Input Validation and Sanitization Strategies

While output encoding is the primary defense against XSS, input validation provides an additional security layer. Implement strict validation rules based on what data should contain rather than trying to block malicious patterns. For example, if a field should contain only alphanumeric characters, reject anything else rather than trying to filter out dangerous characters. This positive validation approach is more secure than blacklisting specific patterns that attackers might bypass.

When rich content is necessary, such as in comment systems or content management platforms, use established sanitization libraries rather than building custom solutions. Libraries like DOMPurify, Google's Closure Library, or OWASP's Java HTML Sanitizer have been battle-tested against numerous bypass techniques. Configure these libraries according to your needs, typically using strict whitelists of allowed tags and attributes. Remember that sanitization is complex, and even well-tested libraries occasionally have bypasses discovered, so stay updated with security patches.

Be particularly careful with features that process URLs, as these are common XSS vectors. Validate URL schemes to allow only safe protocols like HTTP and HTTPS, blocking potentially dangerous schemes like javascript: or data:. Parse URLs properly rather than using regular expressions, as URL parsing is surprisingly complex and regex-based validation often has bypasses. When displaying user-provided URLs, consider warning users they're leaving your site or opening links in sandboxed iframes.