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Input Validation Principles - Security Fundamentals | System Design Interview Prep | Repovive

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System Design Interview Prep22 sections · 691 units23h total

Open in Course

Input Validation Principles

Defending against bad input

Never trust user input. Validate at every boundary:

Whitelist over blacklist: Define what's allowed, reject everything else. Blacklists always miss something.

Validate type, length, format, range:

email: string, max 254 chars, RFC 5322 format
age: integer, 0-150 range
username: alphanumeric, 3-20 chars

Validate at multiple layers:

Client-side (UX only, not security)
API gateway (schema validation)
Application code (business rules)
Database (constraints)

Defense in depth. If one layer fails, others catch it.