Security Checklist

Input validation

• All user input is validated server-side (client validation is UX, not security)
• SQL queries use parameterized statements (NEVER string concatenation)
• HTML output is escaped to prevent XSS (use framework defaults)
• File uploads validate type, size, and are stored outside webroot
• URLs and redirects are validated against allowlist
• JSON/XML parsers have entity expansion limits

Secrets management

• No secrets in source code or git history
• Environment variables or secrets manager for all credentials
• Different secrets for development/staging/production
• API keys have minimum required permissions
• Secrets can be rotated without code deployment

Database security

• Database not exposed to public internet
• Application uses dedicated database user (not root/admin)
• Database user has minimum required permissions
• Row-level security (RLS) enabled where applicable
• Backups encrypted and tested for restoration
• Connection strings use SSL/TLS

Network and transport

• HTTPS only (redirect HTTP to HTTPS)
• TLS 1.2+ (disable older versions)
• HSTS header enabled
• Secure cookie flags set (Secure, HttpOnly, SameSite)
• CORS configured for specific origins (not * in production)

Rate limiting and DDoS

• API endpoints rate-limited per user/IP
• Login endpoints have stricter limits
• CDN or DDoS protection in front of application
• Request size limits configured
• Timeout limits on all external calls

Logging and monitoring

• Authentication events logged (login, logout, failed attempts)
• No sensitive data in logs (passwords, tokens, PII)
• Logs stored securely with retention policy
• Alerts configured for suspicious patterns
• Error messages don't leak system information

Infrastructure

• Firewall configured (deny by default)
• Unnecessary ports closed
• SSH key authentication (no password auth)
• Dependencies updated (check for known vulnerabilities)
• Admin interfaces not publicly accessible

Common vulnerabilities by framework

Node.js/Express

// BAD: SQL injection
db.query(`SELECT * FROM users WHERE id = ${req.params.id}`);

// GOOD: Parameterized query
db.query('SELECT * FROM users WHERE id = $1', [req.params.id]);

// BAD: XSS vulnerability
res.send(`<h1>Hello ${req.query.name}</h1>`);

// GOOD: Use template engine with auto-escaping
res.render('greeting', { name: req.query.name });

// BAD: Secrets in code
const API_KEY = 'sk_live_abc123';

// GOOD: Environment variables
const API_KEY = process.env.API_KEY;

Python/Django/Flask

# BAD: SQL injection
cursor.execute(f"SELECT * FROM users WHERE id = {user_id}")

# GOOD: Parameterized query
cursor.execute("SELECT * FROM users WHERE id = %s", [user_id])

# BAD: Pickle deserialization (RCE vulnerability)
data = pickle.loads(user_input)

# GOOD: Use JSON for untrusted data
data = json.loads(user_input)

# BAD: Debug mode in production
app.run(debug=True)

# GOOD: Debug off, proper error handling
app.run(debug=False)

React/Frontend

// BAD: XSS via dangerouslySetInnerHTML
<div dangerouslySetInnerHTML={{ __html: userContent }} />

// GOOD: Let React escape content
<div>{userContent}</div>

// BAD: Storing tokens in localStorage (XSS accessible)
localStorage.setItem('token', authToken);

// BETTER: HttpOnly cookies (not accessible via JS)
// Set via server response header

// BAD: Exposing API keys in frontend code
const API_KEY = 'sk_live_abc123';

// GOOD: Proxy through your backend
const response = await fetch('/api/proxy/external-service');

Password hashing reference

Node.js with bcrypt

const bcrypt = require('bcrypt');

// Hashing (on registration)
const saltRounds = 12;
const hashedPassword = await bcrypt.hash(plainPassword, saltRounds);

// Verification (on login)
const isValid = await bcrypt.compare(plainPassword, hashedPassword);

Python with bcrypt

import bcrypt

# Hashing
hashed = bcrypt.hashpw(password.encode('utf-8'), bcrypt.gensalt(rounds=12))

# Verification
is_valid = bcrypt.checkpw(password.encode('utf-8'), hashed)

Environment variables setup

.env file (development only, never commit)

# .env
DATABASE_URL=postgresql://user:pass@localhost:5432/myapp
JWT_SECRET=your-256-bit-secret-here
API_KEY=sk_test_xxxxx

.gitignore (mandatory)

.env
.env.local
.env.*.local
*.pem
*.key
credentials.json
secrets/

Loading environment variables

// Node.js with dotenv
require('dotenv').config();
const dbUrl = process.env.DATABASE_URL;

// Fail fast if missing
if (!process.env.JWT_SECRET) {
  throw new Error('JWT_SECRET environment variable is required');
}

Database row-level security (Supabase/PostgreSQL)

-- Enable RLS on table
ALTER TABLE documents ENABLE ROW LEVEL SECURITY;

-- Users can only read their own documents
CREATE POLICY "Users can read own documents" ON documents
  FOR SELECT USING (auth.uid() = user_id);

-- Users can only insert documents as themselves
CREATE POLICY "Users can insert own documents" ON documents
  FOR INSERT WITH CHECK (auth.uid() = user_id);

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

-- Users can only delete their own documents
CREATE POLICY "Users can delete own documents" ON documents
  FOR DELETE USING (auth.uid() = user_id);

CORS configuration

Express.js

const cors = require('cors');

// BAD: Allow all origins
app.use(cors());

// GOOD: Specific origins
app.use(cors({
  origin: ['https://myapp.com', 'https://www.myapp.com'],
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  allowedHeaders: ['Content-Type', 'Authorization'],
  credentials: true
}));

Security headers

Express.js with Helmet

const helmet = require('helmet');

app.use(helmet({
  contentSecurityPolicy: {
    directives: {
      defaultSrc: ["'self'"],
      scriptSrc: ["'self'"],
      styleSrc: ["'self'", "'unsafe-inline'"],
      imgSrc: ["'self'", "data:", "https:"],
    },
  },
  hsts: {
    maxAge: 31536000,
    includeSubDomains: true,
    preload: true
  }
}));

Manual headers

// Essential security headers
res.setHeader('X-Content-Type-Options', 'nosniff');
res.setHeader('X-Frame-Options', 'DENY');
res.setHeader('X-XSS-Protection', '1; mode=block');
res.setHeader('Strict-Transport-Security', 'max-age=31536000; includeSubDomains');
res.setHeader('Content-Security-Policy', "default-src 'self'");

What NOT to log

// NEVER log these:
// - Passwords (plain or hashed)
// - Session tokens / JWTs
// - API keys
// - Credit card numbers
// - Social security numbers
// - Full request bodies with user data

// BAD
console.log('Login attempt:', { email, password });
console.log('Request:', req.body);

// GOOD
console.log('Login attempt:', { email, success: false, reason: 'invalid_password' });
console.log('Request:', { endpoint: req.path, method: req.method, userId: req.user?.id });

Quick fixes for common issues

"I stored passwords in plain text"

1. Add password hashing immediately
2. Force password reset for all users
3. Invalidate all existing sessions
4. Check if database was ever exposed/leaked

"My API key is in the git history"

1. Rotate the key immediately (generate new one)
2. Revoke the old key
3. Use git filter-branch or BFG Repo-Cleaner to remove from history
4. Force push (coordinate with team)

"I don't know what data I'm logging"

1. Search codebase for console.log, logger., print(
2. Review what's being logged
3. Implement structured logging with field allowlists
4. Set up log rotation and retention

"My database is publicly accessible"

1. Change database credentials immediately
2. Configure firewall rules (allow only application server IPs)
3. Enable SSL/TLS for connections
4. Review access logs for unauthorized queries

Compliance quick reference

You likely need to care about:

• GDPR (EU users): Data deletion rights, consent, breach notification
• CCPA (California users): Similar to GDPR
• PCI DSS (credit cards): Don't store card numbers, use payment processor
• HIPAA (health data): Encryption, access controls, audit logs
• SOC 2 (enterprise sales): Security controls documentation

Minimum for any user data:

1. Privacy policy explaining data collection
2. Way for users to request data deletion
3. Encryption in transit (HTTPS) and at rest
4. Access logs and audit trail
5. Incident response plan (even if basic)

Resources

• OWASP Top 10: https://owasp.org/www-project-top-ten/
• OWASP Cheat Sheets: https://cheatsheetseries.owasp.org/
• Have I Been Pwned (check for breaches): https://haveibeenpwned.com/
• Mozilla Observatory (test your headers): https://observatory.mozilla.org/
• SSL Labs (test your TLS): https://www.ssllabs.com/ssltest/