A 51% attack is a potential security threat to a blockchain where a single miner or group gains control of more than half of the network’s mining or staking power. With that majority, they can override the normal consensus process.

On a Proof of Work network, honest participants are expected to confirm transactions and secure the chain. If one party controls most of the power, they can influence which transactions are included and in what order.

How it works

With majority control, an attacker can build a competing version of the chain faster than everyone else combined. Because the network follows the longest valid chain, the attacker’s version can become the accepted one.

This lets them reverse some of their own recent transactions and spend the same coins twice, or block certain transactions from confirming. It does not let them create new coins or move funds from wallets they do not control.

Limitations and likelihood

The deeper a transaction sits in the chain, the more work is needed to undo it, which is why services wait for several confirmations before treating a payment as final. Sustaining the attack is also very expensive, since the attacker must keep out-mining the whole network.

On large networks like Bitcoin, the cost makes a successful attack highly unlikely. Smaller networks with less mining or staking power are more exposed.