If a hacker gains enough computing power to control more than half of the total computing power on the network, (POW) can solve the required cryptographic puzzles faster than other nodes.
Gaining 51% of the hashrate is not hacking, it just means the miner has a lot of money to invest or it is a very popular mining pool. You do not need more than half of the hashrate to be able to solve blocks than other
nodes miners, with enough hashrate 20-30% that pool or mining rig is going to be confirming many of the blocks.
Note that this node uses special hardware and software representing the transaction verification method of the POW algorithm. Once hackers have a higher hash rate (computing power), they can manipulate transaction history and cause twofold spending.
Nodes validate transactions for confirmation. It is miners that actually confirm the transactions and use the high end hardware.
A malicious user who has 51% control can manipulate transactions and the valid block, but it is more profitable for them to use that hashrate to confirm blocs.
- Jay -