Incomplete Denylist to Cross-Site Scripting

Preventing XSS is trickier than it seems because modern web browsers and HTML parsers handle content in many different ways. An incomplete denylist, which only blocks a known set of dangerous patterns, cannot account for all the possible encoding tricks, HTML variations, and browser quirks that attackers can exploit. This creates a false sense of security. Attackers frequently use resources like the 'XSS Cheat Sheet' to find bypass techniques that evade common denylists. To effectively prevent XSS, developers should move away from denylists and instead adopt positive security models like context-aware output encoding and strict Content Security Policies (CSP).