What is Input Validation and why is it critical for interface security?

Input validation checks data at every interface boundary so only valid, expected input gets through — it's the choke point that stops payloads turning into code.

It's critical because interface attacks (SQL injection, XSS, command injection) all work by smuggling code in disguised as data; validation is where you re-assert "this field is a name, not a SQL statement". Doing it at every boundary — not just the outer one — is deliberate:

Defense in depth approach:

User ---> [Input Validation] ---> Application ---> [Input Validation] ---> System

Validation must happen:

• At the user-facing interface
• At every internal interface between components
• Before data reaches critical systems (databases, OS)

Why at EVERY interface?

• Defense in depth - multiple layers of protection
• Different components may have different vulnerabilities
• Attackers may bypass outer validation