When discussing blockchain security, the term "51% attack" frequently arises, painting a picture with technical hues that actually describes a potential threat. This occurs when a malicious individual or organization gains control of over half of a blockchain network's total computational power, also known as hash rate, potentially disrupting the network's integrity.


Envision blockchain as a public ledger documenting every transaction, maintained through a competitive puzzle-solving process by numerous miners using computing devices. The victors add new transaction records to the chain, ensuring data immutability and decentralization. However, if one entity possesses over 50% of this puzzle-solving capacity, complexities arise.


In such an event, attackers can manipulate transactions, like executing "double spends." For instance, an attacker might send some Bitcoin to a seller's digital wallet in an offline transaction, receiving equivalent cash value. With blockchain typically viewed as unalterable, once confirmed, the seller assumes completion and hands over the cash. The attacker then exploits their network control, reverting to a pre-confirmation block and creating a new blockchain version without the initial transfer. As they command the majority of the network's power, this new chain is eventually accepted, making it seem as though the first transaction never occurred, thus enabling the reuse of the same Bitcoin.


However, 51% attacks are not omnipotent. Attackers can't prevent others from broadcasting transactions, reverse transactions of other users, or create/steal tokens out of thin air. Moreover, as transactions are confirmed by more blocks, reversing them becomes exponentially harder—reasoning behind waiting for six confirmations before considering a Bitcoin transaction secure.


For large blockchains like Bitcoin, a 51% attack is nearly impossible due to its vast scale, making it unrealistic for any single entity to accumulate overwhelming computational power. Thus, major blockchains, Bitcoin in particular, are deemed among the most secure in the cryptocurrency realm. Nonetheless, smaller blockchain projects occasionally fall victim to these attacks; Bitcoin Gold suffered a 51% attack in May 2018, resulting in the theft of around $18 million worth of BTG.


Preventing 51% attacks hinges on enhancing decentralization. This includes encouraging more participants to join and run nodes, as a larger network participant pool complicates single entity dominance. Implementing consensus mechanisms like Proof of Work (PoW) or Proof of Stake (PoS), requiring node consensus before adding transactions to the blockchain, raises the economic and technical barriers for attackers. Lastly, promoting global distribution of nodes prevents any single entity from amassing excessive control, a crucial measure against 51% attacks.