Agent Skills: XSS Prevention

XSS Prevention

Table of Contents

• Overview
• When to Use
• Quick Start
• Reference Guides
• Best Practices

Overview

Implement comprehensive Cross-Site Scripting (XSS) prevention using input sanitization, output encoding, CSP headers, and secure coding practices.

When to Use

• User-generated content display
• Rich text editors
• Comment systems
• Search functionality
• Dynamic HTML generation
• Template rendering

Quick Start

Minimal working example:

// xss-prevention.js
const createDOMPurify = require("dompurify");
const { JSDOM } = require("jsdom");
const he = require("he");

const window = new JSDOM("").window;
const DOMPurify = createDOMPurify(window);

class XSSPrevention {
  /**
   * HTML Entity Encoding - Safest for text content
   */
  static encodeHTML(str) {
    return he.encode(str, {
      useNamedReferences: true,
      encodeEverything: false,
    });
  }

  /**
   * Sanitize HTML - For rich content
   */
  static sanitizeHTML(dirty) {
    const config = {
      ALLOWED_TAGS: [
// ... (see reference guides for full implementation)

Reference Guides

Detailed implementations in the references/ directory:

| Guide | Contents | |---|---| | Node.js XSS Prevention | Node.js XSS Prevention | | Python XSS Prevention | Python XSS Prevention | | React XSS Prevention | React XSS Prevention | | Content Security Policy | Content Security Policy |

Best Practices

✅ DO

• Encode output by default
• Use templating engines
• Implement CSP headers
• Sanitize rich content
• Validate URLs
• Use HTTPOnly cookies
• Regular security testing
• Use secure frameworks

❌ DON'T

• Trust user input
• Use innerHTML directly
• Skip output encoding
• Allow inline scripts
• Use eval()
• Mix contexts (HTML/JS)