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A 51% attack is an assault on a blockchain network in which a singular entity or organization is able to take control of the majority of the overall hash rate.
This is problematic as it can potentially causes a network disruption, whereby granting the attacker enough mining power to modify the ordering of transactions.
As its name suggests, a 51% attack surmises that control of over 50% of computing power is hijacked.
In extreme situations, the resulting attack could even reverse transactions, potentially leading to a double-spending problem.
Understanding 51% Attacks
Crypto miners that have successfully engaged a 51% attack can also monopolize the addition of new blocks (bundles of data) to a blockchain.
This means that they can also add transactions and receive all of the associated rewards.
However, it would be nearly impossible for 51% attackers to alter transactions that had been processed prior to the point that they took control of a network.
There have been several points at which various mining pools, including Gash.io and Bitmain, have controlled more than 50% of the mining power on the Bitcoin network.
However, they did not use their power maliciously.
While there is always a risk of a 51% attack, the overall scale of a network is itself a defense mechanism.
For example, a 51% attack on Bitcoin is unlikely given the magnitude of the network.
Once a blockchain becomes large enough, the chances of a single miner or entity obtaining enough computing power to overwhelm all the other participants drops substantially.
Overall this axiom holds true, as larger networks are more resilient to this nature of attack.
As a blockchain is maintained by a distributed network of nodes, all participants within the network cooperate in the process of reaching consensus.
This helps with the level security inherent in the network.