Incomplete List of Disallowed Inputs

Denylist-based security is fundamentally fragile because it requires developers to predict and block every possible bad input. This approach often fails against sophisticated attacks, as attackers constantly evolve their techniques to use new encodings, character sequences, or command variations that aren't on the list. Relying solely on a denylist creates a false sense of security and is difficult to maintain over time. For stronger protection, prioritize 'allowlisting' (only permitting known-good patterns) over denylisting. If a denylist is unavoidable, complement it with other defenses like strict input validation, output encoding, and security libraries designed to handle the specific threat. Always assume your denylist is incomplete and design your system to fail safely when an unexpected input is encountered.