Security practices

Purpose

Comprehensive guide for security in Python projects including input validation, injection prevention, secure defaults, and threat modeling.

When to use

This skill activates when:

• Validating user input
• Handling file paths
• Executing system commands
• Working with sensitive data
• Reviewing code for vulnerabilities
• Designing security architecture

Core principles

Secure by default

• Default configuration should be the secure configuration
• Require explicit opt-in for less secure behavior
• Validate input at system boundaries

Defense in depth

• Use multiple layers of validation
• Never rely on a single security control
• Validate at multiple levels

Input validation

String input validation

import re

def validate_identifier(value: str) -> str:
    """Validate identifier string."""
    if not value:
        raise ValueError("Identifier cannot be empty")

    if not re.match(r'^[a-zA-Z][a-zA-Z0-9_]*$', value):
        raise ValueError(f"Invalid identifier: {value}")

    if len(value) > 255:
        raise ValueError("Identifier too long")

    return value

def sanitize_for_display(value: str) -> str:
    """Sanitize string for safe display."""
    # Remove control characters
    return ''.join(c for c in value if c.isprintable() or c in '\n\t')

Path validation

from pathlib import Path

def validate_path(user_path: str, allowed_base: Path) -> Path:
    """Validate path is within allowed directory."""
    # Resolve to absolute path
    path = (allowed_base / user_path).resolve()

    # Check path is within allowed base
    if not path.is_relative_to(allowed_base):
        raise ValueError("Path traversal detected")

    return path

def validate_filename(filename: str) -> str:
    """Validate filename is safe."""
    # Check for path separators
    if '/' in filename or '\\' in filename:
        raise ValueError("Invalid filename: contains path separator")

    # Check for special names
    if filename in ('.', '..'):
        raise ValueError("Invalid filename: special directory name")

    # Check for null bytes
    if '\x00' in filename:
        raise ValueError("Invalid filename: contains null byte")

    return filename

Command injection prevention

Safe command execution

import subprocess
import shlex

def run_safe_command(args: list[str]) -> subprocess.CompletedProcess:
    """Run command safely without shell."""
    # Never use shell=True with user input
    return subprocess.run(
        args,
        capture_output=True,
        text=True,
        timeout=30,
        check=True,
    )

# Good: List of arguments
run_safe_command(['git', 'log', '--oneline', '-n', '10'])

# BAD: Never do this with user input
# subprocess.run(f"git log {user_input}", shell=True)  # DANGEROUS

Quote user input

import shlex

def quote_for_shell(value: str) -> str:
    """Quote value for safe shell usage."""
    return shlex.quote(value)

# Usage
filename = quote_for_shell(user_provided_filename)

Sensitive data handling

Environment variables

import os

def get_secret(name: str) -> str:
    """Get secret from environment."""
    value = os.environ.get(name)
    if value is None:
        raise ValueError(f"Required secret not set: {name}")
    return value

# Never log secrets
def process_with_secret(data: str) -> None:
    secret = get_secret("API_KEY")
    # Use secret, never print/log it
    result = call_api(data, auth=secret)

Avoid information leakage

class SafeError(Exception):
    """Error with safe message for users."""

    def __init__(self, user_message: str, internal_details: str | None = None):
        super().__init__(user_message)
        self._internal = internal_details

    @property
    def internal_details(self) -> str | None:
        """Details for logging only, never expose to users."""
        return self._internal

def process_request(data: dict) -> Result:
    try:
        return _process_impl(data)
    except DatabaseError as e:
        # Log full error internally
        logger.error(f"Database error: {e}")
        # Return safe message to user
        raise SafeError("Processing failed", str(e)) from e

OWASP considerations

Injection flaws

• Never interpolate user input into commands
• Use parameterized queries for databases
• Validate and sanitize all input

Authentication

• Use secure password hashing (bcrypt, argon2)
• Implement rate limiting
• Use secure session management

Sensitive data exposure

• Never log sensitive data
• Encrypt data at rest and in transit
• Use environment variables for secrets

Security checklist

Before completing security-sensitive code:

• All user input validated
• No command injection vectors
• No path traversal vectors
• Secrets not logged or exposed
• Error messages don't leak internals
• Using latest dependency versions
• Following principle of least privilege

Threat modeling

When designing features, consider:

1. What could go wrong? - Identify attack vectors
2. What are we doing about it? - Document mitigations
3. Did we do a good job? - Verify implementation
4. Can we do better? - Continuous improvement

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Additional resources:

• OWASP Top 10
• Python Security Best Practices