authentication-best-practices

name	authentication-best-practices
description	Authentication and Authorization mastery. Best practices for OAuth2, OpenID Connect, JWT (JSON Web Tokens), session management, password hashing, MFA (Multi-Factor Authentication), RBAC/ABAC, SSO, and secure credential storage. Use when auditing or implementing login flows, identity systems, or access control.
allowed-tools	Read, Write, Edit, Glob, Grep
version	2.0.0
last-updated	"2026-04-02T00:00:00.000Z"
applies-to-model	gemini-2.5-pro, claude-3-7-sonnet
routing	{"domain":"general","tier":"basic"}

Authentication & Authorization — Identity Mastery

---

Passwords & Hashing

// ❌ BAD: md5, sha1, sha256 (too fast, vulnerable to brute force/rainbow tables)
const hash = crypto.createHash("sha256").update(password).digest("hex");

// ✅ GOOD: Argon2 (memory-hard, ASIC resistant) or bcrypt
import * as argon2 from "argon2";

async function hashPassword(password: string): Promise<string> {
  // Argon2 hashes include the salt inherently in the resulting string
  return await argon2.hash(password, {
    type: argon2.argon2id, // recommended variant
    memoryCost: 2 ** 16, // 64 MB
    timeCost: 3, // iterations
    parallelism: 1, // threads
  });
}

async function verifyPassword(hash: string, password: string): Promise<boolean> {
  return await argon2.verify(hash, password);
}

Password Policies

• Length over complexity: Require minimum 12 characters. Stop requiring arbitrary symbols (e.g., !@#).
• Check against breaches: Use HaveIBeenPwned API or similar to reject compromised passwords during signup.
• Never expire passwords arbitrarily: Only force resets if there is evidence of a breach.

---

Session Management vs. JWT

1. Stateful Sessions (Cookies)

Best for: Monolithic web apps, SSR apps (Next.js, Remix).

• Server stores session ID mapped to user data in Redis/DB.
• Client stores session ID in an HttpOnly, Secure, SameSite=Lax/Strict cookie.
• Pros: Immediate revocation, server-side truth, invisible to XSS.
• Cons: Requires DB lookup per request.

2. Stateless JWT (JSON Web Tokens)

Best for: Distributed APIs, Microservices, Native mobile apps.

• Server signs a token containing user claims.
• Client passes it in Authorization: Bearer <token> header.
• Pros: No DB lookup needed, easy cross-origin sharing.
• Cons: Cannot be easily revoked before expiration.

The JWT "Refresh Token" Pattern

// Scenario: API authentication
// 1. Access Token (Short-lived: 15 mins)
const accessToken = jwt.sign({ userId: user.id }, JWT_SECRET, {
  expiresIn: "15m",
  algorithm: "HS256", // ALWAYS explicitly specify
});
// 2. Refresh Token (Long-lived: 7 days, opaque string in DB)
const refreshToken = crypto.randomBytes(40).toString("hex");
await db.refreshTokens.create({ token: refreshToken, userId: user.id, expires: addDays(7) });

// Client flow:
// - Access token kept in memory (JS variable) to prevent XSS theft.
// - Refresh token kept in HttpOnly cookie.
// - When Access Token expires, endpoint reads cookie, validates DB, issues new Access Token.

---

OAuth2 & OIDC (OpenID Connect)

Roles:
1. Resource Owner (User)
2. Client (Your App)
3. Authorization Server (Google/GitHub/Auth0)
4. Resource Server (API)

Flow (Authorization Code + PKCE):
1. User clicks "Login with Google".
2. App generates `code_verifier` and `code_challenge`.
3. App redirects user to Google with `code_challenge`.
4. User logs in, Google redirects back to App with an authorization `code`.
5. App sends `code` + `code_verifier` to Google backend.
6. Google returns `id_token` (OIDC identity) and `access_token` (OAuth permissions).

// ❌ HALLUCINATION TRAP: Implicit Flow is deprecated.
// Never use Implicit Flow (response_type=token) where the token is returned in the URL hash.
// Always use Authorization Code Flow with PKCE, even for Single Page Apps (SPAs).

---

Multi-Factor Authentication (MFA)

• SMS: Deprecated by NIST due to SIM swapping vulnerabilities. (Better than nothing, but avoid as primary MFA).
• TOTP (Authenticator Apps): Standard implementations use HMAC-SHA1. Keep the secret key heavily encrypted at rest.
• WebAuthn / Passkeys: The modern gold standard. Replaces passwords entirely using hardware enclaves (FaceID, TouchID, YubiKey).

---

Authorization Models

RBAC (Role-Based Access Control)

• Users have Roles (admin, editor, viewer).
• Roles have Permissions (create:post, delete:user).

// ✅ Check permissions, not roles directly (more flexible)
if (!user.permissions.includes("delete:user")) {
  throw new ForbiddenError();
}

ABAC (Attribute-Based Access Control)

• Access based on context (e.g., "User can edit Document if Document.department == User.department").

// Example Policy
function canEditPost(user: User, post: Post): boolean {
  if (user.role === "admin") return true;
  if (post.authorId === user.id) return true;
  if (post.status === "draft" && user.department === "content") return true;
  return false;
}

---

---

AI coding assistants often fall into specific bad habits when dealing with this domain. These are strictly forbidden:

1. Over-engineering: Proposing complex abstractions or distributed systems when a simpler approach suffices.
2. Hallucinated Libraries/Methods: Using non-existent methods or packages. Always // VERIFY or check package.json / requirements.txt.
3. Skipping Edge Cases: Writing the "happy path" and ignoring error handling, timeouts, or data validation.
4. Context Amnesia: Forgetting the user's constraints and offering generic advice instead of tailored solutions.
5. Silent Degradation: Catching and suppressing errors without logging or re-raising.

---

Slash command: /review or /tribunal-full Active reviewers: logic-reviewer · security-auditor

❌ Forbidden AI Tropes

1. Blind Assumptions: Never make an assumption without documenting it clearly with // VERIFY: [reason].
2. Silent Degradation: Catching and suppressing errors without logging or handling.
3. Context Amnesia: Forgetting the user's constraints and offering generic advice instead of tailored solutions.

Review these questions before confirming output:

✅ Did I rely ONLY on real, verified tools and methods?
✅ Is this solution appropriately scoped to the user's constraints?
✅ Did I handle potential failure modes and edge cases?
✅ Have I avoided generic boilerplate that doesn't add value?

🛑 Verification-Before-Completion (VBC) Protocol

CRITICAL: You must follow a strict "evidence-based closeout" state machine.

• ❌ Forbidden: Declaring a task complete because the output "looks correct."
• ✅ Required: You are explicitly forbidden from finalizing any task without providing concrete evidence (terminal output, passing tests, compile success, or equivalent proof) that your output works as intended.

Pre-Flight Checklist

• Have I reviewed the user's specific constraints and requests?
• Have I checked the environment for relevant existing implementations?

VBC Protocol (Verification-Before-Completion)

You MUST verify existing code signatures and variables before attempting to modify or call them. No hallucination is permitted.

---

🤖 LLM-Specific Traps

AI coding assistants often fall into specific bad habits when dealing with this domain. These are strictly forbidden:

1. Over-engineering: Proposing complex abstractions or distributed systems when a simpler approach suffices.
2. Hallucinated Libraries/Methods: Using non-existent methods or packages. Always // VERIFY or check package.json / requirements.txt.
3. Skipping Edge Cases: Writing the "happy path" and ignoring error handling, timeouts, or data validation.
4. Context Amnesia: Forgetting the user's constraints and offering generic advice instead of tailored solutions.
5. Silent Degradation: Catching and suppressing errors without logging or re-raising.

---

🏛️ Tribunal Integration (Anti-Hallucination)

Slash command: /review or /tribunal-full Active reviewers: logic-reviewer · security-auditor

❌ Forbidden AI Tropes

1. Blind Assumptions: Never make an assumption without documenting it clearly with // VERIFY: [reason].
2. Silent Degradation: Catching and suppressing errors without logging or handling.
3. Context Amnesia: Forgetting the user's constraints and offering generic advice instead of tailored solutions.

✅ Pre-Flight Self-Audit

Review these questions before confirming output:

✅ Did I rely ONLY on real, verified tools and methods?
✅ Is this solution appropriately scoped to the user's constraints?
✅ Did I handle potential failure modes and edge cases?
✅ Have I avoided generic boilerplate that doesn't add value?

🛑 Verification-Before-Completion (VBC) Protocol

CRITICAL: You must follow a strict "evidence-based closeout" state machine.

• ❌ Forbidden: Declaring a task complete because the output "looks correct."
• ✅ Required: You are explicitly forbidden from finalizing any task without providing concrete evidence (terminal output, passing tests, compile success, or equivalent proof) that your output works as intended.