The ZenCash network was attacked earlier today via a 51% attack. The attacker managed to reorganize the blockchain multiple times, successfully double spending two transactions.
Over $550,000 Double Spent
ZenCash is a Proof of Work cryptocurrency based on the popular Equihash mining algorithm. Equihash is used by several other popular currencies, most notably ZCash, meaning that there is a very large pool of total Equihash hash rate in the world that can be used on a variety of networks. ZEN is a fork of ZClassic that launched in late May 2017, using the same codebase as ZCash.
Yesterday, the ZenCash network experienced a 51% attack, where one person or party controls 51% of the hash rate. The hash rate influx allowed the attacker to reorganize the blockchain several times, with the largest rollback reversing 38 blocks. The attacker also was able to double spend two massive transactions – 13,000 and 6,600 ZEN – worth more than $550,000 at current prices, according to a statement made by the development team. Mining pool operators notified the ZenCash team of a potential attack and exchanges were quickly contacted and warned to increase confirmation times.
It was possible for the attacker to roll back transactions, but it is unclear what he did with the initial transactions. If he sent them to an exchange and traded the coins for Bitcoin or some other cryptocurrency, the attacker made out like a bandit. The attack occurred over a span of around 110 blocks, making the total attack length under four hours. According to 51Crypto, a website which estimates the theoretical cost of a 51% attack on various cryptocurrencies, it cost the attacker around $30,000.
GPU Mined Currency Under Attack
This is now the fourth major 51% attack launched in the last few months, Bitcoin Gold and Verge being among them. These attacks have sparked much debate within the crypto community as the ASIC vs GPU argument rages on.
Last week, a website was created showing the costs incurred in renting enough hash power via NiceHash to attack specific networks. Shockingly, several crypto-currencies not only were cheap to attack but also had plenty of hash rate for sale on NiceHash with which such an attack could take place. When 51% attacks were considered in the past, most calculations included the cost of hardware, electricity, and maintenance. But this new “rent-a-attack” method is proving dangerous for smaller networks.