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If an attacker wishes to alter a block in the Bitcoin blockchain, they must recreate the Proof-of-Work, not only for the block they wish to change, but for all subsequent blocks as well. In addition, they must also produce new blocks faster than all honest miners combined in order to convince nodes that their altered chain is valid. This is due to the fact that nodes always consider the longest chain—the chain with the most blocks—to be the valid chain.
To achieve all of this, the malicious miner must control 51% of the mining power of the Bitcoin network. In other words, they must have more computing power than all other miners combined. The difficulty of this attack makes Bitcoin secure against fraud and transaction reversals.
Fear of a 51% attack is also why measuring hash rate is so important. Hash rate is a measure of the total mining capacity of the Bitcoin network, and the higher this number, the more expensive a 51% attack is. Hash rate is thus a measure of security against a 51% attack.