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How to add Content-Security-Policy on Next.js

Last updated · Reviewed by a CISSP-certified security practitioner

To add Content-Security-Policy on Next.js, set the header in next.config.js with the value default-src 'self'; script-src 'self'; style-src 'self'; …. Controls which scripts, styles and frames a page may load. Restart dev server or redeploy. Then confirm it with curl -I or a free FixSecurityHeaders scan.

What does Content-Security-Policy do?

CSP is an allow-list for where your page may load scripts, styles, images, frames and connections from. It is the strongest defence against cross-site scripting.

Risk if missing: Without a CSP, any injected script (XSS, a compromised third‑party tag, a Magecart skimmer) runs with full access to your users' sessions and payment forms.

What is the Next.js configuration?

Add this to next.config.js:

next.config.js
// next.config.js
module.exports = {
  poweredByHeader: false,
  async headers() {
    return [
      {
        source: "/(.*)",
        headers: [
          { key: "Content-Security-Policy", value: "default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' data: https:; object-src 'none'; base-uri 'self'; frame-ancestors 'none'; form-action 'self'; upgrade-insecure-requests" },
        ],
      },
    ];
  },
};

How do I add it step by step?

  1. Open next.config.js.
  2. Paste the Content-Security-Policy configuration shown below.
  3. Restart dev server or redeploy.
  4. Run curl -I https://yourdomain.com and look for the content-security-policy line.
  5. Scan your site on FixSecurityHeaders to confirm the grade improved.

How do I verify it worked?

terminal
curl -sI https://yourdomain.com | grep -i "content-security-policy"
# expected:
# content-security-policy: default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' data: https:; object-src 'none'; base-uri 'self'; frame-ancestors 'none'; form-action 'self'; upgrade-insecure-requests

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What are the common mistakes?

  • Deploy as Content-Security-Policy-Report-Only first and watch reports before enforcing.
  • 'unsafe-inline' and 'unsafe-eval' in script-src remove most of the XSS protection. Use nonces or hashes.
  • Analytics, chat widgets and payment iframes need their hosts added explicitly.
  • Only one CSP header should be sent; multiple policies are intersected, which often blocks more than expected.
  • For nonce-based CSP use middleware.ts instead of static headers.
  • poweredByHeader: false removes X-Powered-By.

Quick reference

HeaderContent-Security-Policy
Recommended valuedefault-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' data: https:; object-src 'none'; base-uri 'self'; frame-ancestors 'none'; form-action 'self'; upgrade-insecure-requests
StackNext.js
Wherenext.config.js
CompliancePCI DSS 4.0 6.4.3, PCI DSS 4.0 11.6.1, OWASP ASVS V14.4.3, SOC 2 CC6.8, NIST 800-171 3.13.1

FAQ

What value should I use for Content-Security-Policy?

A safe default is default-src 'self'; script-src 'self'; style-src 'self'; img-src 'self' data: https:; object-src 'none'; base-uri 'self'; frame-ancestors 'none'; form-action 'self'; upgrade-insecure-requests. Deploy as Content-Security-Policy-Report-Only first and watch reports before enforcing.

Why isn't Content-Security-Policy showing up on Next.js?

For nonce-based CSP use middleware.ts instead of static headers. Also clear any CDN or page cache, and check you edited the config that actually serves your domain.

Does Content-Security-Policy affect SEO or performance?

No measurable performance cost — it's a few bytes per response. It doesn't affect rankings directly, but a misconfigured policy can break page resources, so test before enforcing.

Which compliance frameworks reference Content-Security-Policy?

PCI DSS 4.0 6.4.3, PCI DSS 4.0 11.6.1, OWASP ASVS V14.4.3, SOC 2 CC6.8, NIST 800-171 3.13.1.

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