What is a 51% attack?

Picture a shared payment ledger: everyone keeps a copy, but one group gains the power to impose its own version of the last few pages. That is the basic idea behind a 51% attack, which mainly applies to networks that use Proof of Work.

On these networks, participants in mining dedicate computing power to adding new blocks. If an attacker controls more than half of the total mining power, known as the hashrate, they can secretly build a competing chain. If that chain accumulates more total computational work, the network may accept it, and it can replace recent blocks.

For example, the attacker pays a merchant in crypto and receives a product. Meanwhile, they build another version of the chain in which they send the same coins to an address of their own. If that version wins out, the payment to the merchant is no longer valid. This is called double spending.

Why it matters

How safe a payment is also depends on how hard it is to reverse its record. Confirmations, meaning the additional blocks added after the transaction, make it harder to reverse, but they don’t neutralize an attacker who keeps the majority of the power.

We think this is where the practical value of decentralization becomes clear: what matters is who controls the power, not just how many computers take part. The attacker can also delay transactions or exclude them from the blocks they produce.

What to watch out for

The 51% refers to mining power, not the number of nodes or coin holdings. It doesn’t let the attacker sign payments from other people’s wallets, create coins at will, or bypass the validity rules that nodes enforce.

Also, 51% isn’t a magic switch: reorganization attacks can be attempted with a smaller share too, with uncertain success. On Proof of Stake networks, the thresholds and attack capabilities differ depending on the consensus mechanism; we don’t automatically carry the same rule over.