51% Attack

Key points

• Happens when an agent or group controls more than half of the mining power of a proof-of-work network.
• Allows the attacker to reorganize recent blocks and spend the same coins twice, known as double-spending, but not to create coins out of thin air.
• Large networks like Bitcoin make the attack economically unfeasible, since the cost of hardware and energy outweighs any likely gain.

What is a 51% attack?

A 51% attack is a security flaw in which a participant, or a coalition of participants, gathers more than half of the computational power of a blockchain that uses proof of work. With that majority, the attacker becomes able to determine which version of the transaction history the network accepts as valid, undermining the chain's core guarantee of immutability.

How it works

In a proof-of-work network, miners compete to add the next block by solving a cryptographic problem. Whoever has more processing power finds solutions more often. An agent with a majority of the mining power can secretly build a parallel chain that is longer than the public chain.

By releasing this longer chain, the network adopts it as valid, discarding the previous blocks. Transactions that appeared confirmed are reverted, and the attacker can resend the same coins to a different destination. This is double-spending.

The attack has clear limits. It does not allow stealing coins from other people's wallets, altering old blocks with many confirmations, or generating units beyond the protocol's rules. Its reach is confined to recent transactions, which reduces the damage on chains with deep history.

The deeper the block in which a transaction is recorded, the more blocks the attacker would need to redo to revert it. That is why the number of confirmations works as a practical security measure: each new block chained on top of a transaction raises the cost of undoing it.

Why it matters

The cost of accumulating a majority of mining power grows with the size of the network. On small, lightly mined chains, the attack has already been carried out more than once in practice. That is why operators who accept crypto deposits adjust the number of confirmations required according to the security of the source network: the smaller the chain, the more confirmations before considering an amount settled.

Risks and limitations

The risk is concentrated in networks with low aggregate mining power, where renting enough capacity is cheap. The usual defense is waiting for more confirmations and monitoring abnormal reorganizations. The attack does not break the cryptography of private keys, nor does it expose funds held outside the attacked chain.