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skills/software-security/rules/codeguard-0-authentication-mfa.md

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+---
+description: Authentication and MFA best practices (passwords, MFA, OAuth/OIDC, SAML, recovery, tokens)
+languages:
+- c
+- go
+- java
+- javascript
+- kotlin
+- matlab
+- php
+- python
+- ruby
+- swift
+- typescript
+alwaysApply: false
+---
+
+rule_id: codeguard-0-authentication-mfa
+
+## Authentication & MFA
+
+Build a resilient, user-friendly authentication system that resists credential attacks, protects secrets, and supports strong, phishing-resistant MFA and secure recovery.
+
+### Account Identifiers and UX
+- Use non-public, random, and unique internal user identifiers. Allow login via verified email or username.
+- Always return generic error messages (e.g., "Invalid username or password"). Keep timing consistent to prevent account enumeration.
+- Support password managers: `<input type="password">`, allow paste, no JS blocks.
+
+### Password Policy
+- Accept passphrases and full Unicode; minimum 8 characters; avoid composition rules. Set only a reasonable maximum length (64+).
+- Check new passwords against breach corpora (e.g., k‑anonymity APIs); reject breached/common passwords.
+
+### Password Storage (Hashing)
+- Hash, do not encrypt. Use slow, memory‑hard algorithms with unique per‑user salts and constant‑time comparison.
+- Preferred order and parameters (tune to your hardware; target <1s on server):
+  - Argon2id: m=19–46 MiB, t=2–1, p=1 (or equivalent security trade‑offs)
+  - scrypt: N=2^17, r=8, p=1 (or equivalent)
+  - bcrypt (legacy only): cost ≥10, be aware of 72‑byte input limit
+  - PBKDF2 (FIPS): PBKDF2‑HMAC‑SHA‑256 ≥600k, or SHA‑1 ≥1.3M
+- Optional pepper: store outside DB (KMS/HSM); if used, apply via HMAC or pre‑hashing. Plan for user resets if pepper rotates.
+- Unicode and null bytes must be supported end‑to‑end by the library.
+
+### Authentication Flow Hardening
+- Enforce TLS for all auth endpoints and token transport; enable HSTS.
+- Implement rate limits per IP, account, and globally; add proof‑of‑work or CAPTCHA only as last resort.
+- Lockouts/throttling: progressive backoff; avoid permanent lockout via resets/alerts.
+- Uniform responses and code paths to reduce oracle/timing signals.
+
+### Multi‑Factor Authentication (MFA)
+- Adopt phishing‑resistant factors by default for sensitive accounts: passkeys/WebAuthn (FIDO2) or hardware U2F.
+- Acceptable: TOTP (app‑based), smart cards with PIN. Avoid for sensitive use: SMS/voice, email codes; never rely on security questions.
+- Require MFA for: login, password/email changes, disabling MFA, privilege elevation, high‑value transactions, new devices/locations.
+- Risk‑based MFA signals: new device, geo‑velocity, IP reputation, unusual time, breached credentials.
+- MFA recovery: provide single‑use backup codes, encourage multiple factors, and require strong identity verification for resets.
+- Handle failed MFA: offer alternative enrolled methods, notify users of failures, and log context (no secrets).
+
+### Federation and Protocols (OAuth 2.0 / OIDC / SAML)
+- Use standard protocols only; do not build your own.
+- OAuth 2.0/OIDC:
+  - Prefer Authorization Code with PKCE for public/native apps; avoid Implicit and ROPC.
+  - Validate state and nonce; use exact redirect URI matching; prevent open redirects.
+  - Constrain tokens to audience/scope; use DPoP or mTLS for sender‑constraining when possible.
+  - Rotate refresh tokens; revoke on logout or risk signals.
+- SAML:
+  - TLS 1.2+; sign responses/assertions; encrypt sensitive assertions.
+  - Validate issuers, InResponseTo, timestamps (NotBefore/NotOnOrAfter), Recipient; verify against trusted keys.
+  - Prevent XML signature wrapping with strict schema validation and hardened XPath selection.
+  - Keep response lifetimes short; prefer SP‑initiated flows; validate RelayState; implement replay detection.
+
+### Tokens (JWT and Opaque)
+- Prefer opaque server‑managed tokens for simplicity and revocation. If using JWTs:
+  - Explicitly pin algorithms; reject "none"; validate iss/aud/exp/iat/nbf; use short lifetimes and rotation.
+  - Store secrets/keys securely (KMS/HSM). Use strong HMAC secrets or asymmetric keys; never hardcode.
+  - Consider binding tokens to a client context (e.g., fingerprint hash in cookie) to reduce replay.
+  - Implement denylist/allowlist for revocation on logout and critical events.
+
+### Recovery and Reset
+- Return the same response for existing and non‑existing accounts (no enumeration). Normalize timing.
+- Generate 32+ byte, CSPRNG tokens; single‑use; store as hashes; short expiry.
+- Use HTTPS reset links to pinned, trusted domains; add referrer policy (no‑referrer) on UI.
+- After reset: require re‑authentication, rotate sessions, and do not auto‑login.
+- Never lock accounts due to reset attempts; rate‑limit and monitor instead.
+
+### Administrative and Internal Accounts
+- Separate admin login from public forms; enforce stronger MFA, device posture checks, IP allowlists, and step‑up auth.
+- Use distinct session contexts and stricter timeouts for admin operations.
+
+### Monitoring and Signals
+- Log auth events (failures/successes, MFA enroll/verify, resets, lockouts) with stable fields and correlation IDs; never log secrets or raw tokens.
+- Detect credential stuffing: high failure rates, many IPs/agents, impossible travel. Notify users of new device logins.
+
+### Implementation Checklist
+- Passwords: Argon2id (preferred) with per‑user salt, constant‑time verify; breached password checks on change/set.
+- MFA: WebAuthn/passkeys or hardware tokens for high‑risk; TOTP as fallback; secure recovery with backup codes.
+- Federation: Authorization Code + PKCE; strict redirect URI validation; audience/scope enforced; token rotation.
+- Tokens: short‑lived, sender‑constrained where possible; revocation implemented; secrets in KMS/HSM.
+- Recovery: single‑use, hashed, time‑boxed tokens; consistent responses; re‑auth required after reset; sessions rotated.
+- Abuse: rate limits, throttling, and anomaly detection on auth endpoints; uniform error handling.
+- Admin: isolated flows with stricter policies and device checks.
+
+### Test Plan
+- Unit/integration tests for login, MFA enroll/verify, resets, and lockouts with uniform errors.
+- Protocol tests: PKCE, state/nonce, redirect URI validation, token audience/scope.
+- Dynamic tests for credential stuffing resistance and token replay; validate revocation after logout and role change.