Auth0 ASP.NET Core Web API Integration

Protect ASP.NET Core Web API endpoints with JWT access token validation using Auth0.AspNetCore.Authentication.Api.

Prerequisites

• .NET 8.0 SDK or higher
• Auth0 API configured (not Application - must be API resource)
• If you don't have Auth0 set up yet, use the auth0-quickstart skill first

When NOT to Use

• Server-rendered web applications - Use session-based auth (Auth0.AspNetCore.Authentication) for MVC/Razor Pages apps
• Single Page Applications - Use auth0-react, auth0-vue, or auth0-angular for client-side auth
• Mobile applications - Use auth0-react-native for React Native/Expo
• Blazor WebAssembly - Requires different auth approach (OIDC client-side)

Quick Start Workflow

1. Install SDK

dotnet add package Auth0.AspNetCore.Authentication.Api

2. Create Auth0 API

You need an API (not Application) in Auth0.

STOP — ask the user before proceeding.

Ask exactly this question and wait for their answer before doing anything else:

"How would you like to create the Auth0 API resource?

1. Automated — I'll run Auth0 CLI scripts that create the resource and write the exact values to your appsettings.json automatically.
2. Manual — You create the API yourself in the Auth0 Dashboard (or via auth0 apis create) and provide me the Domain and Audience.

Which do you prefer? (1 = Automated / 2 = Manual)"

Do NOT proceed to any setup steps until the user has answered. Do NOT default to manual.

If the user chose Automated, follow the Setup Guide for complete CLI scripts. The automated path writes appsettings.json for you — skip Step 3 below and proceed directly to Step 4.

If the user chose Manual, follow the Setup Guide (Manual Setup section) for full instructions including User Secrets and environment variable options. Then continue with Step 3 below.

Quick reference for manual API creation:

# Using Auth0 CLI
auth0 apis create \
  --name "My ASP.NET Core API" \
  --identifier https://my-api.example.com

Or create manually in Auth0 Dashboard → Applications → APIs

3. Configure appsettings.json

{
  "Auth0": {
    "Domain": "your-tenant.auth0.com",
    "Audience": "https://my-api.example.com"
  }
}

Important: Domain must NOT include https://. The library constructs the authority URL automatically.

4. Configure Program.cs

var builder = WebApplication.CreateBuilder(args);

// Register Auth0 JWT validation
builder.Services.AddAuth0ApiAuthentication(options =>
{
    options.Domain = builder.Configuration["Auth0:Domain"];
    options.JwtBearerOptions = new JwtBearerOptions
    {
        Audience = builder.Configuration["Auth0:Audience"]
    };
});

builder.Services.AddAuthorization();

var app = builder.Build();

// Middleware order matters: authentication before authorization
app.UseAuthentication();
app.UseAuthorization();

// Add your endpoints here (see Step 5)
app.MapGet("/api/public", () => Results.Ok(new { message = "Public" }));

app.Run();

5. Protect Endpoints

Minimal API:

// Public endpoint - no authentication
app.MapGet("/api/public", () => Results.Ok(new { message = "Hello from a public endpoint!" }));

// Protected endpoint - requires valid JWT
app.MapGet("/api/private", (HttpContext ctx) =>
{
    var userId = ctx.User.FindFirst("sub")?.Value;
    return Results.Ok(new { message = "Hello from a protected endpoint!", userId });
}).RequireAuthorization();

Controller-based:

[ApiController]
[Route("api")]
public class MessagesController : ControllerBase
{
    [HttpGet("public")]
    public IActionResult Public() =>
        Ok(new { message = "Hello from a public endpoint!" });

    [Authorize]
    [HttpGet("private")]
    public IActionResult Private() =>
        Ok(new { message = "Hello from a protected endpoint!", userId = User.FindFirst("sub")?.Value });
}

6. Test API

Test public endpoint:

curl http://localhost:5000/api/public

Test protected endpoint (requires access token):

curl http://localhost:5000/api/private \
  -H "Authorization: Bearer YOUR_ACCESS_TOKEN"

Get a test token via Client Credentials flow or Auth0 Dashboard → APIs → Test tab.

Common Mistakes

Scope-Based Authorization

See Integration Guide for defining and enforcing scope policies.

DPoP Support

Built-in proof-of-possession token binding per RFC 9449. See Integration Guide for configuration.

Related Skills

• auth0-quickstart - Basic Auth0 setup
• auth0-mfa - Add Multi-Factor Authentication
• auth0-cli - Manage Auth0 resources from the terminal

Quick Reference

Configuration Options:

• options.Domain - Auth0 tenant domain, no https:// prefix (required)
• options.JwtBearerOptions.Audience - API Identifier from Auth0 API settings (required)
• options.JwtBearerOptions - Full access to underlying Microsoft JWT Bearer options

User Claims:

• ctx.User.FindFirst("sub")?.Value - User ID (subject)
• ctx.User.FindFirst("scope")?.Value - Space-separated scopes
• ctx.User.FindAll("scope") - All scope claims

Common Use Cases:

• Protect Minimal API routes → .RequireAuthorization() (see Step 5)
• Protect controller actions → [Authorize] attribute (see Step 5)
• Scope enforcement → Integration Guide
• DPoP token binding → Integration Guide
• Advanced JWT Bearer config → API Reference

Detailed Documentation

• Setup Guide - Auth0 CLI setup, environment configuration
• Integration Guide - Scope policies, DPoP, controller patterns, error handling
• API Reference - Complete configuration options and extension methods

References

• Auth0 ASP.NET Core Web API Quickstart
• SDK GitHub Repository
• API Documentation
• Access Tokens Guide