Security Patterns

Essential security patterns for web applications.

OWASP Top 10 Quick Reference

Input Validation

# WRONG - Trust user input
def search(query):
    return db.execute(f"SELECT * FROM users WHERE name = '{query}'")

# CORRECT - Parameterized query
def search(query):
    return db.execute("SELECT * FROM users WHERE name = ?", [query])

Validation Rules

Always validate:
- Type (string, int, email format)
- Length (min/max bounds)
- Range (numeric bounds)
- Format (regex for patterns)
- Allowlist (known good values)

Never trust:
- URL parameters
- Form data
- HTTP headers
- Cookies
- File uploads

Output Encoding

// WRONG - Direct HTML insertion
element.innerHTML = userInput;

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

// CORRECT - Template with escaping
render(`<div>${escapeHtml(userInput)}</div>`);

Encoding by Context

Authentication

# Password hashing (use bcrypt, argon2, or scrypt)
import bcrypt

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

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

Auth Checklist

• Hash passwords with bcrypt/argon2 (cost factor 12+)
• Implement rate limiting on login
• Use secure session tokens (random, long)
• Set secure cookie flags (HttpOnly, Secure, SameSite)
• Implement account lockout after failed attempts
• Support MFA for sensitive operations

Authorization

# WRONG - Check only authentication
@login_required
def delete_post(post_id):
    post = Post.get(post_id)
    post.delete()

# CORRECT - Check authorization
@login_required
def delete_post(post_id):
    post = Post.get(post_id)
    if post.author_id != current_user.id and not current_user.is_admin:
        raise Forbidden("Not authorized to delete this post")
    post.delete()

Secrets Management

# WRONG - Hardcoded secrets
API_KEY = "sk-1234567890abcdef"

# CORRECT - Environment variables
API_KEY = os.environ["API_KEY"]

# BETTER - Secrets manager
API_KEY = secrets_client.get_secret("api-key")

Secret Handling Rules

DO:
- Use environment variables or secrets manager
- Rotate secrets regularly
- Use different secrets per environment
- Audit secret access

DON'T:
- Commit secrets to git
- Log secrets
- Include secrets in error messages
- Share secrets in plain text

Security Headers

Content-Security-Policy: default-src 'self'; script-src 'self'
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
Strict-Transport-Security: max-age=31536000; includeSubDomains
Referrer-Policy: strict-origin-when-cross-origin
Permissions-Policy: geolocation=(), camera=()

Quick Security Audit

# Find hardcoded secrets
rg -i "(password|secret|api_key|token)\s*=\s*['\"][^'\"]+['\"]" --type py

# Find SQL injection risks
rg "execute\(f['\"]|format\(" --type py

# Find eval/exec usage
rg "\b(eval|exec)\s*\(" --type py

# Check for TODO security items
rg -i "TODO.*security|FIXME.*security"

Additional Resources

• ./references/owasp-detailed.md - Full OWASP Top 10 details
• ./references/auth-patterns.md - JWT, OAuth, session management
• ./references/crypto-patterns.md - Encryption, hashing, signatures
• ./references/secure-headers.md - HTTP security headers guide

Scripts

• ./scripts/security-scan.sh - Quick security grep patterns
• ./scripts/dependency-audit.sh - Check for vulnerable dependencies