Defense in Depth

Core Principle

Make bugs structurally impossible through multiple independent layers of validation.

Overview

Single-layer protection is insufficient. Defense in depth adds multiple independent validation layers so that if one fails, others catch the problem. Different code paths require different validations.

When to Use This Skill

• Implementing data validation
• Adding safety checks
• Preventing invalid state bugs
• Handling user input
• Processing external data
• Critical operations that must not fail

The Four Layers

Layer 1: Entry Point Validation

Purpose: Reject bad input at API boundaries

Where: Controllers, API endpoints, function entry points

What to validate:

• Type correctness
• Required fields present
• Format validity (email, phone, etc.)
• Basic constraints (min/max, length)

Example:

// Laravel Controller
public function createUser(Request $request)
{
    // Layer 1: Entry point validation
    $validated = $request->validate([
        'email' => 'required|email|max:255',
        'password' => 'required|min:8',
        'age' => 'required|integer|min:18',
    ]);

    // Continue with business logic...
}

Layer 2: Business Logic Validation

Purpose: Enforce operational requirements

Where: Service classes, domain logic

What to validate:

• Business rules
• State transitions
• Relationships
• Domain constraints

Example:

// UserService
public function createUser(array $data): User
{
    // Layer 2: Business logic validation
    if ($this->userRepository->emailExists($data['email'])) {
        throw new ValidationException('Email already registered');
    }

    if ($data['age'] < 18) {
        throw new ValidationException('Must be 18 or older');
    }

    // Create user...
}

Layer 3: Environment Guards

Purpose: Context-specific safety checks

Where: Before operations, database queries, external calls

What to check:

• Database connection exists
• Required services available
• File permissions correct
• Network connectivity
• Resource limits

Example:

public function processPayment(Payment $payment): void
{
    // Layer 3: Environment guards
    if (app()->environment('production') && !config('payment.gateway_enabled')) {
        throw new RuntimeException('Payment gateway not configured for production');
    }

    if ($payment->amount > config('payment.max_amount')) {
        throw new RuntimeException('Payment exceeds maximum allowed amount');
    }

    // Process payment...
}

Layer 4: Debug Instrumentation

Purpose: Forensic logging for debugging

Where: Throughout critical paths

What to log:

• Input values
• State transitions
• Decision points
• Output values

Example:

public function transferFunds(Account $from, Account $to, float $amount): void
{
    // Layer 4: Debug instrumentation
    Log::info('Transfer initiated', [
        'from_account' => $from->id,
        'to_account' => $to->id,
        'amount' => $amount,
        'from_balance_before' => $from->balance,
        'to_balance_before' => $to->balance,
    ]);

    // Perform transfer...

    Log::info('Transfer completed', [
        'from_balance_after' => $from->balance,
        'to_balance_after' => $to->balance,
    ]);
}

Complete Example: User Registration

// Layer 1: Entry Point Validation (Controller)
public function register(RegisterRequest $request)
{
    // FormRequest handles basic validation:
    // - email format
    // - password strength
    // - required fields

    $validated = $request->validated();

    $user = $this->userService->register($validated);

    return response()->json(['user' => $user], 201);
}

// Layer 2: Business Logic Validation (Service)
public function register(array $data): User
{
    // Business rule: Email must be unique
    if ($this->userRepository->emailExists($data['email'])) {
        throw new DuplicateEmailException('Email already registered');
    }

    // Business rule: Domain not blacklisted
    $domain = substr($data['email'], strpos($data['email'], '@') + 1);
    if ($this->isBlacklistedDomain($domain)) {
        throw new ValidationException('Email domain not allowed');
    }

    // Business rule: Age requirement
    if (isset($data['birthdate'])) {
        $age = Carbon::parse($data['birthdate'])->age;
        if ($age < 18) {
            throw new ValidationException('Must be 18 or older to register');
        }
    }

    return $this->createUser($data);
}

// Layer 3: Environment Guards (Repository/Model)
protected function createUser(array $data): User
{
    // Environment guard: Database connection
    if (!DB::connection()->getPdo()) {
        throw new DatabaseException('Database connection not available');
    }

    // Environment guard: Email service available
    if (!app('email')->isAvailable()) {
        throw new ServiceException('Email service unavailable');
    }

    // Layer 4: Debug instrumentation
    Log::info('Creating user', [
        'email' => $data['email'],
        'timestamp' => now(),
    ]);

    $user = User::create([
        'email' => $data['email'],
        'password' => Hash::make($data['password']),
    ]);

    Log::info('User created', [
        'user_id' => $user->id,
        'email' => $user->email,
    ]);

    return $user;
}

Why All Layers Matter

Scenario: Layer 1 Only

// Only entry point validation
public function createUser(Request $request)
{
    $validated = $request->validate(['email' => 'required|email']);

    // ❌ Problem: Email could be duplicate
    // ❌ Problem: Domain could be blacklisted
    // ❌ Problem: Database could be down
    // ❌ Problem: No logging for debugging

    User::create($validated);
}

Scenario: All Layers

// Layer 1: Entry validation
$validated = $request->validate(['email' => 'required|email']);

// Layer 2: Business validation
if ($this->emailExists($validated['email'])) {
    throw new ValidationException('Duplicate email');
}

// Layer 3: Environment guard
if (!DB::connection()->getPdo()) {
    throw new DatabaseException('Database unavailable');
}

// Layer 4: Instrumentation
Log::info('Creating user', ['email' => $validated['email']]);

// ✅ Now protected against multiple failure modes
User::create($validated);

Common Validation Patterns

Email Validation (All Layers)

// Layer 1: Format
'email' => 'required|email'

// Layer 2: Business rules
- Unique in database
- Domain not blacklisted
- Not a disposable email service

// Layer 3: Environment
- Email service available
- SMTP configured

// Layer 4: Logging
- Log email (sanitized)
- Log validation results

Payment Processing (All Layers)

// Layer 1: Input validation
'amount' => 'required|numeric|min:0.01'
'currency' => 'required|in:USD,EUR,GBP'

// Layer 2: Business rules
- Amount within limits
- Account has sufficient funds
- Payment method valid

// Layer 3: Environment
- Payment gateway available
- SSL certificate valid
- Fraud detection service up

// Layer 4: Logging
- Log all payment attempts
- Log amounts and currencies
- Log success/failure

File Upload (All Layers)

// Layer 1: Input validation
'file' => 'required|file|max:10240|mimes:jpg,png,pdf'

// Layer 2: Business rules
- User has upload quota remaining
- File name not duplicate
- Content passes virus scan

// Layer 3: Environment
- Disk space available
- Directory writable
- Virus scanner available

// Layer 4: Logging
- Log file details
- Log storage location
- Log processing results

Real-World Impact

Example from Production:

Before Defense in Depth:

// Only basic validation
public function updateProfile(Request $request, User $user)
{
    $data = $request->validate(['bio' => 'string|max:500']);
    $user->update($data);
}

// Bug: User could set bio to another user's email, causing privacy leak
// Bug: Bio could contain SQL injection (if used raw elsewhere)
// Bug: No logging meant debugging was impossible

After Defense in Depth:

public function updateProfile(Request $request, User $user)
{
    // Layer 1: Input validation
    $data = $request->validate([
        'bio' => 'string|max:500',
    ]);

    // Layer 2: Business validation
    if ($this->containsSensitiveData($data['bio'])) {
        throw new ValidationException('Bio contains restricted content');
    }

    // Layer 3: Environment guard
    if (!$user->can('update', $user)) {
        throw new UnauthorizedException();
    }

    // Layer 4: Instrumentation
    Log::info('Profile update', [
        'user_id' => $user->id,
        'old_bio' => $user->bio,
        'new_bio' => $data['bio'],
    ]);

    $user->update($data);

    Log::info('Profile updated successfully', ['user_id' => $user->id]);
}

// ✅ Protected against multiple attack vectors
// ✅ Logging helps debug issues

Integration with Other Skills

Use with:

• test-driven-development - Write tests for each layer
• code-review - Verify all layers present
• systematic-debugging - Logs help identify which layer failed

Complements:

• database-backup - Another safety layer
• verification-before-completion - Validate defenses work

Checklist for Defense in Depth

For any data processing or critical operation:

• Layer 1: Entry point validation implemented?
• Layer 2: Business logic validation implemented?
• Layer 3: Environment guards in place?
• Layer 4: Debug logging added?
• Tested each layer independently?
• Tested with invalid data at each layer?
• Verified logging captures useful information?

Common Mistakes

Mistake 1: Only One Layer

// ❌ Only validates at entry
public function createOrder(Request $request)
{
    $validated = $request->validate(['product_id' => 'required']);
    Order::create($validated);
}

// Missing: Business rules, environment checks, logging

Mistake 2: Duplicate Validation Logic

// ❌ Same validation in every layer (violates DRY)
// Layer 1
$request->validate(['email' => 'email']);

// Layer 2
if (!filter_var($email, FILTER_VALIDATE_EMAIL)) { ... }

// ✅ Different validations per layer
// Layer 1: Format
// Layer 2: Business rules (uniqueness, blacklist)

Mistake 3: No Logging

// ❌ No instrumentation
public function criticalOperation()
{
    // Complex logic
    // No logs
}

// ✅ Add logging
Log::info('Critical operation started');
// Complex logic
Log::info('Critical operation completed');

Authority

This skill is based on:

• Security best practice: Defense in depth principle
• Industry standard: Multiple validation layers prevent bugs
• Real production experience: Single-layer validation fails
• Evidence-based: Reduces bugs by catching at multiple points

Social Proof: Major companies (Google, Amazon, Microsoft) use layered validation.

Your Commitment

When implementing validation:

• I will implement ALL four layers
• I will not rely on a single validation point
• I will add appropriate logging
• I will test each layer independently
• I will make bugs structurally impossible

---

Bottom Line: One layer of validation is not enough. Different layers catch different problems. Implement all four layers to make bugs structurally impossible.