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hardBlockchain Developer interview question

Why have cross-chain bridges been a common target for large hacks, and how would you design a safer cross-chain system?

What the interviewer wants to hear

  • Bridges hold large pooled funds and rely on message verification
  • Weak validator sets or key compromise
  • Bugs in message verification logic
  • Prefer native or proof-based verification, or burn-and-mint designs
  • Rate limits, delays, monitoring and defence in depth

Model answer

Bridges usually lock assets on one chain and mint wrapped versions on another, so the lock contract holds huge pooled funds, making it a prime target. Security depends entirely on how the destination chain verifies that a deposit really happened. Many hacks came from small validator sets or multisigs where attackers stole enough keys, like the Ronin hack, or from bugs in signature and message verification, like Wormhole and Nomad. For a safer design, I would prefer verification that does not depend on a few trusted signers: the canonical rollup bridge for L2s, light clients or ZK proofs of source-chain state, or established messaging protocols with independent verifier networks. Where possible, use burn-and-mint for tokens we control, which avoids a single big honeypot. I would add rate limits per time window, delays for large transfers, independent monitoring with automatic pause, and audits focused on message encoding and replay protection.

Can you answer it under time pressure?

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

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