Next.js Security Guide for Production Apps

Next.js applications have a unique security surface because they run code on both the server and the client. Server Components, API Routes, Server Actions, and Middleware all execute on the server where they can access databases and secrets directly. This power comes with responsibility, as a single mistake can expose sensitive data to the browser. Understanding the boundary between server and client code is fundamental to Next.js security.

Server Components are a powerful Next.js feature that render on the server and send HTML to the client. While they can safely access databases and secrets, you must be careful about what data you pass to Client Components as props. Never pass sensitive information like full database records or internal IDs that should not be exposed. Use Data Transfer Objects to strip sensitive fields before sending data to the client.

Every API route and Server Action is a public endpoint that anyone can call. Always authenticate and authorize requests before performing any operations. Validate all input data using a schema validation library like Zod. Implement rate limiting to prevent abuse and brute force attacks. Never trust data coming from the client, even if your frontend validation seems comprehensive, because API endpoints can be called directly.

Next.js middleware runs before every request and is perfect for implementing security headers and authentication checks. Add security headers like Content-Security-Policy, X-Frame-Options, and Strict-Transport-Security to protect against common attacks. Use middleware to verify authentication tokens before requests reach your pages or API routes. Configure your Content-Security-Policy carefully to prevent XSS while allowing legitimate scripts.

Next.js has a clear convention for environment variables. Variables prefixed with `NEXT_PUBLIC_` are exposed to the browser bundle, while unprefixed variables are only available on the server. Never prefix sensitive values with `NEXT_PUBLIC_`. Store your `.env` files outside of version control and use different values for development, staging, and production. Audit your environment variables regularly to ensure no secrets are accidentally exposed.

Cross-Site Request Forgery attacks trick authenticated users into performing unwanted actions. Next.js Server Actions include CSRF protection by default, but custom API routes need explicit protection. Implement CSRF tokens for form submissions and state-changing operations. Use SameSite cookie attributes and verify the Origin header on incoming requests. For authentication, leverage proven solutions like NextAuth.js rather than building custom authentication.

The complexity of Next.js with its server and client boundaries makes security auditing particularly important. AI-generated Next.js code often blurs these boundaries, accidentally exposing server-side logic or credentials. SpringCode performs comprehensive security audits of Next.js applications, checking server component data leakage, API route authorization, middleware configuration, and dependency vulnerabilities.