Discovered: August 18, 2003
Updated: February 13, 2007 12:05:18 PM
Also Known As: Sobig.F [F-Secure], W32/Sobig.f@MM [McAfee], WORM SOBIG.F [Trend], W32/Sobig-F [Sophos], Win32.Sobig.F [CA], I-Worm.Sobig.f [KAV]
Type: Worm
Systems Affected: Windows

Due to a decreased rate of submissions, and the hard coded deactivation date, Symantec Security Response has downgraded this threat to a Category 2 from a Category 4 as of September 15, 2003.

W32.Sobig.F@mm is a mass-mailing, network-aware worm that sends itself to all the email addresses it finds in the files that have the following extensions:

  • .dbx
  • .eml
  • .hlp
  • .htm
  • .html
  • .mht
  • .wab
  • .txt

The worm uses its own SMTP engine to propagate. It also attempts to create a copy of itself on accessible network shares, but fails due to bugs in the code.

Email routine details
The email message has the following characteristics:

From: Spoofed address (which means that the sender in the "From" field is most likely not the real sender). The worm may also use the address,, as the sender.
    • The spoofed addresses and the Send To addresses are both taken from the files found on the computer. Also, the worm may use the settings of the infected computer's settings to check for an SMTP server to contact.
    • The choice of the domain appears to be arbitrary and does not have any connection to the actual domain or its parent company.

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

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

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

  • The worm de-activates on September 10, 2003. The last day after which the worm should stop spreading is September 9, 2003. However, computers with out of date system clocks are still vulnerable to the worm and may contribute to its spread after the de-activation date.
  • The aforementioned de-activation date applies only to the mass-mailing, network propagation, and email address collection routines. This means that a W32.Sobig.F@mm-infected computer will still attempt to download the updates from the respective list of master servers during the associated trigger period, even after the infection de-activation date. Previous variants of Sobig exhibited similar behavior.
  • Outbound udp traffic was observed on August 22nd, coming from systems infected with both Sobig.E and Sobig.F. However, the target IP addresses were either not responding, taken offline, or contained non-executable content; that is, a link to an adult site.
  • W32.Sobig.F@mm uses a technique known as "email spoofing," by which the worm randomly selects an address it finds on an infected computer. For more information on email spoofing, see the "Technical Details" section below.

Symantec Security Response has developed a removal tool to clean the infections of W32.Sobig.F@mm.

Due to the nature of the email spoofing, a substantial amount of extraneous traffic is generated as a result of virus notifications being sent to invalid email addresses. One solution to alleviate this problem would be to disable the Virus Notification messages that gateway and server-based mail products send.

Antivirus Protection Dates

  • Initial Rapid Release version August 19, 2003
  • Latest Rapid Release version July 03, 2018 revision 040
  • Initial Daily Certified version August 19, 2003
  • Latest Daily Certified version July 04, 2018 revision 002
  • Initial Weekly Certified release date August 19, 2003

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Technical Description

When W32.Sobig.F@mm is executed, it performs the following actions:

  1. Copies itself as %Windir%\winppr32.exe.

    NOTE: %Windir% is a variable. The worm locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.

  2. Creates the file, %Windir%\winstt32.dat.

  3. Adds the value:

    "TrayX"="%Windir%\winppr32.exe /sinc"

    to the registry key:


    so that the worm runs when you start Windows.

  4. Adds the value:

    "TrayX"="%Windir%\winppr32.exe /sinc"

    to the registry key:


    so that the worm runs when you start Windows.

  5. Enumerates any network shares to which the infected computer has write access. The worm uses standard Windows APIs to do this.

    NOTE: Due to a bug in the code, the worm does not copy over network shares.

Sobig.F can download arbitrary files to an infected computer and execute them. The author of the worm has used this functionality to steal confidential system information and to set up spam relay servers on infected computers.

This functionality may also be used as a worm self-update feature. Under the correct conditions, Sobig.F attempts to contact one of the list of master servers, which the author of the worm controls. Then, the worm retrieves a URL that it uses to determine where to get the Trojan file, downloads the Trojan file to the local computer, and then executes it.

In Sobig.F, the conditions for this download attempt are:
  • According to UTC time, the day of the week must be Friday or Sunday.
  • According to UTC time, the time of day must be between 7 P.M. and 10 P.M.

The following list contains the IP addresses correlating to the master servers:

Sobig.F obtains the UTC time through the NTP protocol, by contacting one of several possible servers on port 123/udp (the NTP port).

The worm starts the download attempt by sending a probe to port 8998/udp of the master server. Then, the server replies with a URL, where the worm can download the file to execute.

Unlike W32.Sobig.E@mm, Sobig.F will not open the following ports to listen for incoming UDP datagrams, as was previously reported.
  • 995/udp
  • 996/udp
  • 997/udp
  • 998/udp
  • 999/udp

Network administrators should do the following:
  • Block outbound traffic on port 8998/udp.
  • Monitor NTP requests (port 123/udp), as these could be coming from infected computers. (The frequency of such checks for an infected computer should be once per hour.)

Email spoofing
W32.Sobig.F@mm uses a technique known as "spoofing," by which the worm randomly selects an address it finds on an infected computer. The worm uses this address as the "From" address when it performs its mass-mailing routine. Numerous cases have been reported in which users of uninfected computers received complaints that they sent an infected message to another individual.

For example, Linda Anderson is using a computer infected with W32.Sobig.F@mm. Linda is neither using an antivirus program nor has the current virus definitions. When W32.Sobig.F@mm performs its email routine, it finds the email address of Harold Logan. The worm inserts Harold's email address into the "From" portion of an infected message, which it then sends to Janet Bishop. Then, Janet contacts Harold and complains that he sent her an infected message; however, when Harold scans his computer, Norton AntiVirus does not find anything, because his computer is not infected.

Norton Internet Security/Norton Internet Security Professional
On August 23, 2003, Symantec released updated IDS signatures via LiveUpdate.

Symantec Host IDS
On August 21, 2003, Symantec released an update for Symantec Host IDS 4.1.

Intruder Alert
On August 21, 2003, Symantec released Intruder Alert 3.6 W32_SobigF_Worm Policy .

Symantec ManHunt
Security Update 8 has been released to provide signatures specific to W32.Sobig.F.Worm.

Symantec Client Security
On August 22, 2003, Symantec released IDS signatures via LiveUpdate to detect W32.Sobig.F@mm activity.

Symantec Gateway Security
On August 22, 2003, Symantec released an update for Symantec Gateway Security 1.0.


Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

  • Use a firewall to block all incoming connections from the Internet to services that should not be publicly available. By default, you should deny all incoming connections and only allow services you explicitly want to offer to the outside world.
  • Enforce a password policy. Complex passwords make it difficult to crack password files on compromised computers. This helps to prevent or limit damage when a computer is compromised.
  • Ensure that programs and users of the computer use the lowest level of privileges necessary to complete a task. When prompted for a root or UAC password, ensure that the program asking for administration-level access is a legitimate application.
  • Disable AutoPlay to prevent the automatic launching of executable files on network and removable drives, and disconnect the drives when not required. If write access is not required, enable read-only mode if the option is available.
  • Turn off file sharing if not needed. If file sharing is required, use ACLs and password protection to limit access. Disable anonymous access to shared folders. Grant access only to user accounts with strong passwords to folders that must be shared.
  • Turn off and remove unnecessary services. By default, many operating systems install auxiliary services that are not critical. These services are avenues of attack. If they are removed, threats have less avenues of attack.
  • If a threat exploits one or more network services, disable, or block access to, those services until a patch is applied.
  • Always keep your patch levels up-to-date, especially on computers that host public services and are accessible through the firewall, such as HTTP, FTP, mail, and DNS services.
  • Configure your email server to block or remove email that contains file attachments that are commonly used to spread threats, such as .vbs, .bat, .exe, .pif and .scr files.
  • Isolate compromised computers quickly to prevent threats from spreading further. Perform a forensic analysis and restore the computers using trusted media.
  • Train employees not to open attachments unless they are expecting them. Also, do not execute software that is downloaded from the Internet unless it has been scanned for viruses. Simply visiting a compromised Web site can cause infection if certain browser vulnerabilities are not patched.
  • If Bluetooth is not required for mobile devices, it should be turned off. If you require its use, ensure that the device's visibility is set to "Hidden" so that it cannot be scanned by other Bluetooth devices. If device pairing must be used, ensure that all devices are set to "Unauthorized", requiring authorization for each connection request. Do not accept applications that are unsigned or sent from unknown sources.
  • For further information on the terms used in this document, please refer to the Security Response glossary.


Removal using the W32.Sobig.F@mm Removal Tool
Symantec Security Response has developed a removal tool to clean the infections of W32.Sobig.F@mm. This is the easiest way to remove this threat and should be tried first.

Manual Removal
As an alternative to using the removal tool, you can manually remove this threat.

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

NOTE: If you are on a network or have a full-time connection to the Internet, disconnect the computer from the network and the Internet. Remove this threat from all the computers on the network before reconnecting to it. Disable or password-protect file sharing before reconnecting the computers to the network or to the Internet. For instructions, see your Windows documentation, or the document, "How to configure shared Windows folders for maximum network protection ."

IMPORTANT: Do not skip this step. Disconnect the computer from the network before attempting to remove this worm.

  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Do one of the following:
    • Windows 95/98/Me: Restart the computer in Safe mode.
    • Windows NT/2000/XP: End the Trojan process.
  4. Run a full system scan and delete all the files detected as W32.Sobig.F@mm.
  5. Delete the value that was added to the registry.
For specific details on each of these steps, read the following instructions.

1. Disabling System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.

2. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

    The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. Restarting the computer in Safe mode or ending the Trojan process
    Windows 95/98/Me
    Restart the computer in Safe mode. All the Windows 32-bit operating systems, except for Windows NT, can be restarted in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."

    Windows NT/2000/XP
    To end the Trojan process:
    1. Press Ctrl+Alt+Delete once.
    2. Click Task Manager.
    3. Click the Processes tab.
    4. Double-click the Image Name column header to alphabetically sort the processes.
    5. Scroll through the list and look for Winppr32.exe.
    6. If you find the file, click it, and then click End Process.
    7. Exit the Task Manager.

4. Scanning for and deleting the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected as infected with W32.Sobig.F@mm, click Delete.

5. Deleting the value from the registry

WARNING: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified keys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
  1. Click Start, and then click Run. (The Run dialog box appears.)
  2. Type regedit

    Then click OK. (The Registry Editor opens.)

  3. Navigate to the key:


  4. In the right pane, delete the value:

    "TrayX"="%Windir%\winppr32.exe /sinc"

  5. Navigate to the key:


  6. In the right pane, delete the value:

    "TrayX"="%Windir%\winppr32.exe /sinc"

  7. Exit the Registry Editor.

Writeup By: Benjamin Nahorney