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This guide ensures all code follows security best practices and identifies potential vulnerabilities before they reach production.

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When to Apply

Authentication & Authorization

Implementing login, user roles, permissions

User Input & File Uploads

Handling forms, file uploads, API parameters

API Endpoints

Creating REST APIs, webhooks, external integrations

Sensitive Data

Working with secrets, payments, personal information

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Pre-Commit Security Checklist

Before ANY commit, verify:
  • No hardcoded secrets (API keys, passwords, tokens)
  • All user inputs validated
  • SQL injection prevention (parameterized queries)
  • XSS prevention (sanitized HTML)
  • CSRF protection enabled
  • Authentication/authorization verified
  • Rate limiting on all endpoints
  • Error messages don’t leak sensitive data

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1. Secrets Management

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Never Hardcode Secrets

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

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Environment Variables

1

Create .env.local

OPENAI_API_KEY=sk-proj-xxxxx
DATABASE_URL=postgresql://...
STRIPE_SECRET_KEY=sk_live_xxxxx
2

Add to .gitignore

.env.local
.env*.local
3

Validate at Startup

const requiredEnvVars = [
  'OPENAI_API_KEY',
  'DATABASE_URL',
  'STRIPE_SECRET_KEY'
]

for (const envVar of requiredEnvVars) {
  if (!process.env[envVar]) {
    throw new Error(`Missing ${envVar}`)
  }
}
4

Deploy to Hosting Platform

Add secrets to:
  • Vercel: Project Settings → Environment Variables
  • Railway: Project → Variables
  • Render: Environment → Environment Variables

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Verification Checklist

  • 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

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2. Input Validation

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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
  }
}

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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 Checklist

  • 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

Never concatenate SQL queries with user input.
// SQL Injection vulnerability
const query = `SELECT * FROM users WHERE email = '${userEmail}'`
await db.query(query)

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Verification Checklist

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

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4. Authentication & Authorization

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JWT Token Handling

localStorage.setItem('token', token)

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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 } })
}

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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);

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Verification Checklist

  • 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

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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 }} />
}

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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()
  }
]

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Verification Checklist

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

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6. CSRF Protection

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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
}

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SameSite Cookies

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

​
Verification Checklist

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

​
7. Rate Limiting

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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)

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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 Checklist

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

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8. Sensitive Data Exposure

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Logging

console.log('User login:', { email, password })
console.log('Payment:', { cardNumber, cvv })

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Error Messages

catch (error) {
  return NextResponse.json(
    { error: error.message, stack: error.stack },
    { status: 500 }
  )
}

​
Verification Checklist

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

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9. Dependency Security

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Regular Updates

# Check for vulnerabilities
npm audit

# Fix automatically fixable issues
npm audit fix

# Update dependencies
npm update

# Check for outdated packages
npm outdated

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Lock Files

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

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

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Verification Checklist

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

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Security Testing

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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 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)

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Security Response Protocol

1

STOP Immediately

If security issue found, stop all work.
2

Use security-reviewer Agent

/security-scan
Or delegate to security-reviewer agent.
3

Fix CRITICAL Issues

Address all critical vulnerabilities before continuing.
4

Rotate Exposed Secrets

If any secrets were exposed, rotate immediately.
5

Review Entire Codebase

Search for similar issues across the project.

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ECC Security Tools

security-reviewer Agent

Specialized agent for vulnerability detection and security audits.

/security-scan Command

Run AgentShield security auditor (1282 tests, 102 rules).

security-review Skill

Comprehensive security checklist and patterns.

AgentShield

External tool: npx ecc-agentshield scan --fix

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Resources

Security is not optional. One vulnerability can compromise the entire platform. When in doubt, err on the side of caution.