Initial commit: 9 security patterns for code review

Fundamentals: secure-defaults, input-validation, credential-handling, audit-logging
Identity: authentication, authorization
Attack Prevention: injection-prevention, dos-prevention, prompt-injection

README.md

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+# Security Patterns
+
+Scannable patterns for security code review. Each file has:
+- **Rule** — what to do
+- **Correct Pattern** — code that works (Python)
+- **Incorrect Pattern** — common mistakes
+- **Edge Cases** — gotchas
+
+## Patterns
+
+### Fundamentals
+
+| File | Topic |
+|------|-------|
+| [secure-defaults.md](secure-defaults.md) | Fail closed, deny by default, defense in depth |
+| [input-validation.md](input-validation.md) | Allowlist > blocklist, validate at boundaries |
+| [credential-handling.md](credential-handling.md) | No hardcoded secrets, environment/secret manager |
+| [audit-logging.md](audit-logging.md) | What to log, what not to log |
+
+### Identity
+
+| File | Topic |
+|------|-------|
+| [authentication.md](authentication.md) | Passwords, tokens, MFA, brute force protection |
+| [authorization.md](authorization.md) | Permission checks, IDOR prevention, privilege escalation |
+
+### Attack Prevention
+
+| File | Topic |
+|------|-------|
+| [injection-prevention.md](injection-prevention.md) | SQL, command, template, path traversal |
+| [dos-prevention.md](dos-prevention.md) | Rate limiting, resource bounds, algorithmic complexity |
+| [prompt-injection.md](prompt-injection.md) | LLM security, data/instruction separation |
+
+## Sources
+
+- [OWASP Cheat Sheet Series](https://cheatsheetseries.owasp.org/)
+- [OWASP Top 10](https://owasp.org/Top10/)
+- [OWASP LLM Top 10](https://owasp.org/www-project-top-10-for-large-language-model-applications/)
+- [CWE (Common Weakness Enumeration)](https://cwe.mitre.org/)
+
+## Usage
+
+Reference these patterns when building or reviewing systems. Code examples are in Python for universal model comprehension; concepts apply to any language.