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hardEmbedded Systems Engineer interview question

Design a secure, fail-safe over-the-air firmware update system for a fleet of IoT devices.

What the interviewer wants to hear

  • A/B slots or bootloader with a separate download slot
  • Signature verification with keys anchored in immutable storage
  • Anti-rollback version checks
  • Trial boot with health check and automatic rollback
  • Resumable, power-loss-safe download
  • Staged rollout and fleet monitoring

Model answer

I use a small, immutable bootloader such as MCUboot with two image slots. The running image downloads the new one into the inactive slot in resumable chunks, so power loss or a dropped connection only means continuing later. Images are signed, and the bootloader verifies the signature with a public key stored in protected flash or OTP, plus a security version counter to block rollback to vulnerable releases. After switching, the new image boots in trial mode and must pass a health check, such as connecting to the server and running self-tests, before confirming itself. If it crashes or the watchdog fires before confirmation, the bootloader reverts to the previous slot. On the server side I roll out in stages, starting with a small percentage of the fleet, monitor crash and reconnect rates, and pause automatically if metrics get worse. Delta updates can reduce bandwidth.

Can you answer it under time pressure?

Take a timed hard round and get scored on every answer.

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