Discovered: February 23, 2004
Updated: February 13, 2007 12:17:56 PM
Type: Worm
Systems Affected: Windows

W32.Welchia.D.Worm is a minor variant of W32.Welchia.C.Worm .

If the operating system of an infected computer is Chinese, Korean, or English, the worm will attempt to download the Microsoft Workstation Service Buffer Overrun and Microsoft Messenger Service Buffer Overrun patches from the Microsoft Windows Update Web site, install it, and then restart the computer.

The worm also attempts to remove the W32.Mydoom.A@mm , W32.Mydoom.B@mm , W32.HLLW.Doomjuice , and W32.HLLW.Doomjuice.B worms.

W32.Welchia.D.Worm exploits multiple vulnerabilities, including:

  • The DCOM RPC vulnerability (described in Microsoft Security Bulletin MS03-026) using TCP port 135. The worm specifically targets Windows XP machines using this exploit.
  • The WebDav vulnerability (described in Microsoft Security Bulletin MS03-007) using TCP port 80. The worm specifically targets machines running the Microsoft IIS 5.0 using this exploit. The worm's use of this exploit will impact Windows 2000 systems, and may impact Windows NT/XP systems.
  • The Workstation service buffer overrun vulnerability (described in Microsoft Security Bulletin MS03-049) using TCP port 445.
  • The Locator service vulnerability using TCP port 445 (described in Microsoft Security Bulletin MS03-001). The worm specifically targets Windows 2000 machines using this exploit.

In addition, W32.Welchia.D.Worm attempts to exploit the W32.Mydoom.A@mm backdoor (port 3127) to spread.

The presence of the file, %Windir%\System32\Drivers\Svchost.exe, is an indication of a possible infection.

This threat is compressed with UPX.

Antivirus Protection Dates

  • Initial Rapid Release version February 23, 2004
  • Latest Rapid Release version September 06, 2017 revision 035
  • Initial Daily Certified version February 23, 2004
  • Latest Daily Certified version September 07, 2017 revision 001
  • Initial Weekly Certified release date February 23, 2004

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

Technical Description

When W32.Welchia.D.Worm runs, it does the following:

  1. Creates the mutex, "WksPatch_Mutex." This mutex allows only one instance of the worm to execute in memory.

  2. Copies itself as %System%\Drivers\Svchost.exe.

      • %System% is a variable. The worm locates the System folder and copies itself to that location. By default, this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).
      • There is a legitimate system file, %System%\Svchost.exe, which has the same file size as the worm on the Windows XP system.

  3. Creates the following service:

    Service name: WksPatch
    Service binary: %System%\Drivers\Svchost.exe
    Service display name: Constructed in the form of %string1% %string2% %string3%, where:
    1. %string1% is one of the following:
      • System
      • Security
      • Remote
      • Routing
      • Performance
      • Network
      • License
      • Internet

    2. %string2% is one of the following:
      • Logging
      • Manager
      • Procedure
      • Accounts
      • Event

    3. and %string3% is one of the following:
      • Provider
      • Sharing
      • Messaging
      • Client

        For example, the service display name can be "Security Logging Sharing."

  4. Deletes the service named "RpcPatch," if it exists.

    W32.Welchia.Worm created this service.

  5. Checks for the existence of the W32.Mydoom.A@mm, W32.Mydoom.B@mm, W32.HLLW.Doomjuice, and W32.HLLW.Doomjuice.B worms.

  6. If any of the worms exist, it will attempt to remove them from the system by following these steps:
    1. Ends the respective processes by name:
      • %System%\intrenat.exe (W32.HLLW.Doomjuice)
      • %System%\Regedit.exe (W32.HLLW.Doomjuice.B)
      • %System%\Taskmon.exe (W32.Mydoom.A@mm)
      • %System%\Explorer.exe (W32.Mydoom.B@mm)

    2. Attempts to deletes the files associated with the above processes, as well as the following files:
      • %System%\shimgapi.dll (The .dll associated with W32.Mydoom.A@mm)
      • %System%\ctfmon.dll (The .dll associated with W32.Mydoom.B@mm)

        The worm attempts to delete the files before ending the worm processes, so this action will fail.

    3. Removes the following values from the registry:
      • HKLM/Software/Microsoft/Windows/CurrentVersion/Run/Gremlin (W32.HLLW.Doomjuice)
      • HKLM/Software/Microsoft/Windows/CurrentVersion/Run/NeroCheck (W32.HLLW.Doomjuice.B)
      • HKLM/Software/Microsoft/Windows/CurrentVersion/Run/Taskmon (W32.Mydoom.A@mm)
      • HKCU/Software/Microsoft/Windows/CurrentVersion/Run/Taskmon (W32.Mydoom.A@mm)
      • HKLM/Software/Microsoft/Windows/CurrentVersion/Run/Explorer (W32.Mydoom.B@mm)
      • HKCU/Software/Microsoft/Windows/CurrentVersion/Run/Explorer (W32.Mydoom.B@mm)

    4. Restores the following value in the registry (that W32.Mydoom.a@mm and W32.Mydoom.B@mm modify to point to their own DLL):

      HKCR/CLSID/{E6FB5E20-DE35-11CF-9C87-00AA005127ED}/InProcServer32 = %System\webcheck.dll

    5. Overwrites the HOSTS file, which Mydoom.A and .B modify, with the following text:

      # localhost

  7. Generates random IP addresses, and then sends exploit data to the IP addresses, in an attempt to infect the systems:
    • Sends data to TCP port 135 to exploit the DCOM RPC vulnerability. It uses a randomized IP address when doing so.
    • Sends data to TCP port 80 to exploit the WebDav vulnerability.
    • Sends data to TCP port 445 to exploit the Workstation Service vulnerability.
    • Sends data to TCP port 445 to exploit the Locator service vulnerability.
    • Sends data to TCP port 135 to exploit the DCOM RPC vulnerability. It uses an IP address that is close to the same class B subnet of the IP address of an infected host.

  8. Runs an HTTP server on a random TCP port, so that the vulnerable computers can reconnect to an infected computer, then locally download and execute the worm as WksPatch.exe.

  9. If the operating system is Japanese, it searches for the files in the IIS Virtual Roots and %Windir%\Help\\IISHelp\common folders if the files have the following extensions:
    • .shtml
    • .shtm
    • .stm
    • .cgi
    • .php
    • .html
    • .htm
    • .asp

      Note: The Virtual Roots and IIS Help folders are installed as part of Microsoft's Internet Information Services server.

  10. Overwrites the files that it finds with the following .htm file:

  11. Downloads one of the following patches from Microsoft's Windows Update Web site, if the version of the operating system of the infected computer is Chinese, Korean, or English:

  12. Installs the patch, and then restarts the computer.

The worm will self terminate on June 1, 2004, or after running 120 days, whichever comes first.

Some minor variants of W32.Welchia.D.Worm may self terminate on July 1, 2004.


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.


The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Disable System Restore (Windows XP).
  2. Update the virus definitions.
  3. Restart the computer in Safe mode or VGA mode.
  4. Run a full system scan and delete all the files detected as W32.Welchia.D.Worm.
For details on each of these steps, read the following instructions.

1. To disable System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.

2. To update the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

    The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. To restart the computer in Safe mode or VGA mode

Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
  • For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
  • For Windows NT 4 users, restart the computer in VGA mode.
4. To scan for and delete the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected as infected with W32.Welchia.D.Worm, click Delete.

Writeup By: Frederic Perriot