What is Finney's attack?

This is one of the types of double spending. The bottom line is that an attacker controls a certain amount of network hashes. Let's say an attacker sent some quantity of coins from his address A to his other address B. He will try to find a block with this transaction and, as soon as he finds it, he will buy from someone a product for the same coins, that is, make a transaction from his address A to the address of the seller C. If the seller does not wait for confirmation of the transaction and will ship the goods, then the attacker, if sufficient capacities are available, will select the unit with its own operation and confirm it. And the one that was sent to the seller, will remain invalid.

Therefore, it is always necessary to wait at least 6 confirmations of the transaction by the miners.

The possibility of such an attack was suggested by programmer Hel Finney at the BitcoinTalk forum (https://bitcointalk.org/index.php?topic=3441.msg48384#msg48384), so it was named in his honor. Finney, by the way, is one of the first users of bitcoin, which Satoshi Nakamoto with great gratitude recalls in his supposedly new book. We will talk about this later today.