Firo (formerly known as Zcoin) has announced that it is currently experiencing a 51% attack. 

• In a tweet, the Firo account said that it would recommend that users not make transactions during this time, and that it was working with exchanges and pools.
• The account also tweeted that “chainlocks that would have prevented this were being tested on testnet and was weeks away from deployment.”
• Chainlocks are a secondary validation layer that make 51% attacks much more difficult.
• A 51% attack occurs when a miner (or miners) acquires more than 50% of the network's mining hash power and takes control of the network.
• The price of the privacy coin has dropped around 17% in the last 24 hours.
• Firo recently launched the new privacy protocol Lelantus on its mainnet.