Input Validation

Overview

Never trust input. Validate everything at system boundaries.

All external data is potentially malicious or malformed. Validate at the point of entry, fail fast on invalid input.

When to Use

• Handling HTTP request bodies, params, headers
• Reading files or environment variables
• Accepting data from external APIs
• Any function that receives untrusted input

The Iron Rule

NEVER use external input without explicit validation.

No exceptions:

• Not for "the frontend validates it"
• Not for "it's an internal API"
• Not for "we trust the source"
• Not for "it's just a simple field"

Detection: Trust Smell

If you're using input directly, STOP:

// ❌ VIOLATION: Trusting input
app.post('/users', async (req, res) => {
  const { email, age } = req.body;  // Direct destructure - no validation
  await db.users.create({ email, age });
  res.json({ success: true });
});

Problems:

• email could be empty, null, or not a string
• age could be negative, a string, or missing
• SQL injection, XSS possible depending on usage

The Correct Pattern: Validate at Boundary

// ✅ CORRECT: Validate with schema
import { z } from 'zod';

const createUserSchema = z.object({
  email: z.string().email('Invalid email format'),
  age: z.number().int().min(13).max(120),
  name: z.string().min(1).max(100),
});

app.post('/users', async (req, res) => {
  // Validate
  const result = createUserSchema.safeParse(req.body);
  
  if (!result.success) {
    return res.status(400).json({ 
      error: 'Validation failed',
      details: result.error.flatten().fieldErrors 
    });
  }
  
  // Now data is typed and validated
  const { email, age, name } = result.data;
  await db.users.create({ email, age, name });
  res.status(201).json({ success: true });
});

Validation Rules

Always Validate:

• Type: Is it a string, number, boolean?
• Format: Valid email? Valid UUID?
• Range: Min/max length? Numeric bounds?
• Required: Is it present?
• Sanitize: Strip dangerous characters if needed

Validation Library Examples:

// Zod (recommended for TypeScript)
const schema = z.object({
  id: z.string().uuid(),
  email: z.string().email(),
  age: z.number().min(0).max(150),
  role: z.enum(['user', 'admin']),
  tags: z.array(z.string()).max(10),
});

// Yup
const schema = yup.object({
  email: yup.string().email().required(),
  age: yup.number().positive().integer(),
});

// class-validator
class CreateUserDto {
  @IsEmail()
  email: string;
  
  @IsInt()
  @Min(13)
  age: number;
}

Pressure Resistance Protocol

1. "Frontend Validates It"

Pressure: "The form already validates this data"

Response: Frontend validation is for UX. Backend validation is for security. Attackers bypass frontends.

Action: Validate on backend regardless of frontend validation.

2. "It's an Internal API"

Pressure: "Only our services call this endpoint"

Response: Internal services have bugs too. Defense in depth requires validation everywhere.

Action: Validate internal API inputs too.

3. "We Trust the Source"

Pressure: "This data comes from our partner's API"

Response: Their API can have bugs, be compromised, or change format. Trust no one.

Action: Validate external API responses before using.

4. "Just a Simple Field"

Pressure: "It's just a string, what could go wrong?"

Response: Strings can be empty, too long, contain scripts, SQL, or null bytes.

Action: Define what "valid" means and enforce it.

Red Flags - STOP and Reconsider

• const { x } = req.body without validation
• Using any type for input
• No error response for invalid input
• "The client sends this correctly"
• Parsing input without try/catch

All of these mean: Add validation.

Quick Reference

Input Type	Validate
Email	Format, length, required
Password	Length, complexity
ID	Format (uuid/int), exists
Number	Type, range, integer?
String	Length, pattern, sanitize
Array	Length, item validation
Object	Schema validation

Common Rationalizations (All Invalid)

Excuse	Reality
"Frontend validates"	Attackers bypass frontends.
"Internal API"	Internal bugs exist too.
"We trust the source"	Sources can be compromised.
"Simple field"	Simple fields cause complex bugs.
"Validation is slow"	Validation is faster than breaches.
"TypeScript types are enough"	Types disappear at runtime.

The Bottom Line

Validate all input. Trust nothing. Fail fast on invalid data.

Every external boundary should have explicit validation. Use schema validation libraries. Return clear error messages for invalid input. Never let unvalidated data into your system.