tokenHandler = new JwtSecurityTokenHandler();
            var key = Encoding.ASCII.GetBytes(_appSettings.Secret);
            tokenHandler.ValidateToken(token, new TokenValidationParameters
            {
                ValidateIssuerSigningKey = true,
                IssuerSigningKey = new SymmetricSecurityKey(key),
                ValidateIssuer = false,
                ValidateAudience = false,
                // set clockskew to zero so tokens expire exactly at token expiration time (instead of 5 minutes later)
                ClockSkew = TimeSpan.Zero
            }, out SecurityToken validatedToken);

            var jwtToken = (JwtSecurityToken)validatedToken;
            var userId = int.Parse(jwtToken.Claims.First(x => x.Type == "id").Value);

            // attach user to context on successful jwt validation
            context.Items["User"] = userService.GetById(userId);
        }
        catch
        {
            // do nothing if jwt validation fails
            // user is not attached to context so request won't have access to secure routes
        }
    }
}
 

.NET JWT Authenticate Request Model

Path: /Models/AuthenticateRequest.cs

The authenticate request model defines the parameters for incoming requests to the /users/authenticate route, it is attached to the route as the parameter to the Authenticate action method of the users controller. When an HTTP POST request is received by the route, the data from the body is bound to an instance of the AuthenticateRequest class, validated and passed to the method.

.NET Data Annotations are used to automatically handle model validation, the [Required] attribute sets both the username and password as required fields so if either are missing a validation error message is returned from the api.

namespace WebApi.Models;

using System.ComponentModel.DataAnnotations;

public class AuthenticateRequest
{
    [Required]
    public string Username { get; set; }

    [Required]
    public string Password { get; set; }
}
 

.NET JWT Authenticate Response Model

Path: /Models/AuthenticateResponse.cs

The authenticate response model defines the data returned after successful authentication, it includes basic user details and a JWT access token.

namespace WebApi.Models;

using WebApi.Entities;

public class AuthenticateResponse
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Username { get; set; }
    public string Token { get; set; }


    public AuthenticateResponse(User user, string token)
    {
        Id = user.Id;
        FirstName = user.FirstName;
        LastName = user.LastName;
        Username = user.Username;
        Token = token;
    }
}
 

.NET JWT User Service

Path: /Services/UserService.cs

The user service contains methods for authenticating user credentials and returning a JWT token, getting all users in the application and getting a single user by id.

I hardcoded the array of users in the example to keep it focused on JWT authentication, in a production application it is recommended to store user records in a database with hashed passwords. For an extended example that includes support for user registration and stores data with Entity Framework check out .NET 6.0 - User Registration and Login Tutorial with Example API.

The top of the file contains an interface that defines the user service, below that is the concrete user service class that implements the interface.

On successful authentication the Authenticate() method generates a JWT (JSON Web Token) using the JwtSecurityTokenHandler class which generates a token that is digitally signed using a secret key stored in appsettings.json. The JWT token is returned to the client application which must include it in the HTTP Authorization header of subsequent requests to secure routes.

namespace WebApi.Services;

using Microsoft.Extensions.Options;
using Microsoft.IdentityModel.Tokens;
using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using System.Text;
using WebApi.Entities;
using WebApi.Helpers;
using WebApi.Models;

public interface IUserService
{
    AuthenticateResponse Authenticate(AuthenticateRequest model);
    IEnumerable<User> GetAll();
    User GetById(int id);
}

public class UserService : IUserService
{
    // users hardcoded for simplicity, store in a db with hashed passwords in production applications
    private List<User> _users = new List<User>
    {
        new User { Id = 1, FirstName = "Test", LastName = "User", Username = "test", Password = "test" }
    };

    private readonly AppSettings _appSettings;

    public UserService(IOptions<AppSettings> appSettings)
    {
        _appSettings = appSettings.Value;
    }

    public AuthenticateResponse Authenticate(AuthenticateRequest model)
    {
        var user = _users.SingleOrDefault(x => x.Username == model.Username && x.Password == model.Password);

        // return null if user not found
        if (user == null) return null;

        // authentication successful so generate jwt token
        var token = generateJwtToken(user);

        return new AuthenticateResponse(user, token);
    }

    public IEnumerable<User> GetAll()
    {
        return _users;
    }

    public User GetById(int id)
    {
        return _users.FirstOrDefault(x => x.Id == id);
    }

    // helper methods

    private string generateJwtToken(User user)
    {
        // generate token that is valid for 7 days
        var tokenHandler = new JwtSecurityTokenHandler();
        var key = Encoding.ASCII.GetBytes(_appSettings.Secret);
        var tokenDescriptor = new SecurityTokenDescriptor
        {
            Subject = new ClaimsIdentity(new[] { new Claim("id", user.Id.ToString()) }),
            Expires = DateTime.UtcNow.AddDays(7),
            SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
        };
        var token = tokenHandler.CreateToken(tokenDescriptor);
        return tokenHandler.WriteToken(token);
    }
}
 

.NET JWT App Settings

Path: /appsettings.json

Root configuration file containing application settings for all environments.

IMPORTANT: The "Secret" property is used by the api to sign and verify JWT tokens for authentication, update it with your own random string to ensure nobody else can generate a JWT to gain unauthorised access to your application.

{
    "AppSettings": {
        "Secret": "THIS IS USED TO SIGN AND VERIFY JWT TOKENS, REPLACE IT WITH YOUR OWN SECRET, IT CAN BE ANY STRING"
    },
    "Logging": {
        "LogLevel": {
            "Default": "Information",
            "Microsoft.AspNetCore": "Warning"
        }
    }
}
 

OmniSharp Config

Path: /omnisharp.json

This file contains configuration options for the C# extension in VS Code. The useBundledOnly option tells the C# extension to use the bundled version of MSBuild instead of the global version to prevent errors if you have an older version of MSBuild installed globally (e.g. as part of Visual Studio).

{
    "msbuild": {
        "useBundledOnly": true
    }
}
 

.NET JWT Program

Path: /Program.cs

The .NET 6 Program file contains top-level statements which are converted by the new C# 10 compiler into a Main() method and class for the .NET program. The Main() method is the entry point for a .NET application, when an app is started it searches for the Main() method to begin execution. The top-level statements can be located anywhere in the project but are typically placed in the Program.cs file, only one file can contain top-level statements within a .NET application.

The WebApplication class handles app startup, lifetime management, web server configuration and more. A WebApplicationBuilder is first created by calling the static method WebApplication.CreateBuilder(args), the builder is used to configure services for dependency injection (DI), a WebApplication instance is created by calling builder.Build(), the app instance is used to configure the HTTP request pipeline (middleware), then the app is started by calling app.Run().

I wrapped the add services... and configure HTTP... sections in curly brackets {} to group them together visually, the brackets are completely optional.

Internally the WebApplicationBuilder class calls the ConfigureWebHostDefaults() extension method which configures hosting for the web app including setting Kestrel as the web server, adding host filtering middleware and enabling IIS integration. For more info on the default builder settings see https://docs.microsoft.com/aspnet/core/fundamentals/host/generic-host#default-builder-settings.

using WebApi.Helpers;
using WebApi.Services;

var builder = WebApplication.CreateBuilder(args);

// add services to DI container
{
    var services = builder.Services;
    services.AddCors();
    services.AddControllers();

    // configure strongly typed settings object
    services.Configure<AppSettings>(builder.Configuration.GetSection("AppSettings"));

    // configure DI for application services
    services.AddScoped<IUserService, UserService>();
}

var app = builder.Build();

// configure HTTP request pipeline
{
    // global cors policy
    app.UseCors(x => x
        .AllowAnyOrigin()
        .AllowAnyMethod()
        .AllowAnyHeader());

    // custom jwt auth middleware
    app.UseMiddleware<JwtMiddleware>();

    app.MapControllers();
}

app.Run("http://localhost:4000");
 

.NET JWT Web Api csproj

Path: /WebApi.csproj

The csproj (C# project) is an MSBuild based file that contains target framework and NuGet package dependency information for the application. The ImplicitUsings feature is enabled which tells the compiler to auto generate a set of global using directives based on the project type, removing the need to include a lot of common using statements. The global using statements are auto generated when you build the project and can be found in the file /obj/Debug/net6.0/WebApi.GlobalUsings.g.cs.

For more info on the C# project file see .NET + MSBuild - C# Project File (.csproj) in a Nutshell.

<Project Sdk="Microsoft.NET.Sdk.Web">
    <PropertyGroup>
        <TargetFramework>net6.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
    </PropertyGroup>
    <ItemGroup>
        <PackageReference Include="Microsoft.AspNetCore.Authentication.JwtBearer" Version="6.0.0" />
        <PackageReference Include="System.IdentityModel.Tokens.Jwt" Version="6.15.0" />
    </ItemGroup>
</Project>

 


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