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Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Input Validation Deep Dive

Introduction

Input validation is the first line of defense against injection attacks. Every piece of data entering an application — form fields, HTTP headers, URL parameters, file uploads, API payloads — must be validated before processing. The principle is simple: never trust user input.

Whitelist vs Blacklist

Whitelist (Allowlist) Validation

Whitelist validation defines what is allowed and rejects everything else. It is far more secure than blacklisting.

import re

Whitelist: only allow specific characters

def validate_username_whitelist(username):

"""Allow only alphanumeric, underscore, and hyphen."""

pattern = r'^[a-zA-Z0-9_-]{3,32}$'

if not re.match(pattern, username):

raise ValueError(

f"Username '{username}' contains invalid characters. "

"Only letters, numbers, underscores, and hyphens are allowed."

)

return username

Whitelist for country codes

ALLOWED_COUNTRIES = {'US', 'CA', 'GB', 'DE', 'FR', 'JP'}

def validate_country_code(code):

if code.upper() not in ALLOWED_COUNTRIES:

raise ValueError(f"Country '{code}' is not in the allowed list")

return code.upper()

Blacklist (Blocklist) Validation

Blacklist validation attempts to block known malicious patterns. It is inherently fragile because attackers constantly discover new bypass techniques.

WEAK: Blacklist approach (easily bypassed)

def validate_input_blacklist(input_string):

Easily bypassed — attacker uses alternative syntax

blocklist = ['

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