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Discovered: March 25, 2004
Updated: February 13, 2007 12:16:55 PM
Type: Worm
Systems Affected: Windows

W32.Expobot.Worm is a Trojan horse that steals system and user information. The Trojan also has the ability to spread through network shares, when instructed.

Antivirus Protection Dates

  • Initial Rapid Release version March 26, 2004
  • Latest Rapid Release version September 07, 2017 revision 022
  • Initial Daily Certified version March 26, 2004
  • Latest Daily Certified version September 08, 2017 revision 001
  • Initial Weekly Certified release date March 26, 2004

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

Writeup By: Tony Lee

Discovered: March 25, 2004
Updated: February 13, 2007 12:16:55 PM
Type: Worm
Systems Affected: Windows

When W32.Expobot.Worm is executed, it performs the following actions:

  1. Copies itself to the following file:


    Note: %System% is a variable. The Trojan locates the System folder and copies itself to that location. By default, this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).

  2. Creates the following Browser Helper Object file:


  3. Deletes all the subkeys in the registry key:

    Explorer\Browser Helper Objects

  4. Creates a subkey:

    Browser Helper Objects\{88C20E0E-A302-11D7-AFFC-00D009EFCB77}

  5. Adds the values:
    • "Default" = "%System%\ntdll38.dll"
    • "ThreadingModel" = "Apartment"  

      in the registry key:


  6. Adds the value:

    "Shelldll" = "ntdll38.dll"

    to the registry key:


  7. Executes Explorer.exe, which executes %System%\ntdll38.dll.

  8. Creates the mutex "ONLYONETIMEFOREXPLOREPEXEDELL," which ensures that only one instance of Explorer.exe is loaded with the .dll file.

  9. Searches through all available drives on the local system, and copies files to the %System%\cache folder as files with the following extensions:
    • .doc
    • .txt
    • .ppt
    • .xls
    • .mdb
    • .zip
    • .rar
    • .eml
    • .njx
    • .pgp

  10. Continuously attempts to connect to on TCP port 80.

    Once a connection is established, an attacker can do the following:
    • Steal system information (client version, computer name, user name, OS type)
    • Upload files to the remote server.
    • Collect information on each shared resource and its file system, and send it to the remote server.
    • Enumerate all the network resources, then do the following to each resource:
      1. Creates the Microsoft Visual Basic file R:\Recycled.sys
      2. Attempts to copy itself to %System%\~temp~.exe, and executes the executable file
      3. Replaces R:\Recycled.dll with %System%\Wshell32.dll and set the hidden, read only, and system file attributes.
      4. Creates R:\Desktop.ini and set the file to be a hidden read-only system file. The .ini file configures how explorer displays folders, which sets the default template for an Explorer window to Recycled.sys.

        Note: The Trojan does not enumerate network resources with name R:. This is the drive letter the Trojan uses as local representation of the remote resource.

    • Download a file from a location instructed by the remote server.

      The worm then sets the following registry value:

      "Shelldll" = "<downloaded file name>"

      in the following registry key,


    • Collect information pertaining to files in the following locations:
      • %System%\cache
      • %Personal%
      • %Desktop%

        Note: %Personal% and %Desktop% are paths obtained in the following registry values:
      • Personal
      • Desktop

        in the registry key

        Explorer\Shell Folder

  11. Creates a thread that does the following:
    • Steals user information, such as SMTP server, POP3 server, and email account name, and logs this information in %System%Hpmon128.dll.
    • Logs keystrokes and Window messages in %System%Hpmon128.dll.
    • Continuously monitors the file size of Hpmon128.dll. Once it is over 1 KB, the worm copies Hpmon128.dll to %System%\Hpmon256.dll, and empties Hpmon128.dll. The worm then sends %System%\Hpmon256.dll to a predetermined email address.


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.

Writeup By: Tony Lee

Discovered: March 25, 2004
Updated: February 13, 2007 12:16:55 PM
Type: Worm
Systems Affected: Windows

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

  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Restart the computer in Safe mode or VGA mode.
  4. Run a full system scan, and delete all the files detected as W32.Expobot.Worm.
  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:
Note: When you are completely finished with the removal procedure and are satisfied that the threat has been removed, re-enable System Restore by following the instructions in the aforementioned documents.

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).

    Note: If this Trojan is running, you may not be able to run LiveUpdate. If you cannot, download the definitions using the Intelligent Updater (see the next bulleted item).

  • 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 VGA mode
Shut down the computer and turn off the power. Wait for at least 30 seconds, and then restart the computer in Safe mode or VGA mode.
  • For Windows 95, 98, Me, 2000, or XP users, restart the computer in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode."
  • For Windows NT 4 users, restart the computer in VGA mode.

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.Expobot.Worm, write down the location and file name, and then 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:

    Explorer\Browser Helper Objects

  4. delete the subkey


  5. Navigate to the key:


  6. delete the subkey


  7. Exit the Registry Editor.

Writeup By: Tony Lee