Agent skill

security

Application security patterns - authentication, secrets management, input validation, OWASP Top 10. Use when: auth, JWT, secrets, API keys, SQL injection, XSS, CSRF, RLS, security audit, pen testing basics.

View SKILL.md on GitHub Repository

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Forks 31

Install this agent skill to your Project

npx add-skill https://github.com/majiayu000/claude-skill-registry/tree/main/skills/data/security-skill

SKILL.md

Security is not optional - it's a fundamental requirement. This skill helps you build secure applications from the start, not bolt on security as an afterthought.

<quick_start> Security essentials for any new project:

Secrets: Never commit to git, validate at startup
typescript
```
// .env (gitignored) + envSchema.parse(process.env)
```
Auth: Short-lived JWTs + httpOnly cookies
typescript
```
jwt.sign(payload, secret, { expiresIn: '15m' })
```

Input: Validate everything with schemas

typescript

const data = z.object({ email: z.string().email() }).parse(input)

SQL: Always use parameterized queries (ORMs handle this)
RLS: Enable on all Supabase tables with user-scoped policies </quick_start>

<success_criteria> Security implementation is successful when:

All secrets in environment variables, validated at startup
No secrets in version control (verified with gitleaks)
JWT tokens short-lived (≤15 min) with refresh token rotation
All user input validated with Zod or similar schema validation
RLS enabled on all database tables with appropriate policies
CSP headers configured (no unsafe-inline where possible)
Security checklist completed before deployment </success_criteria>

<security_mindset>

The Security Mindset

Core Principles

Defense in depth - Multiple layers of security, not one wall
Least privilege - Grant minimum access required
Never trust input - Validate everything from users and external systems
Fail secure - Errors should deny access, not grant it
Keep secrets secret - API keys never in code or logs

Security Questions to Ask

Before shipping any feature:

What data does this expose?
Who can access this endpoint/page?
What happens if the user sends malicious input?
Are secrets properly protected?
Is sensitive data logged? </security_mindset>

JWT vs Session

Aspect	JWT	Session
Storage	Client (localStorage/cookie)	Server (DB/Redis)
Scalability	Stateless, easy to scale	Requires shared session store
Revocation	Hard (need blacklist)	Easy (delete from store)
Size	Larger (contains claims)	Small (just session ID)
Best for	APIs, microservices	Traditional web apps

JWT Best Practices

typescript

// DO: Short-lived access tokens + refresh tokens
const accessToken = jwt.sign(
  { userId: user.id, role: user.role },
  process.env.JWT_SECRET,
  { expiresIn: '15m' } // Short-lived!
);

const refreshToken = jwt.sign(
  { userId: user.id },
  process.env.JWT_REFRESH_SECRET,
  { expiresIn: '7d' }
);

// DON'T: Long-lived tokens with sensitive data
// Bad example - never do this:
// { expiresIn: '365d' } // Too long!
// Including PII like SSN in token payload

Token Storage

Method	XSS Safe	CSRF Safe	Recommendation
localStorage	No	Yes	Avoid for auth
httpOnly cookie	Yes	No (needs CSRF token)	Recommended
Memory (variable)	Yes	Yes	Best for SPAs

Refresh Token Flow

┌─────────┐                    ┌─────────┐                    ┌─────────┐
│ Client  │                    │  Server │                    │   DB    │
└────┬────┘                    └────┬────┘                    └────┬────┘
     │                              │                              │
     │  Login (email, password)     │                              │
     │─────────────────────────────>│                              │
     │                              │  Verify credentials          │
     │                              │─────────────────────────────>│
     │                              │<─────────────────────────────│
     │  Access token (15m)          │                              │
     │  Refresh token (7d)          │  Store refresh token hash    │
     │<─────────────────────────────│─────────────────────────────>│
     │                              │                              │
     │  API call + access token     │                              │
     │─────────────────────────────>│                              │
     │  Response                    │                              │
     │<─────────────────────────────│                              │
     │                              │                              │
     │  [Access token expired]      │                              │
     │  Refresh token               │                              │
     │─────────────────────────────>│  Verify refresh token        │
     │                              │─────────────────────────────>│
     │  New access token            │                              │
     │<─────────────────────────────│                              │

Password Handling

typescript

import bcrypt from 'bcrypt';

// DO: Hash with sufficient rounds
const SALT_ROUNDS = 12; // ~300ms on modern hardware
const hash = await bcrypt.hash(password, SALT_ROUNDS);

// DO: Constant-time comparison
const isValid = await bcrypt.compare(inputPassword, storedHash);

// DON'T: Use weak hashing algorithms like MD5 or SHA1 for passwords

Password Requirements

typescript

const passwordSchema = z.string()
  .min(8, 'Minimum 8 characters')
  .max(128, 'Maximum 128 characters')
  .regex(/[a-z]/, 'Must contain lowercase')
  .regex(/[A-Z]/, 'Must contain uppercase')
  .regex(/[0-9]/, 'Must contain number')
  .regex(/[^a-zA-Z0-9]/, 'Must contain special character');

// Check against common passwords (haveibeenpwned API)
async function isPasswordPwned(password: string): Promise<boolean> {
  const sha1 = crypto.createHash('sha1').update(password).digest('hex').toUpperCase();
  const prefix = sha1.slice(0, 5);
  const suffix = sha1.slice(5);

  const response = await fetch(`https://api.pwnedpasswords.com/range/${prefix}`);
  const text = await response.text();

  return text.includes(suffix);
}

<secrets_management>

Secrets Management

The Golden Rule

NEVER commit secrets to version control

bash

# .gitignore - Always include
.env
.env.local
.env.*.local
*.pem
*.key
credentials.json
secrets.yaml

Environment Variables

bash

# .env (local development)
DATABASE_URL=postgresql://localhost:5432/myapp
JWT_SECRET=dev-secret-change-in-production
STRIPE_SECRET_KEY=sk_test_...

# .env.example (commit this!)
DATABASE_URL=postgresql://localhost:5432/myapp
JWT_SECRET=generate-secure-secret
STRIPE_SECRET_KEY=sk_test_your_key_here

Loading Secrets

typescript

// DO: Validate at startup
import { z } from 'zod';

const envSchema = z.object({
  DATABASE_URL: z.string().url(),
  JWT_SECRET: z.string().min(32),
  STRIPE_SECRET_KEY: z.string().startsWith('sk_'),
});

const env = envSchema.parse(process.env);

// DON'T: Use undefined secrets
// Always validate that required env vars exist

Secret Rotation

typescript

// Support multiple secrets during rotation
const JWT_SECRETS = [
  process.env.JWT_SECRET_NEW,    // Current
  process.env.JWT_SECRET_OLD,    // Previous (for validation)
].filter(Boolean);

function verifyToken(token: string): JWTPayload {
  for (const secret of JWT_SECRETS) {
    try {
      return jwt.verify(token, secret);
    } catch {
      continue;
    }
  }
  throw new Error('Invalid token');
}

Never Log Secrets

typescript

// DO: Mask sensitive data in logs
logger.info('User login', {
  userId: user.id,
  email: maskEmail(user.email), // t***@example.com
});

// DON'T: Log tokens or credentials
// Never log: authorization headers, request bodies with passwords, API keys

</secrets_management>

<input_validation>

Input Validation

Validate at System Boundaries

┌─────────────────────────────────────────────────────────┐
│                    Your Application                      │
│                                                          │
│   ┌──────────┐     VALIDATE      ┌──────────────────┐  │
│   │  User    │ ───────────────>  │  Business Logic   │  │
│   │  Input   │                   │  (trusted data)   │  │
│   └──────────┘                   └──────────────────┘  │
│                                                          │
│   ┌──────────┐     VALIDATE      ┌──────────────────┐  │
│   │ External │ ───────────────>  │  Services         │  │
│   │   API    │                   │                   │  │
│   └──────────┘                   └──────────────────┘  │
└─────────────────────────────────────────────────────────┘

Schema Validation (Zod)

typescript

import { z } from 'zod';

// Define schemas
const createUserSchema = z.object({
  email: z.string().email(),
  password: z.string().min(8).max(128),
  name: z.string().min(1).max(100),
  age: z.number().int().min(13).max(120).optional(),
});

// Validate input
export async function createUser(input: unknown) {
  const data = createUserSchema.parse(input); // Throws if invalid
  // data is now typed and validated
  return db.users.create(data);
}

// API handler
export async function POST(req: Request) {
  try {
    const body = await req.json();
    const user = await createUser(body);
    return Response.json(user, { status: 201 });
  } catch (error) {
    if (error instanceof z.ZodError) {
      return Response.json({ errors: error.errors }, { status: 400 });
    }
    throw error;
  }
}

Sanitization

typescript

import DOMPurify from 'dompurify';
import { JSDOM } from 'jsdom';

const window = new JSDOM('').window;
const purify = DOMPurify(window);

// Sanitize HTML (for rich text fields)
const cleanHtml = purify.sanitize(userInput, {
  ALLOWED_TAGS: ['b', 'i', 'em', 'strong', 'a', 'p'],
  ALLOWED_ATTR: ['href']
});

// For plain text - escape or strip HTML
const plainText = purify.sanitize(userInput, { ALLOWED_TAGS: [] });

</input_validation>

<sql_injection>

SQL Injection Prevention

The Problem

Attackers can manipulate SQL queries through unsanitized input. Example attack payload: '; DROP TABLE users; --

The Solution: Parameterized Queries

typescript

// SAFE: Parameterized query (Prisma)
const user = await prisma.user.findUnique({
  where: { email: email }
});

// SAFE: Parameterized query (raw SQL)
const user = await db.query(
  'SELECT * FROM users WHERE email = $1',
  [email]
);

// SAFE: Supabase
const { data } = await supabase
  .from('users')
  .select()
  .eq('email', email);

Supabase RLS (Row Level Security)

sql

-- Enable RLS
ALTER TABLE posts ENABLE ROW LEVEL SECURITY;

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

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

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

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

</sql_injection>

<xss_prevention>

XSS Prevention

The Problem

Attackers inject malicious scripts that execute in victim's browser, stealing cookies/data.

The Solution: Auto-escaping + CSP

typescript

// React auto-escapes by default - this is safe
return <div>Welcome, {userName}</div>;

// AVOID rendering raw HTML from user input
// If you absolutely must render user HTML, ALWAYS sanitize with DOMPurify first
import DOMPurify from 'dompurify';
const sanitizedContent = DOMPurify.sanitize(userContent);

Content Security Policy

typescript

// next.config.js
const securityHeaders = [
  {
    key: 'Content-Security-Policy',
    value: [
      "default-src 'self'",
      "script-src 'self'", // Avoid 'unsafe-inline' if possible
      "style-src 'self' 'unsafe-inline'",
      "img-src 'self' data: https:",
      "font-src 'self'",
      "connect-src 'self' https://api.supabase.co",
    ].join('; ')
  }
];

</xss_prevention>

<csrf_protection>

CSRF Protection

The Problem

Attackers trick authenticated users into submitting malicious requests to your site.

The Solution: CSRF Tokens + SameSite Cookies

typescript

// Server: Generate token
import { randomBytes } from 'crypto';

function generateCsrfToken(): string {
  return randomBytes(32).toString('hex');
}

// Store in session
session.csrfToken = generateCsrfToken();

// Client: Include in forms as hidden field
// Server: Validate token matches session

// Modern approach: SameSite cookies (most effective)
res.cookie('session', sessionId, {
  httpOnly: true,
  secure: true,
  sameSite: 'strict', // or 'lax'
});

</csrf_protection>

Topic	Reference File	When to Load
Auth patterns	`reference/auth-patterns.md`	JWT, OAuth, sessions
Secrets	`reference/secrets-management.md`	API keys, env vars
Input validation	`reference/input-validation.md`	Sanitization, schemas
Supabase RLS	`reference/rls-policies.md`	Row level security
OWASP Top 10	`reference/owasp-top-10.md`	Vulnerability checklist

To load: Ask for the specific topic or check if context suggests it.

Before deploying:

Authentication

Passwords hashed with bcrypt (12+ rounds)
JWT tokens short-lived (15 min max)
Refresh tokens stored securely
Session cookies httpOnly + secure + sameSite

Secrets

No secrets in code or version control
Environment variables validated at startup
Secrets not logged

Input

All user input validated with schemas
SQL uses parameterized queries
HTML sanitized before rendering
File uploads validated (type, size, name)

Headers

HTTPS enforced
CSP header configured
CORS restricted to allowed origins
Security headers set (HSTS, X-Frame-Options)

Authorization

RLS enabled on all tables
API endpoints check permissions
Admin routes protected
Rate limiting on auth endpoints

Maintainer

majiayu000 Core maintainer

Source details

Full Name: majiayu000/claude-skill-registry
Branch: main
Path in repo: skills/data/security-skill
License: MIT License

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Install this agent skill to your Project

SKILL.md

The Security Mindset

Core Principles

Security Questions to Ask

JWT vs Session

JWT Best Practices

Token Storage

Refresh Token Flow

Password Handling

Password Requirements

Secrets Management

The Golden Rule

Environment Variables

Loading Secrets

Secret Rotation

Never Log Secrets

Input Validation

Validate at System Boundaries

Schema Validation (Zod)

Sanitization

SQL Injection Prevention

The Problem

The Solution: Parameterized Queries

Supabase RLS (Row Level Security)

XSS Prevention

The Problem

The Solution: Auto-escaping + CSP

Content Security Policy

CSRF Protection

The Problem

The Solution: CSRF Tokens + SameSite Cookies

Authentication

Secrets

Input

Headers

Authorization

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