A working sign-in screen proves that your app can recognize a customer. It does not prove that the customer can see only their own projects, invoices, or files.
For a solo founder, the safest route is to let a trusted login service handle passwords and account recovery. This service is often called a login provider. Your app should focus on one question: “May this person access this exact item?”
What does a safe login system check?
| Check | Plain-English question |
|---|---|
| Identity | Who is making this request? |
| Session | How does the app keep short-lived proof of who signed in? |
| Permission | May this person use this exact record or action? |
The third check is the one most often missed. If a signed-in customer changes /projects/41 to /projects/42, the app must check the owner before returning anything.
Should you build password security yourself?
Usually, no. Safe password storage is only one part of login. The system also needs verification emails, reset links, time limits on sign-ins, limits on repeated guesses, and two-step verification. Unless login is your product, use a well-established service that supports your app and final domain.
Keep the provider's private keys in the hosting platform's protected settings. Before launch, tell the login service the exact live pages people should return to after sign-in, sign-out, and account recovery.
How should every private request work?
- Confirm that the session is valid.
- Find the account number your app saved for that sign-in.
- Load the requested record together with its owner or team.
- Allow the action only when the permission rule passes.
- Return a simple “not allowed” message without revealing another customer's details.
Hiding a button changes the screen. Blocking an unauthorized request protects the data.
Does every record have an owner?
When a project, order, upload, report, or subscription is created, save who owns it or which team may use it. Adding ownership later is risky because old records may already be visible to the wrong account.
For team products, keep the first roles simple: owner, editor, and viewer are often enough. Start with the rule “block access unless a rule allows it.” This stops a new feature from becoming public just because someone forgot to add a rule.
What should a session contain?
A session is the app's temporary proof that a person signed in. Send it only over HTTPS. Give it an end time, replace it after login, and make it stop working after logout.
A session token may look like a long, random string. That does not make it a safe place for private customer records or service keys. Store only the minimum information the app needs to confirm identity and permissions.
How will customers recover their accounts?
Test the verification email, forgotten-password flow, and expired reset link. Also test a changed email, logout on another device, and an account that an owner has turned off. Responses should not reveal whether a particular email address belongs to a customer.
For a sensitive change—such as a password, email, payout destination, or billing setting—ask the customer to prove their identity again.
How do you run the two-customer test?
- Use two different browsers or separate browser profiles. Sign in as test Customer A in one and test Customer B in the other. Two private windows in the same browser may share a sign-in.
- Create one private record and one upload for each customer.
- Copy A's record address and open it while signed in as B.
- Try the same test for viewing, editing, deleting, and downloading.
- Confirm that B receives no data and learns nothing private from the error.
Repeat the test for team invitations and owner-only actions. If you cannot state the expected result for a role, the role rules are not finished.
What should you ask your coding AI?
Use a concrete request: “For every private action, check who is signed in. Confirm that this customer owns the record or belongs to its team. Do not rely on hidden buttons. Add tests in which two customers try to open each other's data.”
Then run those tests yourself. Login is ready only when identity, sessions, ownership, and recovery work together.
What should you read next?
- Where Should API Keys Go?
- Where Does Your AI App Save Customer Data?
- AI App Launch Checklist for Solo Founders
When the two-customer test passes, use the JustDeploy Quickstart to publish and test the same flow at the live address.