Security Patterns Skill

Security best practices and patterns for secure development.

See: code-examples.md for Python implementations
See: templates.md for checklists and config templates

When This Activates

• API key handling
• User input validation
• File operations
• Security-sensitive code
• Keywords: "security", "api key", "secret", "validate", "input"

---

API Keys & Secrets

Environment Variables (REQUIRED)

Rule: Never hardcode secrets. Always use environment variables via .env files.

# ✅ CORRECT
api_key = os.getenv("ANTHROPIC_API_KEY")

# ❌ WRONG
api_key = "sk-ant-1234567890abcdef"  # NEVER!

See: code-examples.md#api-keys--secrets for full validation code

---

Input Validation

Path Traversal Prevention

Rule: Always validate paths are within allowed directories.

# Use is_relative_to() to prevent ../ attacks
if not file_path.is_relative_to(base_dir):
    raise ValueError("Path traversal detected")

Command Injection Prevention

Rule: Never use shell=True. Pass arguments as lists.

# ✅ CORRECT
subprocess.run([command] + args, shell=False)

# ❌ WRONG
subprocess.run(f"ls {user_input}", shell=True)  # Injection risk!

SQL Injection Prevention

Rule: Always use parameterized queries.

# ✅ CORRECT
cursor.execute("SELECT * FROM users WHERE username = ?", (username,))

# ❌ WRONG
cursor.execute(f"SELECT * FROM users WHERE username = '{username}'")

See: code-examples.md#input-validation for complete examples

---

File Operations Security

Secure Permissions

Use Case	Permission	Octal
Sensitive files	rw-------	0o600
Sensitive dirs	rwx------	0o700
Public files	rw-r--r--	0o644

File Upload Validation

• Validate extensions (whitelist only)
• Check file size limits
• Reject executable files

See: code-examples.md#file-operations-security

---

Cryptographic Operations

Secure Random

Rule: Use secrets module for security-sensitive random values.

# ✅ CORRECT
token = secrets.token_hex(32)

# ❌ WRONG
token = str(random.randint(0, 999999))  # Not cryptographically secure!

See: code-examples.md#cryptographic-operations for password hashing

---

Logging Security

Rule: Never log full secrets. Mask sensitive values.

# ✅ CORRECT
masked_key = api_key[:7] + "***" + api_key[-4:]
logging.info(f"Using key {masked_key}")

# ❌ WRONG
logging.info(f"Using key {api_key}")  # Exposes full key!

---

Dependencies Security

# Check for vulnerabilities
pip install safety && safety check
# OR
pip install pip-audit && pip-audit

---

Key Takeaways

1. Never hardcode secrets - Use environment variables
2. Validate all inputs - User data, file paths, commands
3. Prevent path traversal - Use is_relative_to()
4. No shell=True - Use list arguments with subprocess
5. Parameterized queries - Never string interpolation
6. Secure random - Use secrets module
7. Restrict permissions - Files 0o600, dirs 0o700
8. Mask secrets in logs - Show only first/last few chars
9. Scan dependencies - Use safety/pip-audit
10. .gitignore secrets - .env, *.key, *.pem

---

Related Files

• code-examples.md - Complete Python code examples
• templates.md - .env, .gitignore, and security checklists

OWASP Top 10 Quick Reference

See: templates.md#owasp-top-10-quick-reference