Defense Against Viruses - Awareness by Examples

Author: Uzi Paz
E-Mail: For e-mail contact: user is uzi4wg and domain is my e-mail address as a picture
First version date: 29 Aug 2002
This version: 3 Nov 2002
Legal Notice: While as far as I know all the information here is exact and
correct, because I'm giving this information as a free service, I'm taking no
responsibility. Any comments and  corrections, are welcomed.
Parent Page:

1. Introduction

There are so many tricks which worms, and viruses use in order to penetrate and activate themselves on the victim's computer. While antivirus programs are very important, and usually, if they are always kept updated, they can defend from the more common viruses, this is not enough. Sometimes viruses spread so fast, that they might reach your computer before the antivirus programs are able to identify them. It is thus not recommended to count solely on the antivirus programs to provide the proper defense, and to defend your computer from viruses by not activating them on your computer. A virus can do its deeds only if it is activated (executed). This might happen because you doubleclicked an attachment, or because it used some security hole in your e-mail program to activate itself automatically. In order to prevent a virus from activating itself automatically, you need to apply security updates, security patches, or to configure your programs so that risky features will be disabled. Yet, many viruses will try to convince you to execute them by using various tricks, including many psychological tricks. In this article, we try to raise your awareness to this issue by providing representive examples, and explaining them, so that you will be more cautious to identify suspicious files and thus not to activate them.

The most common channel of infection is e-mail. It could be an innocent looking message, sometimes from someone you know - might be your boss or a good friend. Could be someone you truly trust.
In many cases, the content of the message looks unsuspicious. You may or may not be requested politely to open an attachment. Not something suspicious, only a word document or a picture, or a text attachment, nothing that a typical user will suspect as being a virus. Viruses, in many cases, are sending themselves from infected computers, usually under the e-mail address of the owner of that computer, to e-mail addresses that the virus finds either in the addressbook on that computer, or in cached web pages, or in anywhere in the computer. Sometimes, under another forged e-mail address that the virus found in the infected computer.
This way, a virus use an infected computer to spread further into other computers and infect them as well.

We believe that a few examples, will clarify the various tricks.

Since all the examples below are of famous viruses, those viruses are already detected and removed by all updated antivirus programs. Yet, their methods will clarify to you how it is possible to trick you, and new viruses might try to trick you in similar ways, and yet, not be identified by your antivirus program. We shall first discuss those few examples of ways in which viruses succeed in penetrating into your computer and activating themselves, and then, in the last section, we shall make some more generic recommendations.

2. What is the meaning of infection

Viruses, worms, and Trojan horses, are executable codes. Namely, they contain instructions for the computer to do unwelcomed or malicious things. The code of a worm or a virus also has instructions as to how to send copies of it to other computers. This way, viruses and worms are spread: they simply instruct the infected computer to send copies of them to other computers - either directly or indirectly. Trojan horses, on the other hand, do not have the ability to self-replicate. We will not enter here further into the differences between worms, viruses, and Trojan horses, but will just comment that the exact distinction between viruses and worms, is not within a consensus, and throughout this article, we shall use the term "virus" as a generic term referring to either a virus or a worm.

Of course, the code is meaningless if the computer does not act according to it. Look, for example, at the following line:

DEL c:\Windows\System\*.dll

If typed in an MSDOS window and you press <enter>, or if is written in a batch file (a file with an extension .bat) that will be executed, it will delete many crucial files from your operating system directory, and will thus force you to reinstall your operating system. Certainly a harmful act. But if this line is written as a part of a web page (just as I did in this page) it is harmless, because your computer does not treat those lines here, as instructions for it to follow, but just as lines that should be shown as part of a web page.

It is not the lines themselves that are harmful then, but rather the lines of code, plus the environment which tells the computer to follow the instructions within, which is the harmful thing.
There is a semantical question as to whether the lines themselves being on your computer should be considered as an infection, or only if they are in the right environment to instruct the computer what to do. In any case, this is only a semantical question. What should be clear though, is that a virus code, is harmless, if it is located in an environment which will not cause the computer to follow the instructions within.

So a virus code, once is able to instruct an infected computer, will try to spread itself by telling the infected computer to send replicas of it, to new computers, but it must send copies of itself, in a special format, that will cause the new computers to activate the replicas (follow the instructions within), either directly, by using a security hole in the remote computers, or indirectly, by convincing the user of the remote computer, to innocently tell the computer to follow the instructions within that code. Once it is activated in the new computer, the code will instruct this computer to spread it further in a similar way to more computers.
Most viruses will prefer to remain active also after the user shuts down or restarts the computer, so they use one of various techniques to tell the computer to execute their code once it restarts.
The virus may also include some payload, namely, besides spreading itself, it may include instructions to do some other harm to the computer. Sometimes only in very specific dates or situations.
We will now learn this issue further, by using various examples.

3. Example: The "Loveletter" worm - e-mail infection

You get an e-mail message from a friend from work. The subject is "ILOVEYOU".
"interesting" - you think to yourself. You look at the message and there is a message inviting you to look at an attachment with the filename: loveletter-for-you.txt . You become curious and open it. The icon was not the typical one for Notepad messages, but seems like something that represents text, and you haven't pay attention to it.  Nothing seems to happen when you opened the attachment, but in fact, at this stage, you are already infected by the Loveletter virus, and within a short time your computer starts to send messages in your name to all addresses in your addressbook, and, yes, the subject of those messages is "ILOVEYOU". This is just a message similar to what you got.

The full name of the attached file on that message was: loveletter-for-you.txt.vbs . The VBS filename extension is for Visual Basic Script, and once you doubleclick a file with this attachment it is normally opened by the Microsoft Virtual Scripting Host, which will execute the code within. Former versions of Microsoft Outlook and of Outlook Express, and also some other e-mail programs, under the default configuration of Windows, will not show the extension of a file. The existence of the ".TXT" extension, had to alert you but didn't (sorry! my choice of the script). The icon of that attachment, is not the same as a typical text file which is opened by Notepad, but the icon looks innocent, and symbolize something of a text nature. E-mail is not the only way this virus is spread, see the next example.

4. Example: The "Loveletter" worm - IRC infection

You use mIRC to chat in IRC. This is a very popular chatting set of networks. You enter an IRC channel (a chat room). Then you get from one of the permanent participants in this chatroom a private message named:
love-letter-for-you.htm , i.e. a web page. You become curious and open it, Internet Explorer loads to open this HTML message, and Internet Explorer says something about the need to enable ActiveX to open this page. "The heck" - you think to yourself - "if Internet Explorer tells me that it needs something to open the file, why should I argue". You confirm IE to enable ActiveX. Yet, nothing seems to be in that file, so you continue to use IRC. What you don't know, is that when you opened the HTML page and allowed IE to use ActiveX, you became infected with the Loveletter virus, and yes, a short time later, your computer starts to send under your name those "loveletter" e-mail messages to all people in your addressbook, and when you use IRC, it will also send that WWW page to other members of that IRC channels when they enter the chat.

HTML pages, may contain embedded objects, and try to automatically open them when they are opened. Internet Explorer has a very complicated architecture which defines what is secured and can automatically be opened, and what should not. ActiveX content is usually executable, but Microsoft has a way to decide whether it can trust it to be decent or not. Under default configuration of Internet settings (Internet Explorer), if an HTML message asks for your permission to open an ActiveX object, you should be suspicious. In this case, this was the virus spreading itself, under the name of another member of the IRC channel, because his computer was infected.

5. Example: The "Sircam" virus

You get an e-mail message from your boss. In this message he requests for your advice regarding some document which is attached as a Word document. You are accustomed to exchanging MS-Word documents with your boss and colleagues at work, so there is nothing suspicious in it. In your case, you already set Windows to show you the filename extensions of known filetypes, so that you expect to see the right filename extension in that attachment, and the document has the name: "A response to Wigner's suggestions.doc" which seems quite reasonable, because Wigner corp. are big customers in your company. You open the word document, and it is opened in MS-Word, and looks really innocent. You just don't understand what was the advise your boss wanted from you, as this document is a letter that was already sent to Wigner 6 months ago.
Of course, this attachment was infected with the Sircam virus, and once you opened it, besides showing you the original word document, it also started to take Word and Excel files from computer, to add to them the virus code, and to post them to addresses that it finds in your computer, with a request for advice.

The real filename of the attachment that you got, was not DOC, but rather something like .pif or .lnk (might be other similar types). So that, for example, in the case mentioned above, the virus on your boss computer, arbitrarily chose some file "A response to Wigner's suggestions.doc" and appended this document to its code in a file with the name "a response to Wigner's suggestions.doc.pif". When you opened that attachment, the virus first activated itself, and then launched the original Word document to MS-Word, so that it will look as if the attachment was really that Word document. The interesting thing about these filename extensions, is that they are not showed, even if you configured your operating system to show filename extensions also for known filetypes.
These extensions are set by default to override that configuration. In another document, security and filename extensions [1], I enter much deeper into this topic than in this document, where I just give (at the end of this article) advices how to suspect that a file is a virus.

6. Example: The "Concept" virus

You got a Word document from your boss or from a colleague, doesn't matter. You got it via e-mail or in a floppy disk, doesn't matter either. They gave it to you. They did gave you this file, so it was not the virus who sent it to you. Yet, since their computer was infected by a macro virus, the virus infected all Word documents, in their PC. They weren't aware of that. But once you opened the document on your computer, the viral code has been activated, and  infected all other Word documents on your computer. If you send one of those by e-mail or via a diskette or put it in some electronic library, and other will read it on their own computer, they'll become infected too. When you read the document you weren't worry much about macro viruses, because you instructed MS-Word not to run unsafe macros. The concept virus used a security hole that allowed the viral macros to override MS-Word's security settings, and to thus activate itself.

The document here is indeed a Word document. Just that it contains Word macros, and those macro commands can execute arbitrary commands. As we said, there is a security setting in MS-Word to ignore (not run) those macro commands, but this setting had a security hole, and if you didn't update your office tools with the relevant security updates, you could get infected. The Concept virus, attack only Word6, and Word 7 and Mac word documents, but similar security holes existed in newer versions of Microsoft Office as well. There are security fixes for them, and if you use Office updated as suggested above, then your MS-Word application will ignore those macros and will thus not activate the virus. Similar macros exists also for other Office files, such as Excel spreadsheets. In this example we discussed a legitimate document, which was infected. You cannot simply discard it. You should, however, read it in a safe environment, namely, with MS-Office updated with the latest security updates, and only after you scanned it with an antivirus program. Antivirus programs, may be able to remove viruses from files while restoring the files to their uninfected state.

7. Example: The "SubSeven" Trojan

script #1:
You are browsing a newsgroup dedicated to posting of some movies, one of the movies is not viewable, and some people complain, and in return someone posts a viewer which he claims is able to view also this movie.
You try to install it, but for some reason it seems that the installation didn't succeed. What you installed, was in fact a Subseven Trojan, which opens a backdoor on your computer to allow others to get full access to your computer.

script #2:
You are looking for a cracked version of a program (an illegal version of a program) on Kazaa. by searching for a program with that name, and indeed it is offered in a few places. You take one of them and try to install on your computer. What you are not aware, is that this was not that program, but rather the Subseven Trojan.

script #3:
You are using IRC, and someone you are familiar with on that IRC channel - a known helper, is offering a nice program, that is related to a discussion held there. Since you trust this person, you download and install the application, which does not work. What you don't know what you installed was the Subseven Trojan, and that the "person" that posted it, was not the one who usually use that nickname, but rather someone who forged his name to that name of the person you trusted.

script #4:
You are reading a forum on the Web. One of the more reliable people on that forum sends an executable file to that forum. He uses the name player.jpg for that file, but instruct people to rename it to player.exe before running it (the forum does not allow sending executables). You need the application he was speaking about, and you are thus running it. You didn't take into account the fact, that in this specific forum system, it is very easy for someone to disguise himself as someone else, and that in this specific case, the person who posted this Trojan, forged his nickname, and described this Trojan program as if it does something different.

script #5:
You are looking for some defense program, and you look at a web site, that allow readers to contribute their own programs to it. You find something that according to the description fits your needs, and try to run it at your own computer. nothing seems to happen, but in fact, you are not aware that you have just installed a Subseven Trojan on your computer, and that this Trojan opens a secret backdoor for others to get free access to your computer, without your consent.

The common in all those examples, is that the poster of the file or the person who put it in the place where you took it from could be anonymous, or could easily forged his identity. This allowed that person to put a malicious program, and to present it as a useful one. Trojan horses are not capable of spreading themselves automatically. But they can still be put by people, if those people can remain anonymous (e.g. by giving false identification). The Subseven Trojan is just an example of such a malicious program, although a very common one. There are other similar programs, that can be posted under any executable name, and can do their deeds secretly.

8. Example: The "Nimda" worm - e-mail first type of infection

You get some e-mail message from someone that you know, and open it using your Outlook Express, or Microsoft Outlook (but you didn't patch them with the latest security fixes). The body of the message seems empty and you don't see any attachment. What you are not aware, is that once you opened the message, a virus embedded in a hidden attachment exploited a security hole in your mail program and Internet Explorer in order to activate itself on your computer.

The Nimda virus, is a very sophisticated virus which uses various tricks to spread itself. It uses a security hole in Outlook Express, Microsoft Outlook and Internet Explorer, in order to activate itself automatically upon opening the message. If the trick works, the attachment will not be visible.
This security hole is one of a few security holes that allow automatic activation of content in Outlook or Outlook Express. Another virus which is called KAK, uses a different security hole to do a similar thing. All of those security holes have patches which should be installed. In all cases, it is enough that the infected message will be opened in the Preview Pane of Outlook or Outlook Express in order for the virus to activate itself.

9. Example: The "Nimda" worm - e-mail second type of infection

You get a message from someone that you know and you open it with your e-mail program. This time you may use any of various e-mail programs including Outlook and Outlook Express that are fully fixed with the latest security fixes. Inside that message you find a file with an icon of HTML message (Internet Explorer), and which is called "Readme". Certainly looks like an HTML message. You open it, and this activates the Nimda virus.

This time your e-mail did not open the attachment automatically. The real name of the attachment was "Readme.exe" but your e-mail program is configured to hide filename extensions for known filetypes. A file of type .exe can set its icon to an icon that it holds internally, and this virus tricks users to think that it is a legitimate HTML message.

HTML messages and the way many e-mail programs and web browsers deal with them, are a source for security holes. New security holes are found from time to time, and new security patch to fix them are released later. Usually viruses which use security holes in the way HTML (and HTML extensions) are handled, became common after security fixes were released. There is no guaranty that this will remain true.

Anyway, in this case, it is not an HTML message, but rather an executable code which holds the virus.

10. Example: The "Nimda" worm - infection while browsing

You visit a web site that you trust with the use of Internet Explorer version 5.00 with the latest security fixes. You are not aware that while visiting it, you got infected with the Nimda virus.

Version 5.00 of Internet Explorer is no longer updated with fixes to security holes. You need to upgrade it to version 5.01 and above. A security hole that allows a web site to automatically execute a code on your computer regardless of your security settings, exists there. In this case, the person who updates the web pages on the remote site, got infected with the Nimda virus. Then he worked on some HTML web page, and after finishing it, he loaded the web page to the remote site not knowing that the worm has infected the web page, by adding itself as an embedded code to that web page.
When you browse the web page, with a version of Internet Explorer which is not fixed with the security patch relevant for this security hole, it will execute the internally embedded viral code on your computer.

11. Example: The "Opaserv" worm

You have just bought a new laptop, and wished to be able to connect it to your Desktop computer. You connect them, and in order to allow the moving of files from one computer to another, you enable file and print sharing on both computers. A few days later you find out that there is a high volume communication going through your desktop computer although you have no idea what is it. You run an online antivirus program and clean it, but shortly later you find that it again infected your computer. You didn't use e-mail during that time.

What you were not aware when you enabled file and print sharing, is that you enabled it not only to/from your laptop computer but also sharing them with the rest of the Internet. Your computer is opened wide for anyone with a bit of knowledge.
The importance to limit or disable "file and print sharing" over the Internet, is very high. The Bymer is another worm that uses this. The Nimda, among other means of infections also uses this way to spread.
It is also common among intruders to take control or to access the victim's computer by first putting a backdoor program in the victim's computer with the use of such a sharing.

12. Example: The "Benjamin" worm

Kazaa is a popular file sharing program which is popular for sharing (mainly illegal) music and program files between different computer which are connected to its network.
You use Kazaa to locate some program, and find it on some remote computer. You download and install the program but it seems not to work. You are then try to locate a different place for this program, but at the same time you notice that your hard disk becomes rather fulll, and that the folder which is used for sharing files to others starts to grow with more and more files that you have never put there.

The "Benjamin" worm, on infected computers, fills the sharing folders with many copies of itself. Many of them have attractive names, and just like you got infected with it by taking it from a remote computer because you thought that it is what its name suggests, also other people will download it from your Kazaa shared folder, thinking that these are the programs that they wish to install.

13. Example: the "klez.h" worm

The infection methods of the Klez.h worm are similar to the e-mail infection methods of the Nimda worm, and for this reason, I skipped the "script" part here. The main difference, is that the Klez.h worm spread from infected computers, to addresses that it finds there, under forged e-mail addresses that it also finds there. This means that you might get an infected e-mail that seems to be coming from someone that you either know or don't know, but in fact, it came from someone else.

14. Example: infection with some PE infector

script and discussion:
You take a diskette which contains files in an executable file format (any file format which on your environment, might execute commands). You use the diskette on a friend's computer and then go back and use it on your own computer. You don't have to execute anything in the other computer, but you weren't used the write-protect tab on that diskette. While you put the diskette on the remote (infected) computer, and, say, copied something from it, the virus has infected the files on your diskette. When you are back to your computer, and open one of those files, even though you believe that they are the same ones that you put on it on your own computer before you used it on other's computers, those files are already infected, and will infect your computer. The moral here, is that it is important to slide the write-protect tab on the diskettes (so that it will have two opened windows in it), before putting it into a computer which you cannot fully trust for being cleaned. Unless of course you need to copy things to that diskette. Of course, if you copy something to that diskette, even if the copied files are of no executable format, you can trust the files on the diskette, only as much as you can trust the remote computer to be cleaned from viruses.

15. Example: boot sector infections

script and discussion:
Such viruses are no longer common, but I still wish you to be aware of the method that they use.
Each diskette (and also most of the other medias) has besides files, also what is called "boot record" or "boot sectore" (the term "boot sector" is the one used in the case of floppy diskettes). Not only files in the diskette can be executed, but also the boot sector can as well. The boot sector commands will be executed only if the computer tries to boot from the floppy diskette. This means that once your diskette has "visited" a non-trusty computer (i.e. a computer which you cannot fully trust to be free from viruses) if you by mistake left in in your floppy drive, and booted, and your PC is configured to first try to boot from the floppy, you might become infected even if the diskette is not bootable, and has no files in it.
Our recommendation here, are that if it is possible, to configure your BIOS so that your PC will boot only from C:\, and to change this setting only if needed and for the task needed. If the BIOS is configured to first boot from the floppy, and you need to leave it unchanged, then at least don't forget the floppy inside it. As in the former case, when using your diskette on other's computer, write-protect the diskette unless needed otherwise.

16. Example: the truncation trick (e.g. shoho)

You use a recent version of Outlook Express, and it does show the filename extensions for all filetypes.
You receive a message with the following attachment "readme.txt". You know that this version of your e-mail program, shows you all filetypes. You try to open the attachment, and become infected with the Shoho virus.

If the filename was "readme.txt" then there was no problem. Yet, the trick was to use a name which begins with, say:
readme.txt but after the .txt there are many blank characters followed by, say ".scr".
something like:
"readme.txt                                                                         .scr"
but with more blanks in between the false extension (.txt) and the true one (.scr). Many programs including recent version of Microsoft Outlook and Outlook Express, and various other kinds of programs (not only e-mail), will truncate lines longer than some size, and in this way you will not see the true filename extension but only part of it.
An easy trick to know whether the filename continue beyond its visible part, is by starting to mark it for copying (copying the name). If the line will be marked also after the visible part it means that there are blanks after the visible part of the name. If the text marking will refuse to go beyond the apparent filename, it means that the text really ends there.

17. Example: Aplore infections through IRC

You have just entered to an IRC chat, and got a message referring you to some link for free stuff. Your browser asks you if you wish to run the content from its current location and you refuse. Then the web page, seems to load, and you get a message that in order to view the web page your browser needs to install a plugin. It gives you some information about the certification for the plugin, and asks you whether you wish to install the plugin. You confirm. At this stage, your computer became infected with the Aplore worm.

The message about the need to install a plugin did not initiated by your browser, but rather part of the web page, which the virus runs at the infected computer. The certification is bogus, and the instruction to install the plugins are no more than instructions to install the worm.

18. A fully legal and yet immoral worm (FriendGreet)

You have just gor some greeting card from a friend. The message had a link to a web site which looks decent.
In order to see the greeting card, the web-site asks you to install a plugin. You open it, and you see a long "end user license agreement (EULA)" for the plugin. You do not bother to read the license in its entire, but press "I agree". Then you are able to view this greeting card.
The next day, your friends will thank you for sending them greeting cards. You do not remember sending them anything.

When you accept a legal contract without reading it first, don't blame anyone else. Yes - we know that these end user license agreements are boring and extremely long. Yes, we are aware that CAPITALIZED BLOCK of text is harder to read carefully although formally it is considered as an emphasis. Nevertheless more and more companies are finding the fact that many people do not bother to read these EULAs before accepting them, as a real gold mine.

19. Example: The PrnDial Trojan

I must apologize for not putting the "o" between the P and the R in the name of the Trojan, but had I put it there, then web search engines might have filter this article off.

You are visiting some "gray area" web page, and among the many warning and confirmation message that you get from your browser, you hit by mistake OK on one that offered you to install a program for viewing some pictures (guess what).

Since then your computer became filled with this type of pictures and related advertisements, but it get much worse when you see the phone bill. It is outrageously high, might become a two months salary.

The issue has to do mainly with those who may hit the "OK" on variouas dialogues automatially, without bothering to see what is written there. It might also be relevant to those who do not know English very well, and assume that if  Internet Explorer requests something, then it is OK. This subsection is also relevant to those who believe that the worse thing that can happen to you if  your computer has become infected, is that you will have to reinstall it. What was installed here, is the PRNDial Trojan (I ommited the O as usual here), which makes a phone connections to international ISPs through premium services. This allows the company which you have visited its web site, to earn extra money (taken via your phone bills). This might be a Trojan. But it also might be a program which tells you how it works, (but you didn't notice the fact that it dials).
Be careful, and educate your teenage kid to do these things safe regardless of the fact that some of you also educate not to do it at all.

20. Example: The CIH virus

The CIH virus is also known as the Chernobyl virus.   The reason for mentioning it here, is not the way it infects your computer, but rather to let you know of things that a virus can do. For quite some time, the virus is only busy with spreading. On certain dates, it may activate its payload. Basically it will try to delete part of your hard drives. This by itself can cause you quite a problem, and if you cannot recover that part, the worst thing is that you lost some information, and have to reinstall everything. I'm not underestimating this damage, but there are many destructive viruses, and this is not new. The other thing that it tries to do, is to erase the BIOS. Before we exaplain what is the BIOS and how should we tackle that, let me tell you that once it erased the BIOS, in many cases, we cannot even reinstall the operating system, and even many computer labs cannot fix it, but only send it to bigger labs or replace hardware parts.

The BIOS is in fact the first program that the computer executes. It does not lie in the hard disk, because when the computer executes it, it still cannot even recognize the hard disk. In fact, one of the tasks of this program is to help the computer recognize the various parts, such as display card, memory, CD-ROM drive, and hard disks. Only after running this program, the computer is able to run the operating system from the hard disk or to access the CD-ROM or floppy. In the past, this was a fixed program which was installed as part of the motherboard. Later, in order to allow the upgrade of the BIOS to newer so that it is possible to add new features, and support of newer hardware, companies started to build their motherboard (the component of the computer which connects all the other parts of the computer and help them to "talk" with each other), so that it is possible to change the BIOS program. This puts our computer also at a risk that programs can delete the BIOS. If this happens, then even the first step of the boot will fail. Many modern computers have the ability to write-protect the EPROM (the component in the motherboard which holds the BIOS program) so that it is impossible to rewrite the BIOS or erase it iwithout first removing the write-protection. The instructions may vary from one motherboard to another.  Some motherboards have dual-bios, which contains besides the rewriteable BIOS also another BIOS which is more nbasic but fixed, and which allows you to reinstall the other BIOS in case it was erased or corrupted.

21. Overall discussion

In this article we showed some examples of viruses that use security holes to activate themselves automatically, while in other examples we saw how viruses try to cheat us to activate them on our computer. Once a virus is active, it may instruct the computer to load it every time the computer restarts. They may instruct the computer to spread them further, and they may even contain some malicious and harmful payload which will be activated at a specific day, or when a specific condition is met.
Applying security fixes, or upgrading the relevant programs to versions without a security hole, is very important. When it comes to Microsoft, applying the patches can be easily done by using the Windows Update feature, and applying the "critical updates" mentioned there. The exploitation of security holes in Microsoft's products is very popular among modern viruses.
Another subject which is not discussed here is the configuration of the software so that risky features are disabled.
Using an updated antivirus program is important, because we cannot trust ourselves to know all the tricks, and to fix all security holes.

Of no less importance is our own knowledge and awareness about the possible tricks that viruses use.
As we saw, the fact that an e-mail message came from someone we trust is not enough for us to trust its content. It might be that a virus posted itself under this person's name, and it might also be that that person did post the file, but without being aware that the file contained a virus. Being suspicious is always important.
Of course, we cannot recommend people not to accept attachments via e-mail or software from the web, because, this might prevent you from using many legitimate services, but being suspicious is still important.
If you are not sure that this is a genuine attachment/file sent by its claimed poster, you should ask. Even if you know that you should expect to receive such a file from someone, it is still wise to first save it in a file and then scan it with an updated antivirus program before opening it. While a virus must be executable, and if the attachment/message is not in an executable format, it can safely be opened, it is important to understand the various tricks used by viruses/Trojans to hide their executable nature. I would recommend to read my document "Security and Filename Extensions" on .
You should be aware hw each file will be opened and about the way your application treat different files.

You should know that unless you know that a filename of some properties will be opened by a program in a safe environment, that will not allow it to run harmful commands, you should treat it with enough suspiciousness. Files with filenames such as .jpg, .tiff, .txt, etc. are likely to be opened by programs which will not be able to execute any viral code if exists in them. If you need to use one of them, and you believe it to be safe, still check it with your antivirus program(s).

Although in this article we do not wish to give detailed recommendations on how to secure your PC against viruses and Trojans, we shall still provide some recommendations:

1) If you have Internet Explorer version 5.0 please update it first to version 5.01, or 5.5 or 6.0 and then upgrade to the latest service pack and only then update with the latest security patches. All of those upgrades can be done via "Windows Update".
2) If you have Outlook Express version 6 or above, you may, from the Tools/Options/Security tab, set it to warn you whenever an application tries to use Outlook Express in order to post a message under your name. This might be useful in any case you became infected with a virus which uses Outlook Express to spread itself via e-mail.
3) If you use Microsoft Outlook or Outlook Express, it is recommended to consider setting it to run HTML in the "Restricted sites" security zone.

22. References for viruses mentioned here
Loveletter: .
Sircam: .
Concept: .
SubSeven: .
Nimda: .
Opaserv: .
Benjamin: .
Klez.h: .
Shoho: .
Aplore: .
FriendGreet: .
PrnDial: .
CIH: .
Other viruses: .