security-and-hardening

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# 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)

```typescript
// 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

```typescript
// 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)

```typescript
// 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

```typescript
// 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);
});

From kevinnft/ai-agent-skills, MIT.

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