In the early hours of Jan 20, 2021, Project Steward at Firo (formerly known as Zcoin), Reuben Yap, highlighted that the cryptocurrency has experienced a 51% attack. This occurs when a single entity controls the majority of the hashrate and have enough mining power to intentionally exclude other miners from completing blocks, monopolising the reward system.
In this case, the attacker had reorganised the blockchain approximately 306 blocks deep which means that previously confirmed transactions over the past 48 hours have been unrolled. From a Proof-of-Work (PoW) perspective, it can be seen that they are trying to extend their own private chain and accumulate more transactions to make it longer than Firo’s legitimate chain.
This came weeks before Firo’s intention to deploy Chainlocks which would have required an attacker to not only have 51% of the mining hash rate but also control at least half of all Firo master nodes, possibly deterring these forms of attacks.
It comes against a backdrop of increasingly more attacks on blockchain networks such as the Ethereum Classic in 2020 where over 7,000 blocks were reorganized. Hopefully, future security implementations and system updates can fortify these systems and foster trust in the crypto industry.