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   1. Introduction to Computer Viruses The person might have a computer virus infection when the computer starts acting differently. For instance getting slow or when they turn the computer on, it says that all the data is erased or when they start writing a document, it looks different, some chapters might be missing or something else ubnormal has happened. The next thing usually the person whose computer might be infected with virus, panics. The  person might think that all the work that have been done is missing. That could be true, but in most cases viruses have not done any harm jet, but when one start doing something and are not sure what you do, that might be harmful. When some people try to get rid of viruses they delete files or they might even format the whole hard disk like my cousin did. That is not the best way to act when the person think that he has a virus infection. What people do when they get sick? They go to see a doctor if they do not know what is wrong with them. It is the same way with viruses, if the person does not know what to do they call someone who knows more about viruses and they get professional help. If the person read email at their PC or if they use diskettes to transfer files between the computer at work and the computer at home, or if they just transfer files between the two computers they have a good possibility to get a virus. They might get viruses also when they download files from any internet site. There was a time when people were able to be sure that some sites we secure, that those secure sites did not have any virus problems, but nowadays the  people can not be sure of anything. There has been viruses even in Microsoft's download sites. In this report I am going to introduce different malware types and how they spread out and how to deal with them. Most common viruses nowadays are macro viruses and I am going to spend a little more time with them. I am going to give an example of trojan horses stealing  passwords. 2. General information about computer viruses 2.1 Different malware types Malware is a general name for all programs that are harmful; viruses, trojan, worms and all other similar programs [1].   2.1.1 Viruses A computer virus is a program, a block of executable code, which attach itself to, overwrite or otherwise replace another program in order to reproduce itself without a knowledge of a PC user. There are a couple of different types of computer viruses: boot sector viruses, parasitic viruses, multi-partite viruses, companion viruses, link viruses and macro viruses. These classifications take into account the different ways in which the virus can infect different parts of a system. The manner in which each of these types operates has one thing in common: any virus has to be executed in order to operate. [2] Most viruses are pretty harmless. The user might not even notice the virus for years. Sometimes viruses might cause random damage to data files and over a long period they might destroy files and disks. Even benign viruses cause damage by occupying disk space and main memory, by using up CPU processing time. There is also the time and expense wasted in detecting and removing viruses. 2.1.2 Trojan A Trojan Horse is a program that does something else that the user thought it would do. It is mostly done to someone on purpose. The Trojan Horses are usually masked so that they look interesting, for example a saxophone.wav file that interests a person collecting sound samples of instruments. A Trojan Horse differs from a destructive virus in that it doesn't reproduce. There has been a password trojan out in AOL land (the American On Line). Password30 and Pasword50 which some people thought were wav. files, but they were disguised and people did not know that they had the trojan in their systems until they tried to change their passwords. [9] According to an administrator of AOL, the Trojan steals passwords and sends an E-mail to the hackers fake name and then the hacker has your account in his hands. 2.1.3 Worms A worm is a program which spreads usually over network connections. Unlike a virus which attach itself to a host program, worms always need a host program to spread. In practice, worms are not normally associated with one person computer systems. They are mostly found in multi-  user systems such as Unix environments. A classic example of a worm is Robert Morrisis Internet-worm 1988. [1,5] Picture 1 An example of a worm. 2.2 Macro virus Macro viruses spread from applications which use macros. The macro viruses which are receiving attention currently are specific to Word 6, WordBasic and Excel. However, many applications, not all of them Windows applications, have potentially damaging and infective macro capabilities too. A CAP macro virus, now widespread, infects macros attached to Word 6.0 for Windows, Word 6.0.1 for Macintosh, Word 6.0 for Windows NT, and Word for Windows 95 documents.   What makes such a virus possible is that the macros are created by WordBASIC and even allows DOS commands to be run. WordBASIC in a program language which links features used in Word to macros. A virus, named "Concept," has no destructive payload; it merely spreads, after a document containing the virus is opened. Concept copies itself to other documents when they are saved, without affecting the contents of documents. Since then, however, other macro viruses have been discovered, and some of them contain destructive routines. Microsoft suggests opening files without macros to prevent macro viruses from spreading, unless the user can verify that the macros contained in the document will not cause damage. This does  NOT work for all macro viruses. Why are macro viruses so successful? Today people share so much data, email documents and use the Internet to get programs and documents. Macros are also very easy to write. The problem is also that Word for Windows corrupts macros inadvertently creating new macro viruses. Picture 2 New macro virus by corruption [12] Corruption's also creates "remnant" macros which are not infectious, but look like viruses and cause false alarms. Known macro virus can get together and create wholly new viruses.   Picture 3 Macro virus growth, July 1995 to May 1997 [12] There have been viruses since 1986 and macro viruses since 1995. Now about 15 percent of virus are macro viruses. There are about 2.000 macro viruses and about 11.000 DOS viruses, but the  problem is that macro viruses spreads so fast. New macro viruses are created in the work-place, on a daily basis, on typical end-user machines, not in a virus lab. New macro virus creation is due to corruption, mating, and conversion. Traditional anti-virus programs are also not good at detecting new macro viruses. Almost all virus detected in the Helsinki University of Technology have been macro viruses, according to Tapio Keihänen, the virus specialist in HUT. Before macro viruses it was more easy to detect and repair virus infections with anti-virus  programs. But now when there are new macro viruses, it is harder to detect macro viruses and  people are more in contact with their anti-virus vendor to detect an repair unknown macro viruses, because new macro viruses spread faster than new anti-virus program updates come up. 2.3 Virus sources


Apr 28, 2018
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