Implementing JWT Auth in Next.js 15 App Router

"Authentication is one of the most consequential engineering decisions in any web application, and it is one of the areas most frequently implemented incorrectly by developers who piece together solutions from outdated tutorials. Next.js 15's App Router introduces significant architectural changes — server components, server actions, and middleware at the edge — that fundamentally alter how authentication state is managed, validated, and propagated through the application, compared to patterns that worked in Pages Router applications. JSON Web Tokens (JWT) remain one of the most widely adopted authentication mechanisms for SaaS applications in 2026, offering stateless session management that scales horizontally without requiring a shared session store. However, the implementation details matter enormously. Storing JWTs in localStorage is widely considered insecure because it exposes the token to Cross-Site Scripting (XSS) attacks. Storing them in cookies requires careful configuration of the HttpOnly, Secure, and SameSite attributes to prevent Cross-Site Request Forgery (CSRF) and interception over insecure connections. In the Next.js 15 App Router architecture, the recommended pattern is to issue your JWT as an HttpOnly cookie from a server action or Route Handler after successful credential validation. Middleware running at the edge then intercepts every incoming request, validates the JWT signature and expiry without hitting your database, and either passes the request through or redirects to the login page. Because this validation happens at the CDN edge — not in a Lambda or container — it adds minimal latency to protected routes. This tutorial covers the complete implementation: setting up Jose for JWT signing and verification (it runs in the Edge Runtime, unlike jsonwebtoken), building the login server action, configuring Next.js middleware for route protection, implementing refresh token rotation to extend sessions without re-authentication, and handling token invalidation for logout flows. All code is TypeScript-first and compatible with the Next.js 15 stable release."