In the context of denial of service attacks, how is a zombie defined?

In the context of denial of service (DoS) attacks, a zombie refers to a compromised system that is used to carry out a distributed denial-of-service (DDoS) attack without the knowledge or consent of the system's owner. These compromised systems, often referred to as "bots," are typically part of a larger network of infected computers known as a botnet. Attackers use these zombies to flood a target server with traffic or to exploit its resources, making it unavailable to legitimate users.

This definition is crucial in understanding the nature of DDoS attacks, where control over a large number of devices allows attackers to amplify their assault on targeted systems. The system owner's lack of awareness is a key characteristic of zombies, as it highlights the stealthy nature of the compromise and the malicious intent behind the DDoS attack.

The other choices describe situations that do not align with the function or definition of a zombie in the context of cyber-attacks. Ethical testing, system isolation, and multiple user accounts are not related to the nature of a compromised system participating in an attack without the owner's knowledge, which underscores why the correct answer focuses on the covert involvement of the compromised system in such attacks.