A team of miners who hold more than 50% of the network’s mining hash rate is said to be conducting a 51% attack when they target a cryptocurrency blockchain. The controlling parties have the power to change the blockchain because they control 51% of the network’s nodes.
The attacker’s benefit is that they can stop payments between some or all users by preventing new transactions from receiving confirmations. Additionally, they would be able to undo activities taken while in charge of the network. Reversing transactions would enable them to double-spend coins, which is one of the problems that proof-of-work consensus systems were designed to avoid.
On smaller cryptocurrency networks, this is feasible due to lower hash rates and reduced participation. However, it is almost tough to introduce a modified blockchain into large networks.
Due to the expensive cost of acquiring that much hashing power, major cryptocurrencies like Bitcoin or Ethereum are unlikely to be subject to a 51% attack. Because of this, 51% attacks are typically only successful against coins with low user and hashing power.
Even if a group of miners successfully launches a 51% attack to take over a crypto network, they won’t be able to add new coins or change any of the previous blocks. Therefore, a 51% attack won’t be able to completely destroy a crypto network, even though it can seriously harm it.
