Imagine a fitness app that lets users track workouts, connect wearables, and monitor their progress.

The login system works. Users can view their workout history, update their goals, and synchronize activity data.

But behind the scenes, three potential problems remain:

An authentication token is stored insecurely on the device.

A backend API allows users to access workout records belonging to another account.

Debug logs accidentally capture authorization headers or sensitive fitness information.

These are hypothetical examples, but they illustrate why a functioning login screen is not proof of application security.

Fitness app security requires coordinated protection across authentication, APIs, local storage, third-party integrations, and application logs.

This guide explains practical approaches developers can use to protect those systems.

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