Cross-Site Scripting (XSS) remains one of the most persistent vulnerabilities on the web. Even with careful input sanitization, an accidental flaw can allow malicious scripts to run in a visitor's browser. Content Security Policy (CSP) provides a powerful secondary layer of defense by dictating exactly which resource sources are trusted.
Building a Staged Policy with Report-Only Mode
Deploying an overly strict CSP immediately can break third-party analytics, fonts, and inline styles. The safe deployment path is to start with the `Content-Security-Policy-Report-Only` HTTP response header.
This mode monitors page assets and reports violations to a designated reporting endpoint without blocking any resources. Reviewing these logs reveals which inline scripts or CDN assets need to be explicitly whitelisted or updated before enforcement begins.
Core Directives to Include
A solid production CSP begins with restrictive defaults: `default-src 'self'`. Layer specific rules such as `script-src 'self' https://trusted-cdn.com` and `img-src 'self' data: https:` to cover necessary external resources. Avoid using `'unsafe-inline'` whenever possible. Instead, use cryptographic hashes (`sha256-...`) or single-use nonces for inline scripts.
Security action plan
- Enable multi-factor authentication for privileged accounts and use unique passwords.
- Remove unused accounts and grant only the permissions each person needs.
- Patch dependencies and remove components that are no longer maintained.
- Keep separate backups and periodically test restoring them.
- Decide who receives incident reports and how access can be revoked quickly.
Respond to suspicious activity
Secure affected accounts, rotate exposed credentials, preserve relevant logs, and follow the affected provider's recovery guidance. Avoid sharing sensitive incident details publicly while investigating.