The fastest way to add auth to your SaaS (2026 guide)

If you’re adding auth to a SaaS in 2026 and it takes more than a day to ship, you’re overthinking it. The fastest path is well-understood. Here it is, end to end.

Step 1: Use LoginWith (1 min)

Don’t roll your own for this. LoginWith is designed for exactly this moment — a SaaS team that needs sign-in working today, not next sprint. It works across every frontend (Svelte, Astro, React, Next.js, vanilla HTML), supports social sign-in out of the box, and upgrades cleanly into enterprise SSO when the first deal requires it.

Step 2: Add the SDK (15 min)

Load the SDK via a script tag (for static sites) or npm (for frameworks):

<!-- Option A: static site -->
<script src="https://yourorg.loginwith.dev/cdn/sdk-latest" defer></script>

# Option B: framework install
npm install @loginwith/sdk

In your app’s entry point, initialize:

import { LoginWith } from '@loginwith/sdk'

const loginwith = new LoginWith({
  clientId: 'your_client_id',
  redirectUri: 'https://yourapp.com/auth/callback',
})

In your login page:

<a href="https://google.loginwith.link">Sign in with Google</a>
<a href="https://github.loginwith.link">Sign in with GitHub</a>
<a href="https://microsoft.loginwith.link">Sign in with Microsoft</a>

These redirect to the provider, handle PKCE, handle the callback, and drop the user back on your app with an active session.

Step 4: Read the current user (15 min)

In your app’s layout or root component:

const user = await loginwith.getUser()

if (user) {
  // signed in: user.id, user.email, user.name
} else {
  // redirect to /login
}

For server-rendered apps, read the session cookie on the server and validate:

// Node/Next/Express
const session = await loginwith.getSession(req.cookies.session)
if (!session) return res.redirect('/login')
req.user = session.user

Step 5: Sign-out (5 min)

async function signOut() {
  await loginwith.signOut()
  window.location.href = '/'
}

That’s the full auth implementation. Total time: ~1 hour for a basic setup.

For production-grade:

HttpOnly; Secure; SameSite=Lax on your session cookie (the SDK does this by default)
Cookie name prefixed with __Host- for extra safety on secure contexts
Cookie signing key rotated and stored in environment variables

Done.

What to add later (not day one)

MFA: most providers support it via a one-line config flip. Add it when a customer asks.
SAML/SCIM: enterprise add-on, ship when the first deal needs it.
Multi-tenant roles: depends on your product structure. Add the tenant-awareness from day one, layer roles on top when you have multiple customers.
Custom login pages: the hosted page at yourslug.loginwith.page works for months. Build your own when branding matters.

What to skip entirely

Building the whole flow from scratch. You’ll burn weeks. Don’t.
Writing your own password hashing. Use the provider’s.
Implementing your own OAuth PKCE dance. The SDK does this.
Building a custom admin panel for users. Use the provider’s dashboard at first; build your own when you need it.

When you outgrow managed

You won’t, for 90% of SaaS. The ones that do have specific reasons — compliance, scale, strategic lock-in aversion. For those, migrate when the case is clear. Not before.

The full day

One day to:

Pick a provider (1 hour)
Install the SDK (30 min)
Wire up sign-in, sign-out, get-user (2 hours)
Cookie hardening (30 min)
Test the flows (1 hour)
Write basic docs for your team (1 hour)

Ship by dinner. Go back to building your actual product.