Poloniex faced the challenge of protecting customers from replay attacks following the Bitcoin Cash hard fork, where the SV chain initially lacked replay protection. Replay attacks occur when transactions intended for one blockchain are unintentionally duplicated on another due to identical outputs existing on both chains immediately after a fork. This can lead to unintentional fund transfers across the new chains if proper measures are not taken. To address this, Poloniex implemented a strategy involving the use of post-fork outputs, or UTXOs, which are unique to each chain after the fork. By including at least one post-fork output in transactions, Poloniex ensured that transactions were only recognized on the intended chain, thereby preventing replay attacks and safeguarding customer funds. This solution was crucial for maintaining the integrity and security of transactions on their platform during the transition period following the hard fork.