There are all sorts of things you
can find in your
e-mail box. In the "destructive" and/or "annoying" category go e-mail
attachments that contain:
- Trojan horses
In many cases, e-mail viruses are not "true" viruses because they cannot
replicate without human interaction. Nonetheless, they have been very
effective at shutting down major e-mail systems. See
How Computer Viruses
Work for details on viruses.
A Trojan horse, aptly named after the seemingly harmless tool of
destruction in Homer's Iliad, secretly carries often-damaging
software in a "plain wrapper." The plain wrapper is normally an e-mail file
attachment from someone you may or may not know. The file attachment name
itself can also be very misleading. When you run the attachment, it
can do all sorts of things, from erasing files to changing your desktop. It
then sends itself along to other people in your address book so that it can
Here are two examples to help you understand how e-mail viruses work.
this page from Symantec:
Worm.ExploreZip is a worm that contains a
malicious payload. The worm utilizes Microsoft Outlook, Outlook Express,
Exchange to mail itself out by replying to unread messages in your Inbox.
The worm will also search the mapped drives and networked machines for
Windows installations and copy itself to the Windows directory of the
remote machine and modify the WIN.INI accordingly.
The payload of the worm will destroy any file with the extension .h,
.c, .cpp, asm, .doc, .ppt, or .xls on your hard drives, any mapped drives,
and any network machines that are accessible each time it is executed.
This continues to occur until the worm is removed.
You may receive the worm as an attachment called zipped_files.exe,
masquerading itself as the usual self-extracting zip file. But, when run,
this executable will copy itself to your Windows System directory with the
filename Explore.exe or to your Windows directory with the filename _setup.exe.
The worm modifies your WIN.INI or registry such that the file Explore.exe
is executed each time you start Windows.
this page for details.
Symantec offers more technical information and explains what you need to
do if you suspect Worm.ExploreZip is in your system.
In certain special cases, e-mail attachments can execute even without
your interaction. According to
this Symantec Web page:
VBS.BubbleBoy is a worm that works under Windows
98 and Windows 2000. The worm will also work under Windows 95 only if the
Windows Scripting Host is installed. The worm will only work with the
English and Spanish versions of the
systems, and not with Windows NT.
Microsoft Outlook (or Express) with Internet Explorer 5 must be used in
order for the worm to propagate.
The worm utilizes a known security hole in Microsoft Outlook/IE5 to
insert a script file, UPDATE.HTA, when the e-mail is viewed. It is not
necessary to detach and run an attachment.
UPDATE.HTA is placed in Program-StartUp of the Start menu. Therefore,
the infection routine is not executed until the next time you start your
computer. UPDATE.HTA is a script file that uses MS Outlook to send the
worm e-mail message to everyone in the MS Outlook address book. By
patching the known security hole in Microsoft Outlook/IE5, the worm will
no longer propagate.
Microsoft has more information on this worm.
Keep your virus software up-to-date with the latest virus signatures
from the software vendor, since the anti-virus software cannot detect new
viruses without an update. If you use Norton AntiVirus software, ensure that
Auto-Protect is enabled. Current Norton AntiVirus software automatically
alerts you when your virus signature files are over 30 days old. Norton's
LiveUpdate can also automate updating.
If you think a virus has infected your
PC thanks to an
e-mail virus that mails itself to people in your address book, call
those people and tell them not to open the messages or attachments -- that
is the only effective way to stop the spread.