Security Skill

Version: 1.0 Source: Security Standards

Security must be built in from the start, not bolted on later. These standards apply to all code that handles user data, authentication, or external input.

The Problem

AI agents default to making code work, not making it safe. Without explicit security standards, each session takes the shortest path — string concatenation for queries, hardcoded secrets for convenience, broad permissions for speed. These aren't malicious choices; they're the path of least resistance when security isn't in the prompt. These standards make secure patterns the default path.

Consumption

• Builders: Read ## Builder Checklist before writing any code that touches user input, auth, or external services. Security must be designed in, not patched after.
• Refactorers: Use ## Enforced Rules to find security violations. Read narrative sections for remediation guidance.
• Both: Narrative sections are the authoritative standard. Checklist and rules table are compressed views of the same content.

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Core Principles

1. Security by Design — Build security in from the start, not as an afterthought
2. Defense in Depth — Multiple layers of security; no single point of failure
3. Least Privilege — Grant minimum access required for the task
4. Fail Securely — Errors should not expose vulnerabilities or sensitive data
5. Zero Trust — Never trust input, always verify
6. Assume Breach — Design as if attackers will get in; minimize blast radius

---

OWASP Top 10 (2021)

Critical vulnerabilities to prevent:

#	Vulnerability	Prevention
1	Broken Access Control	Check authorization on every request
2	Cryptographic Failures	Use strong encryption, never roll your own
3	Injection	Parameterized queries, input validation
4	Insecure Design	Threat modeling, secure architecture
5	Security Misconfiguration	Secure defaults, proper error handling
6	Vulnerable Components	Keep dependencies updated, audit regularly
7	Auth Failures	Strong auth, MFA, secure session management
8	Integrity Failures	Verify code/data integrity, sign releases
9	Logging Failures	Log security events, protect log data
10	SSRF	Validate and allowlist server-side URLs

---

Input Validation

The Golden Rule

Never trust user input. All input is potentially malicious.

Validation Strategy

1. Validate → 2. Sanitize → 3. Encode (for output context)

SQL Injection Prevention

# ❌ NEVER - String concatenation
query = f"SELECT * FROM users WHERE id = {user_id}"

# ✅ ALWAYS - Parameterized queries
cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))

# ✅ ALWAYS - ORM with parameters
User.objects.filter(id=user_id)

XSS Prevention

// ❌ NEVER - Direct HTML insertion
element.innerHTML = userInput;
document.write(userInput);

// ✅ ALWAYS - Text content (auto-escapes)
element.textContent = userInput;

// ✅ ALWAYS - Template literals with escaping
const escaped = escapeHtml(userInput);

Command Injection Prevention

# ❌ NEVER - Shell execution with user input
os.system(f"ls {user_path}")
subprocess.call(f"convert {filename}", shell=True)

# ✅ ALWAYS - Argument arrays (no shell)
subprocess.run(["ls", user_path], shell=False)
subprocess.run(["convert", filename], shell=False)

Input Validation Patterns

import re

def validate_email(email: str) -> bool:
    """Validate email format."""
    pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
    return bool(re.match(pattern, email)) and len(email) <= 254

def validate_username(username: str) -> bool:
    """Validate username: alphanumeric, 3-30 chars."""
    pattern = r'^[a-zA-Z0-9_]{3,30}$'
    return bool(re.match(pattern, username))

def validate_url(url: str, allowed_domains: list[str]) -> bool:
    """Validate URL against allowlist."""
    from urllib.parse import urlparse
    parsed = urlparse(url)
    return (
        parsed.scheme in ('http', 'https') and
        parsed.netloc in allowed_domains
    )

---

Authentication & Authorization

Password Requirements

Requirement	Minimum
Length	12 characters
Complexity	Not required if length met
Common passwords	Block top 10,000
Breached passwords	Check against HaveIBeenPwned

Password Storage

# ✅ Use bcrypt, Argon2, or scrypt
import bcrypt

def hash_password(password: str) -> bytes:
    """Hash password with bcrypt."""
    salt = bcrypt.gensalt(rounds=12)
    return bcrypt.hashpw(password.encode(), salt)

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

NEVER:

• Store passwords in plain text
• Use MD5 or SHA1 for passwords
• Roll your own hashing

Session Management

# Session security settings
SESSION_CONFIG = {
    "cookie_secure": True,      # HTTPS only
    "cookie_httponly": True,    # No JavaScript access
    "cookie_samesite": "Lax",   # CSRF protection
    "session_lifetime": 3600,   # 1 hour max
    "regenerate_on_login": True # Prevent session fixation
}

Authorization Checks

# ✅ Check authorization on EVERY request
def get_document(user, document_id):
    document = Document.get(document_id)

    # Always verify ownership/access
    if document.owner_id != user.id and not user.is_admin:
        raise PermissionError("Access denied")

    return document

---

Data Protection

Encryption Requirements

Data Type	At Rest	In Transit
Passwords	Hashed (bcrypt/Argon2)	TLS 1.2+
PII (name, email, address)	AES-256	TLS 1.2+
Payment data	AES-256 + PCI DSS	TLS 1.3
Health data	AES-256 + HIPAA	TLS 1.3
Session tokens	N/A	TLS 1.2+
API keys	AES-256	TLS 1.2+

Sensitive Data Handling

# ✅ Mask sensitive data in logs
def log_user_action(user, action, data):
    safe_data = {
        "user_id": user.id,
        "email": mask_email(user.email),  # j***@example.com
        "action": action,
        # Never log: passwords, tokens, full credit cards
    }
    logger.info("User action", extra=safe_data)

def mask_email(email: str) -> str:
    """Mask email for logging."""
    local, domain = email.split("@")
    return f"{local[0]}***@{domain}"

def mask_card(card: str) -> str:
    """Show only last 4 digits."""
    return f"****-****-****-{card[-4:]}"

Data Retention

• Delete data when no longer needed
• Implement data expiration policies
• Provide user data export/deletion (GDPR)
• Securely wipe deleted data (not just mark deleted)

---

API Security

Rate Limiting

# Implement rate limiting per endpoint
RATE_LIMITS = {
    "/api/login": "5/minute",      # Prevent brute force
    "/api/register": "3/minute",   # Prevent spam
    "/api/password-reset": "3/hour",
    "/api/*": "100/minute",        # General limit
}

CORS Configuration

# ✅ Specific origins only
CORS_CONFIG = {
    "origins": [
        "https://app.example.com",
        "https://admin.example.com",
    ],
    "methods": ["GET", "POST", "PUT", "DELETE"],
    "allow_credentials": True,
    "max_age": 3600,
}

# ❌ NEVER in production
CORS_CONFIG = {
    "origins": "*",  # Allows any origin!
}

API Response Security

# ❌ Never expose internal errors
def handle_error(error):
    # Don't send: stack traces, SQL errors, file paths
    return {"error": "An error occurred"}, 500

# ✅ Log details, return generic message
def handle_error(error):
    logger.error(f"Internal error: {error}", exc_info=True)
    return {"error": "Internal server error"}, 500

Security Headers

SECURITY_HEADERS = {
    "Strict-Transport-Security": "max-age=31536000; includeSubDomains",
    "X-Content-Type-Options": "nosniff",
    "X-Frame-Options": "DENY",
    "X-XSS-Protection": "1; mode=block",
    "Content-Security-Policy": "default-src 'self'",
    "Referrer-Policy": "strict-origin-when-cross-origin",
}

---

Secrets Management

Environment Variables

# ✅ Store secrets in environment
export DATABASE_URL="postgres://..."
export API_KEY="..."
export JWT_SECRET="..."

# ❌ NEVER commit secrets to code
DATABASE_URL = "postgres://user:password@host/db"  # In code!

Secret Requirements

Secret Type	Rotation Period	Storage
API keys	90 days	Vault/env vars
Database passwords	90 days	Vault/env vars
JWT secrets	30 days	Vault/env vars
Encryption keys	1 year	HSM/KMS
User passwords	On compromise	Hashed in DB

.gitignore

# Secrets - NEVER commit
.env
.env.local
.env.*.local
*.pem
*.key
credentials.json
secrets.yaml
config/secrets/*

---

File Upload Security

Validation Checklist

def validate_upload(file) -> bool:
    """Validate uploaded file."""

    # 1. Check file size
    MAX_SIZE = 10 * 1024 * 1024  # 10MB
    if file.size > MAX_SIZE:
        raise ValueError("File too large")

    # 2. Check extension against allowlist
    ALLOWED_EXTENSIONS = {'.jpg', '.jpeg', '.png', '.pdf'}
    ext = Path(file.name).suffix.lower()
    if ext not in ALLOWED_EXTENSIONS:
        raise ValueError("File type not allowed")

    # 3. Verify magic bytes (don't trust extension)
    magic_bytes = file.read(8)
    file.seek(0)
    if not is_valid_magic(magic_bytes, ext):
        raise ValueError("File content doesn't match extension")

    # 4. Generate safe filename
    safe_name = generate_safe_filename(file.name)

    # 5. Store outside web root
    storage_path = UPLOAD_DIR / safe_name  # Not in /public!

    return True

Safe Filename Generation

import uuid
import re
from pathlib import Path

def generate_safe_filename(original: str) -> str:
    """Generate safe filename, preserving extension."""
    ext = Path(original).suffix.lower()
    # Use UUID, not original filename
    return f"{uuid.uuid4()}{ext}"

---

Logging & Monitoring

What to Log

# ✅ Log these security events
SECURITY_EVENTS = [
    "login_success",
    "login_failure",
    "logout",
    "password_change",
    "password_reset_request",
    "permission_denied",
    "invalid_token",
    "rate_limit_exceeded",
    "suspicious_input",
    "admin_action",
]

What NOT to Log

# ❌ NEVER log these
NEVER_LOG = [
    "password",
    "password_hash",
    "credit_card",
    "ssn",
    "api_key",
    "session_token",
    "jwt_token",
    "private_key",
]

Structured Security Logging

def log_security_event(event_type: str, user_id: str, details: dict):
    """Log security event with context."""
    logger.info("security_event", extra={
        "event_type": event_type,
        "user_id": user_id,
        "timestamp": datetime.utcnow().isoformat(),
        "ip_address": get_client_ip(),
        "user_agent": get_user_agent(),
        **sanitize_details(details),
    })

---

Builder Checklist

Before writing code that handles user data, authentication, or external input, verify your plan against these constraints. Builders read this section before writing code; refactorers use the Enforced Rules table and full narrative instead.

Before Every Release

• No secrets in code or version control
• All user input validated and sanitized
• SQL queries use parameterized statements
• Authentication on all protected endpoints
• Authorization checks on all resources
• HTTPS enforced (no HTTP)
• Security headers configured
• Rate limiting in place
• Error messages don't leak sensitive info
• Dependencies updated and audited
• File uploads validated and stored safely
• Logging captures security events
• CORS configured restrictively

Code Review Security Focus

• Input validation at all entry points
• Output encoding for XSS prevention
• Access control checks present
• No hardcoded secrets
• Proper error handling (no stack traces)
• Secure defaults used
• Third-party libraries are necessary and trusted

---

Enforced Rules

These rules are deterministically checked by check.js (clean-team). When updating these standards, update the corresponding check.js rules to match — and vice versa.

Rule ID	Severity	What It Checks
no-document-write	error	document.write() usage (DOM injection risk)
no-hardcoded-secrets	error	API keys, tokens, passwords in string literals
no-innerHTML	warn	.innerHTML assignment (XSS risk)

---

References

• references/owasp-top-10.md — Detailed OWASP vulnerability guide
• references/input-validation.md — Complete input validation patterns
• references/auth-patterns.md — Authentication and authorization patterns

Assets

• assets/security-checklist.md — Pre-release security checklist
• assets/threat-model-template.md — Threat modeling template

Scripts

• scripts/scan_secrets.py — Scan code for accidentally committed secrets
• scripts/check_dependencies.py — Check dependencies for known vulnerabilities