Your BFT Protocol Will Break in Production

A practical breakdown of failure modes encountered while working on Espresso's HotShot, an implementation of HotStuff and HotStuff-2 BFT consensus protocols. Covers three main vulnerability classes: signature replay attacks (where replayed proposals with valid aggregate signatures can disrupt pipelined consensus), deferred validation bugs (where unvalidated proposal data can halt the network), and race conditions in state updates. Each failure mode includes concrete examples and mitigation strategies. The key insight is that these issues rarely surface initially — they emerge under load, integration stress, and system evolution.