XSS Prevention: Complete Cross-Site Scripting Guide

How XSS attacks work and how to prevent them: input validation, output encoding, CSP headers, and sanitization libraries.

Three Types of XSS

• Reflected: Malicious input reflected in the response (search queries, error messages)
• Stored: Malicious input saved to database and displayed to other users (comments, profiles)
• DOM-based: Client-side JavaScript processes untrusted data (innerHTML, document.write)

Prevention

• Escape output: HTML-encode &<>"'
• Use CSP headers: Content-Security-Policy: default-src 'self'
• Use textContent instead of innerHTML
• Use DOMPurify for user HTML

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