W32.Opaserv.Worm is a network-aware worm that attempts to replicate across open network shares. It copies itself to the remote computer as a file named Scrsvr.exe. This worm also attempts to download updates from www.opasoft.com, although the site may have already been shut down. Indicators of infection include:

• The existence of the files Scrsin.dat and Scrsout.dat in the root of drive C. This indicates a
  local infection (that is, the worm was executed on the local computer).
• The existence of the Tmp.ini file in the root of drive C. This indicates a remote infection (that is,
  the computer was infected by a remote host).
• The registry key HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Run
  contains the string value ScrSvr or ScrSvrOld, which is set to c:\tmp.ini.

Wild:

• Number of infections: More than 1000
• Number of sites: More than 10
• Geographical distribution: High
• Threat containment: Easy
• Removal: Moderate

Distribution:

• Shared drives: Attempts to spread to non-password protected shares

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

It checks for the value

ScrSvrOld

in the registry key

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run

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

ScrSvr

exists in the registry key

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run

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

HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run

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 whether it is currently executing 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\Crsvr.exe.

The worm modifies C:\Windows\Win.ini by adding the line

run= c:\tmp.ini

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.

Recommendations

Symantec Security Response encourages all users and administrators to adhere to the following basic security "best practices":

• Turn off and remove unneeded services. By default, many operating systems install auxiliary services that are not critical, such as an FTP server, telnet, and a Web server. These services are avenues of attack. If they are removed, blended threats have less avenues of attack and you have fewer services to maintain through patch updates.
• If a blended 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.
• 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.
• Configure your email server to block or remove email that contains file attachments that are commonly used to spread viruses, such as .vbs, .bat, .exe, .pif and .scr files.
• Isolate infected computers quickly to prevent further compromising your organization. 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.

Removal

1. Update the virus definitions.
2. Run a full system scan, and delete all files that are detected as W32.Opaserv.Worm
3. Delete the values

ScrSvr %windir%\ScrSvr.exe

and

ScrSvrOld <original worm name>

from the registry key

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

4. (Windows 95/98/Me only ) Delete the line

run= c:\tmp.ini

from C:\Windows\Win.ini.

For details on how to do this, read the following instructions.

To update the virus definitions:
All virus definitions receive full quality assurance testing by Symantec Security Response before
being posted to our servers. There are two ways to obtain the most recent virus definitions:

• Run LiveUpdate, which is the easiest way to obtain virus definitions. These virus definitions are posted to the LiveUpdate servers one time each week (usually Wednesdays) unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, look at the Virus Definitions (LiveUpdate) line at the top of this write-up.
• Download the definitions using the Intelligent Updater. Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). They must be downloaded from the Symantec Security Response Web site and installed manually. To determine whether definitions for this threat are available by the Intelligent Updater, look at the Virus Definitions (Intelligent Updater) line at the top of this write-up.
  

  Intelligent Updater virus definitions are available here. For detailed instructions on how to
  download and install the Intelligent Updater virus definitions from the Symantec Security
  Response Web site, click here.

To scan for and delete the infected files:

1. Start your Symantec antivirus program, and make sure that it is configured to scan all files.
   
   • Norton AntiVirus consumer products: Read the document How to configure Norton
     AntiVirus to scan all files.
   • Symantec enterprise antivirus products: Read the document How to verify a
     Symantec Corporate antivirus product is set to scan All Files.
2. Run a full system scan.
3. If any files are detected as infected with W32.Opaserv.Worm, click Delete.

To remove the value that the worm added to the registry:

CAUTION: Symantec strongly recommends that you back up the registry before you make any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify only the keys that are specified. Read the document How to make a backup of the Windows registry for instructions.

1. Click Start, and click Run. The Run dialog box appears.
2. Type regedit and then click OK. The Registry Editor opens.
3. Navigate to the key

   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

4. In the right pane, delete these value:

ScrSvr %windir%\ScrSvr.exe
ScrSvrOld <original worm name>

5. Exit the Registry Editor.

To delete the line that the worm added to the Win.ini file:

This is necessary only on Windows 95/98/Me-based computers.

NOTE: (For Windows Me users only) Due to the file-protection process in Windows Me, a
backup copy of the file that you are about to edit exists in the C:\Windows\Recent folder.
Symantec recommends that you delete this file before you continue with the steps in this section.
To do this using Windows Explorer, go to C:\Windows\Recent, and in the right pane select the
Win.ini file and delete it. It will be regenerated as a copy of the file that you are about to edit when you save your changes to that file.

1. Click Start, and click Run.
2. Type the following, and then click OK.

edit c:\windows\win.ini
The MS-DOS Editor opens.

NOTE: If Windows is installed in a different location, make the appropriate path substitution.

3. In the [windows] section of the file, look for an entry similar to the following:

   run= c:\tmp.ini

4. Select the entire line. Be sure that you have not selected any other text in the file, and then
   press Delete.
5. Click File, and click Save.
6. Click File, and click Exit.