1. Symantec/
  2. Security Response/
  3. W32.Welchia.Worm

W32.Welchia.Worm - Removal

Risk Level 2: Low

August 18, 2003
February 13, 2007 12:05:08 PM
Also Known As:
W32/Welchia.worm10240 [AhnLab], W32/Nachi.worm [McAfee], WORM_MSBLAST.D [Trend], Lovsan.D [F-Secure], W32/Nachi-A [Sophos], Win32.Nachi.A [CA], Worm.Win32.Welchia [Kaspersky]
Systems Affected:
Microsoft IIS, Windows
CVE References:
CAN-2003-0109 CAN-2003-0352

Removal using the W32.Welchia.Worm Removal Tool
Symantec Security Response has developed a removal tool to clean the infections of W32.Welchia.Worm. This is the easiest way to remove this threat and should be tried first. To obtain the W32.Welchia.Worm removal tool, read the document, "W32.Welchia.Worm Removal Tool."

Manual Removal
As an alternative to using the removal tool, you can manually remove this threat. 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 or stop the Worm.
  4. Run a full system scan and delete all the files detected as W32.Welchia.Worm.
  5. Delete the values from the registry.
  6. Delete the Svchost.exe file.
For details on each of these steps, read the following instructions.

1. Disabling 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. Updating 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. Restarting the computer in Safe mode or stopping the services of the worm
    Windows 95/98/Me
    Restart the computer in Safe mode. All the Windows 32-bit operating systems, except for Windows NT, can be restarted in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."

    Windows NT/2000/XP
    To stop the Worm services:
    1. Open Services in the Administrative Tools located in the Control Panel.
    2. Scroll through the list in the right pane and look for the following names:
      • Network Connections Sharing
      • WINS Client
    3. If you find the services, right-click them, and then click Stop.
    4. Exit the Services.
4. Scanning for and deleting 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.Worm, click Delete.

5. Deleting the values from the registry

WARNING: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry," for instructions.
  1. Click Start, and then click Run. (The Run dialog box appears.)
  2. Type regedit

    Then click OK. (The Registry Editor opens.)

  3. Navigate to the key:


  4. Delete the subkeys:




  5. Exit the Registry Editor.

6. Deleting the Svchost.exe file
Navigate to the %System%\Wins folder and delete the Svchost.exe file.

Writeup By: Frederic Perriot

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