A 51% attack on a blockchain refers to an event where a group of miners attempts to take over more than 50% of a network’s mining power. If successful, they can manipulate the transactions made on-chain. They can either prevent the entry of new transactions or confirm inaccurate ones for their own gains.
A 51% attack is commonly used to reverse transactions that have already been recorded. These attacks circumvent the security of the blockchain against double-spends as they allow attackers to regain their cryptocurrency even after they have already used them for a transaction.
Here’s how it works: attackers create an inaccurate version of the blockchain and confirm it. However, they can only be successful if they can acquire the computing power needed to overcome other existing miners, which in this case, is 51% of the total hash power.