In what kind of attack can attackers make use of millions of computers under their control in an attack against a single server or network?

Distributed DOS attacks.

Explanation:

Computers in a network or the network itself is are prone to cyber attacks. A cyber attack is a concept of taking advantage of computer network vulnerabilities, to retrieve information from the target source. An example of these types of security attacks is the denial of service (or DOS) attacks.

The DOS or the distributed DOS attack prevents users form accessing or requesting a service that they are legitimately allowed, by flooding the target system with superfluous resource request from millions of computers, controlled by the attacker.