For example, cyber criminals might use their botnets to commit advertising fraud. A botnet of thousands of computers could generate millions of false clicks on ads hosted on websites owned by the criminals or their partners. They could launch distributed denial of service (DDoS) attacks against business or even government websites. A DDoS attack occurs when the servers of a website are overwhelmed by the sheer numbers of botnet traffic. This denies access to the site by legitimate users.

Botnets are also used for launching email spam campaigns. In addition to these activities, the owner of the botnet can also collect sensitive information such as login credentials, credit card numbers, and personal information from the owners of the enslaved computers. These zombie computers are often used to grow the botnet further.

Between selling or exploiting personal information, advertising fraud, and renting their botnets to other criminals, the owners of large botnets can generate very lucrative revenue streams.

Eight Signs Your Computer Is Part of a Botnet

Some of the signs of a recruited botnet computer resemble those of old computers with severely fragmented storage space or which lack sufficient available memory. Other signs are more conclusive regardless of your computer's age. Eight of these symptoms are:

• Your computer runs slowly even when using few or no applications.
• Your computer frequently crashes for no apparent reason.
• Some of your outgoing emails weren't sent by you.
• Your computer takes too long to shut down and/or start up.
• Your web browser crashes for no apparent reason.
• Inexplicable loss of free hard drive space.
• Unexplained CPU usage spikes.
• High CPU usage by suspicious processes. On Windows systems, these can be viewed in the Task Manager. Check the legitimacy of unfamiliar processes by researching them on the Internet.

Cyber criminals recruit computers into their botnets with the same techniques used for spreading other forms of malware. Therefore, the same cyber security precautions that mitigate the spread of malware in general will also help to protect your computer against botnet recruitment. These include keeping your antivirus and firewall up to date, keeping your browser and their plugins updated, and avoiding suspicious links and email attachments.