The XSS Checker is a security auditing utility that analyzes HTML markup, URL query strings, and data payloads for known Cross-Site Scripting (XSS) attack vectors. It inspects direct script tags, inline DOM event listeners (onerror, onload), dangerous URL schemes (javascript:, vbscript:), base64 data URIs, SVG/MathML injections, and insecure DOM sinks according to OWASP A03 security guidelines.

Reflected XSS occurs when malicious input from a request (e.g. search parameter) is immediately reflected into an HTTP response without sanitization. Stored XSS happens when an attacker stores an injection permanently in a database or comment field. DOM-Based XSS occurs entirely client-side when browser JavaScript reads untrusted data from a source (like location.hash) and feeds it into an active DOM execution sink (such as innerHTML or eval).

XSS prevention depends strictly on where user input is placed in a document. In HTML Body context, angle brackets (< and >) must be encoded into &lt; and &gt;. In Attribute context, quotes (", ') must be escaped to prevent breakout into event attributes. In JavaScript string context, dynamic data must be Unicode-escaped (\u0022) to prevent string delimiters from breaking.

HTML Entity Encoding (OWASP Rule 1) converts special markup characters (&, <, >, ", ', /) into harmless text entities so the browser interprets them as literal display strings rather than executable code. HTML Sanitization (like DOMPurify) parses the markup and strips dangerous tags (<script>, <iframe>) and attributes (onerror) while allowing safe styling tags (<b>, <i>, <p>, <a>) to render safely.

No. The primary scanning engine, defense generators, and report creators execute 100% locally inside your web browser using JavaScript. No payloads, proprietary code, or cookies are sent across the network or logged to any database, guaranteeing complete privacy for security audits.

Content Security Policy (CSP) is an HTTP header that restricts what resources (scripts, images, stylesheets, frames) a browser is allowed to load and execute. A strict CSP (e.g. script-src 'self' 'nonce-...'; object-src 'none') prevents inline scripts and unapproved third-party domains from executing even if an attacker successfully injects a <script> tag into the page HTML.

Inline event attributes (onerror, onload, onclick, onfocus, onmouseover) can be appended to innocent-looking elements like <img>, <svg>, or <body>. If an attacker injects <img src=x onerror=alert(1)>, the browser attempts to fetch the invalid image, triggers an error, and immediately executes the attacker's script without needing a separate <script> tag.

Yes. Base64 data URIs (data:text/html;base64,...) can wrap complete HTML documents containing scripts that execute when loaded in an <iframe> or clicked. Similarly, SVG and MathML are XML formats that natively support embedded <script> tags, which naive text filters frequently overlook.

In React and Next.js, JSX curly braces ({userInput}) automatically escape text content for HTML body context; for rich HTML, use DOMPurify.sanitize(). In Node.js, use sanitize-html and helmet. In Python, use bleach.clean() and html.escape(). In Go, use the html/template package. In PHP, use htmlspecialchars(..., ENT_QUOTES | ENT_HTML5, 'UTF-8').

Press '/' to instantly focus the input workspace, 'Alt+P' for DevToys Smart Clipboard Auto-Detect, 'Alt+D' to toggle star/favorite, 'Alt+S' to open the Developer Remediation Code Studio across 8 languages with standalone script downloads, 'Alt+V' to toggle 2-Column Split Studio vs Stacked View, 'Alt+R' to load random attack vector presets, 'Alt+1..6' to quickly copy individual defense formats, 'Alt+K' for the shortcuts guide, and 'Escape' to dismiss dialogs. You can export results as JSON Audit Reports, RFC 4180 CSV matrices, GitHub Markdown summaries, standalone code scripts (.tsx, .py, .go, .php, etc.), or sanitized HTML files.