Sub-Skill: Security Review Depth

Purpose: Deep security rules for authentication, authorization, injection, session management, and API hardening. Complements code-quality rules 20–23 with IDOR, SSRF, and access-control depth.

Rules

Authentication vs. Authorization

1. AuthN/AuthZ separation. Always treat authentication (who are you?) and authorization (what can you do?) as distinct checks; never collapse them into a single boolean is_logged_in guard.
2. Resource ownership check. Always verify resource ownership in addition to role-based access on mutation endpoints — a valid role does not imply ownership of the target record. Reference: ERR-2026-019
3. IDOR prevention. Never expose sequential or predictable resource IDs in URLs without a server-side ownership assertion; use opaque UUIDs and always re-fetch the record to confirm the caller owns it.
4. Privilege escalation guard. Never allow a user to elevate their own role or grant permissions they do not already hold; enforce privilege changes through a separate admin-only path.
5. Token scope enforcement. Always validate that the token's declared scope covers the requested operation before executing it; reject tokens with insufficient scope with 403, not 401.

Session Management

1. Session fixation prevention. Always regenerate the session identifier immediately after a successful login to prevent session fixation attacks.
2. Idle and absolute timeouts. Ensure sessions carry both an idle timeout (e.g., 15 min) and an absolute expiry (e.g., 8 h); never issue non-expiring session tokens.
3. Secure cookie attributes. Always set HttpOnly, Secure, and SameSite=Strict (or Lax) on session cookies; never omit any of these three attributes.
4. Logout invalidation. Always invalidate the server-side session record on logout; never rely solely on deleting the client-side cookie.

Injection and Input Handling

1. Parameterized queries everywhere. Never construct database queries by string concatenation with user-supplied values; always use parameterized statements or an ORM that enforces binding.
2. Output encoding context. Always encode output in the context where it will be rendered (HTML entity encoding for HTML, JSON encoding for JSON responses, URL encoding for query strings); never apply a single generic escape.
3. SSRF mitigation. Before making any server-side HTTP request to a URL derived from user input, validate the resolved IP against an allowlist; block private RFC-1918 ranges and loopback addresses.
4. File path traversal. Never concatenate user input into file system paths; always resolve the canonical path and assert it falls within the expected base directory before opening.

API and Transport Hardening

1. CORS allowlist. Avoid wildcard Access-Control-Allow-Origin: * on endpoints that return authenticated data; always enumerate trusted origins explicitly.
2. CSP header. Ensure every HTML response includes a Content-Security-Policy header that restricts script-src to known origins; never use unsafe-inline without a nonce or hash.
3. Rate limiting on auth endpoints. Always apply rate limiting and exponential back-off to login, password-reset, and OTP endpoints; never expose them without request throttling.
4. Sensitive data in logs. Never log passwords, tokens, full credit-card numbers, or other PII; mask or omit them before writing to any log sink.

Secrets and Dependencies

1. Secret rotation readiness. Always design secret consumption so that rotating a credential requires only an environment variable update with no code change; never hard-code secrets or embed them in config files committed to version control.
2. Dependency CVE scan. Before merging any PR that adds or upgrades a dependency, run a CVE scan (e.g., pip-audit, npm audit, trivy); block merge if high-severity findings are unresolved.
3. Audit logging on sensitive actions. Always emit a structured audit log entry (actor, action, resource ID, timestamp, outcome) for every privileged or destructive operation; never skip audit logging for admin endpoints.

See also

• skills/code-quality/SKILL.md — rules 20–23 cover sanitization, parameterized queries, and secret hygiene at a general level; this skill deepens those with IDOR, SSRF, and session specifics.
• skills/error-log/SKILL.md — ERR-2026-019 motivated rule 2 (resource ownership check).