The Bitcoin network is protected by the consensus of the network. Valid transactions and changes to the software must be accepted by the majority of the miners on the network. Let's discuss the attacks that could occur:
It is theoretically possible for a large amount of computing power to overtake the network and accept double spending or prevent confirmations. This is called the 51 percent attack.
Although theoretically possible, the opposing argument to the attack notes the amount of computing power needed to perform the attack. Some argue that it is not financially feasible to execute the attack as the network could quickly reject the malicious blocks. It would be difficult for a large pool of nodes to produce a segment of the blockchain of more than six blocks in a row with a corrupt transaction.