Secure Authentication in Next.js: Building a Production-Ready Login System

Every great SaaS product begins at the same point: the login page. It is the gatekeeper of your user data and the first interaction your customers have with your professional application. Yet, for many developers, setting up authentication feels like a high-stakes puzzle where a single mistake can lead to security vulnerabilities or a frustrated user base.

If you have ever struggled with session management, wondered how to securely store user credentials, or felt overwhelmed by the complexity of OAuth providers, you are in the right place. In this lesson, we are going to strip away the confusion and build a robust, secure authentication system using Auth.js (NextAuth v5) within the Next.js App Router framework.

The Problem: The "Homegrown" Auth Trap

Many developers start by trying to build their own authentication logic. They create a users table in MongoDB, hash passwords with bcrypt, and try to manage JWTs (JSON Web Tokens) manually in cookies. While this is a great academic exercise, it is often a recipe for disaster in a production SaaS environment.

Manual auth systems frequently suffer from:

• Security Gaps: Improperly configured cookies or CSRF (Cross-Site Request Forgery) vulnerabilities.
• Maintenance Burden: Keeping up with changing security standards and API updates from providers like Google or GitHub.
• UX Friction: Hard-to-implement features like "Forgot Password," "Magic Links," or social logins.

The Shift: Moving to Auth.js

The professional way to handle this in 2026 is by using a library that does the heavy lifting for you. Auth.js is the standard for anyone wanting to Learn Next.js for SaaS. It handles session management, multi-provider support, and database integration out of the box, allowing you to focus on your core product features instead of reinventing the security wheel.

By shifting to an established library, you gain the confidence that your sessions are handled via encrypted, server-only cookies. You also get an easy path to adding "Login with Google," which significantly increases conversion rates for modern SaaS products.

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Deep Dive: Setting Up Your Auth Workflow

To build a complete SaaS, we need a flexible system. We will implement two main strategies: Email/Password (Credentials) for traditional users and Google OAuth for a frictionless experience.

1. The Architecture of Auth.js in the App Router

In the Next.js App Router, authentication happens primarily on the server. We use a combination of:

• The Auth Configuration File: Where we define our providers and callbacks.
• Middleware: To protect routes before they even hit the browser.
• Server Actions: To handle login and signup logic securely.

2. Initial Setup and Environment Variables

First, we need to install the necessary packages. In your terminal, run:

npm install next-auth@beta mongodb @auth/mongodb-adapter bcryptjs

Before writing code, we must define our environment variables. These are secrets that should never be committed to GitHub. Create a .env.local\ file:

AUTH_SECRET=your_super_secret_random_string
NEXT_PUBLIC_APP_URL=http://localhost:3000

AUTH_GOOGLE_ID=your_google_client_id
AUTH_GOOGLE_SECRET=your_google_client_secret

MONGODB_URI=your_mongodb_connection_string

3. Configuring the Auth Library

We will create a central configuration file. This is the heart of your security system. It tells Next.js how to talk to your database and how to verify users.

File: auth.ts (Root directory)

import NextAuth from "next-auth";
import Google from "next-auth/providers/google";
import Credentials from "next-auth/providers/credentials";
import { MongoDBAdapter } from "@auth/mongodb-adapter";
import clientPromise from "@/lib/mongodb";
import bcrypt from "bcryptjs";

export const { handlers, auth, signIn, signOut } = NextAuth({
  adapter: MongoDBAdapter(clientPromise),
  providers: [
    Google,
    Credentials({
      name: "credentials",
      credentials: {
        email: { label: "Email", type: "email" },
        password: { label: "Password", type: "password" },
      },
      async authorize(credentials) {
        if (!credentials?.email || !credentials?.password) return null;

        const dbClient = await clientPromise;
        const user = await dbClient.db().collection("users").findOne({ 
          email: credentials.email 
        });

        if (!user || !user.password) return null;

        const isValid = await bcrypt.compare(
          credentials.password as string, 
          user.password
        );

        return isValid ? { id: user._id.toString(), email: user.email } : null;
      },
    }),
  ],
  session: { strategy: "jwt" },
  pages: {
    signIn: "/login",
  },
  callbacks: {
    async session({ session, token }) {
      if (token.sub && session.user) {
        session.user.id = token.sub;
      }
      return session;
    },
  },
});

4. Creating the Login UI with Tailwind and DaisyUI

A SaaS needs a professional-looking login page. Using Tailwind CSS and DaisyUI, we can build a clean, responsive form that works on any device.

File: app/(auth)/login/page.tsx

import { signIn } from "@/auth";

export default function LoginPage() {
  return (
    <div className="flex items-center justify-center min-h-screen bg-base-200">
      <div className="card w-full max-w-md shadow-2xl bg-base-100">
        <div className="card-body">
          <h2 className="text-3xl font-bold text-center mb-6">Welcome Back</h2>

          <form
            action={async () => {
              "use server";
              await signIn("google", { redirectTo: "/dashboard" });
            }}
          >
            <button className="btn btn-outline w-full flex items-center gap-2">
              Continue with Google
            </button>
          </form>

          <div className="divider text-xs uppercase text-base-content/50">or</div>

          <form className="space-y-4">
            <div className="form-control">
              <label className="label">
                <span className="label-text">Email</span>
              </label>
              <input type="email" placeholder="[e-mail]" className="input input-bordered" required />
            </div>
            <div className="form-control">
              <label className="label">
                <span className="label-text">Password</span>
              </label>
              <input type="password" placeholder="••••••••" className="input input-bordered" required />
            </div>
            <button className="btn btn-primary w-full">Sign In</button>
          </form>

          <p className="text-center mt-4 text-sm">
            Don't have an account? <a href="/signup" className="link link-primary">Sign up</a>
          </p>
        </div>
      </div>
    </div>
  );
}

5. Protecting Routes with Middleware

In a SaaS application, you don't want unauthorized users accessing the dashboard or settings pages. Instead of checking for a session on every single page, we use Next.js Middleware to handle this globally.

File: middleware.ts (Root directory)

import { auth } from "@/auth";

export default auth((req) => {
  const isLoggedIn = !!req.auth;
  const { nextUrl } = req;

  const isAuthPage = nextUrl.pathname.startsWith("/login") || 
                     nextUrl.pathname.startsWith("/signup");
  const isDashboardPage = nextUrl.pathname.startsWith("/dashboard");

  if (isDashboardPage && !isLoggedIn) {
    return Response.redirect(new URL("/login", nextUrl));
  }

  if (isAuthPage && isLoggedIn) {
    return Response.redirect(new URL("/dashboard", nextUrl));
  }
});

export const config = {
  matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
};

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Key Benefits and Learning Outcomes

By following this workflow, you achieve several critical milestones in your development journey:

• Centralized Security: You have a single source of truth for your authentication logic.
• Database Synchronization: Your user accounts are automatically saved to MongoDB whenever someone logs in via Google.
• Improved Conversions: Providing OAuth options reduces the friction of creating an account, which is vital for any Build SaaS with Next.js project.
• Type Safety: Using TypeScript ensures that your session data is predictable throughout your components.

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Common Mistakes to Avoid

1. Exposing the Secret: Never leave your AUTH_SECRET empty or use a simple string in production. Use a tool like openssl rand -base64 32 to generate a strong key.
2. Client-Side Protection Only: Never rely solely on hiding UI elements to secure your app. Always verify the session on the server or through middleware.
3. Forgetting Secure Cookies: In production, ensure your AUTH_URL uses HTTPS, otherwise Auth.js will not set secure cookies, and your login will fail.

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Pro Tips and Best Practices

• Use Server Components for Auth Checks: Whenever possible, check the session in a Server Component using the auth() function. It is faster and more secure than checking on the client.
• Custom Session Data: If you need to store extra info (like a user's subscription status), extend the session callback in auth.ts to include those fields from your MongoDB database.
• Graceful Error Handling: Redirect users to a custom error page if Google login fails, rather than letting the app crash or show a generic error.

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How This Fits Into the Zero to SaaS Journey

Authentication is the foundation of the user experience. Once you have established who the user is, you can:

1. Store their specific data in MongoDB.
2. Link their account to a Stripe Customer ID for billing.
3. Provide a personalized Build SaaS Dashboard Next.js Tailwind.

Without a secure auth system, your SaaS cannot function because you cannot identify who to charge or whose data to display.

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Real-World Use Case: The Productivity Tool

Imagine you are building a SaaS called TaskFlow. A user arrives at your landing page and clicks Get Started.

1. They click Continue with Google.
2. Auth.js redirects them to Google's secure portal.
3. After they approve, Google sends a token back to your auth.ts handler.
4. Auth.js checks your MongoDB. Since this is a new user, it automatically creates a new record in your users collection.
5. The user is redirected to /dashboard, where your server component greets them: "Welcome!"

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Action Plan: What to Build Next

To master this lesson, I want you to complete these four tasks:

1. Initialize the Project: Set up a fresh Next.js project and install the dependencies.
2. Configure Google Cloud: Go to the Google Cloud Console, create a project, and get your OAuth credentials.
3. Build the Login Page: Use the Tailwind/DaisyUI code provided to create your own branded login screen.
4. Test the Middleware: Create a protected /dashboard page and try to access it while logged out to ensure you are redirected.

Take Your SaaS to the Next Level

Building a secure login system is just the beginning. If you want to skip the trial and error and follow a proven path to a launched product, check out our comprehensive Zero to SaaS Next.js Course. We dive deep into advanced patterns, multi-tenant security, and production-ready deployments.