How to add Content-Security-Policy on WordPress

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 WordPress configuration?

Add this to functions.php (child theme) or a headers plugin:

<?php
// Child theme functions.php (or a small mu-plugin)
add_action('send_headers', function () {
    header('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');
});

How do I add it step by step?

1. Open functions.php (child theme) or a headers plugin.
2. Paste the Content-Security-Policy configuration shown below.
3. Clear page caches (plugin + CDN) and curl -I your homepage.
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?

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

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.
• Prefer setting headers at the web server (nginx/Apache) — PHP headers don't apply to static files.
• Page-cache plugins may serve cached responses without your headers until purged.

Quick reference

FAQ

Related guides

• Content-Security-Policy explained
• Content-Security-Policy on Nginx
• Content-Security-Policy on Apache
• Content-Security-Policy on IIS
• Content-Security-Policy on Cloudflare
• Strict-Transport-Security on WordPress
• X-Content-Type-Options on WordPress
• X-Frame-Options on WordPress
• Referrer-Policy on WordPress