Final answer:
The type of attack was a Zero-day exploit, exploiting an unknown vulnerability in the software or operating system.
Step-by-step explanation:
The attack in question is known as a Zero-day exploit. This type of cyberattack occurs when an attacker takes advantage of a previously unknown vulnerability in software or an operating system, which the vendor has not yet had the chance to fix or patch.
In this scenario, because the operating system vendor confirmed that the vulnerability was not known prior to the attack, it clearly fits the definition of a zero-day exploit.