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Parent class: VirWare

Viruses and worms are malicious programs that self-replicate on computers or via computer networks without the user being aware; each subsequent copy of such malicious programs is also able to self-replicate. Malicious programs which spread via networks or infect remote machines when commanded to do so by the “owner” (e.g. Backdoors) or programs that create multiple copies that are unable to self-replicate are not part of the Viruses and Worms subclass. The main characteristic used to determine whether or not a program is classified as a separate behaviour within the Viruses and Worms subclass is how the program propagates (i.e. how the malicious program spreads copies of itself via local or network resources.) Most known worms are spread as files sent as email attachments, via a link to a web or FTP resource, via a link sent in an ICQ or IRC message, via P2P file sharing networks etc. Some worms spread as network packets; these directly penetrate the computer memory, and the worm code is then activated. Worms use the following techniques to penetrate remote computers and launch copies of themselves: social engineering (for example, an email message suggesting the user opens an attached file), exploiting network configuration errors (such as copying to a fully accessible disk), and exploiting loopholes in operating system and application security. Viruses can be divided in accordance with the method used to infect a computer:
  • file viruses
  • boot sector viruses
  • macro viruses
  • script viruses
Any program within this subclass can have additional Trojan functions. It should also be noted that many worms use more than one method in order to spread copies via networks.

Class: Email-Worm

Email-Worms spread via email. The worm sends a copy of itself as an attachment to an email message or a link to its file on a network resource (e.g. a URL to an infected file on a compromised website or a hacker-owned website). In the first case, the worm code activates when the infected attachment is opened (launched). In the second case, the code is activated when the link to the infected file is opened. In both case, the result is the same: the worm code is activated. Email-Worms use a range of methods to send infected emails. The most common are: using a direct connection to a SMTP server using the email directory built into the worm’s code using MS Outlook services using Windows MAPI functions. Email-Worms use a number of different sources to find email addresses to which infected emails will be sent: the address book in MS Outlook a WAB address database .txt files stored on the hard drive: the worm can identify which strings in text files are email addresses emails in the inbox (some Email-Worms even “reply” to emails found in the inbox) Many Email-Worms use more than one of the sources listed above. There are also other sources of email addresses, such as address books associated with web-based email services.

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Platform: Win32

Win32 is an API on Windows NT-based operating systems (Windows XP, Windows 7, etc.) that supports execution of 32-bit applications. One of the most widespread programming platforms in the world.


The Klez.h variant of the Klez worm family is very similar to Klez.e. The differences are:

  1. This variant has no payload and doesn't destroy files.
  2. It brings with it additional variants of infected Messages, Subjects and Bodies.

Example of a Klez.h email message Subject and Body content:

Worm Klez.E immunity

Klez.E is the most common world-wide spreading worm.

It's very dangerous by corrupting your files.

Because of its very smart stealth and anti-anti-virus technic,

most common AV software can't detect or clean it.

We developed this free immunity tool to defeat the malicious virus.

You only need to run this tool once,and then Klez will never come into your PC.

NOTE: Because this tool acts as a fake Klez to fool the real worm,

some AV monitor maybe cry when you run it.

If so,Ignore the warning,and select 'continue'.

If you have any question,please mail to me.

This worm looks for files with the following extensions:

.txt .htm .html .wab .asp .doc .rtf .xls .jpg .cpp .c .pas .mpg .mpeg .bak .mp3 .pdf

Depending on several conditions Klez.h attaches a file with one of the above listed extensions to infected emails (as the second attached file). As a result, confidential or personal information may be sent out and made public.

Another example of Klez.h email message content:

Win32 Klez V2.01 & Win32 Foroux V1.0

Copyright 2002,made in Asia

About Klez V2.01:

1,Main mission is to release the new baby PE virus,Win32 Foroux

2,No significant change.No bug fixed.No any payload.

About Win32 Foroux (plz keep the name,thanx)

1,Full compatible Win32 PE virus on Win9X/2K/NT/XP

2,With very interesting feature.Check it!

3,No any payload.No any optimization

4,Not bug free,because of a hurry work.No more than three weeks from

having such idea to accomplishing coding and testing

How do I delete the Klez virus?

1) disconnect the infected PC from the local network (if exists)

2) run file

If the program says "nothing to clean" - run it from the command line with the paramrter /scanfiles, for example: /scanfiles

3) re-boot your PC in Safe Mode

4) run again

5) reinstall the anti-virus package and update the anti-virus database

6) run Kaspersky AV Scanner and check all the hard drives

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