W97M.Suppl.A / W95.Suppl.16384.A

SUMMARY

W97M/W95.Suppl.16384.A is a worm that spreads a Microsoft Word 97 macro virus that hooks Winsock32 APIs, by replacing the Wsock32.dll system file. The payload overwrites the files with certain filename extensions, as described in detail in the "Technical Details" section.

The worm arrives as a MIME-encoded email/newsgroup attachment, named Suppl.doc.

Protection

• Initial Rapid Release version December 19, 2000
• Latest Rapid Release version December 19, 2000
• Initial Daily Certified version December 19, 2000
• Latest Daily Certified version December 19, 2000
• Initial Weekly Certified release date pending

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Threat Assessment

Wild

• Wild Level: Low
• Number of Infections: 0 - 49
• Number of Sites: 0 - 2
• Geographical Distribution: Low
• Threat Containment: Easy
• Removal: Easy

Damage

• Damage Level: Low

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

When W97M/W95.Suppl.16384.A is executed from within Microsoft Word, it performs the following actions:

1. The Worm checks whether Wsock33.dll exists in the %System% folder. If the file exists, it ends its infection routine.
   
   NOTE: %System% is a variable. The worm locates the System folder and copies itself to that location. By default, this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).
   
   
2. Copies the active document, which is usually the attachment, into %Windir% folder as Anthrax.ini.
   
   NOTE: %Windir% is a variable. The worm locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.
   
   
3. Accesses an LZ-packed .dll that is appended to the end of the Suppl.doc, saves it as Dll.lzh in the %Windir% folder, and then unpacks the file as Dll.tmp.
   
   
4. Creates a Winninit.ini file. This file is executed on the next Windows startup to delete the dropped Dll.lzh file, renames Wsock32.dll to Wsock33.dll in the %System% folder, and then renames the Dll.tmp to Wsock32.dll.
   
   
5. Intercepts two Winsock APIs, Send and Connect. All the other APIs are redirected to Wsock33.dll, which is the renamed Wsock32.dll file.
   
   
6. Every time a user sends an email from an infected system, the worm sends a second email with a MIME-encoded Suppl.doc attachment.
   
   
7. The payload is triggered after the worm has been active on the system for a week. The worm only executes its payload if the default language of the system is in English.
   
   
8. The payload searches non-removable drives for the files with the extensions .doc, .xls, .txt, rtf, .dbf, .zip, .arj, and .rar, overwriting them with a zero-byte file.
   
   NOTE: These files can be recovered if Norton Unerase is installed on the system before the worm's payload is activated.

Recommendations

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.

REMOVAL

Symantec antivirus products detect and repair the macro from the Suppl.doc as W97M.Suppl.A, but this file can safely be deleted. The worm's .dll, the bad Wsock32.dll, is detected as W95.Suppl.16384.A.

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

1. Disable System Restore (Windows Me/XP).
2. Update the virus definitions.
3. Restart the computer in Safe mode.
4. Run a full system scan and delete all the files detected as W97M.Suppl.A and W95.Suppl.16384.A.
5. Restore the Wsock32.dll file.

For specific details on each of these steps, read the following instructions.

1. Disabling System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:

• "How to disable or enable Windows Me System Restore"
• "How to turn off or turn on Windows XP System Restore"

For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder," Article ID: Q263455.

2. Updating the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:

• Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
• Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted on U.S. business days (Monday through Friday). You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).
  
  The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. Scanning for and deleting the infected files

1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
   • For Norton AntiVirus consumer products: Read the document, "How to configure Norton AntiVirus to scan all files."
   • For Symantec AntiVirus Enterprise products: Read the document, "How to verify that 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 W97M.Suppl.A or W95.Suppl.16384.A., click Delete.

4. Restarting the computer in Safe mode
Restart the computer in Safe mode. All the Windows 32-bit operating systems, except for Windows NT, can be restarted in Safe mode. For instructions on how to do this, read the document, "How to start the computer in Safe Mode."

5. Restoring the Wsock32.dll file

To restore the Wsock32.dll file, configure Windows to show the Hidden and System files, then restore Wsock33.dll to its original name, Wsock32.dll.

Configuring Windows to show all the files

1. Start Windows Explorer.
2. Click the View menu (Windows 95/98) or the Tools menu (Windows Me), and then click Options or Folder options.
3. Click the View tab.
4. Uncheck "Hide file extensions for known file types."
5. Do one of the following:
   • Windows 95/NT: Click "Show all files."
   • Windows 98: In the Advanced settings box, under the "Hidden files" folder, click Show all files.
   • Windows Me/2000: Uncheck "Hide protected operating system files," and under the "Hidden files" folder, click "Show hidden files and folders."
     
6. Click Apply, and then click OK.

Renaming Wsock33.dll file back to Wsock32.dll

Using Windows Explorer, navigate to the %System% folder and rename the Wsock33.dll file to Wsock32.dll.