Auth Flow Implementation

When to Use

Activate when adding authentication or authorization to a fullstack application -- login/signup pages, protected routes, social login, token management, or role-based access control.

Auth Library Selection

Library	Framework	Session Store	OAuth Built-in	MFA Support
NextAuth.js / Auth.js	Next.js, SvelteKit	DB / JWT	Yes	Via adapter
Lucia	Any	DB	Manual	Manual
Clerk	Any	Managed	Yes	Yes
Supabase Auth	Any	Managed	Yes	Yes

Login / Signup Flow

1. Form UI -- use react-hook-form + zod for validation. Fields: email, password, confirm password (signup).
2. API endpoint -- validate input with Zod, hash password with bcrypt (cost 12+) or argon2, store user in DB.
3. Session creation -- issue a session token (stored in httpOnly, secure, sameSite=lax cookie) or JWT pair.
4. Redirect -- on success redirect to the dashboard or the originally requested page (store callbackUrl).

JWT + Refresh Token Strategy

• Access token -- short-lived (15 minutes), stored in memory (not localStorage). Contains user ID, role, permissions.
• Refresh token -- long-lived (7-30 days), stored in httpOnly cookie. Rotate on every use (one-time use tokens).
• Silent refresh -- use an interceptor (Axios/fetch wrapper) that catches 401, calls /api/auth/refresh, replays the original request.
• Token revocation -- maintain a deny-list in Redis for immediate logout, or use short-lived tokens and accept the window.

Social Login (OAuth 2.0)

1. Configure OAuth apps in Google Cloud Console, GitHub Settings, Apple Developer.
2. Set redirect URIs to {APP_URL}/api/auth/callback/{provider}.
3. Use NextAuth.js providers or implement the OAuth flow manually (authorization code + PKCE).
4. On first social login, create a local user record. On subsequent logins, link to the existing account by email.
5. Handle edge case: user signs up with email/password, then tries social login with the same email -- prompt to link accounts.

MFA / 2FA Implementation

• TOTP -- use otpauth or speakeasy library. Generate a secret, show QR code, verify 6-digit code.
• Recovery codes -- generate 10 single-use codes at MFA setup. Hash and store them. Show once, never again.
• Flow -- after password verification, redirect to MFA challenge page. Only issue session after MFA passes.

Protected Routes (Middleware)

// Next.js middleware.ts
export function middleware(request: NextRequest) {
  const token = request.cookies.get('session-token');
  if (!token && request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', request.url));
  }
}
export const config = { matcher: ['/dashboard/:path*', '/api/protected/:path*'] };

Role-Based UI Rendering

• Define roles in a shared enum: admin, editor, viewer.
• Server-side: check role in API handlers and middleware before returning data.
• Client-side: use a <Can action="edit" resource="post"> component or usePermissions() hook to conditionally render UI elements.
• Never rely solely on client-side role checks -- always enforce on the server.

Common Pitfalls

• Storing JWTs in localStorage (XSS vulnerable) -- use httpOnly cookies or in-memory storage.
• Not rate-limiting login endpoints -- add rate limiting (5 attempts per minute per IP).
• Returning different error messages for "user not found" vs "wrong password" -- always use a generic message.
• Forgetting to invalidate sessions on password change.