Next.js Security

Priority: P0 (CRITICAL)

Workflow: Secure a Next.js App

1. Add auth middleware — Create middleware.ts to verify JWT/session on protected routes.
2. Validate Server Actions — Parse all inputs with Zod schemas; call await auth() first.
3. Set security headers — Add CSP, HSTS, X-Frame-Options in middleware response.
4. Use server-only — Import in modules containing secrets to prevent client bundling.
5. Taint sensitive objects — Use taintObjectReference to block server objects from reaching client.

Secure Server Action Example

See implementation examples

Implementation Guidelines

• Next.js Middleware: Use middleware.ts for edge-side authentication, role-based access control (RBAC), and enforcing Security Headers (e.g., Content-Security-Policy (CSP), X-XSS-Protection).
• Server Actions: Always sanitize all inputs from FormData or JSON using Zod. Perform authentication checks (await auth()) inside every action to verify the caller.
• Data Tainting: Use the experimental_taint API (taintObjectReference) to ensure sensitive server objects (e.g., User with passwordHash) never leak into a Client Component.
• Route Handlers (route.ts): Implement rate limiting to prevent brute-force or DoS attacks. Verify Origin/Referer headers to mitigate CSRF (Cross-Site Request Forgery).
• Auth Tokens: strictly use HttpOnly, Secure cookies with SameSite: 'Lax' for session management. Never store tokens in localStorage.
• Logic Isolation: use the server-only package to prevent backend-specific logic from being included in the client bundle.
• Component Purity: Escape all user-provided content rendered in components. Never use dangerouslySetInnerHTML without a sanitizer like DOMPurify.

Anti-Patterns

• No leaking DB fields to client: Use DTOs; never pass raw model objects.
• No process.env in client bundles: Mark as NEXT_PUBLIC_ only if safe to expose.
• No unvalidated Server Action inputs: Always validate with Zod schema.
• No auth checks in shared Layouts: Auth in layouts is insecure; use Middleware.

References

• Secure App Router Patterns