A botnet is a number of Internet-connected devices, each of which is running one or more bots. Botnets can be used to perform distributed denial-of-service attack (DDoS attack), steal data, send spam, and allow the attacker access to the device and its connection. The owner can control the botnet using command and control (C&C) software. Users are often unaware of a botnet infecting their system. Infected devices are controlled remotely by cybercriminals, and are used for specific functions, so the malicious operations stay hidden to the user.
The botnet malware typically looks for vulnerable devices across the internet, rather than targeting specific individuals, companies or industries. The objective for creating a botnet is to infect as many connected devices as possible, and to use the computing power and resources of those devices for automated tasks that generally remain hidden to the users of the devices.
For example, an ad fraud botnet that infects a user's PC will take over the system's web browsers to divert fraudulent traffic to certain online advertisements. However, to stay concealed, the botnet won't take complete control of the web browsers, which would alert the user. Instead, the botnet may use a small portion of the browser's processes, often running in the background, to send a barely noticeable amount of traffic from the infected device to the targeted ads.
On its own, that fraction of bandwidth taken from an individual device won't offer much to the cybercriminals running the ad fraud campaign. However, a botnet that combines millions of devices will be able to generate a massive amount of fake traffic for ad fraud, while also avoiding detection by the individuals using the devices.
