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Interview question · Cryptography, PKI & TLS

Where should mutual TLS terminate (app, load balancer, sidecar), and how does the client's identity reach the application safely?

Mutual TLS proves who connected to whatever terminated the TLS. If that is a load balancer, the application only knows what a header tells it, and headers can be forged.

hardNewPreparation tags: Google, Amazon, MicrosoftUpdated Nov 2026

Mutual TLS proves who connected to whatever terminated the TLS. If that is a load balancer, the application only knows what a header tells it, and headers can be forged.

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A service mesh terminates mutual TLS at sidecars and forwards the client identity to applications in a header.

  1. How does the server validate a client certificate?
  2. Where can mTLS terminate?
  3. What is the risk with identity headers?
  4. How do you make sure only the sidecar reaches the app?
  5. How should the app use the identity?
  6. How would you secure the design?

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