Bitcoin Gold Suffers Rare ‘51% Attack’

Hackers compromised the cryptocurrency Bitcoin Gold—a lesser known offshoot of the original Bitcoin—this month, using superior computing power to falsify the currency’s ledger and swindle at least $18 million from online exchanges.

The hacking incident, which was reported in a blog post earlier this month, is significant because it shows how a so-called 51% percent attack, which poses an existential threat to any Bitcoin-like currency, is not just a theoretical concern.

The threat is known as a “51% attack” because it stems from a malicious actor obtaining more than half of the mining power on a cryptocurrency network.

This dominant level of computer power makes it possible to manipulate the blockchain ledger on which transactions are recorded, and to spend the same digital coins more than once. As an analogy, it’s as if a fraudster got access to the clearing records of a stock exchange and…