Auth0 ASP.NET Core API

Use when securing ASP.NET Core Web API endpoints with JWT Bearer token validation, scope/permission checks, or stateless auth - integrates Auth0.AspNetCore.A...

Security Scan

The skill's instructions, required inputs, and suggested commands are coherent with its stated purpose of securing ASP.NET Core Web APIs with Auth0; it doesn't ask for unrelated credentials or perform unexpected actions in the provided documentation.

Purpose & Capability

Name/description (Auth0 ASP.NET Core API) match the SKILL.md and reference docs: instructions focus on installing the Auth0 SDK, creating an Auth0 API resource, configuring Domain/Audience, adding middleware, and enforcing scopes. No unrelated services, binaries, or credentials are requested.

Instruction Scope

Runtime instructions stay on-topic: package installation, Program.cs changes, appsettings.json or environment variable configuration, and optional use of the Auth0 CLI. The skill explicitly requires asking the user before running the automated path. The automated path may run CLI commands and write values to appsettings.json (expected for setup) but does not instruct reading unrelated files or exfiltrating data.

Install Mechanism

This is an instruction-only skill (no install spec). The only installs it suggests are normal developer tooling (dotnet add package, optional Auth0 CLI). Nothing is downloaded from untrusted URLs or installed silently by the skill itself.

Credentials

No required environment variables or credentials are declared. The docs show standard options (Auth0__Domain, Auth0__Audience, and examples that mention client_id/client_secret for token retrieval), which are proportional to obtaining test tokens and configuring the SDK.

Persistence & Privilege

always:false (no forced inclusion). The skill suggests commands that may modify the project's appsettings.json if you choose the automated path, but it explicitly instructs to ask the user first. Autonomous invocation is allowed by platform default but not combined with other concerning privileges.

Assessment

This skill appears coherent and focused on configuring Auth0 for ASP.NET Core APIs. Before using the automated path, confirm you want the Auth0 CLI to log in and create API resources and to write values into your appsettings.json (review and commit/backup changes). Prefer the manual path if you want to create the API and paste Domain/Audience yourself. Never paste or share long-lived client_secrets unless needed for a specific token exchange, and verify you trust the source (check the official Auth0 docs or the NuGet package for Auth0.AspNetCore.Authentication.Api) before running CLI commands that authenticate to your tenant.

Auth0 ASP.NET Core Web API Integration

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

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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)

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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.

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Common Mistakes

Mistake	Fix
Domain includes https://	Use your-tenant.auth0.com format only - no scheme prefix
Audience doesn't match API Identifier	Must exactly match the API Identifier set in Auth0 Dashboard
Created Application instead of API in Auth0	Must create API resource in Auth0 Dashboard → Applications → APIs
Wrong middleware order	UseAuthentication() must come before UseAuthorization()
Using ID token instead of access token	Must use access token for API auth, not ID token
HTTPS certificate errors locally	Run dotnet dev-certs https --trust

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Scope-Based Authorization

See Integration Guide for defining and enforcing scope policies.

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DPoP Support

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

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Related Skills

• auth0-quickstart - Basic Auth0 setup
• auth0-mfa - Add Multi-Factor Authentication

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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

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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

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References

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