What is defensive programming and how does it relate to security?

Write code that assumes inputs are hostile and handles every error — it's the implementation-level form of "never trust user input."

Write code that anticipates misuse and handles it gracefully, rather than assuming correct inputs.

Key practices:

• Validate all inputs — Check type, length, format, range (even from "trusted" sources)
• Never trust external data — User input, API responses, file contents, environment variables
• Check return values — Don't assume operations succeeded
• Use assertions — Verify assumptions about program state
• Handle all error cases — No empty catch blocks

Security connection: Most vulnerabilities exist because code doesn't handle unexpected inputs:

• Buffer overflow → Didn't check input length
• SQL injection → Didn't validate/escape input
• Path traversal → Didn't validate file path

Defensive programming is the implementation-level version of "never trust user input."

Go deeper:

• Wikipedia — Defensive programming — anticipating misuse, input canonicalization, and the "all data is tainted" rules.