Table of Contents




Posted by Roy Linker

Sobig.f is a worm spreading via the Internet as a file attached to infected emails. The Sobig.f worm also spreads through shared network resources.

The worm itself is a Windows PE EXE file that is written in Microsoft Visual C++ and is compressed by the TeLock utility. Its file sizes are typically around 70 KB when compressed (TeLock), while its decompressed size is about 100 KB.

The Sobig.f worm activates only when a user double clicks on the attached file. Once the worm is launched it installs itself in the system and runs its spreading routine.


During installation the worm copies itself into the Windows directory under the name winppr32.exe and registers itself in the system registry autorun keys:

 TrayX = %WindowsDir%\winppr32.exe/sinc

 TrayX = %WindowsDir%\winppr32.exe/sinc

Spreading via email

To get victim emails the worm looks for .TXT, .EML, .HTML, .HTM, .DBX, WAB, MHT and HLP files in all directories on all available local drives, scans for e-mail like text strings and sends infected e-mails to these addresses. To send infected messages the worm uses the SMTP engine specified in the system properties.

Below are variations of Sobig.f message content:

The From field has fake email address (found on the infected machine) or admin@internet.com.


Re: That movie
Re: Wicked screensaver
Re: Your application
Re: Approved
Re: Re: My details
Re: Details
Your details
Thank you!
Re: Thank you!

Message Body:

See the attached file for details
Please see the attached file for details.

Attached file name:


The worm also creates the file winstt32.dat in the Windows directory and writes the email addresses that were found on the infected machine to this file.

Spreading via network

The worm scans all accessible network resources (other computers in a network) and copies itself to the auto-start directories (if there are such subdirectories) of each resource (computer) found.


The worm sends UDP packets at random IP addresses to port 8998 and awaits commands from the 'master' machine. The commands contain URLs from which Sobig.f downloads and executes files. Thus, the worm is able to upgrade itself and/or install other applications (Trojans for instance).

Loading additional files
The worm launches a procedure that every 60 minutes checks the current time according to Greenwich meantime. To do this it sends out queries via NTP servers (Network Time Protocol). SoBig.F keeps an internal log from 19 NTP servers; following are the IP addresses of these NTP servers:

When an NTP server does not reply, the worm invokes the system function 
for learning the current time - 'gmtime'. On Fridays and Sundays when the
current GM time is between 19:00 and 23:00, the worm begins to download additional files. 
To download these files it sends out UDP (User Datagram Protocol) packets via the 
IP address assigned to port 8998. SoBig.f maintains a list of IP addresses in an encoded file. 
Currently these sites are blocked and therefore do not respond to queries. 

List of encoded IP addresses:

The SoBig.f worm receives replies to its queries in the form of a UDP packet via port 8998. This packet contains an encoded URL (Uniform Resource Locator) file. The SoBig.f worm downloads this file and executes it.


All worm routines are active until September 10, 2003.

Last Update:06/26/2007


Copyright 1999 - 2012 PC Lifeline