Mr.Smith is a lead incident responder of a small financial enterprise, which has a few branches in Australia. Recently, the company suffered a massive attack losing$5MM through an inter-banking system.
After an in-depth investigation, it was found that the incident occurred because 6 months ago the attackers penetrated the network through a minor vulnerability and maintained the access without any user being aware of it. They then tried to delete users' fingerprints and performed a lateral movement to the computer of a person with privileges in the inter-banking system. The attackers finally gained access and performed the fraudulent transactions.
Based on the above scenario, identify the most accurate kind of attack.