51% Attack

A 51 percent attack is a fictitious scenario in which more than half of a blockchain network’s nodes are controlled by a single party. In such a case, a network’s consensus is no longer sufficiently spread to be functional, leaving the blockchain vulnerable to manipulation. Attackers having more than 50% control of a network would be able to halt, reverse, and duplicate new transactions, which would be disastrous for any blockchain. Notably, as a network expands in size, distribution, and value, a 51 percent assault gets more complex and expensive.