1. Symantec/
  2. Security Response/
  3. W32.Klez.E@mm


Risk Level 2: Low

January 17, 2002
February 13, 2007 11:53:18 AM
Also Known As:
W32/Klez.e@MM [McAfee], WORM_KLEZ.E [Trend], Klez.E [F-Secure], W32/Klez-E [Sophos], Win32.Klez.E [CA], I-Worm.Klez.E [AVP]
Worm, Virus
Systems Affected:
CVE References:

When the worm is executed, it copies itself to %System%\Wink[random characters].exe.

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

Wink[random characters] %System%\Wink[random characters].exe

to the registry key


or it creates the registry key

HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Wink[random characters]

and inserts a value in that subkey so that the worm is executed when you start Windows.

The worm attempts to disable on-access virus scanners and some previously distributed worms (such as W32.Nimda and CodeRed) by stopping any active processes. The worm removes the startup registry keys used by antivirus products and deletes checksum database files including:


The worm copies itself to local, mapped, and network drives as:
  • A random file name with a double extension. For example, filename.txt.exe.
  • A .rar archive with a double extension. For example, filename.txt.rar.

In addition, the worm searches the Windows address book, the ICQ database, and local files (such as .html and text files) for email addresses. The worm sends an email message to these addresses with itself as an attachment. The worm contains its own SMTP engine and attempts to guess at available SMTP servers.

The subject line, message bodies, and attachment file names are random. The from address is randomly chosen from email addresses that the worm finds on the infected computer.

  • Because this worm does use a randomly chosen address that it finds on an infected computer as the "From:" address, numerous cases have been reported in which users of uninfected computers receive complaints that they have sent an infected message to someone else.

    For example, Linda Anderson is using a computer that is infected with W32.Klez.E@mm; Linda is not using a antivirus program or does not have current virus definitions. When W32.Klez.E@mm performs its emailing routine, it finds the email address of Harold Logan. It inserts Harold's email address into the "From:" line of an infected email that it then sends to Janet Bishop. Janet then contacts Harold and complains that he sent her infected email, but when Harold scans his computer, Norton AntiVirus does not find anything--as would be expected--because his computer is not infected.

    If you are using a current version of Norton AntiVirus, have the most recent virus definitions, and a full system scan with Norton AntiVirus set to scan all files does not find anything, you can be confident that your computer is not infected with this worm.
  • There have been several reports that, in some cases, if you receive a message that the virus has sent using its own SMTP engine, the message appears to be a "postmaster bounce message" from your own domain. For example, if your email address is jsmith@anyplace.com, you could receive a message that appears to be from postmaster@anyplace.com, indicating that you attempted to send email and the attempt failed. If this is the false message that is sent by the virus, the attachment includes the virus itself. Of course, such attachments should not be opened.

If the message is opened in an unpatched version of Microsoft Outlook or Outlook Express, the attachment may be automatically executed. Information about this vulnerability and a patch are available at


The worm also infects executables by creating a hidden copy of the original host file and then overwriting the original file with itself. The hidden copy is encrypted, but contains no viral data. The name of the hidden file is the same as the original file, but with a random extension.

The worm also drops the virus W32.Elkern.3587 as the file %System%\wqk.exe and executes it.

Finally, the worm has a payload. On the 6th of every odd numbered month (except January or July), the worm attempts to overwrite with zeroes files that have the extensions .txt, .htm, .html, .wab, .doc, .xls, .jpg, .cpp, .c, .pas, .mpg, .mpeg, .bak, or .mp3. If the month is January or July, this payload attempts to overwrite all files with zeroes, not just those with the aforementioned extensions.


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.
Writeup By: Atli Gudmundsson
Summary| Technical Details| Removal

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