How to Handle User Authentication Securely (Without Building It Yourself)
· 6 min read · Mona Technologies
Authentication is the one part of your app where a small mistake does not just create a bug, it creates a breach. And the uncomfortable truth is that most authentication failures are not sophisticated attacks. They are ordinary, well-documented mistakes that OWASP has been warning developers about for over a decade, repeated on a new codebase. This is a decision-maker's guide to the choices that actually matter, not a code tutorial.
Stop thinking of login as a feature you build
The first decision, and the one with the biggest consequences, is whether your team writes its own login system at all. Rolling your own authentication means your developers are now responsible for password hashing, session management, token expiry, account recovery flows, and every edge case an attacker might probe. Get any one of these wrong and the failure is silent until it isn't. Services like Auth0, Clerk, AWS Cognito, and Firebase Authentication exist specifically because this problem has already been solved, audited, and hardened by teams whose full-time job is finding the holes in it. Unless authentication itself is your product, custom-building it is spending engineering time on something that creates liability instead of differentiation.
- If login is not your core product, use a managed identity provider rather than custom code.
- If you must build custom, budget for a third-party security review before launch, not after an incident.
- Treat any 'we'll just hash it ourselves' comment from a developer as a flag to ask more questions, not a shortcut to approve.
Multi-factor authentication is the highest-leverage decision you will make
If there is one control worth insisting on, it is this one. Microsoft's own security research, based on measurement of real Azure Active Directory accounts, found that enabling multi-factor authentication reduced the risk of account compromise by over 99%, even for accounts whose passwords had already leaked elsewhere. That is not a marginal improvement, it is close to a categorical fix for the most common attack pattern: an attacker reusing a password stolen from a breach at a completely unrelated company. Requiring a second factor, an authenticator app, a push notification, or a hardware key, turns a stolen password from a working key into a dead end.
The business objection is usually friction: won't a second step at login hurt signups or daily engagement? In practice, the fix is to be selective rather than absolute. Require MFA for account owners, admins, anyone with access to payment or customer data, and any password-reset flow. Make it optional but strongly nudged for everyday end users. You get most of the protection without taxing the majority of your login volume.
Passwords are a liability you should minimize, not perfect
A lot of energy gets spent arguing over password complexity rules, when the more consequential decisions are elsewhere. Passwords should never be stored in a form your own database admin can read; they should be run through a slow, purpose-built hashing algorithm, not encrypted, not hashed with a fast general-purpose function like plain SHA-256. This is table stakes covered by OWASP's own password storage guidance, and any provider or contractor should be able to state plainly which algorithm they use and why.
Beyond storage, consider whether your product needs a password at all for every account. Passkeys and magic-link email sign-in remove the password entirely for a meaningful share of users, which also removes the single most common attack surface: credential reuse from some other breach you have no control over.
- Confirm passwords are hashed with a slow algorithm (bcrypt, scrypt, or Argon2), never encrypted or fast-hashed.
- Offer passkey or magic-link sign-in as an alternative path, not just a password field.
- Rate-limit and lock out repeated failed login attempts; this alone stops most automated credential-stuffing attacks.
Sessions and tokens need an expiry date, literally
A login that never expires is a standing invitation. Every session or access token your app issues should have a defined lifetime, and sensitive actions, changing a password, adding a payment method, granting admin access, should re-confirm identity even within an active session. This matters more than it sounds: a stolen laptop, a shared device at a co-working space, or a browser extension with excessive permissions can leak a session token just as easily as a password. If that token works forever, the exposure window is forever too.
Ask your development team directly: what happens when a user's session token is stolen, how long does the attacker have before it expires, and can the user or an admin revoke it remotely? If nobody has a confident answer, that is the gap to close before launch, not after.
The short version
Do not build authentication from scratch unless it is genuinely your product; use a managed provider that has already been through security audits you cannot afford to run yourself. Then spend your remaining attention on one control above all others: require multi-factor authentication, at minimum for every privileged account. That single decision, backed by Microsoft's own measurement of real-world attacks, closes off the vast majority of account takeover attempts before they start.