Security And Hardening

Security and Hardening

Overview

Security-first development practices for web applications. Treat every external input as hostile, every secret as sacred, and every authorization check as mandatory. Security isn't a phase — it's a constraint on every line of code that touches user data, authentication, or external systems.

When to Use

• Building anything that accepts user input
• Implementing authentication or authorization
• Storing or transmitting sensitive data
• Integrating with external APIs or services
• Adding file uploads, webhooks, or callbacks
• Handling payment or PII data

The Three-Tier Boundary System

Always Do (No Exceptions)

• Validate all external input at the system boundary (API routes, form handlers)
• Parameterize all database queries — never concatenate user input into SQL
• Encode output to prevent XSS (use framework auto-escaping, don't bypass it)
• Use HTTPS for all external communication
• Hash passwords with bcrypt/scrypt/argon2 (never store plaintext)
• Set security headers (CSP, HSTS, X-Frame-Options, X-Content-Type-Options)
• Use httpOnly, secure, sameSite cookies for sessions
• Run npm audit (or equivalent) before every release

Ask First (Requires Human Approval)

• Adding new authentication flows or changing auth logic
• Storing new categories of sensitive data (PII, payment info)
• Adding new external service integrations
• Changing CORS configuration
• Adding file upload handlers
• Modifying rate limiting or throttling
• Granting elevated permissions or roles

Never Do

• Never commit secrets to version control (API keys, passwords, tokens)
• Never log sensitive data (passwords, tokens, full credit card numbers)
• Never trust client-side validation as a security boundary
• Never disable security headers for convenience
• Never use eval() or innerHTML with user-provided data
• Never store sessions in client-accessible storage (localStorage for auth tokens)
• Never expose stack traces or internal error details to users

OWASP Top 10 Prevention

1. Injection (SQL, NoSQL, OS Command)

// BAD: SQL injection via string concatenation
const query = `SELECT * FROM users WHERE id = '${userId}'`;

// GOOD: Parameterized query
const user = await db.query('SELECT * FROM users WHERE id = $1', [userId]);

// GOOD: ORM with parameterized input
const user = await prisma.user.findUnique({ where: { id: userId } });

2. Broken Authentication

// Password hashing
import { hash, compare } from 'bcrypt';

const SALT_ROUNDS = 12;
const hashedPassword = await hash(plaintext, SALT_ROUNDS);
const isValid = await compare(plaintext, hashedPassword);

// Session management
app.use(
  session({
    secret: process.env.SESSION_SECRET, // From environment, not code
    resave: false,
    saveUninitialized: false,
    cookie: {
      httpOnly: true, // Not accessible via JavaScript
      secure: true, // HTTPS only
      sameSite: 'lax', // CSRF protection
      maxAge: 24 * 60 * 60 * 1000, // 24 hours
    },
  }),
);

3. Cross-Site Scripting (XSS)

// BAD: Rendering user input as HTML
element.innerHTML = userInput;

// GOOD: Use framework auto-escaping (React does this by default)
return <div>{userInput}</div>;

// If you MUST render HTML, sanitize first
import DOMPurify from 'dompurify';
const clean = DOMPurify.sanitize(userInput);

4. Broken Access Control

// Always check authorization, not just authentication
app.patch('/api/tasks/:id', authenticate, async (req, res) => {
  const task = await taskService.findById(req.params.id);

  // Check that the authenticated user owns this resource
  if (task.ownerId !== req.user.id) {
    return res.status(403).json({
      error: {
        code: 'FORBIDDEN',
        message: 'Not authorized to modify this task',
      },
    });
  }

  // Proceed with update
  const updated = await taskService.update(req.params.id, req.body);
  return res.json(updated);
});

5. Security Misconfiguration

// Security headers (use helmet for Express)
import helmet from 'helmet';
app.use(helmet());

// Content Security Policy
app.use(
  helmet.contentSecurityPolicy({
    directives: {
      defaultSrc: ["'self'"],
      scriptSrc: ["'self'"],
      styleSrc: ["'self'", "'unsafe-inline'"], // Tighten if possible
      imgSrc: ["'self'", 'data:', 'https:'],
      connectSrc: ["'self'"],
    },
  }),
);

// CORS — restrict to known origins
app.use(
  cors({
    origin: process.env.ALLOWED_ORIGINS?.split(',') || 'http://localhost:3000',
    credentials: true,
  }),
);

6. Sensitive Data Exposure

// Never return sensitive fields in API responses
function sanitizeUser(user: UserRecord): PublicUser {
  const { passwordHash, resetToken, ...publicFields } = user;
  return publicFields;
}

// Use environment variables for secrets
const API_KEY = process.env.STRIPE_API_KEY;
if (!API_KEY) throw new Error('STRIPE_API_KEY not configured');

Input Validation Patterns

Schema Validation at Boundaries

import { z } from 'zod';

const CreateTaskSchema = z.object({
  title: z.string().min(1).max(200).trim(),
  description: z.string().max(2000).optional(),
  priority: z.enum(['low', 'medium', 'high']).default('medium'),
  dueDate: z.string().datetime().optional(),
});

// Validate at the route handler
app.post('/api/tasks', async (req, res) => {
  const result = CreateTaskSchema.safeParse(req.body);
  if (!result.success) {
    return res.status(422).json({
      error: {
        code: 'VALIDATION_ERROR',
        message: 'Invalid input',
        details: result.error.flatten(),
      },
    });
  }
  // result.data is now typed and validated
  const task = await taskService.create(result.data);
  return res.status(201).json(task);
});

File Upload Safety

// Restrict file types and sizes
const ALLOWED_TYPES = ['image/jpeg', 'image/png', 'image/webp'];
const MAX_SIZE = 5 * 1024 * 1024; // 5MB

function validateUpload(file: UploadedFile) {
  if (!ALLOWED_TYPES.includes(file.mimetype)) {
    throw new ValidationError('File type not allowed');
  }
  if (file.size > MAX_SIZE) {
    throw new ValidationError('File too large (max 5MB)');
  }
  // Don't trust the file extension — check magic bytes if critical
}

Triaging npm audit Results

Not all audit findings require immediate action. Use this decision tree:

npm audit reports a vulnerability
├── Severity: critical or high
│   ├── Is the vulnerable code reachable in your app?
│   │   ├── YES --> Fix immediately (update, patch, or replace the dependency)
│   │   └── NO (dev-only dep, unused code path) --> Fix soon, but not a blocker
│   └── Is a fix available?
│       ├── YES --> Update to the patched version
│       └── NO --> Check for workarounds, consider replacing the dependency, or add to allowlist with a review date
├── Severity: moderate
│   ├── Reachable in production? --> Fix in the next release cycle
│   └── Dev-only? --> Fix when convenient, track in backlog
└── Severity: low
    └── Track and fix during regular dependency updates

Key questions:

• Is the vulnerable function actually called in your code path?
• Is the dependency a runtime dependency or dev-only?
• Is the vulnerability exploitable given your deployment context (e.g., a server-side vulnerability in a client-only app)?

When you defer a fix, document the reason and set a review date.

Rate Limiting

import rateLimit from 'express-rate-limit';

// General API rate limit
app.use(
  '/api/',
  rateLimit({
    windowMs: 15 * 60 * 1000, // 15 minutes
    max: 100, // 100 requests per window
    standardHeaders: true,
    legacyHeaders: false,
  }),
);

// Stricter limit for auth endpoints
app.use(
  '/api/auth/',
  rateLimit({
    windowMs: 15 * 60 * 1000,
    max: 10, // 10 attempts per 15 minutes
  }),
);

Secrets Management

.env files:
  ├── .env.example  → Committed (template with placeholder values)
  ├── .env          → NOT committed (contains real secrets)
  └── .env.local    → NOT committed (local overrides)

.gitignore must include:
  .env
  .env.local
  .env.*.local
  *.pem
  *.key

Always check before committing:

# Check for accidentally staged secrets
git diff --cached | grep -i "password\|secret\|api_key\|token"

Security Review Checklist

### Authentication

- [ ] Passwords hashed with bcrypt/scrypt/argon2 (salt rounds ≥ 12)
- [ ] Session tokens are httpOnly, secure, sameSite
- [ ] Login has rate limiting
- [ ] Password reset tokens expire

### Authorization

- [ ] Every endpoint checks user permissions
- [ ] Users can only access their own resources
- [ ] Admin actions require admin role verification

### Input

- [ ] All user input validated at the boundary
- [ ] SQL queries are parameterized
- [ ] HTML output is encoded/escaped

### Data

- [ ] No secrets in code or version control
- [ ] Sensitive fields excluded from API responses
- [ ] PII encrypted at rest (if applicable)

### Infrastructure

- [ ] Security headers configured (CSP, HSTS, etc.)
- [ ] CORS restricted to known origins
- [ ] Dependencies audited for vulnerabilities
- [ ] Error messages don't expose internals

See Also

For detailed security checklists and pre-commit verification steps, see references/security-checklist.md.

Common Rationalizations

Red Flags

• User input passed directly to database queries, shell commands, or HTML rendering
• Secrets in source code or commit history
• API endpoints without authentication or authorization checks
• Missing CORS configuration or wildcard (*) origins
• No rate limiting on authentication endpoints
• Stack traces or internal errors exposed to users
• Dependencies with known critical vulnerabilities

Verification

After implementing security-relevant code:

• npm audit shows no critical or high vulnerabilities
• No secrets in source code or git history
• All user input validated at system boundaries
• Authentication and authorization checked on every protected endpoint
• Security headers present in response (check with browser DevTools)
• Error responses don't expose internal details
• Rate limiting active on auth endpoints