A potential vulnerability in a blockchain network, particularly those that use a Proof of Work (PoW) consensus mechanism. In a 51% attack, a single miner or mining pool controls more than 50% of the network’s mining computational power (hashrate). With this majority control, the attacker can:
Double Spend: Spend the same cryptocurrency more than once. By controlling the majority of the computational power, the attacker can create an alternative version of the blockchain (a fork) where they can spend their coins a second time after they’ve already been spent on the original chain.
Block Transactions: Prevent other miners from confirming new transactions by only mining on their own version of the chain.
Reverse Transactions: Change the history of transactions in recent blocks, undoing transactions and making it appear as if they never happened.
It’s worth noting that while 51% attacks are theoretically possible, executing them on major blockchains like Bitcoin would require a vast amount of computational resources and financial investment. However, smaller and newer blockchains with less hashing power can be more vulnerable to such attacks.