Content Security Policy (XSS)

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A Content Security Policy is a set of rules you give the browser that control which scripts, styles, and resources a page is allowed to load. It is one of the best defenses against cross-site scripting attacks. A strong policy makes security a built-in standard rather than an afterthought.

Content Security Policy (XSS)

A Content Security Policy (CSP) is an HTTP header that tells the browser exactly which sources of content your page trusts. It whitelists where scripts, stylesheets, images, fonts, and other resources can be loaded from — and blocks everything else. By restricting what can run on your page, CSP is one of the strongest defenses against cross-site scripting (XSS) and data injection attacks.

Why It Matters

XSS attacks are extremely common. Cross-site scripting remains one of the most frequently exploited web vulnerabilities. An attacker injects malicious code into your page, and without CSP, the browser runs it without question.
CSP blocks unauthorized scripts. Even if an attacker finds a way to inject a <script> tag, a properly configured CSP prevents it from executing because the source is not whitelisted.
It protects your users' data. XSS attacks can steal cookies, session tokens, login credentials, and personal information. CSP helps keep that data safe even if other defenses fail.
It catches unintentional leaks. CSP reports can reveal third-party scripts loading unexpected resources, helping you identify supply chain risks you did not know about.

How to Implement It

Start with report-only mode. Use Content-Security-Policy-Report-Only first instead of enforcing immediately. This logs violations without breaking anything, letting you see what would be blocked.
Define a strict default-src. Set default-src 'self' as your baseline to only allow resources from your own domain. Then add specific exceptions for trusted external sources as needed.
Eliminate inline scripts. CSP is most effective when you ban inline JavaScript with script-src directives that do not include 'unsafe-inline'. Move inline scripts to external files or use nonce-based exceptions.
Use nonces for necessary inline scripts. If you absolutely need inline scripts, generate a unique nonce per page load and include it in both the CSP header and the script tag: <script nonce="abc123">.
Set up violation reporting. Add a report-uri or report-to directive so the browser sends you a report every time a resource is blocked. This helps you catch issues and refine your policy over time.

Common Mistakes

Using 'unsafe-inline' and 'unsafe-eval'. These directives essentially disable CSP's core protections. If your policy includes both, you have a CSP header but almost no actual security benefit.
Whitelisting entire CDN domains. Allowing script-src cdn.example.com trusts every script on that CDN, not just yours. An attacker who can host content on the same CDN bypasses your policy entirely.
Deploying without testing. Going straight to enforcement mode without report-only testing will almost certainly break something — especially third-party analytics, ads, or embedded content.
Setting it and forgetting it. New features, third-party integrations, and code changes can introduce CSP violations. Review your violation reports regularly and update the policy as your site evolves.

Bottom Line: A Content Security Policy is one of the most effective security measures you can add to your site. Start in report-only mode, eliminate inline scripts, whitelist only what you trust, and monitor violation reports. It takes effort to get right, but the protection it provides is worth it.

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Synonyms: CSP, XSS Protection

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Glossary of Terms

Content Security Policy (XSS)