Security Review Skill

This skill ensures all code follows security best practices and identifies potential vulnerabilities.

When to Activate

• Implementing authentication or authorization
• Handling user input or file uploads
• Creating new API endpoints
• Working with secrets or credentials
• Implementing payment features
• Storing or transmitting sensitive data
• Integrating third-party APIs

Security Checklist

1. Secrets Management

NEVER Do This

api_key = "sk-proj-xxxxx"  # Hardcoded secret
db_password = "password123"  # In source code

ALWAYS Do This

import os
from dotenv import load_dotenv

load_dotenv()

api_key = os.environ.get("OPENAI_API_KEY")
db_url = os.environ.get("DATABASE_URL")

# Verify secrets exist
if not api_key:
    raise ValueError("OPENAI_API_KEY not configured")

Using Pydantic Settings (Recommended)

from pydantic_settings import BaseSettings


class Settings(BaseSettings):
    openai_api_key: str
    database_url: str
    secret_key: str

    class Config:
        env_file = ".env"
        env_file_encoding = "utf-8"


settings = Settings()  # Raises ValidationError if missing

Verification Steps

• No hardcoded API keys, tokens, or passwords
• All secrets in environment variables

• .env

  in .gitignore
• No secrets in git history
• Production secrets in hosting platform (Railway, Render, AWS)

2. Input Validation

Always Validate User Input with Pydantic

from pydantic import BaseModel, EmailStr, field_validator


class CreateUserRequest(BaseModel):
    email: EmailStr
    name: str
    age: int

    @field_validator("name")
    @classmethod
    def name_must_be_valid(cls, v: str) -> str:
        if len(v) < 1 or len(v) > 100:
            raise ValueError("Name must be 1-100 characters")
        return v.strip()

    @field_validator("age")
    @classmethod
    def age_must_be_valid(cls, v: int) -> int:
        if v < 0 or v > 150:
            raise ValueError("Age must be 0-150")
        return v


# FastAPI automatically validates
@app.post("/users")
async def create_user(request: CreateUserRequest):
    return await db.users.create(request.model_dump())

File Upload Validation

from pathlib import Path
from fastapi import UploadFile, HTTPException


ALLOWED_EXTENSIONS = {".jpg", ".jpeg", ".png", ".gif"}
ALLOWED_CONTENT_TYPES = {"image/jpeg", "image/png", "image/gif"}
MAX_FILE_SIZE = 5 * 1024 * 1024  # 5MB


async def validate_file_upload(file: UploadFile) -> bool:
    # Content type check
    if file.content_type not in ALLOWED_CONTENT_TYPES:
        raise HTTPException(400, "Invalid file type")

    # Extension check
    extension = Path(file.filename or "").suffix.lower()
    if extension not in ALLOWED_EXTENSIONS:
        raise HTTPException(400, "Invalid file extension")

    # Size check (read content to verify)
    content = await file.read()
    if len(content) > MAX_FILE_SIZE:
        raise HTTPException(400, f"File too large (max {MAX_FILE_SIZE // 1024 // 1024}MB)")

    # Reset file position for later use
    await file.seek(0)
    return True

Verification Steps

• All user inputs validated with Pydantic schemas
• File uploads restricted (size, type, extension)
• No direct use of user input in queries
• Whitelist validation (not blacklist)
• Error messages don't leak sensitive info

3. SQL Injection Prevention

NEVER Concatenate SQL

# DANGEROUS - SQL Injection vulnerability
query = f"SELECT * FROM users WHERE email = '{user_email}'"
await db.execute(query)

ALWAYS Use Parameterized Queries

# Safe - SQLAlchemy ORM
from sqlalchemy import select

stmt = select(User).where(User.email == user_email)
result = await session.execute(stmt)

# Safe - Raw parameterized query
await db.execute(
    "SELECT * FROM users WHERE email = :email",
    {"email": user_email}
)

# Safe - asyncpg
await conn.fetch("SELECT * FROM users WHERE email = $1", user_email)

Verification Steps

• All database queries use parameterized queries
• No string concatenation/f-strings in SQL
• ORM/query builder used correctly
• SQLAlchemy queries properly sanitized

4. Authentication & Authorization

JWT Token Handling

from fastapi import Response
from datetime import datetime, timedelta
import jwt

# WRONG: Storing in response body (client stores in localStorage)
# Vulnerable to XSS

# CORRECT: httpOnly cookies
def set_auth_cookie(response: Response, token: str):
    response.set_cookie(
        key="access_token",
        value=token,
        httponly=True,
        secure=True,  # HTTPS only
        samesite="strict",
        max_age=3600,  # 1 hour
    )

Authorization Checks

from fastapi import Depends, HTTPException, status

async def get_current_user(token: str = Depends(oauth2_scheme)):
    user = await verify_token(token)
    if not user:
        raise HTTPException(
            status_code=status.HTTP_401_UNAUTHORIZED,
            detail="Invalid credentials",
        )
    return user


async def require_admin(user: User = Depends(get_current_user)):
    if user.role != "admin":
        raise HTTPException(
            status_code=status.HTTP_403_FORBIDDEN,
            detail="Admin access required",
        )
    return user


@app.delete("/users/{user_id}")
async def delete_user(user_id: str, admin: User = Depends(require_admin)):
    await db.users.delete(user_id)

Row Level Security (PostgreSQL)

-- Enable RLS on all tables
ALTER TABLE users ENABLE ROW LEVEL SECURITY;

-- Users can only view their own data
CREATE POLICY "Users view own data"
  ON users FOR SELECT
  USING (auth.uid() = id);

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

Verification Steps

• Tokens stored in httpOnly cookies (not localStorage)
• Authorization checks before sensitive operations
• Row Level Security enabled if using PostgreSQL
• Role-based access control implemented
• Session management secure

5. XSS Prevention

Sanitize HTML

import bleach

# ALWAYS sanitize user-provided HTML
def sanitize_html(html: str) -> str:
    return bleach.clean(
        html,
        tags=["b", "i", "em", "strong", "p", "br"],
        attributes={},
        strip=True,
    )

Content Security Policy (FastAPI)

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware
from starlette.middleware import Middleware
from starlette.middleware.base import BaseHTTPMiddleware


class SecurityHeadersMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        response = await call_next(request)
        response.headers["Content-Security-Policy"] = (
            "default-src 'self'; "
            "script-src 'self'; "
            "style-src 'self' 'unsafe-inline'; "
            "img-src 'self' data: https:; "
            "connect-src 'self' https://api.example.com"
        )
        response.headers["X-Content-Type-Options"] = "nosniff"
        response.headers["X-Frame-Options"] = "DENY"
        response.headers["X-XSS-Protection"] = "1; mode=block"
        return response


app = FastAPI()
app.add_middleware(SecurityHeadersMiddleware)

Verification Steps

• User-provided HTML sanitized with bleach
• CSP headers configured
• No unvalidated dynamic content rendering
• Jinja2 auto-escaping enabled (default)

6. CSRF Protection

CSRF Tokens (FastAPI)

from fastapi_csrf_protect import CsrfProtect
from pydantic import BaseModel


class CsrfSettings(BaseModel):
    secret_key: str = "your-secret-key"


@CsrfProtect.load_config
def get_csrf_config():
    return CsrfSettings()


@app.post("/api/transfer")
async def transfer(csrf_protect: CsrfProtect = Depends()):
    await csrf_protect.validate_csrf()
    # Process request

SameSite Cookies

response.set_cookie(
    key="session",
    value=session_id,
    httponly=True,
    secure=True,
    samesite="strict",
)

Verification Steps

• CSRF tokens on state-changing operations
• SameSite=Strict on all cookies
• Double-submit cookie pattern implemented

7. Rate Limiting

API Rate Limiting (FastAPI)

from slowapi import Limiter, _rate_limit_exceeded_handler
from slowapi.util import get_remote_address
from slowapi.errors import RateLimitExceeded

limiter = Limiter(key_func=get_remote_address)
app.state.limiter = limiter
app.add_exception_handler(RateLimitExceeded, _rate_limit_exceeded_handler)


@app.get("/api/endpoint")
@limiter.limit("100/15minutes")
async def endpoint(request: Request):
    return {"status": "ok"}


# Stricter limits for expensive operations
@app.get("/api/search")
@limiter.limit("10/minute")
async def search(request: Request, q: str):
    return await perform_search(q)

Verification Steps

• Rate limiting on all API endpoints
• Stricter limits on expensive operations
• IP-based rate limiting
• User-based rate limiting (authenticated)

8. Sensitive Data Exposure

Logging

import structlog

logger = structlog.get_logger()

# WRONG: Logging sensitive data
logger.info("User login", email=email, password=password)
logger.info("Payment", card_number=card_number, cvv=cvv)

# CORRECT: Redact sensitive data
logger.info("User login", email=email, user_id=user_id)
logger.info("Payment", last4=card.last4, user_id=user_id)

Error Messages

from fastapi import HTTPException
import logging

logger = logging.getLogger(__name__)

# WRONG: Exposing internal details
@app.get("/api/data")
async def get_data():
    try:
        return await fetch_data()
    except Exception as e:
        raise HTTPException(500, detail=str(e))  # Leaks info!

# CORRECT: Generic error messages
@app.get("/api/data")
async def get_data():
    try:
        return await fetch_data()
    except Exception as e:
        logger.exception("Internal error fetching data")
        raise HTTPException(500, detail="An error occurred. Please try again.")

Verification Steps

• No passwords, tokens, or secrets in logs
• Error messages generic for users
• Detailed errors only in server logs
• No stack traces exposed to users

9. Dependency Security

Regular Updates

# Check for vulnerabilities
pip-audit

# Or with safety
safety check

# Update dependencies
pip install --upgrade -r requirements.txt

# Check for outdated packages
pip list --outdated

Lock Files

# ALWAYS commit lock files
git add requirements.txt  # or pyproject.toml + poetry.lock

# Use pip-compile for reproducible builds
pip-compile requirements.in -o requirements.txt

# In CI/CD
pip install -r requirements.txt --no-deps

Verification Steps

• Dependencies up to date
• No known vulnerabilities (pip-audit clean)
• Lock files committed
• Dependabot/Renovate enabled on GitHub
• Regular security updates

10. Path Traversal Prevention

NEVER Trust User Paths

from pathlib import Path

# WRONG: Path traversal vulnerability
@app.get("/files/{filename}")
async def get_file(filename: str):
    return FileResponse(f"/uploads/{filename}")  # ../../../etc/passwd!

# CORRECT: Validate and resolve paths
UPLOAD_DIR = Path("/uploads").resolve()


@app.get("/files/{filename}")
async def get_file(filename: str):
    # Resolve and validate path
    file_path = (UPLOAD_DIR / filename).resolve()

    # Ensure path is within upload directory
    if not file_path.is_relative_to(UPLOAD_DIR):
        raise HTTPException(403, "Access denied")

    if not file_path.exists():
        raise HTTPException(404, "File not found")

    return FileResponse(file_path)

Security Testing

Automated Security Tests

import pytest
from httpx import AsyncClient


@pytest.mark.asyncio
async def test_requires_authentication(client: AsyncClient):
    """Protected endpoints require auth."""
    response = await client.get("/api/protected")
    assert response.status_code == 401


@pytest.mark.asyncio
async def test_requires_admin_role(client: AsyncClient, user_token: str):
    """Admin endpoints require admin role."""
    response = await client.get(
        "/api/admin",
        headers={"Authorization": f"Bearer {user_token}"},
    )
    assert response.status_code == 403


@pytest.mark.asyncio
async def test_rejects_invalid_input(client: AsyncClient):
    """Invalid input is rejected."""
    response = await client.post(
        "/api/users",
        json={"email": "not-an-email"},
    )
    assert response.status_code == 422  # Pydantic validation error


@pytest.mark.asyncio
async def test_enforces_rate_limits(client: AsyncClient):
    """Rate limiting is enforced."""
    responses = [
        await client.get("/api/endpoint")
        for _ in range(101)
    ]
    too_many_requests = [r for r in responses if r.status_code == 429]
    assert len(too_many_requests) > 0

Pre-Deployment Security Checklist

Before ANY production deployment:

• Secrets: No hardcoded secrets, all in env vars
• Input Validation: All user inputs validated with Pydantic
• SQL Injection: All queries parameterized (SQLAlchemy/asyncpg)
• XSS: User content sanitized with bleach
• CSRF: Protection enabled
• Authentication: Proper token handling (httpOnly cookies)
• Authorization: Role checks in place
• Rate Limiting: Enabled on all endpoints (slowapi)
• HTTPS: Enforced in production
• Security Headers: CSP, X-Frame-Options configured
• Error Handling: No sensitive data in errors
• Logging: No sensitive data logged
• Dependencies: Up to date, no vulnerabilities (pip-audit)
• Row Level Security: Enabled if using PostgreSQL
• CORS: Properly configured
• File Uploads: Validated (size, type, path traversal)
• Path Traversal: All file paths validated

Python Security Libraries

pydantic

bleach

python-jose

passlib

slowapi

pip-audit

safety

bandit

python-dotenv

pydantic-settings

Resources

• OWASP Top 10
• FastAPI Security
• Bandit Documentation
• OWASP Python Security

Remember: Security is not optional. One vulnerability can compromise the entire platform. When in doubt, err on the side of caution.