Check a Website for XSS Vulnerabilities

Cross-Site Scripting attacks fall into three distinct categories, and each requires a slightly different defence. Reflected XSS occurs when user-supplied input is echoed back in the HTTP response without sanitisation — for example, a search query that appears verbatim in the page title. An attacker crafts a malicious URL containing a script payload and tricks a victim into clicking it. The script executes in the victim's browser under the site's origin, giving the attacker full access to cookies, localStorage, and DOM content scoped to that domain.

Stored XSS, sometimes called persistent XSS, is considerably more dangerous because the payload lives in the application's database. A comment field, a user profile bio, or a product review that saves raw HTML to the database and renders it later without encoding can infect every visitor who views that content. In 2026, stored XSS attacks frequently target e-commerce checkout flows, SaaS dashboards, and collaborative tools where many users share the same views. A single unsanitised field can expose an entire user base.

DOM-based XSS bypasses the server entirely. The vulnerability exists in client-side JavaScript that reads from attacker-controllable sources — document.location, window.name, or postMessage — and writes that data to dangerous sinks such as innerHTML, eval(), or document.write(). Server-side scanners often miss DOM XSS because the payload never appears in the HTTP response body. Vuln0x's full engine injects instrumented payloads and evaluates the rendered DOM state in a headless browser to catch these client-side flaws alongside traditional reflected and stored variants.

Preventing XSS requires a defence-in-depth approach rather than a single silver bullet. Output encoding is the primary control: every value written into an HTML context must be entity-encoded (& → &amp;, < → &lt;, > → &gt;, " → &quot;). Values written into JavaScript contexts require a different encoding scheme, and URL contexts a third. The second layer is a strict Content-Security-Policy header that uses a nonce or hash approach to allowlist only scripts you control, blocking inline execution and preventing data exfiltration via connect-src. The third layer is input validation — rejecting angle brackets and script keywords at ingestion — which reduces the attack surface even if a rendering path is accidentally missed.

Vuln0x tests your site against all three XSS categories as part of the free 40+ engine full scan. The scanner submits safe probe payloads into GET parameters, POST fields, HTTP headers, and JSON request bodies, then analyses both the raw HTTP response and the fully rendered DOM. Each finding is assigned a severity (Critical, High, Medium, Low) based on exploitability and impact. The report includes the exact request that triggered the finding, the parameter name, the reflected value location, and a step-by-step remediation guide tailored to your framework — whether you are running Next.js, Laravel, Django, Ruby on Rails, or a headless CMS. After you apply the fix, the re-scan feature confirms closure without consuming an additional credit.