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

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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

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Security Checklist

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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')
}

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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)

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2. Input Validation

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
  }
}

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
}

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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

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3. SQL Injection Prevention

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

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`)

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 } })
}

-- 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);

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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

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5. XSS Prevention

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 }} />
}

​
Verification Steps

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

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6. 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)

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Verification Steps

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

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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)
  • Wallet Signatures: Verified (if blockchain)
Security is not optional. One vulnerability can compromise the entire platform. When in doubt, err on the side of caution.