Threat description


Category: Malware
Type: Worm
Platform: W32
Aliases: Welchi.B, Nachi.B, Welchia.B


A new variant of the Welchi RPC worm was found on February 12th, 2004.

This variant is functionally quite similar to the original virus. It copies itself on the infected system as SVCHOST.EXE.

The worm contains links to several security patches and will try to install them. Welchi.B will also try to remove Mydoom.

It might display a text saying "LET HISTORY TELL FUTURE !" and references to the dropping of atomic bombs during seconds world war.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.


More information on scanning and removal options available in your F-Secure product can be found in the Help Center.

You may also refer to the Knowledge Base on the F-Secure Community site for more information.

Contact Support

For further assistance, F-Secure customers can request support online via the Request support or the Chat forms on our Home - Global site.

Technical Details

The worm will start by creating a mutex named "WksPatch_Mutex" after which it will install itself as a service.

It copies itself to the Windows' System folder as "svchost.exe"

Whenever the system's date is within the month of June it will remove itself.

On certain occasions the worm attempts to download one of the following Windows patches:

  • https://download.microsoft.com/download/4/d/3/4d375d48-04c7-411f-959b-3467c5ef1e9a/WindowsXP-KB828035-x86-CHS.exe
  • https://download.microsoft.com/download/a/4/3/a43ea017-9abd-4d28-a736-2c17dd4d7e59/WindowsXP-KB828035-x86-KOR.exe
  • https://download.microsoft.com/download/e/a/e/eaea4109-0870-4dd3-88e0-a34035dc181a/WindowsXP-KB828035-x86-ENU.exe
  • https://download.microsoft.com/download/9/c/5/9c579720-63e9-478a-bdcb-70087ccad56c/Windows2000-KB828749-x86-CHS.exe
  • https://download.microsoft.com/download/0/8/4/084be8b7-e000-4847-979c-c26de0929513/Windows2000-KB828749-x86-KOR.exe
  • https://download.microsoft.com/download/3/c/6/3c6d56ff-ff8e-4322-84cb-3bf9a915e6d9/Windows2000-KB828749-x86-ENU.exe

It will scan the computer for files with extensions from the following list:

.shtml  .shtm  .stm  .cgi  .php  .html  .htm  .asp   

And overwrite them with and HTML contained within the worm's body, the exact HTML code follows:


  1937.12.13 300,000 !

  1945.8.6 Little boy
  1945.8.9 Fatso

  Let history tell future !

Access to the Internet will be checked by attempting to resolve randomly any of:

microsoft.com  intel.com  google.com   

In case of failure, it will wait for 20 minutes before attempting the operation again.


The worm will spread through the backdoor left in the system by the Mydoom.A and Mydoom.B worms.

In addition it will use the same exploits as the original Welchi variant, information about those can be found from the Welchi description:



Detection in F-Secure Anti-Virus was published on February 12th, 2004 in update:

Detection Type: PC
Database: 2004-02-12_01

Description Details: Katrin Tocheva, Mikko Hypponen February 12th, 2004
Technical Details:Ero Carrera, February 16th, 2004


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