• Viruses. Attackers can develop harmful code known as viruses. Using hacking techniques, they can break into systems and plant viruses. Viruses in general are a threat to any environment. They come in different forms and although not always malicious, they always take up time. Viruses can also be spread via e-mail and disks.
• Trojan horses. These are malicious programs or software code hidden inside what looks like a normal program. When a user runs the normal program, the hidden code runs as well. It can then start deleting files and causing other damage to the computer. Trojan horses are normally spread by e-mail attachments. The Melissa virus that caused denial-of-service attacks throughout the world in 1999 was a type of Trojan horse.