What is a 51% attack?

A 51% attack is a situation when a malicious actor gains more than half of mining power and is able to censor transactions or create double spends. Also known as majority attacks, 51% attacks usually befall blockchains that use the proof-of-work (PoW) consensus mechanism.

What are the main risks of the 51% attacks?

The main risk of these attacks is that double spend transactions can be created. For example, an attacker sends cryptocurrency to an exchange, gets another one to his wallet, and then reverts the first transaction with a 51% attack. The attacker gets both cryptocurrencies in his wallet. Another possible implication is that a successful 51% attack undermines trust in the blockchain technology itself.

Are proof-of-stake (PoS) consensus blockchains vulnerable to 51% attacks?

Yes, PoS blockchains are also vulnerable to 51% attacks, however, they are generally considered much less susceptible to these types of attacks. To create a 51% attack an attacker needs to gain more than half of the stake or ownership of the cryptocurrency. However, cryptocurrencies take measures to prevent a single entity from having a big stake, such as limits on the amount that can be staked by a single validator.

How common are 51% attacks?

51% are not common compared, for example, to attacks on vulnerable smart contracts in DeFi. This is because these attacks are expensive and complex to execute. However, there is a record of some successful attacks. Chains which have notably suffered attacks include Bitcoin Gold and Ethereum Classic.

How not to become a victim of a 51% attack?

51% attacks can cancel a sent transaction that already has several confirmations. But the bigger the number of confirmations, the less likely it is that the transaction will be reverted. As a user, you have to make sure that enough blocks are mined after the one in question. The more blocks are mined after yours, the less likely it is to be reverted.