Login and Auth for an MVP: What to Build First

For an MVP, build email-and-password or social sign-in, a session that is handled safely, a password reset that is as strict as sign-up, and an option for multi-factor authentication. Leave single sign-on, role matrices and custom identity features for later. The login page is the easy part. Most of the risk sits in sessions, recovery flows and token handling, and that is where prototypes tend to fail.

I'm Hanad, a senior software engineer in Vienna. I build first real versions of products for founders at a fixed price. This guide covers what I would put into the first release, what I would postpone, and which mistakes I see in prototypes that have hit a wall.

Key takeaways

• Auth is more than a login screen. Sessions, password reset and token handling decide whether your MVP is safe.
• Ship the minimum: sign-up, sign-in, safe sessions, a strict reset flow and optional MFA. Postpone SSO and complex roles until a customer asks.
• Use the modern OAuth pattern (Authorization Code with PKCE) and never put tokens in URLs.
• Don't force password rotation. Offer MFA instead.
• If an AI-built prototype already has auth, have it reviewed before real users or investors touch it.

Table of Contents

• What "login and auth" means in an MVP
• The MVP auth scope I would ship first
• Sessions are where most of the security lives
• Password reset: the flow founders underestimate
• Multi-factor authentication without over-building
• Do not force password rotation
• Social login and OAuth: use the current pattern
• Build, buy or use a managed service?
• Warning signs in a prototype's auth
• A sensible order of work for an auth MVP
• My perspective: auth is where a boring stack pays off
• Need auth that survives real users?
• FAQ
• Sources

What "login and auth" means in an MVP

The two words get blurred, so here is the split. Authentication answers who you are. OWASP defines it as verifying that a person or system is who it claims to be, by checking one or more authenticators such as passwords, fingerprints or security tokens. Authorization answers what you may do once the system knows who you are.

For a first release, you need all of this:

1. Account creation: email and password, a social login, or both.
2. Sign-in: the check that turns credentials into a session.
3. Session management: how the app remembers you between requests.
4. Recovery: what happens when someone forgets a password.
5. Basic authorization: at minimum, "users only see their own data" and "admins can do admin things".

That is the real scope. A prototype built in a visual or AI tool usually has the first two and a thin version of the others. Founders then discover this when a tester logs in as someone else, or when a security questionnaire arrives.

The MVP auth scope I would ship first

An MVP exists to test one core workflow with real users. Auth should not eat your budget, but it should not be improvised either. Here is a practical cut.

The rule behind the table: build what protects users on day one, and leave out what only matters at scale. If you want the bigger picture of what belongs in a first release, my MVP best practices for non-technical founders covers scoping beyond auth.

Sessions are where most of the security lives

Most founders think auth is a login page. The real work happens after the password is accepted. The OWASP Developer Guide puts it plainly: once a user is authenticated, the session token is equivalent to their username and password. If session handling is weak, a strong login protects nothing.

Three habits matter most for an MVP.

Regenerate the session on login

Whenever privileges change, especially at login or when a user becomes an admin, the app should issue a new session ID and destroy the old one. OWASP's session management guidance explains why: if the previous session survives, an attacker who planted or learned that ID can take over the user's session. This is called session fixation. It is a one-line fix when built in from the start and an annoying hunt when found later.

Set timeouts you can explain

The same OWASP guide calls for short, clear inactivity timeouts and for ending active sessions periodically, even when they look busy. For a B2B product holding customer data, I would pick a conservative idle timeout and a hard maximum session length, then write both down. If a customer's security reviewer asks, you can answer in one sentence.

Let users end other sessions

A "sign out everywhere" button costs little and gives users and support a real tool after a suspected compromise. It also connects to the password reset flow below.

Password reset: the flow founders underestimate

Many founders write a custom reset flow in an afternoon. Experts warn that this is a frequent failure point, because a reset is a second way into the account. OWASP's MFA guidance says recovery needs the same level of security and controls as the initial account creation.

A safe MVP reset flow does these things:

• Sends a time-limited, single-use link to the account's email.
• Gives the same response whether or not the email exists, so attackers can't list your customers.
• Notifies the user by email whenever a password or recovery method changes, and offers to end all other active sessions.
• Sends the user back to the normal login after the reset.

That last point surprises people. OWASP's Forgot Password cheat sheet advises against logging a user in automatically after a reset. It feels convenient, but it forces your auth logic to handle odd states that tend to introduce vulnerabilities. One extra login is a small price.

Multi-factor authentication without over-building

MFA is the best return on effort in this whole topic. OWASP's MFA cheat sheet cites analysis that MFA would have stopped 99.9% of account compromises, and it recommends that MFA be required for administrative and high-privilege accounts and offered to all users through TOTP, the six-digit codes from an authenticator app.

For an MVP, I would suggest:

1. Require TOTP for any admin or staff account from day one.
2. Offer it to every user in account settings.
3. Generate recovery codes and show them once, so that losing a phone does not become a support crisis.

What about passkeys? The July 2025 NIST SP 800-63B-4 guidance says that at Authentication Assurance Level 2 and above, applications must offer at least one phishing-resistant option such as FIDO2/WebAuthn. It also says manually typed one-time or SMS codes do not count as phishing-resistant. That guidance is written for US federal systems, so it is not a legal requirement for your startup. It is still a useful signal of where expectations are going, especially if you sell to regulated buyers. I wrote more on this in MFA for SaaS: FIDO2 and session binding.

Do not force password rotation

A common instinct is to make users change passwords every 90 days. OWASP's authentication guidance argues against over-engineering rotation: forced periodic changes push people toward weaker, predictable passwords. Spend that effort on MFA, and require a reset only when you detect a breach or a compromised identity. It is one less feature to build and one less thing to confuse your first users.

Social login and OAuth: use the current pattern

"Sign in with Google" saves users a password and saves you a reset flow. It also brings OAuth, which has changed. RFC 9700, the current best practice for OAuth 2.0 security, formally deprecates the Implicit Grant. As OWASP's OAuth2 cheat sheet summarizes it, modern apps, including single-page apps and mobile apps, should use the Authorization Code flow with PKCE, which protects against authorization code injection and token interception.

Mistakes I see in older tutorials and in generated code:

• Tokens in URL query parameters. RFC 9700 treats this as an anti-pattern, since tokens can leak through browser history and logs.
• Loose redirect URIs. Require an exact match for each redirect URI, and never allow open redirectors.
• Access tokens from the front channel. Tokens should come from the token endpoint, not from the browser redirect.

If your prototype was built from a five-year-old tutorial, check these three first.

Build, buy or use a managed service?

You have three honest options, and the right one depends on your product.

Managed identity service. Fastest to start, and often the right call for a simple product. You trade some control and recurring cost for speed. Check where user data is stored and how the pricing grows with users, especially if you have EU customers.

An established open-source auth library inside your own app. Your data stays in your database and the code is yours. You carry the responsibility to configure it correctly, which is exactly where review pays off.

Hand-rolled auth. I would avoid this. Password hashing, token handling, reset flows and session rules are all easy to get subtly wrong.

Whichever you pick, ask what access any third-party tool gets to your systems. A good vendor documents its access model clearly, the way Koritsu describes its read-only access for infrastructure and billing data. Apply that same standard to your auth provider and to every service you connect.

If an off-the-shelf tool fits your case better than custom code, I will tell you so. That is part of how I work.

Warning signs in a prototype's auth

If your Lovable, Bolt, Replit, Bubble or similar prototype already has a login, test it before real users do. These signs usually mean the auth needs a rebuild or at least a review:

• One user can see another user's data by changing an ID in the address bar.
• The password reset link still works after it has been used once.
• Logging out in one tab leaves other devices signed in with no way to end them.
• Admin pages are hidden in the interface but not protected on the server.
• Nobody on your team can say where passwords or tokens are stored.
• The platform bill grows with each active user, and you can't export the user table.

None of these is a moral failing. A prototype exists to validate an idea. But each one is the kind of thing that stops an investor's technical review or a customer's security questionnaire.

A sensible order of work for an auth MVP

1. Define roles and data ownership. Who are the users, and what may each see? Two roles are usually enough.
2. Choose the sign-in methods. Email and password, plus at most one social provider.
3. Build sessions first. Regeneration on login, timeouts, sign-out everywhere.
4. Add recovery. Reset by email, notification on change, manual login afterward.
5. Add MFA. TOTP for admins, optional for users, with recovery codes.
6. Enforce authorization on the server. Every request checks who is asking, never the interface alone.
7. Test it like an attacker would. Try other users' IDs, reused links and expired sessions.

This fits comfortably inside a two-to-four-week first build alongside the core workflow. For how that workflow gets chosen, see my custom app development guide for startup founders.

My perspective: auth is where a boring stack pays off

I have spent ten years in software, nine of them in engineering, including systems work for Deutsche Bahn, BMW and the Bundesrechenzentrum. The lesson that carries over to every startup is simple: security problems rarely come from exotic attacks. They come from skipped basics.

So I build auth on a deliberately boring stack: TypeScript, Next.js, React, Node, SQL and Cloudflare Workers. Widely known tools mean the next developer can read the code, and a reviewer can check it. Founders own the code from the first commit, which matters most for auth. You should be able to see exactly how your users' sessions and passwords are handled, and you should never depend on someone else's black box to explain it.

I also keep the scope honest. Most MVPs do not need SSO, a permissions matrix or passkeys on day one. They need a login that holds up, a reset flow that does not become a back door, and MFA for the people who can do the most damage. Everything else can wait for a customer who asks.

— Hanad Kubat

Need auth that survives real users?

If you have a prototype whose login is shaky, or you need a first working version with accounts and payments that behave, I offer a fixed-price Prototype Audit at €1,500 (3 to 5 days, credited against the build). Builds start from €12,000. The prices are possible because it is just me: no agency overhead, no project-manager layer, no offshore markup. You see working progress before you pay, and the code is yours from the first commit. Details are on hanadkubat.com.

If you are still deciding what to build first, my piece on why startups need an MVP is a good starting point.

FAQ

What is the minimum login and auth setup for an MVP?

Sign-up and sign-in, sessions that regenerate on login and expire, a strict password reset, and MFA for admins. Add server-side checks so users only reach their own data. That is enough for a first release with real users.

Should I build my own authentication or use a service?

Avoid writing the cryptographic and session parts from scratch. A managed service or a well-maintained library is safer than hand-rolled code. Choose based on where you need your user data to live and how costs scale with users.

Do I need passkeys in my MVP?

Usually not on day one. TOTP-based MFA covers most early needs. Passkeys become more important if you sell to regulated buyers, since NIST's 2025 guidance expects phishing-resistant options at higher assurance levels for US federal systems.

Should users have to change passwords regularly?

No. Forced rotation tends to produce weaker passwords. Offer MFA, and require a reset only when you detect a breach or compromise.

Can a Lovable or Bolt prototype's login go to production?

Sometimes, but test it first. Check data isolation between users, reset links, session handling and server-side authorization. If it fails those checks, rebuilding the auth as maintainable code is usually cheaper than patching it.

Sources

• Authentication Cheat Sheet, OWASP
• Session Management Cheat Sheet, OWASP
• Multifactor Authentication Cheat Sheet, OWASP
• Implement Digital Identity, OWASP Developer Guide
• Forgot Password Cheat Sheet, OWASP
• OAuth2 Cheat Sheet, OWASP
• RFC 9700: Best Current Practice for OAuth 2.0 Security
• NIST SP 800-63B-4, Digital Identity Guidelines

Recommended

• Startup MVP best practices: fast validation for non-tech founders
• Founders' One Page Release Checklist: Staging vs Production for MVPs
• Avoid common MVP pitfalls: 5 steps for founder success
• MVP architecture: Why it matters for SaaS founders