https://qz.com/1287701/bitcoin-golds-51-attack-is-every-cryptocurrencys-nightmare-scenario/

Bitcoin Gold is a fork, or spin-off, of the original cryptocurrency, bitcoin. It shares much of the same code and works in a similar way to bitcoin, with Bitcoin Gold miners contributing computational power to process new transactions. That also means it faces the same vulnerabilities as bitcoin, but without the protections that come from the large, dispersed group of people and organizations whose computers are powering the bitcoin blockchain.

In recent days the nightmare scenario for any cryptocurrency is playing out for Bitcoin Gold, as an attacker has taken control of its blockchain and proceeded to defraud cryptocurrency exchanges. All the Bitcoin Gold in circulation is valued at $786 million, according to data provider Coinmarketcap. Blockchains are designed to be decentralized but when an individual or group acting in concert controls the majority of a blockchain’s processing power, they can tamper with transactions and pave the way for fraud. This is known as a 51% attack.