Claude Skills Security Best Practices

Security is critical when Skills execute real code. Follow these practices to protect your systems.

Key Security Principles

1. Input Validation

Always validate and sanitize inputs:


python
1def process_file(filepath):
2    # Validate file exists
3    if not os.path.exists(filepath):
4        raise ValueError("File not found")
5
6    # Check file type
7    allowed_extensions = ['.pdf', '.docx', '.txt']
8    if not any(filepath.endswith(ext) for ext in allowed_extensions):
9        raise ValueError("Invalid file type")
10
11    # Check file size
12    if os.path.getsize(filepath) > 10 * 1024 * 1024:  # 10MB
13        raise ValueError("File too large")
1def process_file(filepath):
2    # Validate file exists
3    if not os.path.exists(filepath):
4        raise ValueError("File not found")
5
6    # Check file type
7    allowed_extensions = ['.pdf', '.docx', '.txt']
8    if not any(filepath.endswith(ext) for ext in allowed_extensions):
9        raise ValueError("Invalid file type")
10
11    # Check file size
12    if os.path.getsize(filepath) > 10 * 1024 * 1024:  # 10MB
13        raise ValueError("File too large")

2. Sandboxing

Run untrusted code in isolated environments:


python
1import subprocess
2
3def run_sandboxed(script_path):
4    result = subprocess.run(
5        ['docker', 'run', '--rm', '-v', f'{script_path}:/app', 'python:3.9', 'python', '/app/script.py'],
6        capture_output=True,
7        timeout=30
8    )
9    return result.stdout
1import subprocess
2
3def run_sandboxed(script_path):
4    result = subprocess.run(
5        ['docker', 'run', '--rm', '-v', f'{script_path}:/app', 'python:3.9', 'python', '/app/script.py'],
6        capture_output=True,
7        timeout=30
8    )
9    return result.stdout

3. Secrets Management

Never hardcode sensitive data:


python
1import os
2
3# ✅ Good: Use environment variables
4API_KEY = os.getenv('API_KEY')
5
6# ❌ Bad: Hardcoded secrets
7# API_KEY = "sk-1234567890abcdef"
1import os
2
3# ✅ Good: Use environment variables
4API_KEY = os.getenv('API_KEY')
5
6# ❌ Bad: Hardcoded secrets
7# API_KEY = "sk-1234567890abcdef"

4. Principle of Least Privilege

Request minimum permissions:


markdown
## Required Permissions
- File read: /data/input/
- File write: /data/output/
- Network: None
## Required Permissions
- File read: /data/input/
- File write: /data/output/
- Network: None

5. Code Review

Audit Skills before deployment:


bash
# Check for dangerous patterns
grep -r "eval(" skill/
grep -r "exec(" skill/
grep -r "subprocess.call" skill/
# Check for dangerous patterns
grep -r "eval(" skill/
grep -r "exec(" skill/
grep -r "subprocess.call" skill/

Common Vulnerabilities

Path Traversal


python
1# ❌ Vulnerable
2def read_file(filename):
3    return open(filename).read()
4# Attack: read_file("../../etc/passwd")
5
6# ✅ Secure
7import os
8def read_file(filename):
9    base_dir = "/safe/directory"
10    filepath = os.path.join(base_dir, filename)
11    if not filepath.startswith(base_dir):
12        raise ValueError("Invalid path")
13    return open(filepath).read()
1# ❌ Vulnerable
2def read_file(filename):
3    return open(filename).read()
4# Attack: read_file("../../etc/passwd")
5
6# ✅ Secure
7import os
8def read_file(filename):
9    base_dir = "/safe/directory"
10    filepath = os.path.join(base_dir, filename)
11    if not filepath.startswith(base_dir):
12        raise ValueError("Invalid path")
13    return open(filepath).read()

Command Injection


python
# ❌ Vulnerable
os.system(f"convert {filename} output.pdf")

# ✅ Secure
subprocess.run(['convert', filename, 'output.pdf'], check=True)
# ❌ Vulnerable
os.system(f"convert {filename} output.pdf")

# ✅ Secure
subprocess.run(['convert', filename, 'output.pdf'], check=True)

Arbitrary Code Execution


python
1# ❌ Dangerous
2eval(user_input)
3exec(user_code)
4
5# ✅ Use safe alternatives
6import ast
7ast.literal_eval(user_input)  # Only for literals
1# ❌ Dangerous
2eval(user_input)
3exec(user_code)
4
5# ✅ Use safe alternatives
6import ast
7ast.literal_eval(user_input)  # Only for literals

Security Checklist

[ ] All inputs validated
[ ] File paths sanitized
[ ] Commands use subprocess with list args
[ ] No hardcoded secrets
[ ] Timeouts on all operations
[ ] Error messages don't leak sensitive info
[ ] Dependencies from trusted sources
[ ] Regular security audits

Monitoring

Log security-relevant events:


python
import logging

logging.info(f"Skill executed: {skill_name} by {user_id}")
logging.warning(f"Invalid input attempt: {input_data}")
logging.error(f"Security violation: {error_details}")
import logging

logging.info(f"Skill executed: {skill_name} by {user_id}")
logging.warning(f"Invalid input attempt: {input_data}")
logging.error(f"Security violation: {error_details}")

Resources

Reading Time: 4 minutes

Author: ClaudeSkills Team

Claude Skills Security: Best Practices and Protection