Malware
The most common malicious programs include viruses, worms and spyware. Typically, computers are infected with viruses through e-mail, file downloads from the Internet, peer-to-peer networks and instant messenger programs. Viruses are spread also through diskettes, CDs, DVDs and memory sticks.
Malware can make computer use difficult in many ways. Computer operations may slow down, and surfing the Internet and using e-mail may become extremely difficult. Software may malfunction, and the computer may restart without the user doing anything. Other problems include data being lost or changed, the browser start page being changed and other failures in operations. In the most difficult cases, an operator may close the Internet connection if a computer is sending disruptive traffic to the network or relays spam without the user of the computer being aware.
E-mail worms can send copies of themselves to all addresses in a user's address book and may even include in the message information that is stored on the computer. Viruses can also enable spam senders to use a computer for spreading junk mail. Such a computer ("zombie") sends out large amounts of messages automatically, without the user noticing it.
Network worms, such as Sasser and Blaster, search for computers that are connected to a network and do not have the latest patch updates. Worms spread from one computer to another considerably faster than viruses.
In addition to so called traditional viruses and worms, there are also other types of malicious software. Programs known as bots and Trojan horses are the most common types.
Bot comes from the word robot which describes well how this type of malware is spread. A bot is automatic and resembles network worms in how it is spread. Bots may also include other features, such as backdoors and spyware. Bots are malware that allow a computer to be controlled through a network without the user noticing it. Bot computers are typically used for illegal file and network services and for sending spam. Bots that are distributed automatically form so called botnets that can be controlled to perform tasks such as the ones mentioned above. There are tens of thousands of different types of bots.
Trojan horse is malicious software disguised as, for example, an innocent game or other useful program. A Trojan horse may have any kinds of functions and at its worse it can destroy the content of the computer hard drive.
Backdoor is a program that enables external access to an unprotected computer. Backdoors can be used for, for example, stealing a user's personal data from the computer.
Spyware refers to programs that gather information on the ways a computer or its software are used, the data saved by the user and even keystrokes. Spyware can send out the gathered information automatically or allow network access to the computer by installing a backdoor.
Adware is sometimes classified as malware, even though adware applications are often distributed as parts of so called freeware. Adware is used to get a user to visit certain websites.
The most recent types of new malware include, for instance, so called rootkits. A rootkit is a program that can obscure its operations so that both the user and regular anti-virus software are unable to detect it. Rootkits have also been used for hiding bots from anti-virus software. There have also been incidents where a rootkit has been used for enforcing copy protection on CDs, which in itself is legal.
More information on protection against viruses, worms and other malware is available here.
