🔐 Access Token Pattern in .NET Core

🔐 Access Token Pattern in .NET Core

The Access Token pattern is a security mechanism used in modern web applications and APIs, including those built with .NET Core. It provides a stateless and scalable way to handle authentication and authorization. An access token is a credential that a client (such as a user or another application) presents to an API to prove its identity and permission to access a protected resource.

Instead of using stateful, server-side sessions, this pattern relies on a token issued by an identity provider upon successful authentication. The client then sends this token with every subsequent request. The API validates the token to grant or deny access without needing to store session information on its own server. JSON Web Tokens (JWTs) are the most common format for access tokens.

⚙️ How the access token pattern works in .NET Core

• 🔑 Authentication request: The client sends a request with user credentials (e.g., username and password) to an authentication endpoint. This endpoint is typically managed by a secure token service, like IdentityServer, or your own .NET Core-based identity provider.
• 🪙 Token issuance: The identity provider validates the credentials. If valid, it generates and signs an access token (and often a refresh token) and sends it back to the client.
• 📦 Token storage: The client stores the token securely. In a web application, this is typically done using an HTTP-only cookie, whereas single-page applications (SPAs) often use browser storage.
• 🔐 Resource access: For every protected resource, the client includes the access token in the Authorization request header, formatted as Bearer <token>.
• 🧪 Token validation: The .NET Core API's built-in JWT bearer authentication middleware validates the incoming token's signature, expiry, and claims. If the token is valid, the request is processed; otherwise, a 401 Unauthorized or 403 Forbidden response is returned.

🧪 Example: Implementing JWT bearer authentication in .NET Core

1️⃣ Install NuGet package

Install the Microsoft.AspNetCore.Authentication.JwtBearer package in your Web API project.

//shell
dotnet add package Microsoft.AspNetCore.Authentication.JwtBearer

2️⃣ Configure JWT authentication in Program.cs

This code configures the application to validate incoming JWTs. It defines the validation parameters, including the secret key, issuer, and audience, typically loaded from appsettings.json.

This setup configures JWT bearer authentication with specific token validation parameters.

3️⃣ Protect API endpoints

Apply the [Authorize] attribute to secure controllers or actions. For example, applying [Authorize] to a controller means all actions within that controller require a valid token.

✅ Advantages

• 📡 The access token pattern offers advantages such as being stateless and scalable, making it suitable for microservices and horizontal scaling.
• 🔗 It also provides decoupled authentication, allowing a central identity provider to be used by multiple APIs.
• 🌐 Cross-platform interoperability due to its open standard nature.
• 📉 Reduced database load as validation information is in the token.
• 🔒 Increased security with short-lived tokens and signed keys.

⚠️ Disadvantages

• 🚫 Tokens are not easily revoked, and logout often just involves deleting the token from the client.
• 🕵️‍♂️ Stolen tokens pose a risk if intercepted before expiry, emphasizing the need for short lifetimes and refresh token mechanisms.
• 🧩 Implementing refresh token flows can add complexity.
• 📦 JWTs can also increase payload size.
• 🧪 Debugging can be challenging due to their binary nature.

📌 When to use

• 🧱 This pattern is well-suited for microservices and distributed systems with a central authentication server.
• 📱 Mobile applications and SPAs where cookie-based sessions are less ideal.
• 🌍 Public-facing APIs for standardized authentication.

🚫 When not to use

• 🏛️ It may not be the best choice for traditional monolithic web applications where cookie-based authentication is simpler.
• 💳 Applications requiring strict real-time consistency for immediate revocation, such as high-security financial transactions.

🛡️ Precautions and best practices

• ⏱️ Use short-lived access tokens to limit damage from leaks.
• 🔐 Securely handle refresh tokens, ideally storing them on the server side.
• 🌐 Always use HTTPS for token-based communication.
• 🙈 Avoid putting sensitive data in token payloads.
• 📜 Adhere to OAuth 2.0 and OIDC standards, potentially using frameworks like IdentityServer.
• 🔑 Ensure a secure signing key is used and validated.
• 🧪 Validate all token parameters on the API side, including issuer, audience, and lifetime.