ASP.NET Core Web API: Authentication and Authorization

Introduction to Authentication and Authorization

Securing your ASP.NET Core Web API is crucial to protect your data and resources. Authentication (who are you?) and Authorization (what are you allowed to do?) are fundamental concepts in API security. ASP.NET Core provides a flexible and robust framework for implementing both.

This document explores the core concepts, common patterns, and implementation details for securing your Web APIs using ASP.NET Core.

Authentication

Authentication is the process of verifying the identity of a user or client trying to access your API. Common authentication schemes include:

• Cookie-based authentication: Typically used for browser-based applications where a cookie is sent with each request.
• Bearer token authentication (e.g., JWT): A stateless mechanism where a token is sent in the Authorization header.
• API Keys: A simple mechanism for server-to-server communication.
• OAuth 2.0 / OpenID Connect: Frameworks for delegated authorization and identity verification, often used for third-party access.

ASP.NET Core's authentication middleware allows you to configure multiple authentication handlers.

Authorization

Authorization is the process of determining whether an authenticated user has permission to perform a specific action or access a particular resource. ASP.NET Core supports several authorization models:

• Role-Based Authorization: Grants access based on predefined roles assigned to users (e.g., "Administrator", "User").
• Policy-Based Authorization: A more flexible approach that defines authorization policies based on requirements (e.g., "Must be an Administrator" or "Must have been active for more than 30 days").
• Resource-Based Authorization: Fine-grained control over access to specific instances of resources.

Implementing JWT Bearer Authentication

JSON Web Tokens (JWT) are a popular choice for securing APIs due to their stateless nature. They are self-contained and can carry information about the user and their permissions.

To use JWT bearer authentication, you'll typically:

1. Install the necessary NuGet packages: Microsoft.AspNetCore.Authentication.JwtBearer.
2. Configure the JWT Bearer authentication scheme in your Program.cs (or Startup.cs).
3. Issue JWTs from your authentication server (e.g., after a user logs in).
4. Protect your API endpoints using the [Authorize] attribute.

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
    .AddJwtBearer(options =>
    {
        options.TokenValidationParameters = new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidateAudience = true,
            ValidateLifetime = true,
            ValidateIssuerSigningKey = true,
            ValidIssuer = Configuration["Jwt:Issuer"],
            ValidAudience = Configuration["Jwt:Audience"],
            IssuerSigningKey = new SymmetricSecurityKey(Encoding.UTF8.GetBytes(Configuration["Jwt:Key"]))
        };
    });
                    

[ApiController]
[Route("api/[controller]")]
[Authorize] // Requires authentication
public class ProtectedController : ControllerBase
{
    [HttpGet]
    public IActionResult GetProtectedData()
    {
        return Ok("This is protected data.");
    }
}
                    

OAuth 2.0 and OpenID Connect

OAuth 2.0 is an authorization framework that enables applications to obtain limited access to user accounts on an HTTP service, such as Facebook or Google, without exposing the user's password. OpenID Connect (OIDC) is an identity layer built on top of OAuth 2.0, providing authentication capabilities.

ASP.NET Core supports integration with identity providers that implement OAuth 2.0 and OIDC. You can add external authentication providers like Google, Facebook, Microsoft Identity, or custom OIDC providers.

services.AddAuthentication().AddGoogle(googleOptions =>
{
    googleOptions.ClientId = Configuration["Authentication:Google:ClientId"];
    googleOptions.ClientSecret = Configuration["Authentication:Google:ClientSecret"];
});
                    

Policy-Based Authorization

Policy-based authorization offers a more granular and descriptive way to manage permissions. You define policies, which are sets of requirements that a user must meet to be authorized.

Defining a Policy:

services.AddAuthorization(options =>
{
    options.AddPolicy("RequireAdminRole", policy =>
        policy.RequireRole("Administrator"));

    options.AddPolicy("MinimumAgePolicy", policy =>
        policy.RequireAssertion(context =>
        {
            // Example: Check if user's age meets a minimum requirement
            var ageClaim = context.User.FindFirst(c => c.Type == "Age");
            if (ageClaim == null) return false;
            return int.Parse(ageClaim.Value) >= 18;
        }));
});
                    

Applying a Policy:

[ApiController]
[Route("api/[controller]")]
[Authorize(Policy = "RequireAdminRole")] // Apply the policy
public class AdminController : ControllerBase
{
    [HttpGet]
    public IActionResult GetAdminStuff()
    {
        return Ok("Admin access granted.");
    }
}
                    

Role-Based Authorization

Role-based authorization is a simpler form of authorization where users are assigned to roles, and access is granted based on these roles.

Enabling Role-Based Authorization:

Ensure your authentication scheme supports roles (e.g., when creating JWTs, include a role claim).

Applying Role-Based Authorization:

[ApiController]
[Route("api/[controller]")]
[Authorize(Roles = "Manager,Supervisor")] // Requires user to be in either role
public class ManagementController : ControllerBase
{
    [HttpGet]
    public IActionResult GetManagementInfo()
    {
        return Ok("Management information.");
    }
}
                    

Custom Authentication and Authorization

For more complex scenarios or specialized requirements, ASP.NET Core allows you to implement custom authentication and authorization handlers.

• Custom Authentication: You can write your own IAuthenticationHandler to implement unique authentication logic.
• Custom Authorization: Implement IAuthorizationHandler to create custom authorization logic that can be evaluated as part of a policy.

This is typically done by registering your custom services in the dependency injection container and configuring them in the authentication/authorization setup.

Best Practices for API Security

• Use HTTPS: Always use TLS/SSL to encrypt communication between clients and your API.
• Validate and Sanitize Input: Protect against injection attacks by validating all incoming data.
• Least Privilege: Grant only the necessary permissions to users and services.
• Secure Secrets: Store sensitive information (API keys, database credentials, JWT signing keys) securely, not directly in code or configuration files. Use environment variables, Azure Key Vault, or other secure secret management solutions.
• Regularly Update Dependencies: Keep your ASP.NET Core framework and NuGet packages up to date to patch security vulnerabilities.
• Implement Rate Limiting: Protect your API from abuse and denial-of-service attacks.
• Log and Monitor: Implement robust logging to track authentication and authorization events for auditing and incident response.