Cryptocurrency miners commit their computer processing power—or hash power—to adding new transactions to a coin’s blockchain. They are rewarded in units of the coin in return. The idea is that these incentives create competition among miners to add more hash power to the chain. The more hash power is added, the better the chances of winning a reward.

So what’s a 51% attack? It’s when a single miner controls more than half of the hash power on a particular blockchain. When this happens, that miner can mess with transactions in a bunch of ways, including spending coins twice. This is the “double-spending problem,” a puzzle surrounding digital money that has vexed computer scientists for years—and which was solved by bitcoin. But the solution only holds if no single miner controls the majority of the hash power on a chain.

Bitcoin Gold has been experiencing double-spending attacks for at least a week, according to forum posts by Bitcoin Gold director of communications Edward Iskra. Someone has taken control of more than half of Bitcoin Gold’s hash rate and is double-spending coins. Since an attacker must spend coins in his or her possession, and can’t conjure up new coins, the attack is somewhat limited.

What’s happening now, according to Iskra, is that exchanges that automatically accept large deposits are being targeted. The fraudster deposits Bitcoin Gold into an account at an exchange, where coins are traded. Once the exchange credits the Bitcoin Gold to the attacker’s account, the attacker trades those coins for another cryptocurrency and withdraws it. The attacker can repeatedly make deposits of the same Bitcoin Gold it deposited in the first exchange and profit in this way.

A bunch of other cryptocurrencies have been attacked in similar ways recently. Something called Verge has been hit twice in the last two months, leading to $2.7 million being stolen. The exotic-sounding coins Monacoin and Electroneum have also suffered from 51% attacks not too long ago.

A 51% attack isn’t likely to hit bitcoin any time soon. Bitcoin Gold has a lot less hash rate securing it than bitcoin. Bitcoin miners are contributing about a million times more processing power than Bitcoin Gold miners at the moment. To give you an idea of how much money would be required to capture more than half of the bitcoin network, in 2013 the processing power on the bitcoin network was already greater than the 500 most powerful supercomputers combined—by eightfold (although it’s worth noting that the comparison isn’t entirely fair, since the specialized chips mining bitcoin can only perform one operation, instead of general computing). In the five years since, bitcoin’s hash rate has increased by about 1.4 million times.

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