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It now costs just north of $21 million per day to sustain a 51% attack on the Bitcoin network for a whole day, according to data from Messari Pro. As it stands, this cost makes Bitcoin almost eight times more expensive to attack than Ethereum, which would set you back almost $2.7 million to attack for a whole day.
In brief, a 51% attack—also known as a majority attack—is arguably one of the biggest threats to many blockchains, since it sees a group of miners, or a single nefarious entity secure more than half of a network's hash rate in order to gain control over the associated proof-of-work (POW) blockchain.
In order to attack Bitcoin, you would need to get enough mining machines to control half the Bitcoin hash rate. And that means matching the hash rate that's already there. Messari estimates the cost of an attack based on how much it would cost to rent this much hash power on NiceHash, a crypto mining marketplace.
Comparatively, an attacker can gain control over the Bitcoin Cash, Bitcoin SV, Litecoin blockchains for well under $1 million each, whereas the Ethereum Classic blockchain could be subverted (again) for just $326,000 at current rates.
If successfully executed and sustained for long enough, a 51% attack could allow a bad entity to selectively block new transactions, reverse recent transactions, and potentially even perform a blockchain reorg—rewriting previously confirmed blocks and causing a temporary chain split. However, it has been argued that the costs involved with securing enough mining power for long enough to accomplish a 51% attack on a well-protected blockchain makes most major POW coins safe from such attacks.
Nonetheless, several less prominent blockchains have been subjected to such an attack in the past, including Bitcoin Gold, Verge, and Vertcoin—all because they lacked sufficient hashrate to make it cost-prohibitive to do so. But at $21 million a pop, someone would need to be pretty motivated to take Bitcoin down.