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51% attack


A 51% attack is a hypothetical scenario in which more than 50% of the nodes of a blockchain network fall under the control of a single group. In such a case, the consensus of a network is no longer sufficiently distributed to be viable and the blockchain is vulnerable to manipulation. Attackers with more than 50% control of a network could stop, reverse and duplicate new transactions - a disastrous situation for any blockchain. A 51% attack becomes more difficult and expensive the larger, more distributed and more valuable a network is.


A 51% attack refers to a malicious actor (or group acting in concert) controlling more than 50% of the total mining power of the blockchain network and disrupting the integrity of the blockchain.Due to the way the blockchain works, consensus is required for transactions to be confirmed or added. A malicious actor that controls the majority of the hashing or mining power can theoretically form the "majority" in this consensus mechanism and disrupt the integrity of the blockchain by changing the order of transactions, preventing transactions from being confirmed or double spending.The risk of a 51% attack is higher on blockchains with less hashing power, as it is easier for a malicious actor to obtain the required majority computing power. The more miners and resources used to mine a blockchain, the more secure the blockchain. The Bitcoin Network is considered the most secure blockchain of all, as it provides the largest amount of hashing power for mining. An example of a 51% attack occurred on the altcoin's Ethereum Classic blockchain in January 2019.

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