Threat Description

Worm: W32/Welchi

Details

Category: Malware
Type: Worm
Platform: W32
Aliases: Sachi, WORM_MSBLAST.D, Nachi, Welchia

Summary


Worm:W32/Welchi is an unusual malware in that it attempts to disinfect the computer system from Worm:W32/Lovsan infections. It also attempts to patch a vulnerability used by the Lovsan worm to propagate.



Removal


Automatic action

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

Manual Disinfection

Caution: Manual disinfection is a risky process; it is recommended only for advanced users.

The worm will stop working when the computer's clock is set to 2004. This feature can be used to remove the worm from the computer. This can be automated with the date and the shutdown commands, for example:

  • DATE 01-01-2004
  • SHUTDOWN -r
More scanning & removal options

More information on scanning or removal options is available in the documentation for your F-Secure security product on the Downloads section of our Home - Global site.

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.

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

Network Disinfection

For general instructions on disinfecting a local network infection, please see Eliminating A Local Network Outbreak.



Technical Details


The virus code contains the following text string, which is never displayed:

=========== 
I love my wife & baby :)
~~~ Welcome Chian
~~~ Notice: 2004 will remove myself:)
~~ sorry zhongli~~~=========== wins

Welchi was first reported on August 18th 2003. The worm will stop working when the computer's clock is set to 2004.

Infection

Welchi only infects Windows XP machines through the RPC hole used by the Lovsan worm.

In addition, Welchi will attempt to infect IIS 5.0 web servers via WebDAV exploit. For more on this vulnerability found in March 2003, see http://www.microsoft.com/technet/security/bulletin/MS03-007.asp.

This variant uses files named DLLHOST.EXE and SVCHOST.EXE. Note that these are names of normal Windows system files.

On infection, the worm drops files at the following locations:

  • %systemDir%\wins\DLLHOST.EXE - The worm's executable file
  • %systemDir%\wins\SVCHOST.EXE - The renamed tftp server
Activity

Unlike most worms, Welchi's effects on the infected system appear to be beneficial. When active, it attempts to disinfect Lovsan.A from the machine by looking for a process with the strings "msblast" in its name (a Lovsan characteristic) and terminating the process if found.

It will then delete the file %systemdir%\msblast.exe, Lovsan's executable copy.

Welchi also attempts to download the Microsoft patch for closing the RPC vulnerability, and tries to download the patch from eight different URLs, corresponding to four language versions (English, Chinese, Simplified Chinese and Korean) for both Windows XP and 2000. This patch routine is not always successful and on clearing Welchi from the machine it is recommended that the patch be checked according to instructions from Microsoft (see the section File Information at http://support.microsoft.com/?kbid=823980).

Registry

When run, Welchi will create a mutex named "RpcPatch_Mutex", to ensure that only one active copy of the worm is ever run.

The worm does not modify the Windows Registry to launch itself automatically.





Description Created: 2009-07-20 13:04:14.0

Description Last Modified: 2010-08-02 08:01:41.0


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