What Is a 51% Attack? When One Miner Controls the Chain

A 51% attack is what happens when one party controls most of the computing power securing a blockchain. With the majority, they can build blocks faster than everyone else combined, and the network follows the longest chain — so they can rewrite recent history. It has never happened to Bitcoin. It has happened repeatedly to smaller coins, including Ethereum Classic and Bitcoin Gold.

What can an attacker actually do?

Less than most people assume. They can spend coins, wait for the payment to be accepted, then publish a version of history in which that payment never happened — keeping both the coins and whatever they bought. This is the double-spend, and it is the whole point of the exercise. They can also block other people's transactions from being included while they hold the majority.

They cannot steal from wallets they do not have the keys to, invent coins out of nothing, or change the rules about how many coins exist. Those limits are enforced by every node on the network, and majority mining power does not override them.

Why is Bitcoin safe from this and small coins are not?

It comes down to what the hardware costs. Renting enough power to outpace the whole Bitcoin network would run into millions of dollars an hour, assuming the machines could even be found. A small coin that shares Bitcoin's mining algorithm is a different story: attackers can rent spare capacity from miners pointed at bigger chains and take over for an afternoon. That is roughly what happened to Bitcoin Gold in 2018 and 2020, and to Ethereum Classic in 2019 and 2020, where exchanges lost coins to reversed deposits.

How is an attack usually noticed?

The tell is a "chain reorganisation" — a stretch of blocks that everyone accepted suddenly being replaced by a longer, different version. Exchanges watch for this, and their usual defence is to make deposits of a risky coin wait for far more confirmations before crediting them, which makes the attack much more expensive to pull off.

A practical note for ordinary users: this is not a threat to coins sitting in your own wallet. Where it touches you is timing — if you are moving a small, low-hash-rate coin, let the confirmations pile up before you treat the payment as final, and expect an exchange to do the same.