Cross-Site Scripting Prevention

Cross-Site Scripting Prevention with Browser-Native CSP Enforcement

Output encoding and secure code review won't catch everything. CSP is the second line of defense. Report URI is how you deploy, monitor, and enforce it without breaking your site.

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Why Code-Level Defenses Aren't Enough

How XSS gets past your existing defenses

XSS doesn't need your whole application to be vulnerable. It needs one gap — and that gap is often somewhere your code-level defenses can't reach.

Input

The missed input

One unencoded field, one overlooked parameter, one edge case in a legacy template. XSS doesn't need your whole application to be vulnerable. It needs one gap.

Dependency

The compromised dependency

A third-party script changes after you approved it. The XSS isn't in your code — it's in a dependency you don't control and didn't review.

Browser

The browser-layer injection

Malicious browser extensions, compromised CDNs, or man-in-the-browser attacks inject scripts into the page. Your code is clean. The execution environment isn't.

CSP is the browser-native mechanism designed for XSS protection when code-level defenses miss. The challenge is deploying and enforcing it at scale.

What Report URI Does

How Report URI delivers cross-site scripting prevention

Client-side security that delivers XSS detection and enforcement through CSP — browser-native, no agents, no injected scripts, no code running on your behalf.

CSP Reporting

Detect XSS attempts in real browsers

Every CSP violation from every user session is logged and searchable. See exactly what's being blocked, what's attempting to execute, and where the violation originated. Catch XSS attempts in production as they happen.

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CSP Wizard

Build a deployable policy without guesswork

Collect seven days of data from real browser sessions, get a complete inventory of every resource your site loads, and build an enforceable CSP in clicks. The hardest part of XSS prevention with CSP is getting the policy right. The Wizard solves that.

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Script Watch

Detect when dependencies change

Continuous monitoring of every script executing across your site. When a third-party dependency is modified, a new script appears, or an approved resource starts behaving differently, you know immediately. Catch compromised dependencies before they become an XSS vector.

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CSP Builder

Maintain your policy as your site evolves

Import your existing policy, update it as resources change, and generate the new header. CSP breaks when policies go stale. The Builder keeps enforcement up to date without manual header editing.

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The Gap in Existing Tools

Why existing XSS protection tools miss the browser layer

Your scanners, code review, and WAF all do important work. None of them can see what's actually executing in a real user's browser right now.

DAST / Scanners

They test code, not live browsers

DAST and vulnerability scanners test your application for XSS vulnerabilities in code. They don't tell you whether an XSS attack is happening right now in a real user's browser.

Code Review

It only covers code you control

Code review and output encoding catch the vulnerabilities you know about in code you control. They can't cover third-party scripts, compromised CDNs, or browser-layer injections.

WAF

It never sees in-browser execution

WAFs filter known attack patterns at the network edge. A script that's already executing in the browser — loaded from an approved but compromised source — never crosses the WAF.

X-XSS-Protection

The old browser filter is gone

The deprecated X-XSS-Protection header was the browser's own XSS filter. Every major browser has removed it. CSP is what replaced it — but a CSP header without tooling to monitor violations and enforce policy is a header nobody maintains.

CSP is the browser's built-in defense against XSS. Report URI is what makes it operational.

See what's executing in your users' browsers
No agent. No proxy. No deployment risk.

Live on your site in minutes

Report URI is browser-native. It works through the Reporting API already built into modern browsers. You add a reporting endpoint to your existing CSP header.

HTTP response header
Content-Security-Policy: default-src 'self';
  report-uri https://your-subdomain.report-uri.com/r/d/csp/enforce

Start in report-only mode. See everything before you enforce anything. Move to enforcement when you're confident in the policy.

No agent to install

No traffic routed through a third party

No changes to your infrastructure

No impact on site performance

Site functions normally even if Report URI is unavailable

Get Started

Start preventing cross-site scripting with CSP that actually works

Browser-native. No infrastructure changes. See everything before you enforce anything.

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30-day free trial  ·  One header  ·  No infrastructure changes  ·  Report-only to enforcement

What it covers. What it doesn't.

Cross-site scripting prevention: what's covered

Report URI is the client-side security layer that makes CSP operational — deployment, monitoring, enforcement, and ongoing management. It's the second line of defense against XSS, not a replacement for secure coding.

Report URI covers Doesn't replace
CSP deployment and enforcement Output encoding / secure coding
XSS attempt detection in real browsers DAST / vulnerability scanning
Script change and dependency monitoring Penetration testing
Policy management and maintenance Secure code review

Vendors who promise to do all of it usually inject their own code into your pages — another attack surface. Report URI doesn't. Nothing runs on your behalf, so nothing can break, slow down, or get compromised.

“Report URI has given us the capability to seamlessly build and roll out new Content Security Policies with a high level of confidence. The unopinionated and technology-agnostic nature of Report URI allowed us to integrate it directly and easily into our existing workflows, and to gain instant visibility into CSP reports.”

Colin Barr, Head of InfoSec and IT  ·  Paddle

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