How to add Content-Security-Policy on Express / Node

Last updated November 11, 2026 · Reviewed by a CISSP-certified security practitioner

To add Content-Security-Policy on Express / Node, set the header in app.js (helmet) with the value default-src 'self'; script-src 'self'; style-src 'self'; …. Controls which scripts, styles and frames a page may load. Restart the Node process and curl -I your app. 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 Express / Node configuration?

Add this to app.js (helmet):

app.js (helmet)

// npm i helmet
const helmet = require("helmet");

app.disable("x-powered-by");
app.use(helmet()); // sensible defaults

// Exact values recommended by FixSecurityHeaders:
app.use((req, res, next) => {
    res.setHeader("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");
  next();
});

How do I add it step by step?

Open app.js (helmet).
Paste the Content-Security-Policy configuration shown below.
Restart the Node process and curl -I your app.
Run curl -I https://yourdomain.com and look for the content-security-policy line.
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

Check your site now

Free scan, instant grade, exact fixes for your stack.

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.
Install with npm i helmet.
Call app.use(helmet(...)) before your routes.

Quick reference

Header	`Content-Security-Policy`
Recommended 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`
Stack	Express / Node
Where	app.js (helmet)
Compliance	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

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 Express / Node?

Install with npm i helmet. 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.