Threat Description



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


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 Disinfection

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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:


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


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

 <body text="#FF0000" bgcolor="#000000"
 <font size="7" face="Arial">
 <p align="center">
 1937.12.13 300,000 !<br>
 1945.8.6 Little boy<br>
 1945.8.9 Fatso<br>
 Let history tell future !<br>

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

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


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