security

name	security
description	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.

Comprehensive security skill covering authentication patterns, secrets management, input validation, and common vulnerability prevention. Focuses on practical patterns for web applications with Supabase, Next.js, and Python backends.

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:

1. Secrets: Never commit to git, validate at startup

   // .env (gitignored) + envSchema.parse(process.env)
   

2. Auth: Short-lived JWTs + httpOnly cookies

   jwt.sign(payload, secret, { expiresIn: '15m' })
   

3. Input: Validate everything with schemas

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

4. SQL: Always use parameterized queries (ORMs handle this)

5. 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

1. Defense in depth - Multiple layers of security, not one wall
2. Least privilege - Grant minimum access required
3. Never trust input - Validate everything from users and external systems
4. Fail secure - Errors should deny access, not grant it
5. 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>

## Authentication Patterns

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

// 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

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

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

Golden rule: Never commit secrets to version control. Use .env files (gitignored), validate all env vars at startup with Zod schemas, support rotation with multiple active secrets, and never log sensitive data.

See reference/secrets-management.md for gitignore patterns, env var templates, Zod validation, rotation patterns, and log masking. </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)

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

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

// 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)

-- 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

// 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

// 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

// 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>

For detailed patterns, load the appropriate reference:

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.

## Security Checklist

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

Emit Outcome Sidecar

As the final step, write to ~/.claude/skill-analytics/last-outcome-security.json:

{"ts":"[UTC ISO8601]","skill":"security","version":"1.0.0","variant":"default",
 "status":"[success|partial|error]","runtime_ms":[estimated ms from start],
 "metrics":{"vulnerabilities_found":[n],"fixes_applied":[n],"audit_checks_passed":[n]},
 "error":null,"session_id":"[YYYY-MM-DD]"}

Use status "partial" if some stages failed but results were produced. Use "error" only if no output was generated.