Free CSP Header Scanner for Next.js App: Find & Fix Issues

Quick answer

A free CSP header scanner for Next.js app checks your deployed site's HTTP response headers to see if a Content-Security-Policy is present and correctly configured. Tools like OverMCP's security headers checker can scan your Next.js app in seconds, flag missing CSP, and highlight common misconfigurations like unsafe-inline or missing directives. For AI-built apps, where security is often skipped, running a free CSP header scanner before launch is a quick win to protect against XSS and data injection.

Why CSP matters for Next.js apps (especially AI-built ones)

Content-Security-Policy is a browser security mechanism that restricts which resources (scripts, styles, images) your app can load. A strong CSP blocks malicious scripts injected via XSS attacks, which are a top vulnerability in AI-generated code. Many AI coding tools generate inline scripts or use dangerouslySetInnerHTML, making CSP even more critical. Without a CSP, your Next.js app is exposed to script injection and data theft.

For Next.js apps, especially those built with Cursor, Bolt.new, or v0, CSP is often missing because AI doesn't prioritize security headers. A free CSP header scanner is the easiest way to check if your app has one.

What to check first

Before you fix anything, run a free CSP header scanner on your Next.js app. Here's a checklist to guide you:

Run these checks with a free CSP header scanner to get a baseline.

Step-by-step fix

Once you've scanned your Next.js app and identified missing or weak CSP, follow these steps to add or fix it.

1. Add CSP via `next.config.js`

The easiest way to set CSP headers in Next.js is using the headers() function in next.config.js. Here's a basic example:

// next.config.js
module.exports = {
  async headers() {
    return [
      {
        source: '/(.*)',
        headers: [
          {
            key: 'Content-Security-Policy',
            value: "default-src 'self'; script-src 'self' 'nonce-abc123'; style-src 'self' 'unsafe-inline'; img-src 'self' data:; object-src 'none'; upgrade-insecure-requests"
          }
        ]
      }
    ]
  }
}

Note: The 'nonce-abc123' is a placeholder. For production, you need a dynamic nonce. Next.js supports nonce generation via middleware or server-side rendering.

2. Use a nonce for inline scripts

AI-generated code often includes inline scripts. To allow them securely, generate a nonce per request and add it to the CSP and the script tag. Here's a simplified middleware approach:

// middleware.js
import { NextResponse } from 'next/server'

export function middleware(req) {
  const nonce = crypto.randomBytes(16).toString('base64')
  const csp = `default-src 'self'; script-src 'self' 'nonce-${nonce}'; style-src 'self' 'unsafe-inline';`
  const res = NextResponse.next()
  res.headers.set('Content-Security-Policy', csp)
  return res
}

Then, in your components, use the nonce on script tags:

<script nonce={nonce} src="/script.js" />

3. Verify with a scanner

After deploying, run a free CSP header scanner again to confirm the header is correct. Use OverMCP's security headers checker to see if your CSP passes common checks.

4. Test in report-only mode first

To avoid breaking your app, start with Content-Security-Policy-Report-Only and monitor violations. Then switch to enforcing once you're confident.

Common mistakes

AI-built Next.js apps often have these CSP pitfalls:

How OverMCP helps

Running a free CSP header scanner on your Next.js app is the first step. OverMCP's security headers checker not only checks for CSP but also gives you a grade. For continuous monitoring, you can connect your Vercel deployment to get alerts when headers change. This is especially useful for vibe-coded apps that update frequently.

FAQ

What is a CSP header scanner?

A CSP header scanner is a tool that analyzes your website's HTTP response headers to check if a Content-Security-Policy is present and correctly configured. It flags missing or weak policies that could leave your site vulnerable to XSS attacks.

Can I use a free CSP header scanner for Next.js apps on localhost?

Yes, but most scanners require a publicly accessible URL. For localhost, you can use browser extensions or run a local proxy. For production, a free online scanner works best.

How do I fix CSP issues in a Next.js app built with AI?

Start by scanning with a free tool to see what's missing. Then add CSP via next.config.js or middleware, using nonces for inline scripts. Test in report-only mode before enforcing, and re-scan to verify.

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Note: Always test your CSP thoroughly to avoid breaking functionality. Use a free CSP header scanner regularly as part of your [continuous security monitoring](https://www.overmcp.com/monitor) routine.