Security Review

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

vs security-scan: This skill audits code patterns — authentication, input validation, secrets handling, SQL injection, XSS, CSRF, OWASP Top 10. Use security-scan when you need to audit your Claude Code configuration (.claude/settings.json, hooks, MCP configs, CLAUDE.md) for injection risks.

When to Activate

• Implementing authentication or authorization (JWT, OAuth, session management)
• Handling user input — form fields, file uploads, query parameters, JSON payloads
• Creating new API endpoints that accept external data
• Working with secrets, credentials, or API keys in code or configuration
• Implementing payment processing or financial transaction logic
• Storing or transmitting PII, health data, or other sensitive user data
• Integrating third-party APIs that receive or return sensitive data
• Writing SQL queries or ORM calls that include user-provided values

Security Checklist

1. Secrets Management

❌ NEVER Do This

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

✅ ALWAYS Do This

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

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

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

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

✅ ALWAYS Use Parameterized Queries

// 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]
)

Verification Steps

• All database queries use parameterized queries
• No string concatenation in SQL
• ORM/query builder used correctly
• Supabase queries properly sanitized

4. Authentication & Authorization

JWT Token Handling

// ❌ 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

export async function deleteUser(userId: string, requesterId: string) {
  // ALWAYS verify authorization first
  const requester = await db.users.findUnique({
    where: { id: requesterId }
  })

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

  // Proceed with deletion
  await db.users.delete({ where: { id: userId } })
}

Row Level Security (Supabase)

-- Enable RLS on all tables
ALTER TABLE users ENABLE ROW LEVEL SECURITY;

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

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

Verification Steps

• Tokens stored in httpOnly cookies (not localStorage)
• Authorization checks before sensitive operations
• Row Level Security enabled in Supabase
• Role-based access control implemented
• Session management secure

5. XSS Prevention

Sanitize HTML

import DOMPurify from 'isomorphic-dompurify'

// ALWAYS sanitize user-provided HTML
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

// 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()
  }
]

Verification Steps

• User-provided HTML sanitized
• CSP headers configured
• No unvalidated dynamic content rendering
• React's built-in XSS protection used

6. CSRF Protection

CSRF Tokens

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 }
    )
  }

  // Process request
}

SameSite Cookies

res.setHeader('Set-Cookie',
  `session=${sessionId}; HttpOnly; Secure; SameSite=Strict`)

Verification Steps

• CSRF tokens on state-changing operations
• SameSite=Strict on all cookies
• Double-submit cookie pattern implemented

7. Rate Limiting

API Rate Limiting

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'
})

// Apply to routes
app.use('/api/', limiter)

Expensive Operations

// Aggressive rate limiting for searches
const searchLimiter = rateLimit({
  windowMs: 60 * 1000, // 1 minute
  max: 10, // 10 requests per minute
  message: 'Too many search requests'
})

app.use('/api/search', searchLimiter)

Verification Steps

• Rate limiting on all API endpoints
• Stricter limits on expensive operations
• IP-based rate limiting
• User-based rate limiting (authenticated)

8. Sensitive Data Exposure

Logging

// ❌ WRONG: Logging sensitive data
console.log('User login:', { email, password })
console.log('Payment:', { cardNumber, cvv })

// ✅ CORRECT: Redact sensitive data
console.log('User login:', { email, userId })
console.log('Payment:', { last4: card.last4, userId })

Error Messages

// ❌ 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 }
  )
}

Verification Steps

• No passwords, tokens, or secrets in logs
• Error messages generic for users
• Detailed errors only in server logs
• No stack traces exposed to users

9. Dependency Security

Regular Updates

# Check for vulnerabilities
npm audit

# Fix automatically fixable issues
npm audit fix

# Update dependencies
npm update

# Check for outdated packages
npm outdated

Lock Files

# ALWAYS commit lock files
git add package-lock.json

# Use in CI/CD for reproducible builds
npm ci  # Instead of npm install

Verification Steps

• Dependencies up to date
• No known vulnerabilities (npm audit clean)
• Lock files committed
• Dependabot enabled on GitHub
• Regular security updates

Security Testing

Automated Security Tests

// Test authentication
test('requires authentication', async () => {
  const response = await fetch('/api/protected')
  expect(response.status).toBe(401)
})

// Test authorization
test('requires admin role', async () => {
  const response = await fetch('/api/admin', {
    headers: { Authorization: `Bearer ${userToken}` }
  })
  expect(response.status).toBe(403)
})

// Test input validation
test('rejects invalid input', async () => {
  const response = await fetch('/api/users', {
    method: 'POST',
    body: JSON.stringify({ email: 'not-an-email' })
  })
  expect(response.status).toBe(400)
})

// Test rate limiting
test('enforces rate limits', async () => {
  const requests = Array(101).fill(null).map(() =>
    fetch('/api/endpoint')
  )

  const responses = await Promise.all(requests)
  const tooManyRequests = responses.filter(r => r.status === 429)

  expect(tooManyRequests.length).toBeGreaterThan(0)
})

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
• Row Level Security: Enabled in Supabase
• CORS: Properly configured
• File Uploads: Validated (size, type)

For authentication and authorization patterns (JWT, OAuth 2.0, RBAC, session management) — see skill auth-patterns.

For security anti-patterns (localStorage token storage, client-side auth checks, error detail leaks, blacklist vs whitelist validation, npm install in CI), resources, and the full reminder — see skill security-review-advanced.