How to Add Security Headers to Next.js (Complete Guide with Code)

This guide covers every essential header, a full working next.config.js configuration, the differences between App Router and Pages Router setups, and how to verify everything is working.

Why Security Headers Matter

HTTP security headers are instructions your server sends to the browser. They tell the browser what it is and is not allowed to do with your content. Without them, browsers apply permissive defaults that attackers can exploit.

Every header in that table takes under 10 lines of code to add.

How next.config.js Headers Work

Next.js lets you define custom HTTP headers via the headers() async function in next.config.js. This function returns an array of header configuration objects, each with a source pattern (which routes get the headers) and a headers array.

The source field uses path-to-regexp syntax. Using "/(.*)" applies headers to every route including API routes.

The Seven Essential Security Headers

1. Strict-Transport-Security (HSTS)

Forces browsers to use HTTPS for all future requests to your domain, even if the user types http://.

• max-age=31536000 — remember this for one year
• includeSubDomains — apply to all subdomains
• preload — eligible for browser HSTS preload list (submit at hstspreload.org)

Warning: Do not add preload until you are certain every subdomain serves HTTPS. This directive is very hard to reverse once in the preload list.

2. Content-Security-Policy (CSP)

Controls which resources (scripts, styles, fonts, images, iframes) the browser is allowed to load. The most powerful and complex header — covered in depth in the CSP guide.

3. X-Frame-Options

Prevents your pages from being embedded in iframes on other domains, blocking clickjacking attacks.

Use DENY unless you specifically need to allow framing from the same origin (SAMEORIGIN).

Note: CSP''s frame-ancestors directive supersedes this header in modern browsers, but X-Frame-Options is still needed for legacy browser compatibility.

4. X-Content-Type-Options

Prevents browsers from MIME-sniffing a response away from the declared Content-Type.

This is a one-value header. Always set it to nosniff. Without it, a browser might execute a JavaScript file that your server declares as text/plain.

5. Referrer-Policy

Controls how much referrer information is sent when users navigate away from your site.

This is the recommended default: sends the full URL as referrer for same-origin requests, but only the origin (no path) for cross-origin requests. This prevents sensitive URL paths (like /dashboard/billing) from appearing in third-party analytics.

6. Permissions-Policy

Restricts access to browser APIs — camera, microphone, geolocation, payment, etc. This replaces the older Feature-Policy header.

The empty parentheses () explicitly disable the feature for all origins. Only grant permissions your application actually uses.

7. Cross-Origin-Opener-Policy (COOP)

Isolates your browsing context so cross-origin windows cannot access your window object.

Required for sites that use SharedArrayBuffer (for Wasm performance) and a good practice for all sites. Prevents attacks via window.opener.

Full Working next.config.js Configuration

This is a production-ready configuration you can drop into a new or existing Next.js project:

App Router vs Pages Router Differences

Pages Router (pages/)

The headers() function in next.config.js applies to all Pages Router routes. CSP with inline scripts is handled by using ''unsafe-inline'' (lower security) or by generating a nonce server-side in _document.js.

App Router (app/)

App Router makes nonce-based CSP significantly easier because you can generate and inject nonces in middleware.ts:

Then in your root layout, read the nonce and pass it to <Script> components:

This approach is more secure than ''unsafe-inline'' because each nonce is unique per request, making it impossible for injected scripts to execute.

Common Mistakes

Mistake 1: Applying headers only to HTML routes

If your source pattern excludes API routes, your API is not covered. Use "/(.*)" to cover everything.

Mistake 2: Setting HSTS before HTTPS is fully working

If you set HSTS and then your certificate expires or you try to revert to HTTP, users will be locked out of your site for the duration of max-age. Verify your HTTPS is solid before setting HSTS.

Mistake 3: CSP that breaks on first load

A CSP that is too restrictive will break your site silently in production. Before enforcing CSP, use Content-Security-Policy-Report-Only to collect violations without blocking anything:

Mistake 4: Not setting frame-ancestors in CSP

If you set X-Frame-Options: DENY but your CSP does not include frame-ancestors ''none'', modern browsers use the CSP directive and ignore X-Frame-Options. Set both for full coverage.

Mistake 5: Forgetting subdomains

Headers configured in next.config.js only apply to your Next.js application. If you have a subdomain running on different infrastructure (a Stripe-hosted payment page, a documentation site), those subdomains need their own header configuration.

How to Test Your Headers

After deploying, there are two fast ways to verify:

Option 1: Run a free ZeriFlow scan

Go to zeriflow.com/free-scan, enter your domain, and the scan will check all seven headers and return a score with specific pass/fail results for each. It takes under 60 seconds and requires no account.

Option 2: curl

Check the response headers manually. Look for all seven headers in the output.

Option 3: Browser DevTools

Open the Network tab, reload your page, click the HTML document request, and inspect the Response Headers section.

What to Expect After Adding Headers

On a typical Next.js SaaS app that had no security headers configured, adding all seven headers moves the security score from around 40–50/100 to 75–85/100 in a single deployment. The remaining gap is usually TLS configuration (controlled by your hosting provider) and cookie settings.

The whole process — adding headers, deploying, verifying — takes under 30 minutes for an experienced developer.

Summary

Next.js security headers are configured in next.config.js via the headers() function and apply to all routes matching your source pattern. The seven essential headers — HSTS, CSP, X-Frame-Options, X-Content-Type-Options, Referrer-Policy, Permissions-Policy, and COOP — each defend against a specific class of attack. App Router enables the more secure nonce-based CSP approach via middleware. Verify your deployment with a free ZeriFlow scan to confirm every header is being served correctly.