A 51 percent attack refers to an individual miner, or group of miners controlling more than 50% of a network’s mining hash rate or computing power. Given the fact that blockchains are open and distributed, theoretically, there would be no central authority to prevent an attacker from amassing over 50% of a network’s computational power. Controlling over 50% of a network’s computing power gives the attacker in control the power to disrupt the network.
An important feature of a cryptocurrency based blockchain is the ability to prevent double spending. For example, double spending is solved on the Bitcoin network when miners collectively confirm that a transaction is not a double-spend. When a miner verifies that a transaction is not a double-spend, it will be included in a block of transactions and then added to the blockchain. However, there is still the possibility that a miner’s confirmation cannot be fully trusted, thus, it is important to wait and see if the block confirmations of other miners are consistent. Any conflict in block confirmations between multiple miners will be identified, with the double-spend transaction being rejected by the network. However, a miner with more than 50% of a network’s computing power could circumvent this security process, because they would have the power to send a transaction, then simply reverse it. Thus, making it appear that they still had the coins that they just spent.
By controlling the majority of a network’s computing power, an attacker could also disrupt the process of adding new blocks to the blockchain. They could prevent miners from completing their blocks, which in theory, could allow the attacker to monopolize the mining of new blocks, making sure the attacker received all of the block rewards.
A 51 percent attack is theoretically feasible, especially with the growth of mining pools, which allows miners to combine computational power. However, amassing over 50% of a network’s computational power is an expensive process, which is likely to deter any potential attempts.