Security Review

# Security Review Skill

This skill ensures all code follows security best practices and identifies potential vulnerabilities.

## When to Activate

- Implementing authentication or authorization
- Handling user input or file uploads
- Creating new API endpoints
- Working with secrets or credentials
- Implementing payment features
- Storing or transmitting sensitive data
- Integrating third-party APIs

## Security Checklist

### 1. Secrets Management

#### NEVER Do This

```typescript
const apiKey = "sk-proj-xxxxx"  // Hardcoded secret
const dbPassword = "password123" // In source code
```

#### ALWAYS Do This

```typescript
const apiKey = process.env.OPENAI_API_KEY
const dbUrl = process.env.DATABASE_URL

// Verify secrets exist
if (!apiKey) {
  throw new Error('OPENAI_API_KEY not configured')
}
```

#### Verification Steps

- No hardcoded API keys, tokens, or passwords
- All secrets in environment variables
- `.env.local` in .gitignore
- No secrets in git history
- Production secrets in hosting platform (Vercel, Railway)

### 2. Input Validation

#### Always Validate User Input

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

// Define validation schema
const CreateUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
  age: z.number().int().min(0).max(150)
})

// Validate before processing
export async function createUser(input: unknown) {
  try {
    const validated = CreateUserSchema.parse(input)
    return await db.users.create(validated)
  } catch (error) {
    if (error instanceof z.ZodError) {
      return { success: false, errors: error.errors }
    }
    throw error
  }
}
```

#### File Upload Validation

```typescript
function validateFileUpload(file: File) {
  // Size check (5MB max)
  const maxSize = 5 * 1024 * 1024
  if (file.size > maxSize) {
    throw new Error('File too large (max 5MB)')
  }

  // Type check
  const allowedTypes = ['image/jpeg', 'image/png', 'image/gif']
  if (!allowedTypes.includes(file.type)) {
    throw new Error('Invalid file type')
  }

  // Extension check
  const allowedExtensions = ['.jpg', '.jpeg', '.png', '.gif']
  const extension = file.name.toLowerCase().match(/\.[^.]+$/)?.[0]
  if (!extension || !allowedExtensions.includes(extension)) {
    throw new Error('Invalid file extension')
  }

  return true
}
```

#### Verification Steps

- All user inputs validated with schemas
- File uploads restricted (size, type, extension)
- No direct use of user input in queries
- Whitelist validation (not blacklist)
- Error messages don't leak sensitive info

### 3. SQL Injection Prevention

#### NEVER Concatenate SQL

```typescript
// DANGEROUS - SQL Injection vulnerability
const query = `SELECT * FROM users WHERE email = '${userEmail}'`
await db.query(query)
```

#### ALWAYS Use Parameterized Queries

```typescript
// Safe - parameterized query
const { data } = await supabase
  .from('users')
  .select('*')
  .eq('email', userEmail)

// Or with raw SQL
await db.query(
  'SELECT * FROM users WHERE email = $1',
  [userEmail]
)
```

### 4. Authentication & Authorization

#### JWT Token Handling

```typescript
// WRONG: localStorage (vulnerable to XSS)
localStorage.setItem('token', token)

// CORRECT: httpOnly cookies
res.setHeader('Set-Cookie',
  `token=${token}; HttpOnly; Secure; SameSite=Strict; Max-Age=3600`)
```

#### Authorization Checks

```typescript
export async function deleteUser(userId: string, requesterId: string) {
  const requester = await db.users.findUnique({
    where: { id: requesterId }
  })

  if (requester.role !== 'admin') {
    return NextResponse.json(
      { error: 'Unauthorized' },
      { status: 403 }
    )
  }

  await db.users.delete({ where: { id: userId } })
}
```

#### Row Level Security (Supabase)

```sql
ALTER TABLE users ENABLE ROW LEVEL SECURITY;

CREATE POLICY "Users view own data"
  ON users FOR SELECT
  USING (auth.uid() = id);

CREATE POLICY "Users update own data"
  ON users FOR UPDATE
  USING (auth.uid() = id);
```

### 5. XSS Prevention

#### Sanitize HTML

```typescript
import DOMPurify from 'isomorphic-dompurify'

function renderUserContent(html: string) {
  const clean = DOMPurify.sanitize(html, {
    ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'p'],
    ALLOWED_ATTR: []
  })
  return <div dangerouslySetInnerHTML={{ __html: clean }} />
}
```

#### Content Security Policy

```typescript
// next.config.js
const securityHeaders = [
  {
    key: 'Content-Security-Policy',
    value: `
      default-src 'self';
      script-src 'self' 'unsafe-eval' 'unsafe-inline';
      style-src 'self' 'unsafe-inline';
      img-src 'self' data: https:;
      font-src 'self';
      connect-src 'self' https://api.example.com;
    `.replace(/\s{2,}/g, ' ').trim()
  }
]
```

### 6. CSRF Protection

```typescript
import { csrf } from '@/lib/csrf'

export async function POST(request: Request) {
  const token = request.headers.get('X-CSRF-Token')

  if (!csrf.verify(token)) {
    return NextResponse.json(
      { error: 'Invalid CSRF token' },
      { status: 403 }
    )
  }
}
```

### 7. Rate Limiting

```typescript
import rateLimit from 'express-rate-limit'

const limiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100, // 100 requests per window
  message: 'Too many requests'
})

app.use('/api/', limiter)
```

### 8. Sensitive Data Exposure

#### Logging

```typescript
// WRONG: Logging sensitive data
console.log('User login:', { email, password })

// CORRECT: Redact sensitive data
console.log('User login:', { email, userId })
```

#### Error Messages

```typescript
// WRONG: Exposing internal details
catch (error) {
  return NextResponse.json(
    { error: error.message, stack: error.stack },
    { status: 500 }
  )
}

// CORRECT: Generic error messages
catch (error) {
  console.error('Internal error:', error)
  return NextResponse.json(
    { error: 'An error occurred. Please try again.' },
    { status: 500 }
  )
}
```

### 9. Dependency Security

```bash
# Check for vulnerabilities
npm audit

# Fix automatically fixable issues
npm audit fix

# Use in CI/CD for reproducible builds
npm ci
```

## Pre-Deployment Security Checklist

Before ANY production deployment:

- **Secrets**: No hardcoded secrets, all in env vars
- **Input Validation**: All user inputs validated
- **SQL Injection**: All queries parameterized
- **XSS**: User content sanitized
- **CSRF**: Protection enabled
- **Authentication**: Proper token handling
- **Authorization**: Role checks in place
- **Rate Limiting**: Enabled on all endpoints
- **HTTPS**: Enforced in production
- **Security Headers**: CSP, X-Frame-Options configured
- **Error Handling**: No sensitive data in errors
- **Logging**: No sensitive data logged
- **Dependencies**: Up to date, no vulnerabilities
- **CORS**: Properly configured
- **File Uploads**: Validated (size, type)

## Resources

- [OWASP Top 10](https://owasp.org/www-project-top-ten/)
- [Next.js Security](https://nextjs.org/docs/security)
- [Web Security Academy](https://portswigger.net/web-security)

**Remember**: Security is not optional. One vulnerability can compromise the entire platform.