Advanced Sanitization Techniques

When rich content is necessary, such as allowing users to format text or embed media, sanitization becomes critical. Sanitization differs from validation in that it modifies input to make it safe rather than rejecting it entirely. This is particularly important for features like comment systems, forums, or content management systems where users expect some HTML formatting capabilities.

HTML sanitization is complex because browsers are extremely forgiving of malformed HTML, often "fixing" it in ways that can introduce vulnerabilities. Use established sanitization libraries like DOMPurify (JavaScript), Bleach (Python), or HTMLPurifier (PHP) rather than attempting to build custom sanitizers. These libraries have been tested against thousands of bypass techniques and are regularly updated as new ones are discovered. Configure them according to your needs, typically starting with a minimal whitelist of allowed tags and attributes.

When configuring sanitization libraries, carefully consider which HTML features to allow. Start with the absolute minimum needed for your use case. Basic formatting might only require tags like p, br, strong, and em. Each additional tag or attribute increases attack surface. Be particularly careful with attributes that can contain URLs (href, src) or event handlers. If you must allow links, validate URL schemes to permit only http and https, blocking javascript:, data:, and other potentially dangerous schemes.