---

name: application-security description: >- OWASP Top 10 with code examples, SAST/DAST tools, dependency scanning, CSP headers, and input validation patterns. Use when hardening applications, reviewing security posture, or implementing defensive coding practices.

Application Security Skill

Secure coding patterns, vulnerability prevention, and security tooling for web applications.

---

OWASP Top 10 (2021) with Code Examples

A01: Broken Access Control

// BAD - No authorization check
app.get('/api/users/:id', async (req, res) => {
  const user = await db.user.findUnique({ where: { id: req.params.id } });
  res.json(user);
});

// GOOD - Verify ownership or role
app.get('/api/users/:id', authenticate, async (req, res) => {
  if (req.user.id !== req.params.id && req.user.role !== 'ADMIN') {
    return res.status(403).json({ error: 'Forbidden' });
  }
  const user = await db.user.findUnique({ where: { id: req.params.id } });
  res.json(user);
});

A02: Cryptographic Failures

// BAD - Weak hashing
import crypto from 'crypto';
const hash = crypto.createHash('md5').update(password).digest('hex');

// GOOD - Use bcrypt with proper rounds
import bcrypt from 'bcrypt';
const hash = await bcrypt.hash(password, 12);
const isValid = await bcrypt.compare(password, hash);

A03: Injection

// BAD - SQL injection
const query = `SELECT * FROM users WHERE email = '${email}'`;

// GOOD - Parameterized queries (Prisma handles this automatically)
const user = await prisma.user.findUnique({ where: { email } });

// GOOD - Parameterized raw SQL when needed
const users = await prisma.$queryRaw`SELECT * FROM users WHERE email = ${email}`;

A07: Cross-Site Scripting (XSS)

// BAD - Rendering raw HTML
element.innerHTML = userInput;

// GOOD - Use textContent or framework escaping
element.textContent = userInput;

// GOOD - React auto-escapes by default
return <div>{userInput}</div>;

// BAD in React - dangerouslySetInnerHTML
return <div dangerouslySetInnerHTML={{ __html: userInput }} />;

---

Content Security Policy (CSP)

Recommended Headers

// Next.js middleware
import { NextResponse } from 'next/server';

export function middleware(request: Request) {
  const nonce = crypto.randomUUID();
  const csp = [
    `default-src 'self'`,
    `script-src 'self' 'nonce-${nonce}'`,
    `style-src 'self' 'unsafe-inline'`,
    `img-src 'self' data: https:`,
    `font-src 'self'`,
    `connect-src 'self' https://api.example.com`,
    `frame-ancestors 'none'`,
    `base-uri 'self'`,
    `form-action 'self'`,
  ].join('; ');

  const response = NextResponse.next();
  response.headers.set('Content-Security-Policy', csp);
  response.headers.set('X-Content-Type-Options', 'nosniff');
  response.headers.set('X-Frame-Options', 'DENY');
  response.headers.set('Referrer-Policy', 'strict-origin-when-cross-origin');
  response.headers.set('Permissions-Policy', 'camera=(), microphone=(), geolocation=()');
  return response;
}

---

SAST/DAST Tooling

Static Analysis (SAST)

Tool	Language	Usage
ESLint security plugins	JS/TS	eslint-plugin-security, @microsoft/eslint-plugin-sdl
Semgrep	Multi	semgrep --config=auto .
Bandit	Python	bandit -r src/
gosec	Go	gosec ./...
cargo-audit	Rust	cargo audit

Dynamic Analysis (DAST)

Tool	Purpose	Usage
OWASP ZAP	Web app scanning	Proxy-based scanner, API scan mode
Nuclei	Vulnerability scanning	Template-based scanner
Burp Suite	Manual + automated	Professional penetration testing

Dependency Scanning

# Node.js
npm audit
npm audit fix

# Python
pip-audit
safety check

# Go
govulncheck ./...

# Rust
cargo audit

---

Input Validation Patterns

Server-Side Validation (Always Required)

import { z } from 'zod';

const CreateUserSchema = z.object({
  email: z.string().email().max(255),
  name: z.string().min(1).max(100).regex(/^[a-zA-Z\s'-]+$/),
  age: z.number().int().min(0).max(150).optional(),
});

function createUser(input: unknown) {
  const validated = CreateUserSchema.parse(input);
  // validated is now typed and safe
}

Rate Limiting

import rateLimit from 'express-rate-limit';

const authLimiter = rateLimit({
  windowMs: 15 * 60 * 1000, // 15 minutes
  max: 5, // 5 attempts per window
  message: 'Too many login attempts, please try again later',
  standardHeaders: true,
  legacyHeaders: false,
});

app.use('/api/auth/login', authLimiter);

---

Security Checklist

• All user input validated server-side
• Parameterized queries for all database operations
• CSP headers configured
• Rate limiting on auth endpoints
• CORS properly restricted
• Secrets in environment variables, not code
• Dependencies scanned for vulnerabilities
• Authentication tokens in httpOnly cookies
• HTTPS enforced in production
• Error messages don't leak internal details

---

Related Resources

• ~/.claude/docs/reference/checklists/security-hardening.md - Security hardening checklist
• ~/.claude/agents/security-auditor.md - Security audit agent
• ~/.claude/skills/authentication-patterns/SKILL.md - Auth patterns

---

Secure by default. Validate at boundaries. Defense in depth.