Bitcoin Gold hit by 51% attacks, $72K in cryptocurrency double-spent
ELI5:Lots of cyptocurrencies use the same mining algorithm, i.e. they require the same type of puzzle to be solved to make money creating blocks. In recent years, lots of online services have sprung up to offer cloud mining, which people usually use to mine blocks on the bigger cryptocurrencies, like Bitcoin Core or Bitcoin Cash.However, since the smaller cryptocurrencies, like Bitcoin Gold, have less users/miners, they also require easier "puzzles" to be solved, which opens an opportunity for any random person to pay some $$$ to hire a bunch of these cloud servers for a limited time and point them at these easier puzzles, which can cause such a smaller blockchain to get confused about account balances. The attacker can then re-spend the same currency multiple times to make a profit.AFAIK there aren't really any good solutions to prevent this problem- For complex game-theoretic reasons, simply changing the mining algorithm to something different doesn't really offer much protection. (Some folks believe if these smaller currencies were to move to "proof of stake" it could help solve this problem, but this is an extremely contentious topic.)