A 51% attack happens when a single group controls more than half of a blockchain network’s mining power. With that much control, they can manipulate the system—reversing transactions, blocking others, or spending the same coins twice. It’s one of the biggest threats to blockchain security.
Large networks like Bitcoin are safer because gaining that much power is extremely expensive and technically difficult. But smaller blockchains with less mining activity are more vulnerable and have been successfully attacked in the past. To prevent these attacks, networks use tools like proof-of-stake, longer confirmation times, and penalties for bad behavior.
