CHAPTER 1. INTRODUCTION
The main source of viruses today is the global Internet. Most virus infections
happen via e-mail. The facts that almost every computer has e-mail client
applications installed and that malicious programs are able to take a full
advantage of software address book in order to find new victims are favorable
factors for the distribution of malware. Without even suspecting it, the user of an
infected computer is sending infected e-mail messages to his or her contacts,
who, in turn, send new waves of infected messages and so on. It is not
uncommon when infected files, due to someone's negligence, enter commercial
mailing lists of large companies. In this case, the virus will affect not just five, but
hundreds or even thousands recipients of such mailings who then will send
infected files to dozens thousands of their contacts.
Apart from the threat of virus or malware infection, there is a problem of
unsolicited e-mail messages (SPAM) and misuse of the Internet resources.
Although not a direct threat, unsolicited e-mail messages increase the load on
the mail servers, fill mailboxes with unwanted messages, cause the loss of
working time and inflict serious financial losses.
Additionally, it is to be noted that the newest malicious programs use the so-
called spamming technologies for efficient mass distribution and the methods of
social psychology to make the user open the message, etc. Therefore, SPAM
filtering is important not only for convenience, but also in order to protect your
computer against some new types of viruses.
It is now acknowledged that information has become an important asset. At the
same time, in order to gain profit through the use of the information, it has to be
available to the company's employees, clients and partners. This raises the issue
of data security and, as its important element, the issue of protection of the
corporate mail servers against the external threats, preventing virus outbreaks
within the corporate networks and filtering out the unsolicited correspondence.
1.1. Threats to the computer
security
There are a vast number of threats that could affect your computer today.
Reading this chapter will give you a general understanding of them.
Worms
This malicious program category largely exploits operating system vulner-
abilities to spread itself. The class was named for the way the worms
