Intro
If you’ve been responsible for the security of a web application, you already know how tricky it is to balance user experience, performance, and defense. Attackers keep looking for new ways to run code in places they shouldn’t, and cross site scripting (XSS) remains one of the most common. Content Security Policy, or CSP, is one of the few tools that can dramatically cut down that risk.
At its core, CSP is just a browser feature. You send a header, the browser reads it, and from that moment forward the browser only allows resources from the places you’ve approved. If you say “scripts can only come from me and my CDN,” then an attacker trying to sneak in a script from somewhere else will get shut down before the user even notices. Simple idea, big impact.
Why CSP Matters
Most modern websites are a patchwork of third party scripts: analytics, chat widgets, personalization, ads, marketing tools. Each one introduces a new door into your environment. If any of them is compromised, your site is suddenly serving code you didn’t write. Without CSP, the browser just runs it. With CSP, you at least have a fighting chance to stop it.
For CISOs, this is less about technical novelty and more about reducing exposure. You don’t need to explain to the board how XSS works. But you can explain how a simple browser control makes it harder for attackers to weaponize those bugs.
Rolling It Out
CSP is best rolled out in stages. A safe first step is to ship it in “report only” mode. Nothing gets blocked yet, but the browser tells you every time something violates the rules. The first report is usually an eye opener. Even on a simple site you’ll see requests flying off to analytics, fonts, APIs, and random third parties you may not have realized were in use.
From there you tune. A minimal starting point might be:
Content-Security-Policy: default-src 'self'
That’s strict, and it will break a lot. But it gives you a baseline. Then you add exceptions carefully—scripts from your CDN, fonts from Google, styles from a trusted source. Each addition is a conscious choice instead of an accident.
Content-Security-Policy: default-src 'self'; script-src 'self' https://cdn.example.com; style-src 'self' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com
The Pitfalls
The two big mistakes are opposite extremes. The first is making it too loose. Teams under pressure sometimes add wildcards or unsafe-inline just to make errors go away. That defeats the whole point. The second mistake is being too strict. A CSP that blocks all third party scripts might look secure, but if it breaks analytics, chat, or marketing tools, the business won’t thank you for it.
CSP also needs to evolve. Sites change. New services get added. Old ones get retired. If the policy isn’t updated, it becomes either uselessly permissive or dangerously out of sync.
Keeping It Practical
The way to keep CSP useful is to treat it like code. Store it in version control. Review changes the same way you review pull requests. Monitor violation reports and adjust as needed. And where possible, combine it with Subresource Integrity (SRI) so that even trusted CDNs can’t slip something in unnoticed.
For leadership, the message is simple. CSP doesn’t replace your other defenses, but it lowers risk in a meaningful way. It prevents a single injected script from escalating into a breach. It shrinks the blast radius if a third party gets compromised. And once in place, it works quietly in the background without needing constant attention.
Final Thoughts
CSP is not glamorous, but it’s effective. It’s a control that sits right where attacks usually happen: in the browser. For CISOs, the business case is clear. The cost is some upfront tuning and the discipline to maintain it. The return is a significant cut in risk from one of the most common classes of web attacks.
In a world where most sites depend on third party code, ignoring CSP is leaving the door wide open. Setting it up right is like putting a guard at the front gate. It won’t solve everything, but it makes the attacker’s job a lot harder.