Content Security Policy (CSP) Builder
Build, test, and validate a Content Security Policy from scratch to protect against XSS attacks.
Overview
Content Security Policy is the most effective browser-level defense against XSS attacks. Building a correct CSP requires understanding your application's resource origins and testing the policy without breaking functionality. This workflow guides you through the full process.
Step-by-Step Implementation
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Quick Summary
Build a Content Security Policy that blocks XSS without breaking your app: declare each resource origin in the builder, deploy in `Content-Security-Policy-Report-Only` mode first, watch for violation reports, then promote to enforcing. CSP is the strongest browser-level XSS defense available.
Key Takeaways
- Always deploy CSP in `Report-Only` mode first, enforcing mode out of the gate will break inline scripts you forgot about.
- `script-src 'self'` blocks inline `<script>` and `onclick` handlers, refactor or use nonces/hashes.
- Avoid `'unsafe-inline'` and `'unsafe-eval'`, they negate most of CSP's XSS protection.
- Use a strict `default-src 'self'` baseline and only loosen specific directives (e.g., `img-src` for a CDN).
- `upgrade-insecure-requests` automatically rewrites HTTP subresources to HTTPS, easy mixed-content fix.
When to use it
- Adding XSS hardening to a legacy app where input sanitization is unreliable.
- Meeting compliance requirements (PCI-DSS 4.0 mandates CSP for payment pages).
- Detecting third-party tag injection or malicious browser-extension behavior via CSP reports.
- Restricting frame-ancestors to prevent clickjacking on sensitive pages.
Common Mistakes
- Using `unsafe-inline` 'just to ship', once it's in production, removing it breaks the site.
- Forgetting that `*.example.com` doesn't cover `example.com` itself, list both if needed.
- Setting CSP via `<meta>` tag for directives that only work as HTTP headers (`frame-ancestors`, `sandbox`, `report-uri`).
- Not setting a `report-uri` or `report-to` endpoint, you'll never know what's being blocked in production.
Content Security Policy (CSP) Builder, Frequently Asked
How do I handle inline scripts I can't remove?
Add a per-request nonce: `script-src 'nonce-xyz123'` and set `<script nonce='xyz123'>...`. The nonce must be unique per response and cryptographically random.
What's the difference between CSP Level 2 and Level 3?
Level 3 adds `strict-dynamic`, `worker-src`, hash-based whitelisting for inline scripts, and improved nonce semantics. Use Level 3 directives where supported with Level 2 fallbacks.
Will CSP slow down my page?
Negligibly. Modern browsers parse CSP once per response and apply it during normal resource loading. The cost is in your build pipeline (generating hashes/nonces), not runtime.