API Security Checklist — Authentication to Encryption

Introduction

APIs are the backbone of modern applications — and a primary attack surface. This checklist covers the essential security controls every API should implement, from authentication to deployment hardening.

1. Authentication

Use Strong Token-Based Authentication

# Bad: API keys passed in query strings (logged by proxies)
GET /data?api_key=abc123

# Good: Bearer tokens in Authorization header
GET /data
Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

Implement JWT Securely

import jwt
from datetime import datetime, timedelta

def create_access_token(user_id, secret, algorithm="HS256"):
    payload = {
        "sub": str(user_id),
        "iat": datetime.utcnow(),
        "exp": datetime.utcnow() + timedelta(minutes=15),
        "jti": str(uuid.uuid4())  # unique token ID for revocation
    }
    return jwt.encode(payload, secret, algorithm=algorithm)

Requirements Checklist

• Use HTTPS everywhere (no HTTP fallback)
• Tokens expire in 15 minutes or less (access tokens)
• Refresh tokens expire in 7-30 days with rotation
• Store tokens securely (HttpOnly cookies for browser clients)
• Reject tokens with weak signatures (none, none256)

2. Authorization

Enforce Least Privilege

# Bad: admin check is missing
def delete_user(user_id):
    db.execute("DELETE FROM users WHERE id = %s", user_id)

# Good: verify authorization before action
def delete_user(requesting_user, target_user_id):
    if not requesting_user.has_role("admin"):
        raise Forbidden("Admin role required")
    if requesting_user.id == target_user_id:
        raise BadRequest("Cannot delete yourself")
    db.execute("DELETE FROM users WHERE id = %s", target_user_id)

Checklist

• Authenticate before authorizing (no auth bypass)
• Validate resource ownership (user A cannot access user B’s data)
• Role-based access control (RBAC) or attribute-based (ABAC)
• Deny by default — explicitly allow, don’t implicitly trust

3. Input Validation

Validate Everything

from pydantic import BaseModel, Field, validator

class CreateUserRequest(BaseModel):
    email: str = Field(..., min_length=5, max_length=254)
    password: str = Field(..., min_length=12, max_length=128)

    @validator("email")
    def validate_email(cls, v):
        if "@" not in v:
            raise ValueError("Invalid email format")
        return v.lower().strip()

Checklist

• Validate type, length, format, and range for every input
• Reject unexpected fields (strict schema validation)
• Sanitize file uploads (extension, MIME type, size limits)
• Use parameterized queries (prevent SQL injection)
• Encode output to prevent XSS

4. Rate Limiting

Prevent Abuse

from flask_limiter import Limiter

limiter = Limiter(
    key_func=lambda: request.headers.get("Authorization"),
    default_limits=["100 per minute"]
)

@app.route("/api/login", methods=["POST"])
@limiter.limit("5 per minute")
def login():
    ...

Checklist

• Different limits per endpoint (stricter for auth, looser for read)
• Per-user and per-IP rate limits
• Return 429 Too Many Requests with Retry-After header
• Log and alert on repeated violations

5. Encryption

Data in Transit

• TLS 1.2+ only
• Strong cipher suites (no RC4, DES, MD5)
• HSTS header to prevent downgrade attacks
• Certificate pinning for mobile clients

Data at Rest

from cryptography.fernet import Fernet

key = Fernet.generate_key()
cipher = Fernet(key)

# Encrypt sensitive fields
encrypted_ssn = cipher.encrypt(b"123-45-6789")
decrypted = cipher.decrypt(encrypted_ssn)

Checklist

• TLS 1.2+ for all API communication
• Encrypt sensitive data at rest (PII, credentials, tokens)
• Hash passwords with bcrypt/Argon2 (never MD5 or SHA1)
• Secure key management (KMS, HSM, or vault — not in code)

6. Error Handling

Don’t Leak Information

# Bad: exposes internal details
except DatabaseError as e:
    return {"error": str(e)}  # reveals schema, query structure

# Good: generic message, log details server-side
except DatabaseError as e:
    logger.error("Database error", exc_info=e, extra={"request_id": request.id})
    return {"error": "Internal server error"}, 500

Checklist

• Generic error messages to clients
• Detailed logs server-side (with correlation IDs)
• Consistent error format (RFC 7807 Problem Details)
• Don’t expose stack traces, file paths, or system info

7. Logging and Monitoring

What to Log

Event	Data to Log	Data to Avoid
Authentication	Success/failure, timestamp, IP	Passwords, tokens
Authorization failures	Resource, action, user	Sensitive payload
Rate limit hits	User/IP, endpoint	Full request body
Errors	Error type, request ID, endpoint	Stack traces in client logs

Checklist

• Log all authentication attempts (success and failure)
• Alert on anomalous patterns (unusual IPs, volume spikes)
• Retain logs for incident investigation (30-90 days)
• Centralized log aggregation (SIEM or equivalent)

8. CORS and Headers

# Strict CORS policy
from flask_cors import CORS

CORS(app, origins=["https://app.example.com"], supports_credentials=True)

# Security headers
@app.after_request
def add_security_headers(response):
    response.headers["X-Content-Type-Options"] = "nosniff"
    response.headers["X-Frame-Options"] = "DENY"
    response.headers["Content-Security-Policy"] = "default-src 'self'"
    response.headers["Strict-Transport-Security"] = "max-age=31536000"
    return response

Checklist

• Restrict CORS to known origins (no * with credentials)
• Set security headers on all responses
• Disable server version banners (nginx, Apache, framework)

9. Deployment Hardening

• Run API in isolated network (VPC, private subnets)
• Use a Web Application Firewall (WAF)
• Keep dependencies updated (automated vulnerability scanning)
• Disable unused endpoints and HTTP methods
• Run with least-privilege OS user (not root)

Common Mistakes

• Trusting client-side validation (always validate server-side)
• Storing secrets in environment variables without rotation
• Using predictable IDs (/user/1, /user/2) without authorization checks
• Missing pagination limits (DoS via huge ?limit=999999)
• CORS set to * in production

Advanced Topics

Scenario: REST API Hardening for Production

System: REST API Node.js, 50 endpoints, 100K users
Goal: Complete API security checklist

API security checklist (40 items):
  Authentication:
  [x] JWT with RS256 (not HS256)
  [x] Token expiry: 15 min access, 7 days refresh
  [x] Refresh token rotation
  [x] MFA for admin endpoints
  [x] Rate limiting on login: 5 attempts -> lock 15 min
  [x] No token in URL
  [x] Logout invalidates token (Redis blacklist)
  [x] Password policy: min 12 chars, complexity

  Authorization:
  [x] RBAC: roles user/admin/super_admin
  [x] Verify ownership on every request (anti-IDOR)
  [x] Scope per resource: user only accesses their data
  [x] Deny by default, allow explicitly
  [x] No auto-increment IDs (use UUID)

  Input:
  [x] Schema validation (Zod) on every endpoint
  [x] Payload size limit (max 1MB)
  [x] String sanitization (no HTML injection)
  [x] Parameterized queries (anti-SQL injection)
  [x] No eval/exec with user input
  [x] File upload: validate type, size, content

  Output:
  [x] No stack traces in prod
  [x] DTO mapping: no internal fields exposed
  [x] Headers: X-Content-Type-Options, X-Frame-Options, CSP, HSTS
  [x] No server/framework version exposed
  [x] Global rate limiting: 100 req/min per user

  Transport:
  [x] TLS 1.3 mandatory (no TLS 1.0/1.1)
  [x] Redirect HTTP -> HTTPS
  [x] HSTS: max-age=31536000; includeSubDomains
  [x] Certificate pinning (mobile apps)

  Configuration:
  [x] CORS: strict origin, no wildcard
  [x] NODE_ENV=production
  [x] Secrets in Secrets Manager (no .env in prod)
  [x] Helmet() configured
  [x] Compression with Brotli (not gzip to avoid BREACH)

  Logging and monitoring:
  [x] Audit log of critical actions
  [x] No logging secrets, passwords, tokens, PII
  [x] Alerts for failed auth attempts
  [x] Alerts for rate limit exceeded
  [x] SIEM integration (ELK + alerting)

  Dependencies:
  [x] npm audit in CI (--audit-level=high)
  [x] Dependabot/Snyk configured
  [x] Lockfile in repo (package-lock.json)
  [x] License check in CI

  CI/CD:
  [x] SAST (semgrep) in pipeline
  [x] DAST (OWASP ZAP) on staging
  [x] Container scan (Trivy) in build
  [x] Secret scan (git-secrets) in pre-commit
  [x] Code review mandatory (1 approver min)

Lessons:
  - 40 items is the minimum for production
  - IDOR is #1: verify ownership always
  - Schema validation on every endpoint, no exceptions
  - Audit log + alerting = early detection
  - SAST + DAST + secret scan in CI is mandatory

How do I prioritize if I have limited time?

Start with authentication (secure JWT + rate limiting), authorization (verify ownership), input validation (Zod on every endpoint), and TLS. These 4 cover 80% of vulnerabilities. Then add headers (helmet), audit log, and dependency scanning. Finally, SAST/DAST in CI. Security is a continuous process, not a one-time project.