What are botnets?

Botnets are essentially networks of devices, often computers, that have been compromised and controlled by an attacker without the knowledge of the owners. This malicious control allows the attacker to use these devices for various illegal activities, such as sending spam emails, conducting distributed denial-of-service (DDoS) attacks, or stealing personal information.

The defining characteristic of a botnet is that the infected machines, referred to as "bots" or "zombies," operate under the command of a central server or group of servers typically controlled by the hacker. This makes the networks utilized by botnets particularly dangerous and insidious, as unsuspecting users may continue to use their devices without realizing they are part of a larger criminal operation.

The other options do not accurately capture the essence of a botnet. For example, networks of computers used for legitimate purposes or private networks for organizational use do not involve unauthorized access or control by an external entity. Cloud-based services for data storage are legitimate web services meant for storing and managing data rather than malicious networks of compromised devices. Thus, the best representation of what botnets are is indeed the networks appropriated by hackers without owners' knowledge.