51% attack happens when a single miner or group of miners hold the majority mining power of a blockchain’s hash power or computational power, allowing them to control the network. Due to how blockchain networks work, for successful attacks to take place, a miner must control at least 51% of the mining pool. In an unlikely event that this happens, the attacker or group of attackers can be said to have the biggest hashing power and consequently, a mining monopoly, and will always get the highest mining reward at all times.
The potential damage of a 51 percent attack goes deeper for any popular cryptocurrency. Such a crypto network runs a substantial risk where attackers can double-spend coins and other crypto assets, reverse transactions, leave lots of unconfirmed transactions pending, and cause damages that could be worth millions of dollars. The bad news continues for any platform that falls prey as valid blocks get corrupted and the legitimate blockchain becomes a fraudulent chain.
Likelihood of Attack
Bitcoin and Ethereum along with other established blockchains are unlikely to ever fall to such attacks due to the high concentration of computational power and high hash difficulty level now protecting the networks. This protection comes from the fact that so many people are mining the networks.
Some examples of Successful 51% attacks ever carried out include the Ethereum classic network attack of January 2019, attack on Bitcoin Gold, the 26th largest cryptocurrency in the world as at the time of the attack, and the 2016 attack on Krypton and Shift – two blockchains on the Ethereum network.