A 51 percent attack on a blockchain occurs when a group of miners controls more than 50% of the network’s mining hash rate — the total computing power allocated to mining and processing transactions.
Blockchain is a distributed ledger, wherein each block is a collection of data that keeps track of all completed transactions throughout a specific time period. A block cannot be changed once it has been finished or mined since a fraudulent version would be rapidly detected and rejected by the network’s users.
An attacker or group of attackers, on the other hand, can prohibit other miners from publishing blocks by controlling the majority of the processing power on the network, theoretically allowing the attackers to monopolize block rewards and reject undesired transactions.
Although a 51 percent attack is highly unlikely, it is a possibility we need to be aware of.
