security-best-practices

Security Best Practices Skill

When writing code, follow these security best practices to prevent common vulnerabilities.

Input Validation

Always validate and sanitize user input:

# ❌ Bad - trusting user input
user_id = request.args.get('id')
query = f"SELECT * FROM users WHERE id = {user_id}"

# ✅ Good - parameterized query
user_id = request.args.get('id')
cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))

Validation Checklist

• Validate type, length, format, and range
• Use allowlists over denylists
• Sanitize for the specific output context
• Never trust client-side validation alone

Authentication

Implement secure authentication:

• Use established libraries (e.g., bcrypt, argon2)
• Never store passwords in plain text
• Implement rate limiting on login attempts
• Use secure session management
• Implement proper logout handling

# ✅ Good - using bcrypt
import bcrypt

def hash_password(password: str) -> bytes:
    return bcrypt.hashpw(password.encode(), bcrypt.gensalt())

def verify_password(password: str, hash: bytes) -> bool:
    return bcrypt.checkpw(password.encode(), hash)

Authorization

Check permissions on every request:

# ✅ Good - checking authorization
def delete_post(post_id: int, user: User):
    post = get_post(post_id)
    if post.author_id != user.id and not user.is_admin:
        raise PermissionError("Not authorized")
    delete_post_from_db(post_id)

Secrets Management

Never hardcode secrets:

# ❌ Bad
API_KEY = "sk-1234567890abcdef"

# ✅ Good
import os
API_KEY = os.environ.get('API_KEY')

Environment Variables

• Use .env files for development (in .gitignore)
• Use secrets managers in production
• Rotate secrets regularly
• Use different secrets per environment

SQL Injection Prevention

Always use parameterized queries:

# ❌ Bad - string concatenation
cursor.execute(f"SELECT * FROM users WHERE name = '{name}'")

# ✅ Good - parameterized
cursor.execute("SELECT * FROM users WHERE name = ?", (name,))

# ✅ Good - ORM
User.query.filter_by(name=name).first()