api-security-auth-pattern

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• SKILL.md

SKILL.md (first lines)

# API Security & Auth Skill

Design complete authentication and security layers for any API.

---

## Auth Pattern Selection Guide

| Use Case | Recommended Pattern |
|----------|-------------------|
| User-facing web/mobile app | OAuth 2.0 + JWT (Authorization Code + PKCE) |
| Server-to-server (M2M) | OAuth 2.0 Client Credentials + JWT |
| Simple 3rd party access | API Key (header) |
| High-security enterprise | mTLS + short-lived JWT |
| Microservices internal | JWT propagation or service mesh (mTLS) |
| Webhook verification | HMAC-SHA256 signature header |

---

## OAuth 2.0 Flow Endpoints

```
POST /auth/oauth/authorize      — redirect user to consent screen
POST /auth/oauth/token          — exchange code for tokens
POST /auth/oauth/token/refresh  — refresh access token
POST /auth/oauth/revoke         — revoke token
GET  /auth/oauth/userinfo       — get user profile from token
```

### Token endpoint request
```json
{
  "grant_type": "authorization_code",
  "code": "AUTH_CODE",
  "redirect_uri": "https://app.example.com/callback",
  "client_id": "CLIENT_ID",
  "code_verifier": "PKCE_VERIFIER"
}
```

### Token response
```json
{
  "access_token": "eyJhbGci...",
  "token_type": "Bearer",
  "expires_in": 3600,
  "refresh_token": "rt_...",
  "scope": "read write"
}
```

---

## JWT Design

**Header**: `{ "alg": "RS256", "typ": "JWT" }`

**Claims payload**:
```json
{
  "sub": "user-uuid",
  "iss": "https://auth.example.com",
  "aud": "https://api.example.com",
  "exp": 1700000000,
  "iat": 1699996400,
  "jti": "unique-token-id",
  "roles": ["admin", "editor"],
  "tenant_id": "org-uuid",
  "scope": "read:users write:posts"
}
```

**Validation checklist**: verify `iss`, `aud`, `exp`, `nbf`; reject `alg: none`; check token revocation list.

---

## RBAC Design

```
Roles:   super_admin > admin > editor > viewer > guest
Resources: users, posts, settings, billing, reports

Permission matrix:
              users   posts  settings  billing  reports
super_admin:  CRUD    CRUD   CRUD      CRUD     R
admin:        CRUD    CRUD   R         R        R
editor:       R       CRUD   -         -        R
viewer:       R       R      -         -        R
guest:        -       R(pub) -         -        -
```

---

## OWASP API Security Top 10 Checklist

| # | Risk | Mitigation |
|---|------|------------|
| 1 | Broken Object Level Auth | Validate ownership on every request |
| 2 | Broken Auth | Strong token validation, short expiry |
| 3 | Broken Object Property Level Auth | Whitelist returned fields |
| 4 | Unrestricted Resource Consumption | Rate limiting, pagination limits |
| 5 | Broken Function Level Auth | RBAC on every route |
| 6 | Unrestricted Access to Sensitive Flows | Step-up auth for critical actions |
| 7 | SSRF | Allowlist outbound URLs |
| 8 | Security Misconfiguration | Disable debug, enforce HTTPS, CORS policy |
| 9 | Improper Inventory Management | Version & deprecate old endpoints |
| 10 | Unsafe Consumption of APIs | Validate & sanitize all 3rd-party data |

---

## Security Headers

```http
Strict-Transport-Security: max-age=63072000; includeSubDomains; preload
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
Content-Security-Policy: default-src 'self'
Referrer-Policy: no-referrer
Permissions-Policy: geolocation=(), microphone=()

From LambdaTest/agent-skills, MIT.

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