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Discovered: October 25, 2001
Updated: February 13, 2007 11:37:35 AM
Also Known As: W32.Poverty.A@mm, W32.Klez.gen@mm
Type: Worm

W32.Klez.A@mm is a mass-mailing email worm. It attempts to copy itself into folders on both local and network drives.

The worm exploits a vulnerability in Microsoft Outlook and Outlook Express in an attempt to execute itself when you open or even preview the message. Information and a patch for the vulnerability can be found at

The worm also inserts the virus W32.ElKern.3326 . W32.ElKern.3326 can also infect W32.Klez.A@mm.

Finally, the worm executes its payload on the 13th of January, March, May, July, September, and November. The payload causes files on local and mapped drives to become zero bytes in size.

Removal tool
Symantec provides a tool to remove infections of all known variants of W32.Klez and W32.ElKern. Click here to obtain the tool. This is the easiest way to remove these threats and should be tried first.

For information about how Klez affects a Macintosh computer, read the document Are Macintoshes affected by the Klez virus?

Antivirus Protection Dates

  • Initial Rapid Release version October 26, 2001
  • Latest Rapid Release version September 28, 2010 revision 054
  • Initial Daily Certified version October 26, 2001
  • Latest Daily Certified version September 28, 2010 revision 036

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Technical Description

When W32.Klez.A@mm is executed, it does the following:

It copies itself to


NOTE: %System% is a variable. The worm locates the \Windows\System folder (by default this is C:\Windows\System or C:\Winnt\System32) and copies itself to that location.

It adds the value

krn132   %System%\krn132.exe

to the registry key


so that it is executed when you start Windows.

The worm attempts to disable on-access virus scanners and searches local, mapped, and network drives. The worm copies itself using a random file name with a variable double extension, such as Filename.txt.exe.

In addition, the worm searches the Windows address book, which is used by Microsoft Outlook, for email addresses. The worm sends an email message to these addresses with itself as an attachment.

The email message has the following characteristics:

Subject: The subject of the email varies. It will usually be one of the following:
How are you?
Can you help me?
We want peace
Where will you go?
Don't cry
Look at the pretty
Some advice on your shortcoming
Free XXX Pictures
A free hot porn site
Why don't you reply to me?
How about have dinner with me together?
Never kiss a stranger

Attachment: The attachment has a random file name with the .exe extension.
I'm sorry to do so,but it's helpless to say sorry.
I want a good job,I must support my parents.
Now you have seen my technical capabilities.
How much my year-salary now? NO more than $5,500.
What do you think of this fact?
Don't call my names,I have no hostility.
Can you help me?

This message may not be visible (this depends on the ability of the email client to display HTML email messages). If the message is received by Microsoft Outlook or Outlook Express, the attachment may be automatically executed. Information about this vulnerability and a patch are available at

Every other month starting in January (January, March, May, and so on), if the date is the 13th of the month, the payload is executed. This causes files on local and mapped drives to become zero bytes in length.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.


Removal using the removal tool
Removal tool
Symantec provides a tool to remove infections of all known variants of W32.Klez and W32.ElKern. Click here to obtain the tool. This is the easiest way to remove these threats and should be tried first.

Manual removal procedure
To remove this worm, delete files that are detected as W32.Klez.A@mm, and remove the value that the worm added to the registry.

To remove W32.Klez.A@mm files:

  1. Run LiveUpdate to make sure that you have the most recent virus definitions.
  2. Start Norton AntiVirus (NAV), and make sure that NAV is configured to scan all files. For instructions on how to do this, read the document How to configure Norton AntiVirus to scan all files.
  3. Run a full system scan.
  4. Delete all files that are detected as W32.Klez.A@mm.

To edit the registry:

CAUTION : Symantec strongly recommends that you back up the registry before you make any changes. Incorrect changes to the registry could result in permanent data loss or corrupted files. Make sure that you modify only the keys that are specified; read the document How to back up the Windows registry before you proceed.
  1. Click Start, and click Run. The Run dialog box appears.
  2. Type regedit and then click OK. The Registry Editor opens.
  3. Navigate to the following key:

  4. In the right pane, delete the following value:

    krn132   %System%\krn132.exe
  5. Click Registry, and click Exit.

Writeup By: Atli Gudmundsson