51% Attack
a.k.a. Majority attack
When one person or group controls most of a blockchain's mining or staking power and uses it to rewrite recent history.
A 51% attack happens when a single player controls more than half of the power that secures a blockchain: more than half the computing power on a proof-of-work chain, or more than half the staked coins on a proof-of-stake one.
With that majority, the attacker can outvote everyone else on which version of history is true. The classic move is a double spend: pay an exchange with coins, withdraw something else, then quietly build a longer chain where that payment never happened. The exchange is left holding nothing. The attacker can also block other people's transactions from getting in.
What they cannot do is forge signatures or steal coins straight out of wallets they don't control. Big networks are brutally expensive to attack this way; small coins with only a trickle of miners are the ones that get hit.
A double spend, step by step
- The attacker deposits 1,000 coins on an exchange and the deposit gets confirmed.
- They swap those coins for bitcoin and withdraw it.
- Meanwhile their private, majority-powered chain, where the deposit never happened, grows longer than the public one.
- They release it. The network follows the longest valid chain, and the deposit vanishes.
Bottom line: The attacker keeps the withdrawn bitcoin and the original 1,000 coins. The exchange eats the loss.
Assuming every blockchain is equally hard to attack. Security comes from how much power protects it, and tiny chains can be rented and overpowered.
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