Blockchains stay honest because no single participant controls the majority of the resource that secures them, computing power on a proof-of-work chain or staked coin on a proof-of-stake one. If one actor amassed more than half of it, they could gain outsized control over which blocks get added. That is a 51% attack. With it, an attacker could potentially exclude or reorder recent transactions and, most damagingly, carry out double-spends, spending coins and then rewriting history to undo the payment.
There are firm limits to what such an attack can do, which is worth stating to avoid overblown fear. Even with a majority, an attacker cannot steal coins from wallets they do not control, create coins out of thin air, or change the network's core rules; the damage is mostly confined to disrupting and reversing recent transactions. Deep, well-confirmed history remains effectively untouchable.
For large networks like Bitcoin, a 51% attack is considered wildly impractical because acquiring that much hash power would cost an astronomical sum, and succeeding would likely destroy the value of the very asset the attacker holds. The real risk lies with smaller coins, where the total mining or staking power is low enough to rent or amass, and several such chains have indeed been attacked. It is a key reason network security and decentralisation matter.
Key takeaways
- A 51% attack is when one party controls most of a chain's mining or staking power and can disrupt or reverse recent transactions.
- Even a majority attacker cannot steal from other wallets, mint coins or rewrite deep history; the damage is limited to recent blocks.
- It is impractical against large networks like Bitcoin but a genuine risk for small, low-security coins.
51% Attack — preguntas frecuentes
What could an attacker actually do with 51%?
Mainly disrupt the network and reverse or reorder recent transactions, enabling double-spends. They could not steal coins from other people's wallets, create new coins, or change the protocol's rules, and older confirmed history stays effectively safe.
Could Bitcoin suffer a 51% attack?
In theory yes, but in practice it is considered wildly impractical. Gaining a majority of Bitcoin's hash power would cost an astronomical amount, and a successful attack would likely crash the value of the bitcoin the attacker holds. Small coins are far more exposed.
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