Threat Descriptons



Category :


Type :


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.


Based on the settings of your F-Secure security product, it will either move the file to the quarantine where it cannot spread or cause harm, or remove it.

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note: You need administrative rights to change the settings.

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:




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:

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:

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