1. Symantec/
  2. Security Response/
  3. W32.Opaserv.Worm


Risk Level 2: Low

September 30, 2002
February 13, 2007 11:40:37 AM
Also Known As:
W32/Opaserv.worm [McAfee], W32/Opaserv-A [Sophos], Win32.Opaserv [CA], WORM_OPASOFT.A [Trend], Worm.Win32.Opasoft [AVP]
Systems Affected:
CVE References:

When W32.Opaserv.Worm runs, it does the following:

It checks for the value


in the registry key


If the value exists, the worm deletes the file that the ScrSvrOld points to.

If the ScrSvrOld value does not exist, then the worm determines whether the value


exists in the registry key


If the value does not exist, the worm adds the value

ScrSvr %windir%\ScrSvr.exe

to that registry key.

Next it checks whether it is being run as the file %windir%\ScrSvr.exe. If it is not, it copies itself to that file name and adds the value

ScrSvrOld  <original worm name>

to the registry key


NOTE: %windir% is a variable. The worm locates the Windows installation folder (by default this is C:\Windows or C:\Winnt) and uses that as a destination folder.

After the worm checks the registry values and the location from which the worm is executing, the worm checks to make sure that only one instance of the worm is running in memory by creating a mutex with the name ScrSvr31415.

If it is not already executing, the worm registers itself as a process under Windows 95/98/Me. Under Windows NT/2000/XP it elevates the priority of the worm process.

The worm then inventories the network looking for "C\" shares. For each share that it finds, it copies itself to C\Windows\Scrsvr.exe.

The worm uses a security vulnerability in Microsoft Windows 95/98/Me. It sends single character passwords to network shares to get access to Windows 95/98/Me file shares without knowing the entire password assigned to the shares. The affected systems include,
  • Microsoft Windows 95
  • Microsoft Windows 98
  • Microsoft Windows 98 Second Edition
  • Microsoft Windows Me

A patch for computers running these operating systems can be found at http://www.microsoft.com/technet/security/bulletin/MS00-072.asp.

So that Windows 95/98/Me computers will run the worm each time that you start Windows, the worm modifies the[windows]section of the C:\Windows\Win.ini file by adding the line

run= c:\ScrSvr.exe

  • The worm modifies the file C:\Windows\Win.ini before it copies itself as %windir%\ScrSvr.exe. Therefore, Symantec antivirus products will find and delete %windir%\ScrSvr.exe after the system has been altered, but not before it modifies the Win.ini file. As a result, when you restart the computer, you may see a message that ScrSvr.exe cannot be found. To fix this, remove the line that the worm added.
  • The worm is apparently coded to add this line to the Win.ini:

    run= c:\tmp.ini

    However, in actual infections or detections, the worm is adding the line run= c:\ScrSvr.exe.

It also creates C:\Tmp.ini, which contains the text

run= c:\windows\scrsvr.exe

The worm also appears to be able to update itself by reading files from a Web site whose URL is hardcoded within the worm. It also attempts to download an update named Scrupd.exe.


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: Douglas Knowles
Summary| Technical Details| Removal

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