如何減少收信時被電腦病毒感染的機會？

單單不任意開啟電郵附件檔案是不夠的，最好還是避免HTML格式信件內的 Script 自動執行。請細閱下文...如有任何意見，歡迎來郵筆者。

Since once my computer was also infected by email viruses I have paid more attention to the protection issue. I hope you will find the following information useful.

What is a Virus ?

Very simply (and unprofessional) speaking, a virus is essentially a small 'program' that exhibits destructive behaviors, one of the most serious results being destroying the data in your hard disk. However, by 'program' that means it needs to be 'activated' before it can spread itself. In order for a computer to 'activate' a virus, the virus-infected file must be first recognized by the computer as either 'directly executable' (e.g. as an .exe [executable] file) or 'indirectly executable' (e.g. the macro function in a .doc [Word document] file).

How to prevent infection when receiving e-mails?

To prevent infection some people may install anti-virus applications to monitor all incoming files. I think this is necessary but not sufficient, because unless you update your virus-definition codes of your anti-virus application regularly, it may not be able to protect you from the newest viruses. Also there is a time- lag between the discovery of a new virus and the release of its 'antidote'. The following precautions should be taken whether you have installed an anti-virus application or not.

Every time I receive an email-attachment, I will first check the file-type of the file. If this is an '. exe', a '. com' (command) or a .bat (batch) file, most of the time I will throw it directly into the trash box, even the file was sent by my best friend. If I must use the file I first scan it with my code-updated anti-virus application, then download it to a separate directory, and pray.

The macro facilities in MSOffice applications, such as Word and Excel, are convenient and powerful mini-programs, but also provide the best habitats for various macro viruses. So whenever I download a .doc (or .dot, .xls, or .xlt) file, I still first scan the file and save it separately before I open it. And I have also checked the 'macro virus protection' option of my Word to provide additional protection. (I would suggest you not to transfer the closed-format file .doc , because this does not respect the freedom of choice of recipients...Well, something not very relevant of the present subject.)

The I LOVE YOU vbs trojan/virus was sent around the world as an email attachment named
'Love-Letter-For-You.txt.vbs', which contained malicious script. Unfortunately, most users had their 'display file extensions' function disabled by Microsoft's default, and the attachment was displayed as 'Love-Letter-For-You.txt'. People just thought the attachments were text files and the files were opened, resulting in extreme damage. Similarly, the attachment "Life_stages.txt.shs" of recent "VBS.Stages" virus was also displayed as "Life_stages.txt". In general, files with double-extensions are very likely to be trojans. You may like to configure your Windows Explorer to show all files and their extensions. In Windows 98, for example, this means double clicking "My Computer" --> "View"|"Folder Options"--> the "View" tab -->UNCHECKing "Hide file extensions for known file types" and CHECKing "Show all files".

'Do not open any attachment unless you are sure what the file-type means and what the file will do" perhaps is always a good way to protect your computer against virus attack. ( Here is a frustratingly long list of unsafe file extensions.)

Scanning email attachments...That's all?

No. Recently viruses threaten us in ways that are more complicated than the above. 'Offline'、'Redirect' and 'Prepend' viruses infected computers not because people had opened .exe or .doc files, but just because they had opened emails (your friend might have just forwarded a web page s/he found on the web [an HTML file] to you)! So the impression 'there are html viruses' spreads out.

The content of Web Pages/ HTML files should be ASCII text only, not binary files, so why do they 'contain' viruses? The secret is that these viruses are written in 'VBScript' or 'Javascript', mobile codes embedded in web pages to enhance the display effects or interactive functions of these pages. Yes, small 'programs', so there is the possibility that they turn out to be viruses. They are now the most difficult kinds of viruses to combat, because they are too invisible, too common, too ordinary, too ...

To increase your protection to these 'scripting' viruses, the essence is to disable the active scripting function of your email program so that it will not automatically execute these included codes when you open your HTML e-mails (Hence, if you are accustomed to 'previewing' your mails without this precaution, your practice is very unsafe). For example,

In Netscape this should be relatively easy:
From the menu bar, choose Edit-->Preference-->Advanced--> UNCHECK 'enable Javascript for mails and news'

For Microsoft Outlook Express, you must adjust the settings BOTH in Outlook Express and Internet Explorer to disable active scripting:
First, in Outlook Express Options Dialog: Security-->Security Zones--> CHECK Restricted sites zone;
then, in Internet Explorer: Internet Options-->Security--> SELECT Restricted sites--> Click Custom Level--> CHECK Prompt (or Disable) 'Active Scripting' under 'Scripting Area' ;
also CHECK Disable 'Script Active X controls not marked as safe for scripting’
& CHECK Prompt 'Script Active X controls marked safe for scripting' (this is to avoid the additional chain reactions when VBScript meets Active X)

The settings in other email programs (if they support scripting), such as Eudora, should be similarly adjusted.

For people using Web-based mail services:

Web-based email (e.g. Yahoo! and Hotmail) is easy to use and convenient. However, besides the fact that you are trusting a third party to store all your archived emails, and that Web-based email messages are especially prone to sniffing by malicious people (due to the existence of multiple capture points on the internet), Web-based emails are also subjected to offensive mobile codes because an account must be accessed by a browser.

The solution is, again, to disable the scripting function either in your Web-based mail, or in your browser. To enhance your protection, I also suggest you to regularly change your password, which should be a strong one too, and to encrypt your login information before sending it out (e.g. using the secure sockets layer (SSL) option when signing in Yahoo!). If other people can access to your computer, you should also make sure that you have not checked the 'remember my ID and password’ box, nor leaving the Web page without logging out.

After all, this message invites your feedbacks and value-addition.