A 51% attack is a potential attack on a blockchain network where a single entity or organization gains control of the majority of the hashrate, potentially disrupting the network. In this scenario, the attacker would have sufficient mining power to intentionally exclude or alter the order of transactions. They could also reverse transactions that have already been made while controlling the network, leading to double-spending.

A successful majority attack could also allow the attacker to prevent some or all transactions from being confirmed (transaction denial of service) or prevent some or all other miners from mining, resulting in so-called mining monopolization.

On the other hand, a majority attack cannot enable the attacker to reverse other users' transactions or prevent others from creating transactions and broadcasting them to the network. Changing block rewards, creating tokens out of thin air, or stealing tokens that do not belong to the attacker are also impossible.

How likely is a 51% attack?

Since blockchains are maintained by a decentralized network of nodes, all participants cooperate in the process of reaching consensus. This is one of the reasons why blockchains are generally highly secure. The larger the network, the stronger its ability to defend against attacks and data corruption.

In the context of proof-of-work blockchains, the higher the hashrate a miner possesses, the greater the chance of finding a valid solution for the next block. This is true because mining involves countless hash attempts, and greater computational power means more attempts can be made per second. Some early miners joined the Bitcoin network, contributing to its development and security. As the price of Bitcoin as a currency continues to rise, many new miners enter the system, aiming to compete for block rewards (currently set at 12.5 Bitcoins per block). This competition is one of the reasons for Bitcoin's security. If not for acting honestly and striving to earn block rewards, miners would have no incentive to invest significant resources.

Therefore, due to the large size of the Bitcoin network, a 51% attack is unlikely to occur. Once a blockchain grows large enough, the possibility of a single individual or group gaining enough computational power to overpower all other participants quickly drops to an extremely low level.

Moreover, as the chain grows, altering previously confirmed blocks becomes increasingly difficult because these blocks are linked through cryptographic proofs. For the same reason, the more confirmations a block has, the higher the cost to change or reverse its transactions. Thus, a successful attack might only be able to modify transactions in the most recent blocks for a short period.

Now, imagine if a malicious entity attacked the Bitcoin network not for profit but solely to destroy it, regardless of cost. Even if the attacker succeeded in disrupting the network, the Bitcoin software and protocol would respond to the attack, quickly making modifications and adjustments. This would require consensus from other network nodes to agree on these changes, but in an emergency, the process could be completed swiftly. Bitcoin has strong resistance to attacks and is considered the most secure and reliable cryptocurrency in existence.

While gaining more computational power than the rest of the Bitcoin network is quite difficult for an attacker, it is much less challenging for smaller cryptocurrencies. Compared to Bitcoin, altcoins have relatively low hashing power protecting their blockchains—low enough to make 51% attacks a practical reality. Some notable examples of cryptocurrencies that have suffered majority attacks include Monacoin, Bitcoin Gold, and ZenCash.