Building Blockchain Projects

51% attack

If the attacker controls more than half of the network hashrate, the attacker can generate blocks faster than the rest of the network. The attacker can simply preserve his private fork until it becomes longer than the branch built by the honest network and then broadcast it.

With more than 50% of hash power, the miner can reverse transactions, prevent all/some transactions from getting mined, and prevent other miners' mined blocks from getting inserted to the blockchain.