51% Attack

The malicious actor could then go back in the blockchain to the block before the BTC transfer was confirmed and mine an alternate chain, in which the BTC transfer is not included. The majority share of the networking power would ensure that this is forced to be adopted by the rest of the network as a valid transaction.

On the other hand, a majority attack does not allow the malicious actor to prevent transactions from being broadcasted nor to reverse transactions from other users. Changing the block’s reward, creating coins out of thin air or stealing coins that never belonged to the attacker are also very improbable scenarios.

The further back a transaction is, the harder it would be to subvert it, as the number of new blocks to be mined to bring the network up to the current level becomes further and further away. This is the reason why Bitcoin transactions usually require a threshold of x number of confirmations before clearing.

A 51% attack on the Bitcoin blockchain is very unlikely because of the magnitude of the network. As the network grows, the possibility of a single person or entity obtaining enough computing power to overwhelm all the other participants gets more and more improbable.