In this chapter I want to talk about attacks on computers on the Internet. You could think of these attacks as attacks against computers, which should be part of Chapter 8. You could also think of these attacks as network attacks, which should be part of Chapter 11. I think they are a different kind of attack, and am separating them in their own chapter.
Malicious software is probably the first interaction most of us had with computer security. Even if no one has access to your computer but you, and it is not attached to a network, you have to worry about viruses. The reason is that you don't really know what is going on in your computer, and trust the software you are running to behave itself. If you run an untrusted piece of software, you are taking a risk.
Malicious software includes viruses, Trojan horses, and worms. Together these are called malware. Malware generally has two components: a payload and a propagation mechanism. The payload is the part that does damage. Traditionally, payloads have been boring; a prototypical virus might display an annoying message on the screen, reformat the victim's hard drive, or do absolutely nothing. It could also do much sneakier things: modify the access control permissions on the computer, steal a secret key and send it via e-mail to someone, and so on. Payloads can be malign, and I expect that we'll see more devious payloads over the next few years. More interesting for this book are ...