Quote from: seoincorporation on December 30, 2019, 04:25:01 PM

Even if someone use this attack to make a double spend that will not work at all because the different services who accept bitcoin always ask for 1 confirmation. So, the double-spend isn't a risk at all nowadays.

You misunderstand what a 51% attack can achieve.

A successful 51% attacker would mine and build a secondary blockchain in secret, with whatever transactions they like included in it. Once their own secret blockchain is longer than the main chain, they broadcast it to the network. Since they now have a longer chain with more proof of work, the rest of the network would automatically switch to this secondary chain, and the previous main chain would be abandoned. Any transactions which were included in the previous main chain but not in the newly accepted secondary chain would be reversed. This can happen after theoretically any number of confirmations, although with each confirmation it becomes exponentially harder and more expensive. However, a successful 51% attack would, at a minimum, be able to reverse or double spend transactions with 1 confirmation.

Most exchanges wait for at least 3 confirmations before confirming bitcoin, and some still wait for 6.