Threat Description

Worm: W32/Sobig


Category: Malware
Type: Worm
Platform: W32
Aliases: Worm:W32/Sobig


The Sobig worm was found in the wild on January 9th 2003. The worm spreads via email and network shared drives. It also tries to download other files from web pages located on a Geocities site.


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

Update 2003-04-23 09:00 GMTIt has been reported that the webpage that controls the trojan downloader component of the worm had been updated for a period of time. The page pointed to a location containing a trojan (detected by F-Secure Anti-Virus as Backdoor.Delf.da). At the time of this update, the control page is no longer available.


When the worm is run on a system for the first time it copies itself to the Windows System Directory using the name winmgm32.exe. After this a new value, pointing to this file is added to the registry as

  • HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run\WindowsMGM

This way the worm will be started every time Windows starts.


Sobig contains a routine that downloads a text file from a website. The content of the file is used as a URL to download some program and run it on the infected machine. At the time of writing this description this feature is inactive, as the file points to a non-exisiting location.

The worm might affect network printers. In such cases printers might start to print garbage.

Propagation (E-mail)

Email addresses are collected from files with various extensions:

  • .WAB
  • .DBX
  • .HML
  • .HTML
  • .EML
  • .TXT

The sender address is fixed, it is always

Subjects are randomly chosen from the following list:

  • Re: Here is that sample
  • Re: Document
  • Re: Sample
  • Re: Movies

The message body says:

  • Attached file:

The message contains an executable attachment. The attachment name can be one of the following:

  • Sample.pif
  • Untitled1.pif
  • Document003.pif
  • Movie_0074.mpeg.pif

The infected emails are sent using the worms own STMP engine that is independent from the users email settings.

Propagation (Local Area Network)

Sobig lists all the network shares available to the infected computer and tries to copy itself to either of these directories:

  • Windows\All Users\Start Menu\Programs\StartUp


  • Documents and Settings\All Users\Start Menu\Programs\Startup

These are the default startup folders for Windows 9x and NT/XP based systems. If the worm is copied there Windows will run it next time the user logs in. This way the system gets infected.


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