The dangerous assumption after sign-in
A founder can add a working sign-in flow and still leave customer data exposed. OWASP distinguishes authentication, which verifies identity, from authorization, which decides whether an action is allowed. A signed-in person is not entitled to every record or function in an application.
This is easy to miss in a small prototype because the builder or developer often tests with one powerful account. Every page loads, every button works and every record is visible. Those observations establish very little about what a second customer can access.
Write rules around resources
List the resources your product stores and the operations users can perform. A document may have an owner, invited readers and invited editors. An administrator may manage accounts without needing to read every private document. Define those differences before implementation.
OWASP recommends least privilege and denying access by default. Give each role the minimum needed for its job, then add explicit allowances. Avoid relying on a growing collection of exceptions around an otherwise open system. A newly introduced action should not become available merely because nobody remembered to forbid it.
Keep enforcement off the screen
Hiding an edit button helps the interface make sense, but it is not the security boundary. OWASP warns that client-side checks are easy to bypass and says authorization checks must run server-side or in the appropriate trusted enforcement layer. The same rule must apply whether a request comes from the ordinary page or a direct API call.
Validate access on every request. A one-time permission check when a user opens a dashboard does not cover a later export, download or delete action. If an account changes role, later requests need to reflect the new permissions rather than trusting an old screen state.
Test the ordinary failures
Create two test accounts with separate records and try to cross the boundary. Attempt to read, edit, export and delete the other account's data. Repeat with a lower-privilege role and with an expired session. Include direct record links rather than navigating only through buttons the interface exposes.
Keep these tests as the product grows. A new endpoint or integration can introduce a path that an earlier test never covered. Access control is part of the product model, not a decorative step added after the login page looks finished.