A 51% attack is a situation where more than half of the computing power on a network is operated by a single individual or concentrated group, which gives them complete and total control over a network. Things that an entity with 51% of the computing power can do include, but are not limited to:
- Halting all mining.
- Halting and manipulating all interpersonal transactions.
- Use singular coins over and over.
A 51% attack, also known as a majority attack, refers to a situation in which a single miner or group of miners control the majority of the network hash rate. If attained, this would enable a bad actor to censor and reverse transactions, allowing them to double spend coins. One of bitcoin’s greatest attributes is its immunity to attacks, be they governmental or technological. With over 31 exahash now concentrated on the bitcoin network, launching a 51% attack would be virtually impossible. And yet the very act of contemplating such an event is critical in mitigating the likelihood of it ever occurring.
However, hitting 51% network control is not a guarantee of success, just the point where success is likely. In fact, you could attempt this sort of attack with much less network control, but your odds of success would be very low.