Discovered: November 21, 2003
Updated: February 13, 2007 12:14:09 PM
Also Known As: W32/Gaobot.worm.gen [McAfee], Backdoor.Agobot [Kaspersky], Phatbot
Type: Worm
Systems Affected: Windows

W32.HLLW.Gaobot.gen is a detection for a large family of worms, which propagate themselves using multiple vulnerabilities including:

Most variants are packed with a run-time packer, such as UPX.

The W32.HLLW.Gaobot.gen removal tool will remove many but not all the variants that are detected as W32.HLLW.Gaobot.gen.

Note: Virus definitions, version 60227t (extended version 2/27/2004 rev. 20) and later, detect the threat known as Phatbot as W32.HLLW.Gaobot.gen.

Antivirus Protection Dates

  • Initial Rapid Release version November 24, 2003
  • Latest Rapid Release version July 18, 2019 revision 001
  • Initial Daily Certified version November 24, 2003 revision 036
  • Latest Daily Certified version July 18, 2019 revision 007
  • Initial Weekly Certified release date November 26, 2003

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

Technical Description

A worm from the W32.HLLW.Gaobot.gen family generally does the following:

  1. Copies itself to the %System% folder. The file names vary and are often selected to resemble the names of legitimate Windows system files. Some examples include Csrrs.exe, Scvhost.exe, System.exe, explored.exe, or lms.exe.

    However, many other file names have been seen. The attacker can program the file names and the actions that the worm performs.

    Note: %System% is a variable. The worm 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. Adds a value in the form:

    "<key name>" = "<the filename of the worm>"

    for example:
    • "Configuration Loader" = "Service.exe"
    • "Windows Login" = "lms.exe"

      to the registry keys:
    • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
    • HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\

      so that the worm runs when you start Windows.

  3. May create a service for the worm and set it to automatically run on startup. To do this, the worm creates a registry key:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\<service name>

    Note: Typical values for <service name> are x4, a3, or MpR. However, the name can be configured to be any value.

  4. Connects to an IRC server, using its own IRC client, and then listens for commands to do any of the following:
    • Download and execute files
    • Steal system information
    • Send the worm to other IRC users
    • Add new accounts
    • Perform Denial of Service (DoS) attacks

  5. Attempts to spread to other computers using numerous vulnerabilities. These could include:
  6. Attempts to connect to network shares. To do this, it uses easily guessed user name/password combinations, including empty passwords.

    Some examples of this are:
    • admin$
    • c$
    • d$
    • e$
    • print$

      Read the W32.HLLW.Gaobot.AA writeup for a sample list of user names and passwords.

  7. Copies itself to any computers that it compromised using the previously mentioned exploits.

  8. Remotely schedules a task to run the worm on a newly infected computer.

  9. Queries the registry to steal the CD keys of various games.

  10. Terminates antivirus and firewall software, as well as the process names associated with other worms.

  11. Recent Gaobot variants may add entries to the %System%\drivers\etc\hosts file to disable access to certain antivirus Web sites.

    Refer to the W32.Gaobot.ZW writeup for a typical list of entries that may be added to the Hosts file.


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.


Before you begin:
If you are running Windows NT/2000/XP, make sure that you do, or have done, the following:

Removal using the Removal Tool
Symantec Security Response has developed a removal tool to clean the infections of W32.HLLW.Gaobot.gen. The removal tool will remove many but not all variants that are detected as W32.HLLW.Gaobot.gen.

If the removal tool cannot remove the variant that has infected your computer, follow the instructions in the next section.

Manual Removal
Perform a manual removal if you cannot obtain the tool, or if the tool does not remove the variant that has infected your computer.

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. Restart the computer in Safe mode or VGA mode.
  3. Restore the Hosts file.
  4. Update the virus definitions.
  5. Run a full system scan and delete all the files detected as W32.HLLW.Gaobot.gen.
  6. Reverse the changes that the worm made to the registry.
For details on each of these steps, read the following instructions.

Before you begin:
If you are running Windows NT/2000/XP, make sure that you do, or have done, the following:

1. To disable 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. To restart 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.

3. To restore the Windows Hosts file

Note: The location of the Hosts file may vary and some computers may not have this file. For example, if the file exists in Windows 98, it will usually be in C:\Windows; and it is located in the C:\WINNT\system32\drivers\etc folder in Windows 2000. There may also be multiple copies of this file in different locations.

Follow the instructions for your operating system:
  • Windows 95/98/Me/NT/2000
    1. Click Start, point to Find or Search, and then click Files or Folders.
    2. Make sure that "Look in" is set to (C:) and that "Include subfolders" is checked.
    3. In the "Named" or "Search for..." box, type:


    4. Click Find Now or Search Now.
    5. For each one that you find, note its location. (This is displayed in the "In Folder" column.)
    6. Right-click each file, and then click "Open With."
    7. Deselect the "Always use this program to open this program" check box.
    8. Scroll through the list of programs and double-click Notepad.
    9. When the file opens, delete all the entries in the Hosts file that begin with "," except for the following line:     localhost

    10. Close Notepad and save your changes when prompted.

  • Windows XP
    1. Click Start, and then click Search.
    2. Click All files and folders.
    3. In the "All or part of the file name" box, type:


    4. Verify that "Look in" is set to "Local Hard Drives" or to (C:).
    5. Click "More advanced options."
    6. Check "Search system folders."
    7. Check "Search subfolders."
    8. Click Search.
    9. Click Find Now or Search Now.
    10. For each one that you find, note its location. (This is displayed in the "In Folder" column.)
    11. Right-click each file, and then click "Open With."
    12. Deselect the "Always use this program to open this program" check box.
    13. Scroll through the list of programs and double-click Notepad.
    14. When the file opens, delete all the entries in the Hosts file that begin with "," except for the following line:     localhost

    15. Close Notepad and save your changes when prompted.

      Note: Some variants of this worm do not modify the hosts file.

4. 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.

5. 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 W32.HLLW.Gaobot.gen, note the path and file names, and then click Delete.

6. Reversing the changes made to 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 any values that refer to the worm files, which were detected in step 5.

  5. Navigate to the key:


    and repeat step d.

  6. Navigate to the key:


  7. Click "Edit," and then click "Find." Search for the value:


  8. If any such value is found, make sure it has the form:

    <key name> = "%System%\<the filename of the worm>" -service

    (where the file name of the worm is the same as the file in step 5.)

    For example:

    "Configuration Loader" = "C:\WINNT\SYSTEM32\Service.exe" -service or
    "ImagePath" = "C:\WINNT\System32\lms.exe" -service

    If this is the case, delete the subkey containing the value.

    For instance:


    Note: All the variants of the worm do not create this key.

    Because there could be more than one such subkey, repeat the search until no more -service values are found.

  9. Exit the Registry Editor.

Writeup By: Heather Shannon