A 51% attack is when a single entity acquires enough hashing power to control the majority of the network and generate blocks. This allows them to manipulate transactions, double-spend coins, or prevent confirmations from happening at all. This is done by either acquiring enough mining hardware to be in this position or by controlling any other computing power such as a botnet.
Some examples of blockchains that have experienced 51% attacks are:
– Bitcoin Gold
A 51% attack is a type of cyber-attack that occurs when an attacker control more than 50% of the total mining power on a blockchain. The attacker can then manipulate transactions to take advantage of other users or to disrupt the network.
If a single bad user, or group of bad users acting together, control more than 50% of the total network hashing rate for a blockchain, they are able to override the consensus mechanism of the network and commit malicious acts such as double-spending. The attacker would have enough mining power to intentionally modify the ordering of transactions, preventing some or all transactions from being confirmed. He would also be able to prevent some or all other miners from mining, leading to a mining monopoly.
On the other hand, a majority attack does not allow the malicious actor to prevent transactions from being broadcasted nor to reverse transactions from other users. Changing the block’s reward, creating coins out of thin air, or stealing coins that never belonged to the attacker are also very not likely to happen.
A 51% attack is not possible on Bitcoin because the mining power is distributed evenly among many miners, and no one miner has enough power to control over half of the network’s mining power.
Therefore, 51% attacks are highly unlikely to happen on big networks, especially on the Bitcoin blockchain, which is considered the most secure cryptocurrency network. While many of the large blockchains have not yet suffered an attack of this kind, the majority of attacks have been seen on other smaller chains.