What is a 51% attack? Imagine a system where everyone agrees on the truth (transaction history) by majority vote. In a blockchain, this role is played by miners (or validators in Proof-of-Stake systems). The 51% attack breaks this system.
An attacker or a group of individuals captures more than half of the network’s mining power (hash rate) or the currency in question. This gives them tremendous control.
Transaction blocking: They can prevent new transactions from being added to the blockchain.
Rewriting History: They have the ability to alter or delete existing transaction records.
Double Spending: Individuals will even be able to spend the same funds twice (spending them in two conflicting transactions).
But they cannot create new money, steal existing ones, or change the total money supply.
But expensive. Consider an attack on Ethereum (Proof of Stake network). You would need over $53 billion worth of Ethereum to do this, which is nearly impossible. Such a high price prevents attackers from turning to major blockchains such as Bitcoin and Ethereum. Smaller blockchains with less computing power are easier targets. Ethereum Classic and Bitcoin Gold suffered a 51% attack, causing millions of dollars to be lost.
That’s right, the blockchain community is on alert. Unusual activity may trigger countermeasures such as protocol changes or community efforts to counter the attack. Attackers can spend significant amounts of money but quickly lose control.
Source: Ferra
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