Secure Code Guardian

Security-focused developer specializing in writing secure code and preventing vulnerabilities.

Role Definition

You are a senior security engineer with 10+ years of application security experience. You specialize in secure coding practices, OWASP Top 10 prevention, and implementing authentication/authorization. You think defensively and assume all input is malicious.

When to Use This Skill

•Implementing authentication/authorization
•Securing user input handling
•Implementing encryption
•Preventing OWASP Top 10 vulnerabilities
•Security hardening existing code
•Implementing secure session management

Core Workflow

•Threat model - Identify attack surface and threats
•Design - Plan security controls
•Implement - Write secure code with defense in depth
•Validate - Test security controls
•Document - Record security decisions

Reference Guide

Load detailed guidance based on context:

Topic	Reference	Load When
OWASP	`references/owasp-prevention.md`	OWASP Top 10 patterns
Authentication	`references/authentication.md`	Password hashing, JWT
Input Validation	`references/input-validation.md`	Zod, SQL injection
XSS/CSRF	`references/xss-csrf.md`	XSS prevention, CSRF
Headers	`references/security-headers.md`	Helmet, rate limiting

Constraints

MUST DO

•Hash passwords with bcrypt/argon2 (never plaintext)
•Use parameterized queries (prevent SQL injection)
•Validate and sanitize all user input
•Implement rate limiting on auth endpoints
•Use HTTPS everywhere
•Set security headers
•Log security events
•Store secrets in environment/secret managers

MUST NOT DO

•Store passwords in plaintext
•Trust user input without validation
•Expose sensitive data in logs or errors
•Use weak encryption algorithms
•Hardcode secrets in code
•Disable security features for convenience

Output Templates

When implementing security features, provide:

•Secure implementation code
•Security considerations noted
•Configuration requirements (env vars, headers)
•Testing recommendations

Knowledge Reference

OWASP Top 10, bcrypt/argon2, JWT, OAuth 2.0, OIDC, CSP, CORS, rate limiting, input validation, output encoding, encryption (AES, RSA), TLS, security headers

Related Skills

•Fullstack Guardian - Feature implementation with security
•Security Reviewer - Security code review
•Architecture Designer - Security architecture

Authentication

Password Hashing

typescript

import bcrypt from 'bcrypt';

const SALT_ROUNDS = 12;

async function hashPassword(password: string): Promise<string> {
  return bcrypt.hash(password, SALT_ROUNDS);
}

async function verifyPassword(password: string, hash: string): Promise<boolean> {
  return bcrypt.compare(password, hash);
}

// Password requirements
const PASSWORD_REGEX = /^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{12,}$/;

function validatePassword(password: string): { valid: boolean; errors: string[] } {
  const errors: string[] = [];

  if (password.length < 12) errors.push('Minimum 12 characters');
  if (!/[a-z]/.test(password)) errors.push('Requires lowercase');
  if (!/[A-Z]/.test(password)) errors.push('Requires uppercase');
  if (!/\d/.test(password)) errors.push('Requires digit');
  if (!/[@$!%*?&]/.test(password)) errors.push('Requires special character');

  return { valid: errors.length === 0, errors };
}

JWT Implementation

typescript

import jwt from 'jsonwebtoken';

const JWT_SECRET = process.env.JWT_SECRET!;
const ACCESS_TOKEN_EXPIRY = '15m';
const REFRESH_TOKEN_EXPIRY = '7d';

interface TokenPayload {
  sub: string;
  type: 'access' | 'refresh';
}

function generateAccessToken(userId: string): string {
  return jwt.sign(
    { sub: userId, type: 'access' },
    JWT_SECRET,
    { expiresIn: ACCESS_TOKEN_EXPIRY }
  );
}

function generateRefreshToken(userId: string): string {
  return jwt.sign(
    { sub: userId, type: 'refresh' },
    JWT_SECRET,
    { expiresIn: REFRESH_TOKEN_EXPIRY }
  );
}

function verifyToken(token: string): TokenPayload {
  return jwt.verify(token, JWT_SECRET) as TokenPayload;
}

Auth Middleware

typescript

function authMiddleware(req: Request, res: Response, next: NextFunction) {
  const header = req.headers.authorization;

  if (!header?.startsWith('Bearer ')) {
    return res.status(401).json({ error: 'Missing token' });
  }

  try {
    const token = header.slice(7);
    const payload = verifyToken(token);

    if (payload.type !== 'access') {
      return res.status(401).json({ error: 'Invalid token type' });
    }

    req.userId = payload.sub;
    next();
  } catch (error) {
    if (error instanceof jwt.TokenExpiredError) {
      return res.status(401).json({ error: 'Token expired' });
    }
    return res.status(401).json({ error: 'Invalid token' });
  }
}

Account Lockout

typescript

const MAX_ATTEMPTS = 5;
const LOCKOUT_DURATION = 15 * 60 * 1000; // 15 minutes

async function handleLoginAttempt(email: string, success: boolean) {
  const key = `login:attempts:${email}`;

  if (success) {
    await redis.del(key);
    return;
  }

  const attempts = await redis.incr(key);
  await redis.expire(key, LOCKOUT_DURATION / 1000);

  if (attempts >= MAX_ATTEMPTS) {
    await redis.set(`login:locked:${email}`, '1', 'PX', LOCKOUT_DURATION);
    throw new Error('Account locked. Try again later.');
  }
}

Quick Reference

Practice	Implementation
Password hash	bcrypt (12+ rounds)
Token expiry	Access: 15m, Refresh: 7d
Lockout	5 attempts, 15min lockout
MFA	TOTP (authenticator apps)

JWT Claim	Purpose
`sub`	User ID
`exp`	Expiration
`iat`	Issued at
`type`	access/refresh

Input Validation

Zod Validation

typescript

import { z } from 'zod';

const UserSchema = z.object({
  email: z.string().email().max(255),
  name: z.string().min(1).max(100).regex(/^[\w\s-]+$/),
  age: z.number().int().min(0).max(150).optional(),
  role: z.enum(['user', 'admin']).default('user'),
});

function validateUser(data: unknown) {
  return UserSchema.parse(data); // Throws on invalid
}

// Safe parse (no throw)
const result = UserSchema.safeParse(data);
if (!result.success) {
  console.error(result.error.issues);
}

SQL Injection Prevention

typescript

// ❌ NEVER do this
const bad = `SELECT * FROM users WHERE id = ${userId}`;
const bad2 = `SELECT * FROM users WHERE name = '${name}'`;

// ✅ Parameterized queries
const good = await db.query(
  'SELECT * FROM users WHERE id = $1 AND name = $2',
  [userId, name]
);

// ✅ Use ORM
const user = await prisma.user.findFirst({
  where: { id: userId, name: name }
});

// ✅ Query builder
const user = await knex('users')
  .where({ id: userId, name: name })
  .first();

Path Traversal Prevention

typescript

import path from 'path';

// ❌ Vulnerable
const vulnerable = path.join('/uploads', userInput);

// ✅ Safe - validate and sanitize
function getSecurePath(baseDir: string, userInput: string): string {
  // Remove any path traversal attempts
  const sanitized = path.basename(userInput);

  // Resolve and verify it's within base directory
  const fullPath = path.resolve(baseDir, sanitized);

  if (!fullPath.startsWith(path.resolve(baseDir))) {
    throw new Error('Invalid path');
  }

  return fullPath;
}

Command Injection Prevention

typescript

import { execFile } from 'child_process';

// ❌ Never use exec with user input
exec(`convert ${userInput}`); // Vulnerable!

// ✅ Use execFile with arguments array
execFile('convert', ['-resize', '100x100', safeFilename], (error, stdout) => {
  // ...
});

// ✅ Better: Use library functions instead of shell
import sharp from 'sharp';
await sharp(inputPath).resize(100, 100).toFile(outputPath);

URL Validation

typescript

function validateUrl(input: string, allowedHosts: string[]): URL {
  const url = new URL(input);

  // Check protocol
  if (!['http:', 'https:'].includes(url.protocol)) {
    throw new Error('Invalid protocol');
  }

  // Check host allowlist
  if (!allowedHosts.includes(url.hostname)) {
    throw new Error('Host not allowed');
  }

  return url;
}

File Upload Validation

typescript

const ALLOWED_TYPES = ['image/jpeg', 'image/png', 'image/gif'];
const MAX_SIZE = 5 * 1024 * 1024; // 5MB

function validateUpload(file: Express.Multer.File) {
  if (!ALLOWED_TYPES.includes(file.mimetype)) {
    throw new Error('Invalid file type');
  }

  if (file.size > MAX_SIZE) {
    throw new Error('File too large');
  }

  // Verify magic bytes (not just extension)
  const buffer = fs.readFileSync(file.path);
  const type = fileType.fromBuffer(buffer);

  if (!type || !ALLOWED_TYPES.includes(type.mime)) {
    throw new Error('Invalid file content');
  }
}

Quick Reference

Input Type	Validation
Email	Regex + max length
URL	Protocol + host allowlist
File path	basename + resolve check
SQL	Parameterized queries
Command	execFile + no shell
File upload	Type + size + magic bytes

OWASP Top 10 Prevention

OWASP Top 10 Quick Reference

#	Vulnerability	Prevention
1	Injection	Parameterized queries, ORMs
2	Broken Auth	Strong passwords, MFA, secure sessions
3	Sensitive Data	Encryption at rest/transit
4	XXE	Disable DTDs, use JSON
5	Broken Access	Deny by default, server-side validation
6	Misconfig	Security headers, disable defaults
7	XSS	Output encoding, CSP
8	Insecure Deserialization	Schema validation, allowlists
9	Known Vulnerabilities	Dependency scanning
10	Insufficient Logging	Log security events

A01: Injection Prevention

typescript

// SQL Injection - Use parameterized queries
// ❌ Bad
const bad = `SELECT * FROM users WHERE id = ${userId}`;

// ✅ Good
const good = await db.query('SELECT * FROM users WHERE id = $1', [userId]);

// ✅ Good - Use ORM
const user = await prisma.user.findUnique({ where: { id: userId } });

// Command Injection - Avoid shell execution
// ❌ Bad
exec(`ls ${userInput}`);

// ✅ Good - Use library functions
const files = fs.readdirSync(safeDirectory);

A02: Broken Authentication

typescript

// Use bcrypt for passwords
const hash = await bcrypt.hash(password, 12);
const isValid = await bcrypt.compare(password, hash);

// Implement account lockout
if (failedAttempts >= 5) {
  await lockAccount(userId, 15 * 60 * 1000); // 15 min
}

// Use secure session configuration
app.use(session({
  secret: process.env.SESSION_SECRET,
  cookie: {
    httpOnly: true,
    secure: true,
    sameSite: 'strict',
    maxAge: 15 * 60 * 1000, // 15 minutes
  },
}));

A03: Sensitive Data Exposure

typescript

// Encrypt sensitive data at rest
import crypto from 'crypto';

function encrypt(text: string, key: Buffer): string {
  const iv = crypto.randomBytes(16);
  const cipher = crypto.createCipheriv('aes-256-gcm', key, iv);
  // ... encryption logic
}

// Use HTTPS only
app.use((req, res, next) => {
  if (!req.secure) {
    return res.redirect(`https://${req.hostname}${req.url}`);
  }
  next();
});

A05: Broken Access Control

typescript

// Always validate on server side
async function getResource(userId: string, resourceId: string) {
  const resource = await db.resource.findUnique({ where: { id: resourceId } });

  // Verify ownership
  if (resource.ownerId !== userId) {
    throw new ForbiddenError('Access denied');
  }

  return resource;
}

// Use role-based access
function requireRole(...roles: string[]) {
  return (req: Request, res: Response, next: NextFunction) => {
    if (!roles.includes(req.user.role)) {
      return res.status(403).json({ error: 'Forbidden' });
    }
    next();
  };
}

A07: XSS Prevention

typescript

// Use Content Security Policy
app.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'"],
    styleSrc: ["'self'", "'unsafe-inline'"],
  },
}));

// Sanitize user input for HTML
import DOMPurify from 'dompurify';
const clean = DOMPurify.sanitize(userInput);

Quick Reference

Attack	Defense
SQL Injection	Parameterized queries
XSS	Output encoding, CSP
CSRF	CSRF tokens
IDOR	Authorization checks
Command Injection	Avoid exec(), validate input

Security Headers

Helmet (Express)

typescript

import helmet from 'helmet';

app.use(helmet()); // Enable all defaults

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

Manual Headers

typescript

app.use((req, res, next) => {
  // Prevent clickjacking
  res.setHeader('X-Frame-Options', 'DENY');

  // Prevent MIME sniffing
  res.setHeader('X-Content-Type-Options', 'nosniff');

  // HSTS (HTTPS only)
  res.setHeader('Strict-Transport-Security', 'max-age=31536000; includeSubDomains');

  // Referrer policy
  res.setHeader('Referrer-Policy', 'strict-origin-when-cross-origin');

  // Permissions policy
  res.setHeader('Permissions-Policy', 'geolocation=(), microphone=(), camera=()');

  next();
});

Rate Limiting

typescript

import rateLimit from 'express-rate-limit';

// General API rate limit
const apiLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 100,
  message: { error: 'Too many requests' },
  standardHeaders: true,
  legacyHeaders: false,
});

app.use('/api/', apiLimiter);

// Strict limit for auth endpoints
const authLimiter = rateLimit({
  windowMs: 15 * 60 * 1000,
  max: 5,
  message: { error: 'Too many login attempts' },
  skipSuccessfulRequests: true,
});

app.post('/api/login', authLimiter, loginHandler);
app.post('/api/register', authLimiter, registerHandler);

CORS Configuration

typescript

import cors from 'cors';

// Strict CORS
app.use(cors({
  origin: ['https://example.com', 'https://app.example.com'],
  methods: ['GET', 'POST', 'PUT', 'DELETE'],
  allowedHeaders: ['Content-Type', 'Authorization'],
  credentials: true,
  maxAge: 86400, // Cache preflight for 24 hours
}));

// Dynamic origin validation
app.use(cors({
  origin: (origin, callback) => {
    const allowedOrigins = ['https://example.com'];
    if (!origin || allowedOrigins.includes(origin)) {
      callback(null, true);
    } else {
      callback(new Error('Not allowed by CORS'));
    }
  },
}));

Cookie Security

typescript

res.cookie('session', token, {
  httpOnly: true,      // No JavaScript access
  secure: true,        // HTTPS only
  sameSite: 'strict',  // CSRF protection
  maxAge: 900000,      // 15 minutes
  path: '/',
  domain: '.example.com',
});

Quick Reference

Header	Value	Purpose
X-Frame-Options	DENY	Clickjacking
X-Content-Type-Options	nosniff	MIME sniffing
Strict-Transport-Security	max-age=31536000	Force HTTPS
Content-Security-Policy	default-src 'self'	XSS
Referrer-Policy	strict-origin-when-cross-origin	Privacy

Cookie Flag	Purpose
httpOnly	No JS access
secure	HTTPS only
sameSite=strict	CSRF protection
maxAge	Expiration

XSS & CSRF Prevention

XSS Prevention

Output Encoding

typescript

// React automatically escapes by default
function SafeComponent({ userInput }: { userInput: string }) {
  return <div>{userInput}</div>; // Safe - auto-escaped
}

// If you must render HTML, sanitize first
import DOMPurify from 'dompurify';

function HtmlContent({ html }: { html: string }) {
  return (
    <div
      dangerouslySetInnerHTML={{
        __html: DOMPurify.sanitize(html)
      }}
    />
  );
}

Content Security Policy

typescript

import helmet from 'helmet';

app.use(helmet.contentSecurityPolicy({
  directives: {
    defaultSrc: ["'self'"],
    scriptSrc: ["'self'"],
    styleSrc: ["'self'", "'unsafe-inline'"],
    imgSrc: ["'self'", "data:", "https:"],
    connectSrc: ["'self'", "https://api.example.com"],
    fontSrc: ["'self'"],
    objectSrc: ["'none'"],
    frameSrc: ["'none'"],
    upgradeInsecureRequests: [],
  },
}));

Input Sanitization

typescript

import DOMPurify from 'dompurify';

// Sanitize HTML
const clean = DOMPurify.sanitize(dirty);

// Sanitize with config
const cleanStrict = DOMPurify.sanitize(dirty, {
  ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a'],
  ALLOWED_ATTR: ['href'],
});

// Strip all HTML
const textOnly = DOMPurify.sanitize(dirty, { ALLOWED_TAGS: [] });

CSRF Prevention

Synchronizer Token Pattern

typescript

import csrf from 'csurf';

const csrfProtection = csrf({ cookie: true });

// Add to forms
app.get('/form', csrfProtection, (req, res) => {
  res.render('form', { csrfToken: req.csrfToken() });
});

// Validate on submission
app.post('/submit', csrfProtection, (req, res) => {
  // Token validated automatically
});

Double Submit Cookie

typescript

// Set CSRF cookie
res.cookie('csrf', token, {
  httpOnly: false, // Must be readable by JS
  secure: true,
  sameSite: 'strict',
});

// Client sends in header
fetch('/api/action', {
  method: 'POST',
  headers: {
    'X-CSRF-Token': getCookie('csrf'),
  },
});

// Server validates
if (req.cookies.csrf !== req.headers['x-csrf-token']) {
  return res.status(403).json({ error: 'CSRF validation failed' });
}

SameSite Cookies

typescript

// Modern CSRF protection
app.use(session({
  cookie: {
    httpOnly: true,
    secure: true,
    sameSite: 'strict', // Or 'lax' for GET requests
  },
}));

HTTP Headers

typescript

// Security headers
app.use((req, res, next) => {
  // Prevent clickjacking
  res.setHeader('X-Frame-Options', 'DENY');

  // Prevent MIME sniffing
  res.setHeader('X-Content-Type-Options', 'nosniff');

  // XSS filter (legacy)
  res.setHeader('X-XSS-Protection', '1; mode=block');

  // Referrer policy
  res.setHeader('Referrer-Policy', 'strict-origin-when-cross-origin');

  next();
});

Quick Reference

Attack	Prevention
Reflected XSS	Output encoding
Stored XSS	Input sanitization + encoding
DOM XSS	Avoid innerHTML, use textContent
CSRF	Tokens + SameSite cookies

Header	Purpose
CSP	Script/resource restrictions
X-Frame-Options	Clickjacking
X-Content-Type-Options	MIME sniffing
SameSite	CSRF protection