Worm:Win32/Esbot is a family of network worms that targets Microsoft Windows 2000 computers by exploiting the Windows Plug-and-Play buffer overflow vulnerability that is fixed with Microsoft Security Bulletin MS05-039. 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 detect and remove this threat and other malicious software that may be installed on your computer, run a full-system scan with an appropriate, up-to-date, security solution. The following Microsoft products will detect and remove this threat:
For more information on antivirus software, see

Threat behavior

Win32/Esbot takes the following actions:
  • Copies itself as a new file to the Windows system folder. 
  • Uses the Service Control Manager to register itself as a service that has a misleading display name.
  • Starts the worm service, if possible. If the service starts, the original worm process exits after starting a new explorer.exe process to delete the original worm file. In this case, the service performs the operations that follow. If the service does not start, the original worm process performs the operations. 
  • Checks for the presence of a particular mutex. If the mutex exists, the service or original worm process exits so that multiple instances of the worm are not running concurrently on the same computer. If the mutex does not exist, the service or original worm process takes the following actions:
    • Creates a mutex to prevent multiple instances of the worm from running on the same computer.
    • Places data in the EnableDCOM value of registry key: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\OLE
    • Places data in the restrictanonymous value 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 random 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.


Win32/Esbot symptoms differ according to the variant. There may be no readily apparent indications that your computer is infected.


Take the following steps to help prevent infection on your computer:
  • Enable a firewall on your computer.
  • Get the latest computer updates for all your installed software.
  • Use up-to-date antivirus software.
  • Limit user privileges on the computer.
  • Use caution when opening attachments and accepting file transfers.
  • Use caution when clicking on links to webpages.
  • Avoid downloading pirated software.
  • Protect yourself against social engineering attacks.
  • Use strong passwords.
Enable a firewall on your computer
Use a third-party firewall product or turn on the Microsoft Windows Internet Connection Firewall.
Get the latest computer updates
Updates help protect your computer from viruses, worms, and other threats as they are discovered. It is important to install updates for all the software that is installed in your computer. These are usually available from vendor websites.
You can use the Automatic Updates feature in Windows to automatically download future Microsoft security updates while your computer is on and connected to the Internet.
Use up-to-date antivirus software
Most antivirus software can detect and prevent infection by known malicious software. To help protect you from infection, you should always run antivirus software, such as Microsoft Security Essentials, that is updated with the latest signature files. For more information, see
Limit user privileges on the computer
Starting with Windows Vista and Windows 7, Microsoft introduced User Account Control (UAC), which, when enabled, allowed users to run with least user privileges. This scenario limits the possibility of attacks by malware and other threats that require administrative privileges to run.
You can configure UAC in your computer to meet your preferences:
Use caution when opening attachments and accepting file transfers
Exercise caution with email and attachments received from unknown sources, or received unexpectedly from known sources. Use extreme caution when accepting file transfers from known or unknown sources.
Use caution when clicking on links to webpages
Exercise caution with links to webpages that you receive from unknown sources, especially if the links are to a webpage that you are not familiar with, unsure of the destination of, or suspicious of. Malicious software may be installed in your computer simply by visiting a webpage with harmful content.
Avoid downloading pirated software
Threats may also be bundled with software and files that are available for download on various torrent sites. Downloading "cracked" or "pirated" software from these sites carries not only the risk of being infected with malware, but is also illegal. For more information, see 'The risks of obtaining and using pirated software'.
Protect yourself from social engineering attacks
While attackers may attempt to exploit vulnerabilities in hardware or software to compromise a computer, they also attempt to exploit vulnerabilities in human behavior to do the same. When an attacker attempts to take advantage of human behavior to persuade the affected user to perform an action of the attacker's choice, it is known as 'social engineering'. Essentially, social engineering is an attack against the human interface of the targeted computer. For more information, see 'What is social engineering?'.
Use strong passwords
Attackers may try to gain access to your Windows account by guessing your password. It is therefore important that you use a strong password – one that cannot be easily guessed by an attacker. A strong password is one that has at least eight characters, and combines letters, numbers, and symbols. For more information, see

Alert level: Severe
This entry was first published on: Aug 19, 2005
This entry was updated on: Apr 17, 2011

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