Worm:Win32/Esbot.A is a network worm that targets computers running Microsoft Windows 2000 that do not have Microsoft Security Bulletin MS05-039 installed. The worm can also infect computers running other Windows operating systems if it is delivered through e-mail, instant messaging, or other routes. The worm has a backdoor component that connects to an IRC server to receive commands from attackers.

What to do now

To manually recover from infection by Worm:Win32/Esbot.A, follow these steps:
  1. Install Windows security update MS05-039.
  2. Disconnect from the Internet.
  3. Restart your computer in safe mode.
  4. Delete the registry key for the worm service.
  5. Delete the worm file.
  6. Restart your computer.
  7. Take steps to prevent re-infection.

Install Windows security update MS05-039

To install MS05-039 using Windows Update
  1. Go to the Windows Update Web site at
  2. On the Windows Update site, click Scan for Updates. Windows Update scans your computer and returns a list of critical updates, including service packs.
  3. In the Pick updates to install list, click Critical Updates and Service Packs. Windows Update creates a list of the updates appropriate for your computer, including MS05-039 if it is not installed. Critical updates are selected for download automatically.
  4. Click Review and install updates, and then click Install Now. You may need to restart your computer after installing the updates.

Disconnect from the Internet

To help ensure that your computer is not actively infecting other computers, disconnect it from the Internet before proceeding. Print this Web page or save a copy on your computer; then unplug your network cable and disable your wireless connection. You can reconnect to the Internet after completing these steps.

Restart your computer in safe mode

To start your the computer in safe mode
  1. Remove all floppy disks and CDs from your computer, and then restart your computer.
  2. When prompted, press F8. If Windows starts without displaying the Please select the operating system to start menu, restart your computer. Press F8 after the firmware POST process completes, but before Windows displays graphical output.
  3. From the Windows Advanced Options menu, select a safe mode option.

Delete the registry key for the worm service

Worm:Win32/Esbot.A creates a key in the Windows registry that registers the worm as a service. This entry should be deleted.
To delete the registry key for the worm service
  1. On the Start menu, click Run.
  2. Type regedit and click OK.
  3. In the left pane, navigate to the key:
  4. Right-click the key and click Delete.
  5. Click Yes to confirm the deletion.
  6. Close the Registry Editor.

Delete the worm file

To delete the worm file from your computer
  1. Click Start, and click Run.
  2. In the Open field, type the name of the Windows system folder, for example, C:\Winnt\System32
  3. Click OK.
  4. Click Name to sort files by name.
  5. If mousebm.exe is in the list, delete it.
  6. On the Desktop, right-click the Recycle Bin and click Empty Recycle Bin.
  7. Click Yes.
If deleting the file fails, follow these steps to verify that mousebm.exe is not running:
  1. Press CTRL+ALT+DEL once and click Task Manager.
  2. Click Processes and click Image Name to sort the running processes by name.
  3. Confirm that mousebm.exe is not in the list.

Restart your computer

To restart your computer
  1. On the Start menu, click Shut Down.
  2. Select Restart from the drop-down list and click OK.

Take steps to prevent re-infection

Do not reconnect your computer to the Internet until the computer is protected from re-infection. See the "Preventing Infection" section for more information.

Threat behavior

When Worm:Win32/Esbot.A runs, it takes the following actions:
  • Copies itself as a new file to <system folder>\mousebm.exe. 
  • Creates registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MouseBM
    This registers the worm as a service named mousebm. This service has display name "Mouse Button Monitor" and executable file name <system folder>\mousebm.exe.
  • Exits after starting worm service mousebm.exe. The service checks for the presence of a mutex named mousebm. If the mutex exists, the worm process mousebm.exe exits. If the mutex does not exist, the process takes the following actions:
    • Creates mutex mousebm to prevent multiple instances of the worm from running on the same computer.
    • Places data: N
      in value: EnableDCOM
      of registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\OLE
    • Places data: 1
      in value: restrictanonymous
      of registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
    • Injects its code into the explorer.exe process space and runs the injected code.
    • Scans randomly-selected IP addresses to exploit the Windows Plug-and-Play vulnerability described in Microsoft Security Bulletin MS05-039. The worm may begin scanning with a random address or an address based on the IP address of the infected computer; it then scans IP addresses consecutively.
    • Connects to a specific IRC server and channel to enable attackers to perform functions such as the following:
      • Download files.  
      • Run commands or executable files.
      • Collect information such as system information and file data.


There are no readily apparent indications that your computer is infected with Backdoor:Win32/Esbot.A. However, the following symptoms may indicate that your computer is infected by this Trojan:
  • Presence of the file <system folder>\mousebm.exe.
  • The Processes tab in Windows Task Manager shows that mousebm.exe is running.
  • The executable file mousebm.exe is running as a service with display name "Mouse Button Monitor."


Follow these steps to help prevent infection on your system:
  • Enable a firewall on your computer.
  • Get the latest computer updates.
  • Use up-to-date antivirus software.

Enable a firewall on your computer

Use a third-party firewall product or turn on the Microsoft Windows XP Internet Connection Firewall.
To turn on the Internet Connection Firewall in Windows XP
  1. Click Start, and click Control Panel.
  2. Click Network and Internet Connections, and click Network Connections. If you do not see Network and Internet Connections, click Switch to Category View.
  3. Highlight a connection that you want to help protect, and click Change settings of this connection.
  4. Click Advanced, and select Protect my computer and network by limiting or preventing access to this computer from the Internet.
  5. Click OK.

Get the latest computer updates

Updates help protect your computer from viruses, worms, and other threats as they are discovered. You can use the Automatic Updates feature in Microsoft Windows XP to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
To turn on Automatic Updates in Windows XP
  1. Click Start, and click Control Panel.
  2. Click Performance and Maintenance. If you do not see Performance and Maintenance, click Switch to Category View.
  3. Click System.
  4. Click Automatic Updates, and select Keep my computer up to date.
  5. Select a setting. Microsoft recommends selecting Automatically download the updates, and install them on the schedule that I specify and setting a regular update time.
  6. If you choose to have Automatic Updates notify you in step 5, you will see a notification balloon when new downloads are available to install. Click the notification balloon to review and install updates.

Use up-to-date antivirus software

Most antivirus software can detect and prevent infection by known malicious software. You should always run antivirus software on your computer that is updated with the latest signature files to automatically help protect you from infection. If you don't have antivirus software installed, you can get it from one of several companies. For more information, see  

Alert level: Severe
This entry was first published on: Aug 15, 2005
This entry was updated on: Sep 19, 2006

This threat is also detected as:
  • W32/IRCbot.gen (McAfee)
  • Esbot.A (Symantec)
  • BKDR_RBOT.BD (Trend Micro)
  • W32/Backdoor.EUR (F-secure)
  • Esbot.A (Sophos)
  • Win32.Esbot.A (CA)