Auth is the first thing you build and the last thing you want to rebuild. Pick wrong and you migrate users, tokens, and sessions under load. That migration is the most expensive code you will ever write. So treat the first choice as a one-way door, mostly.
Here are the three real options for a SaaS. Cognito, Auth0, or roll your own. Each is correct for a specific case. Each fails in a specific way.
Cognito: cheap, AWS-native, sharp edges
Cognito is the default when you already live in AWS. It is cheap at scale and integrates with IAM, API Gateway, and Lambda authorizers directly. Your tokens are standard JWTs. You get user pools, MFA, and social login without running a service.
The failure mode is the developer experience. The SDK is awkward and the docs assume you know the model already. Custom flows mean Lambda triggers, and those triggers are hard to test. Email and SMS delivery through SES and SNS needs setup before a single user signs up.
Cognito fits when you want auth inside your AWS account with no third-party bill. We have shipped it on real products and know where the edges cut. If your stack is already on AWS, this is the cheapest path that holds at scale.
Auth0: fast to ship, priced to punish growth
Auth0 is the fastest way to a working login screen. The docs are good, the rules engine is flexible, and enterprise SSO is close to a config toggle. SAML, OIDC, and SCIM are there when a customer asks. You buy speed and you buy support.
The failure mode is the bill and the lock-in. Pricing climbs hard as monthly active users grow, and enterprise features sit behind the expensive tiers. Their actions and rules become custom logic that lives in their dashboard, not your repo. Migrating off later means exporting users and reworking every login flow.
Auth0 fits when speed beats cost and you need SSO yesterday. Use it when a deal depends on SAML next week. Know the price curve before you sign, because it bends.
Roll-your-own: total control, total ownership of the breach
Building auth yourself is the right call less often than engineers think. You control the schema, the session model, and every byte of the token. No per-user bill and no vendor dashboard holding your logic. For a narrow case, that control is worth real money.
The failure mode is the part you cannot see. Password hashing, token rotation, session fixation, account enumeration, rate limiting on the reset flow. Each one is a known attack with a known fix you now own forever. A missed one is a breach, and the breach is on your name, not a vendor's.
Roll your own when auth is a core product surface, not a checkbox. A B2B tool with strange tenancy rules can justify it. Most SaaS cannot, and the honest answer is to use a provider.
How to actually decide
Start with one question. Will an enterprise customer demand SSO in the next year? If yes, lean Auth0 or budget the SSO work into Cognito early. SSO retrofits are painful, so price it in now.
Then weigh cost against control. If you are AWS-native and cost-sensitive, Cognito wins. If speed and SSO matter more than the bill, Auth0 wins. If auth is your product and you have the security depth to own it, build it. Most teams land on a provider, and that is the right answer.
Whatever you pick, keep tokens standard and keep the login surface thin. Wrap the provider in your own interface so a future migration touches one module, not fifty. That wrapper is the cheapest insurance you will buy on this project.
Where we fit
We build Web App projects at a fixed price, and auth is usually the first decision we make with you. Two engineers, ex-AWS background, 20 cloud certifications between us. We have shipped 17 production products, 10 of them in our first 7 months, so we have made this call across real stacks. Fixed price every time, you own the code, and we exit.
Auth is a small part of the build but it sets the shape of everything after it. Getting it right early is cheaper than fixing it under traffic. If you are weighing these three and want a straight answer for your case, book a free 30-minute call.