Bitcoin Gold (BTG) was subjected to what is known as a double spend attack, resulting in losses of over $18 million worth of tokens.
The miner who attacked the network started depositing BTG at cryptocurrency exchanges and also a wallet under his control. In response, on May 18th, Bitcoin Gold’s Director of Communications Edward Iskra notified the community, and the next day, the BTG blog stated, "[a]n unknown party with access to very large amounts of hashpower is trying to use “51% attacks” to perform “double spend” attacks to steal money from Exchanges. We have been advising all exchanges to increase confirmations and carefully review large deposits.” A total of 388,200 BTC has been stolen, which equals $18.6 million worth of funds.
The other two that have been affected in the last one week include XVG where the attacker manipulated a hashing algorithm to mine more than 35 million XVG. The other is Monacoin which a miner gained as much as 57% of the network’s hashrate.