What is a 51% Attack?

A 51% attack refers to a potential attack on a blockchain network where a single entity or group gains control of the majority of the hashrate, and can then use that control to manipulate the network for their own gain. The name refers to having more than 50% of the computing power on proof-of-work based blockchains like Bitcoin.

Since blockchains rely on decentralized control and verification, taking over 51% of the hashing power enables the attacker to override normal consensus rules. They can reverse transactions, spend coins twice, prevent confirmations and force blockchain forks more easily. By controlling the majority of the network processing capability, the attacker has sufficient power to overwhelm the system.

To initiate a 51% attack, the attacker needs to gain majority control of the hashrate by dominating mining power in a proof-of-work context. This requires a large amount of specialized mining equipment and electricity to achieve.

While called a 51% attack, the threshold to execute one can often be much less than 51% due to the probabilistic nature of mining. The more hashing power amassed, the higher the attack success rate. 51% is not an absolute requirement.

The risk of 51% attacks is one reason decentralization of mining hashrate across many small miners is important for blockchain security models. Networks with heavily concentrated mining are most vulnerable.

Overall, while often theoretically possible, 51% attacks demand major resources and are difficult to successfully orchestrate. But their feasibility underscores the importance of decentralized participation for securing public blockchain networks against domination.