Backdoor.NetTrojan

SUMMARY

Backdoor.NetTrojan is a Backdoor Trojan Horse that allows unauthorized use of an infected computer. Backdoor.NetTrojan allows its creator to configure unauthorized access, as well as the filename and the port in use. This Trojan may also be referred to as the Distributed Trojan Horse Network (DHTN).

NOTE: Definitions dated prior to December 24, 2002 may detect this threat as Backdoor.Trojan.

Behavior

Symptoms

Transmission

Protection

• Virus Definitions (LiveUpdate™ Weekly) December 24, 2002
• Virus Definitions (Intelligent Updater) December 24, 2002

Threat Assessment

Wild

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

Damage

• Damage Level: Medium
• Releases Confidential Info: Allows unauthorized use of affected computers.
• Degrades Performance: May utilize infected computers to perform Denial of Service attacks.

Distribution

• Distribution Level: Low
• Ports: configurable, default is to use a randomly selected port.

TECHNICAL DETAILS

Backdoor.NetTrojan can be configured in many different ways. If the Trojan is set to use its default settings, it does the following:

1. Copies itself as %Windir%Unwise.exe. It also deletes the files from that particular folder.
   
   NOTE: %Windir% is a variable. The virus locates the Windows installation folder (by default, this is C:Windows or C:Winnt) and copies itself to that location. It deletes the files from this folder as well.
   
   
2. Adds the registry value:
   
   WinLoader %windir%UNWISE.EXE
   
   to the following registry keys:
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindows
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunOnce
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunServices
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunServicesOnce
   HKEY_CURRENT_USER.DEFAULTSoftwareMicrosoftWindowsCurrentVersionRun
   HKEY_CURRENT_USER.DEFAULTSoftwareMicrosoftWindowsCurrentVersionRunServices
   HKEY_CURRENT_USER.DEFAULTSoftwareMicrosoftWindowsCurrentVersionRunOnce
   
   
3. Modifies the (Default) values in the following registry keys:
   
   HKEY_LOCAL_MACHINESoftwareCLASSES xtfileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSESexefileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSESatfileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSEScomfileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSESpiffileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSESgiffileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSEShtmlfileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSESjpegfileshellopencommand
   HKEY_LOCAL_MACHINESoftwareCLASSESWord.Document.8shellopencommand
   
   These modifications cause the Trojan to execute when any of the associated file types is launched. The original (Default) value determines the modification to each key. The Trojan saves the original default value under the same key with the value "winampold."
   
   
4. Modifies the shell= line in the System.ini file to:
   
   shell = Explorer.exe %windir%UNWISE.EXE
   
   
5. The Trojan has many plug-ins that perform different functions. Some of these functions may include:
   • Performing a Denial of Service (DoS) attack
   • Sending IRC messages
   • Spreading itself through email
   • Spreading to network drives

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 (for example, all Windows-based computers should have the current Service Pack installed.). Additionally, please apply any security updates that are mentioned in this writeup, in trusted Security Bulletins, or on vendor Web sites.
• 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

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

1. Reverse the changes made to the registry.
2. For Windows 95/98/Me only: Reverse the change made to the Win.ini file.
3. Update the virus definitions.
4. Run a full system scan and delete all the files detected as Backdoor.NetTrojan.

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

1. Reversing changes made to the registry

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

1. Click Start, and then click Run. (The Run dialog box appears.)
2. Type regedit, and then click OK. (The Registry Editor opens.)
3. Navigate to each of these keys:
   
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindows
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunOnce
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunServices
   HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRunServicesOnce
   HKEY_CURRENT_USER.DEFAULTSoftwareMicrosoftWindowsCurrentVersionRun
   HKEY_CURRENT_USER.DEFAULTSoftwareMicrosoftWindowsCurrentVersionRunServices
   HKEY_CURRENT_USER.DEFAULTSoftwareMicrosoftWindowsCurrentVersionRunOnce
   
   NOTE: Some of the keys may not exist on your system.
   
   
4. In the right pane, delete the value:
   
   WinLoader %windir%UNWISE.EXE
   
   NOTE: This value may vary. Look for any value that refers to the files detected as Backdoor.NetTrojan.
   
   
5. Navigate to the following key:
   
   HKEY_LOCAL_MACHINESoftwareCLASSES xtfileshellopencommand
   
   
6. In the right pane, double-click the value:
   
   winampold
   
   
7. Copy the contents of "Value data:", then click OK.
   
8. In the right pane, double-click the value:
   
   (Default)
   
   
9. Paste the text from "winampold" into the "Value data:" text box.
   
10. Repeat steps f through i for each of the following keys:
    
    HKEY_LOCAL_MACHINESoftwareCLASSESexefileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSESatfileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSEScomfileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSESpiffileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSESgiffileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSEShtmlfileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSESjpegfileshellopencommand
    HKEY_LOCAL_MACHINESoftwareCLASSESWord.Document.8shellopencommand
    
    
11. Exit the Registry Editor.
    
12. Reboot the computer.
    
    NOTE: Windows 95/98/Me users should continue with section 2 before rebooting the computer.

2. For Windows 95/98/Me only: Reversing the change made to the Win.ini file

NOTE: If you are not running Windows 95, Windows 98, or Windows Me, skip to section 3.

1. The function you perform first depends on your operating system:
   • Windows 95/98: Go to step b.
   • Windows Me: If you are running Windows Me, the Windows Me file-protection process may have made a backup copy of the Win.ini file that you are to edit. If this backup copy exists, it will be in the C:WindowsRecent folder. Symantec recommends that you delete this file before continuing with the steps in this section. To do this:
     • Start Windows Explorer.
     • Browse to and select the C:WindowsRecent folder.
     • In the right pane, select the Win.ini file and delete it. It will be regenerated when you save your changes to the Win.ini file in step f.
       
       
2. Click Start, and then click Run.
   
3. Type the following, and then click OK.
   
   edit c:windowssystem.ini
   
   (The MS-DOS Editor opens.)
   
   NOTE: If Windows is installed in a different location, make the appropriate path substitution.
   
   
4. In the [boot] section of the file, look for a line similar to the following:
   
   shell = Explorer.exe %windir%UNWISE.EXE
   
   
5. If it exists, delete everything to the right of Explorer.exe. When you are done, it should look like:
   
   shell = Explorer.exe
   
   
6. Click File, and then click Save.
   
7. Click File, and then click Exit.
   
8. Reboot the computer.

3. 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.

4. 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 Backdoor.NetTrojan, click Delete.