Security Patterns

Essential security patterns for web applications.

OWASP Top 10 Quick Reference

Rank	Vulnerability	Prevention
A01	Broken Access Control	Check permissions server-side, deny by default
A02	Cryptographic Failures	Use TLS, hash passwords, encrypt sensitive data
A03	Injection	Parameterized queries, validate input
A04	Insecure Design	Threat modeling, secure defaults
A05	Security Misconfiguration	Harden configs, disable unused features
A06	Vulnerable Components	Update dependencies, audit regularly
A07	Auth Failures	MFA, rate limiting, secure session management
A08	Data Integrity Failures	Verify signatures, use trusted sources
A09	Logging Failures	Log security events, protect logs
A10	SSRF	Validate URLs, allowlist destinations

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

Context	Encoding
HTML body	HTML entity encode
HTML attribute	Attribute encode + quote
JavaScript	JS encode
URL parameter	URL encode
CSS	CSS encode

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