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Interview question · Identity, OAuth & Access Control

Why must a server reject tokens that were not issued for it, and what goes wrong with token passthrough?

A valid token is not valid everywhere. The scored answer explains audience binding, why forwarding a caller's token downstream breaks it, and what MCP's authorisation rules require instead.

hardNewPreparation tags: Microsoft, Google, AnthropicUpdated Nov 2026

A valid token is not valid everywhere. The scored answer explains audience binding, why forwarding a caller's token downstream breaks it, and what MCP's authorisation rules require instead.

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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

An MCP server receives the user's access token from an AI client and forwards it unchanged to a downstream project-management API.

  1. What is token passthrough?
  2. Why does audience validation matter?
  3. How should the MCP server call the downstream API?
  4. What if the token is stolen from the MCP server's logs?
  5. What does the MCP specification say?
  6. How would you fix the server?

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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