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Interview question · Mobile Application Security

A mobile app needs to call your API. How do you handle authentication and secrets when the app cannot keep a secret?

Anything in the app binary belongs to whoever downloads it. The strong answer stops trying to hide a client secret and builds on user tokens, PKCE, device-bound keys and server-side limits.

hardNewPreparation tags: Google, Apple, MetaUpdated Nov 2026

Anything in the app binary belongs to whoever downloads it. The strong answer stops trying to hide a client secret and builds on user tokens, PKCE, device-bound keys and server-side limits.

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What would you say before reading the answer?

  1. 01 State the assumptions your answer depends on.
  2. 02 Explain the mechanism and choose a control.
  3. 03 Name a condition that would change your decision.

The worked answer includes an approach, scoring criteria and follow-up questions. Compare the reasoning with your own.

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What if · the follow-ups in this answer

The answer works through 6 follow-ups like an interviewer would

A mobile app ships with an OAuth client secret in its code and uses it to get tokens.

  1. Why can a mobile app not keep a client secret?
  2. How should the app authenticate users?
  3. How should tokens be stored?
  4. What do you do about the shipped secret?
  5. What about other API keys in the app?
  6. How would you fix the app?

Each rung has a nudge, a model answer, the principle that carries over and the weak answer to avoid. Premium opens every answer and ladder.

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