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.

Installation

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

HKCU\Software\Microsoft\Windows\CurrentVersion\Run
 TrayX = %WindowsDir%\winppr32.exe/sinc

HKLM\Software\Microsoft\Windows\CurrentVersion\Run
 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.

Subject:

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:

movie0045.pif
wicked_scr.scr
application.pif
document_9446.pif
details.pif
your_details.pif
thank_you.pif
document_all.pif
your_document.pif

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.

Updating

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:

 200.68.60.246
 62.119.40.98
 150.254.183.15
 132.181.12.13
 193.79.237.14
 131.188.3.222
 131.188.3.220
 193.5.216.14
 193.67.79.202
 133.100.11.8
 193.204.114.232
 138.96.64.10
 chronos.cru.fr
 212.242.86.186
 128.233.3.101
 142.3.100.2
 200.19.119.69
 137.92.140.80
 129.132.2.21

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:

 67.73.21.6
 68.38.159.161
 67.9.241.67
 66.131.207.81
 65.177.240.194
 65.93.81.59
 65.95.193.138
 65.92.186.145
 63.250.82.87
 65.92.80.218
 61.38.187.59
 24.210.182.156
 24.202.91.43
 24.206.75.137
 24.197.143.132
 12.158.102.205
 24.33.66.38
 218.147.164.29
 12.232.104.221
 68.50.208.96

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.

Other

All worm routines are active until September 10, 2003.