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


Risk Level 2: Low

April 17, 2002
February 13, 2007 11:38:50 AM
Also Known As:
W32/Klez.h@MM [McAfee], WORM_KLEZ.H [Trend], WORM_KLEZ.I [Trend], I-Worm.Klez.h [Kaspersky], Klez.H, W32/Klez-H [Sophos], Win32.Klez.H [Computer Associa, W32/Klez.I [Panda], W32/Klez.H@mm [Frisk]
Systems Affected:
CVE References:

When this worm is executed, it does the following:
  1. 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.

  2. 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 this subkey, so that the worm executes when you start Windows.

  3. 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, which antivirus products use, and deletes the checksum database files, including:
  • Anti-Vir.dat
  • Chklist.dat
  • Chklist.ms
  • Chklist.cps
  • Chklist.tav
  • Ivb.ntz
  • Smartchk.ms
  • Smartchk.cps
  • Avgqt.dat
  • Aguard.dat

Copying local and network drives
The worm copies itself to the 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.

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

For example, if the worm encounters the address, user@abc123.com, it attempts to send email via the server, smtp.abc123.com.

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

The worm will the search files with the following extensions for the email addresses:
  • mp8
  • .exe
  • .scr
  • .pif
  • .bat
  • .txt
  • .htm
  • .html
  • .wab
  • .asp
  • .doc
  • .rtf
  • .xls
  • .jpg
  • .cpp
  • .pas
  • .mpg
  • .mpeg
  • .bak
  • .mp3
  • .pdf

In addition to the worm attachment, the worm may also attach a random file from the computer. The file will have one of the following extensions:
  • mp8
  • .txt
  • .htm
  • .html
  • .wab
  • .asp
  • .doc
  • .rtf
  • .xls
  • .jpg
  • .cpp
  • .pas
  • .mpg
  • .mpeg
  • .bak
  • .mp3
  • .pdf

As a result, the email message would have two attachments, the first being the worm and the second being the randomly selected file.

"Random" strings comprise the email message that this worms sends. The subject can be one of the following:
  • Worm Klez.E immunity
  • Undeliverable mail--"[Random word]"
  • Returned mail--"[Random word]"
  • a [Random word] [Random word] game
  • a [Random word] [Random word] tool
  • a [Random word] [Random word] website
  • a [Random word] [Random word] patch
  • [Random word] removal tools
  • how are you
  • let's be friends
  • darling
  • so cool a flash,enjoy it
  • your password
  • honey
  • some questions
  • please try again
  • welcome to my hometown
  • the Garden of Eden
  • introduction on ADSL
  • meeting notice
  • questionnaire
  • congratulations
  • sos!
  • japanese girl VS playboy
  • look,my beautiful girl friend
  • eager to see you
  • spice girls' vocal concert
  • japanese lass' sexy pictures

The random word is one of the following:
  • new
  • funny
  • nice
  • humour
  • excite
  • good
  • powful
  • WinXP
  • IE 6.0
  • W32.Elkern
  • W32.Klez.E
  • Symantec
  • Mcafee
  • F-Secure
  • Sophos
  • Trendmicro
  • Kaspersky

The body of the email message is random.

Email spoofing
  • This worm often uses a technique called "spoofing." When the worm performs its email routine, it can use a randomly chosen address it finds on an infected computer as the "From:" address. Numerous cases have been reported in which users of uninfected computers received complaints that they sent an infected message to someone else.

    For example, Linda Anderson is using a computer infected with W32.Klez.H@mm. Linda is not using an antivirus program or does not have the current virus definitions. When W32.Klez.H@mm performs its emailing routine, it finds the email address of Harold Logan. The worm inserts Harold's email address into the "From:" portion of an infected message, which the worm then sends to Janet Bishop. Then, Janet contacts Harold and complains that he sent her an infected message, but when Harold scans his computer, Norton AntiVirus (NAV) does not find anything 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, which is set to scan all the files, does not find anything, be assured 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 an email and the attempt failed. If this is the false message sent by the virus, the attachment includes the virus itself. Of course, such attachments should not be opened.
  • The message may be disguised as an immunity tool. One version of this false message is:

    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.

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:

Virus Insertion
This worm inserts the W32.Elkern.4926 virus as a file, with a random name in the \%Program Files% folder, and then executes it.

Note: %Program Files% is a variable. The worm locates the \Program Files folder (by default, this is C:\Program Files) and copies the virus to that location.


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: Neal Hindocha
Summary| Technical Details| Removal

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