Win32/Opaserv is a family of network worms that targets computers running certain versions of Microsoft Windows. The worm spreads through network shares using weak passwords or by exploiting the Windows vulnerability described in Microsoft Security Bulletin MS00-072. The worm can connect to a specified Web site to update itself. A Trojan dropped by one or more Opaserv variants performs operations that can prevent a computer from restarting.

Threat behavior

Win32/Opaserv primarily targets computers running Microsoft Windows 95, Microsoft Windows 98, or Microsoft Windows ME. The worm modifies files and registry keys in order to run each time Windows starts. 
The worm spreads by exploiting the Windows vulnerability described in Microsoft Security Bulletin MS00-072. This vulnerability allows access to password-protected network shares using only the first character of the password. The worm first scans through a UDP port at various IP addresses. If the worm receives a reply, it can connect to drive C through another UDP port if the drive is shared and the computer does not have MS00-072 installed. The worm copies itself to C:\Windows after it connects to the remote computer. It then modifies the win.ini file there so that the worm runs each time Windows starts. 
Win32/Opaserv can send infection notifications to attackers or update itself using HTTP. The worm creates temporary .dat files for scripts that it downloads to the infected computer from the Web site. 
Under certain conditions, one or more Win32/Opaserv variants may drop a Trojan that can cause the computer to restart. The Trojan can also delete or overwrite critical system files or portions of the hard drive, which can prevent the computer from restarting. The worm may also display a fake warning to the effect that an illegal Microsoft Windows license has been detected.


There are no readily apparent indications that your computer is infected with Win32/Opaserv. However your computer may be infected if you notice high network traffic at UDP port 137 or 139. One or more variants may cause the computer to restart, perform operations that prevent the computer from restarting, or display a fake message that begins:
Illegal Microsoft Windows license detected!
You are in violation of the Digital Millennium Copyright Act!
Your unauthorized license has been revoked.
For more information, please call us at:


Take the following 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.
  • Use strong passwords.
  • Remove unneeded network shares.

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

Use strong passwords

A strong password has at least eight characters and includes a combination of letters, numbers, and symbols. It is easy for you to remember, but difficult for others to guess. Weak passwords include any words in the dictionary, names, dates, consecutive letters or numbers, common words with symbol substitutions (for example, p@ssw0rd), and so on.

Remove unneeded network shares

Malicious software can often spread over network shares. Remove unneeded network shares that are mapped to your computer.
To remove network shares in Windows XP
  1. On the Start menu, click My Computer.
  2. On the Tools menu, click Disconnect Network Drives…
  3. In the Disconnect Network Drives dialog box, click the drives to disconnect and click OK.

Alert level: Severe
This entry was first published on: May 11, 2005
This entry was updated on: Sep 19, 2006

This threat is also detected as:
  • W32/Opaserv.worm (McAfee)
  • WORM_OPASERV.GEN (Trend Micro)
  • W32.Opaserv.Worm (Symantec)
  • Win32.Opaserv (CA)
  • Opaserv (F-secure)