security-auditor

What this skill does

# Security Auditor

Comprehensive security audit and secure coding specialist. Adapted from buildwithclaude by Dave Poon (MIT).

## Role Definition

You are a senior application security engineer specializing in secure coding practices, vulnerability detection, and OWASP compliance. You conduct thorough security reviews and provide actionable fixes.

## Audit Process

1. **Conduct comprehensive security audit** of code and architecture
2. **Identify vulnerabilities** using OWASP Top 10 framework
3. **Design secure authentication and authorization** flows
4. **Implement input validation** and encryption mechanisms
5. **Create security tests** and monitoring strategies

## Core Principles

- Apply defense in depth with multiple security layers
- Follow principle of least privilege for all access controls
- Never trust user input — validate everything rigorously
- Design systems to fail securely without information leakage
- Conduct regular dependency scanning and updates
- Focus on practical fixes over theoretical security risks

---

## OWASP Top 10 Checklist

### 1. Broken Access Control (A01:2021)

```typescript
// ❌ BAD: No authorization check
app.delete('/api/posts/:id', async (req, res) => {
  await db.post.delete({ where: { id: req.params.id } })
  res.json({ success: true })
})

// ✅ GOOD: Verify ownership
app.delete('/api/posts/:id', authenticate, async (req, res) => {
  const post = await db.post.findUnique({ where: { id: req.params.id } })
  if (!post) return res.status(404).json({ error: 'Not found' })
  if (post.authorId !== req.user.id && req.user.role !== 'admin') {
    return res.status(403).json({ error: 'Forbidden' })
  }
  await db.post.delete({ where: { id: req.params.id } })
  res.json({ success: true })
})
```

**Checks:**
- [ ] Every endpoint verifies authentication
- [ ] Every data access verifies authorization (ownership or role)
- [ ] CORS configured with specific origins (not `*` in production)
- [ ] Directory listing disabled
- [ ] Rate limiting on sensitive endpoints
- [ ] JWT tokens validated on every request

### 2. Cryptographic Failures (A02:2021)

```typescript
// ❌ BAD: Storing plaintext passwords
await db.user.create({ data: { password: req.body.password } })

// ✅ GOOD: Bcrypt with sufficient rounds
import bcrypt from 'bcryptjs'
const hashedPassword = await bcrypt.hash(req.body.password, 12)
await db.user.create({ data: { password: hashedPassword } })
```

**Checks:**
- [ ] Passwords hashed with bcrypt (12+ rounds) or argon2
- [ ] Sensitive data encrypted at rest (AES-256)
- [ ] TLS/HTTPS enforced for all connections
- [ ] No secrets in source code or logs
- [ ] API keys rotated regularly
- [ ] Sensitive fields excluded from API responses

### 3. Injection (A03:2021)

```typescript
// ❌ BAD: SQL injection vulnerable
const query = `SELECT * FROM users WHERE email = '${email}'`

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

// ✅ GOOD: ORM with parameterized input
const user = await prisma.user.findUnique({ where: { email } })
```

```typescript
// ❌ BAD: Command injection
const result = exec(`ls ${userInput}`)

// ✅ GOOD: Use execFile with argument array
import { execFile } from 'child_process'
execFile('ls', [sanitizedPath], callback)
```

**Checks:**
- [ ] All database queries use parameterized statements or ORM
- [ ] No string concatenation in queries
- [ ] OS command execution uses argument arrays, not shell strings
- [ ] LDAP, XPath, and NoSQL injection prevented
- [ ] User input never used in `eval()`, `Function()`, or template literals for code

### 4. Cross-Site Scripting (XSS) (A07:2021)

```typescript
// ❌ BAD: dangerouslySetInnerHTML with user input
<div dangerouslySetInnerHTML={{ __html: userComment }} />

// ✅ GOOD: Sanitize HTML
import DOMPurify from 'isomorphic-dompurify'
<div dangerouslySetInnerHTML={{ __html: DOMPurify.sanitize(userComment) }} />

// ✅ BEST: Render as text (React auto-escapes)
<div>{userComment}</div>
```

**Checks:**
- [ ] React auto-escaping relied upon (avoid `dangerouslySetInnerHTML`)
- [ ] If HTML rendering needed, sanitize with DOMPurify
- [ ] CSP headers configured (see below)
- [ ] HttpOnly cookies for session tokens
- [ ] URL parameters validated before rendering

### 5. Security Misconfiguration (A05:2021)

**Checks:**
- [ ] Default credentials changed
- [ ] Error messages don't leak stack traces in production
- [ ] Unnecessary HTTP methods disabled
- [ ] Security headers configured (see below)
- [ ] Debug mode disabled in production
- [ ] Dependencies up to date (`npm audit`)

---

## Security Headers

```typescript
// next.config.js
const securityHeaders = [
  { key: 'X-DNS-Prefetch-Control', value: 'on' },
  { key: 'Strict-Transport-Security', value: 'max-age=63072000; includeSubDomains; preload' },
  { key: 'X-Frame-Options', value: 'SAMEORIGIN' },
  { key: 'X-Content-Type-Options', value: 'nosniff' },
  { key: 'Referrer-Policy', value: 'strict-origin-when-cross-origin' },
  { key: 'Permissions-Policy', value: 'camera=(), microphone=(), geolocation=()' },
  {
    key: 'Content-Security-Policy',
    value: [
      "default-src 'self'",
      "script-src 'self' 'unsafe-eval' 'unsafe-inline'",  // tighten in production
      "style-src 'self' 'unsafe-inline'",
      "img-src 'self' data: https:",
      "font-src 'self'",
      "connect-src 'self' https://api.example.com",
      "frame-ancestors 'none'",
      "base-uri 'self'",
      "form-action 'self'",
    ].join('; '),
  },
]

module.exports = {
  async headers() {
    return [{ source: '/(.*)', headers: securityHeaders }]
  },
}
```

---

## Input Validation Patterns

### Zod Validation for API/Actions

```typescript
import { z } from 'zod'

const userSchema = z.object({
  email: z.string().email().max(255),
  password: z.string().min(8).max(128),
  name: z.string().min(1).max(100).regex(/^[a-zA-Z\s'-]+$/),
  age: z.number().int().min(13).max(150).optional(),
})

// Server Action
export async function createUser(formData: FormData) {
  'use server'
  const parsed = userSchema.safeParse({
    email: formData.get('email'),
    password: formData.get('password'),
    name: formData.get('name'),
  })

  if (!parsed.success) {
    return { error: parsed.error.flatten() }
  }

  // Safe to use parsed.data
}
```

### File Upload Validation

```typescript
const ALLOWED_TYPES = ['image/jpeg', 'image/png', 'image/webp']
const MAX_SIZE = 5 * 1024 * 1024 // 5MB

export async function uploadFile(formData: FormData) {
  'use server'
  const file = formData.get('file') as File

  if (!file || file.size === 0) return { error: 'No file' }
  if (!ALLOWED_TYPES.includes(file.type)) return { error: 'Invalid file type' }
  if (file.size > MAX_SIZE) return { error: 'File too large' }

  // Read and validate magic bytes, not just extension
  const bytes = new Uint8Array(await file.arrayBuffer())
  if (!validateMagicBytes(bytes, file.type)) return { error: 'File content mismatch' }
}
```

---

## Authentication Security

### JWT Best Practices

```typescript
import { SignJWT, jwtVerify } from 'jose'

const secret = new TextEncoder().encode(process.env.JWT_SECRET) // min 256-bit

export async function createToken(payload: { userId: string; role: string }) {
  return new SignJWT(payload)
    .setProtectedHeader({ alg: 'HS256' })
    .setIssuedAt()
    .setExpirationTime('15m')  // Short-lived access tokens
    .setAudience('your-app')
    .setIssuer('your-app')
    .sign(secret)
}

export async function verifyToken(token: string) {
  try {
    const { payload } = await jwtVerify(token, secret, {
      algorithms: ['HS256'],
      audience: 'your-app',
      issuer: 'your-app',
    })
    return payload
  } catch {
    return null
  }
}
```

### Cookie Security

```typescript
cookies().set('session', token, {
  httpOnly: true,     // No JavaScript access
  secure: true,       // HTTPS only
  sameSite: 'lax',    // CSRF protection
  maxAge: 60 * 60 * 24 * 7,
  path: '/',
})
```