What Is the Minimum Number of Confirmations an Attacker Needs to Overcome?

The minimum number of confirmations an attacker needs to overcome is the number required by the target exchange before they credit the deposit and allow withdrawal. For small, vulnerable PoW coins, exchanges might require 30 to 60 confirmations, or even more.

The attacker must secretly mine a longer chain that is at least one block longer than the public chain after the exchange's confirmation count has been reached.