Backdoor.SubSeven22

Backdoor.SubSeven22 is a Trojan horse. It is generally UPX packed; however, unpacked versions and versions packed with different executable packing software do exist. When packed with UPX, the Trojan horse's size is usually between 370 KB and 390 KB. Larger variations have been encountered, however, with file sizes ranging from 470 KB to 550 KB .

This Trojan horse does not produce any output while running.

CAUTION: Due to evolving packing technology and possible modifications to the Trojan by hackers, new variations of the Trojan may be encountered. If Symantec antivirus products does not detect the encountered variation, then you should submit the possible Trojan samples to Symantec Security Response for further examination.

Type:	Trojan Horse
Systems Affected:	Windows 95, Windows 98, Windows NT, Windows 2000, Windows XP, Windows Me
Systems Not Affected:	Windows 3.x, Macintosh, OS/2, UNIX, Linux

Virus Definitions (LiveUpdate™ Weekly) April 05, 2000 Virus Definitions (Intelligent Updater) April 05, 2000

Wild Number of infections: More than 1000 Number of sites: More than 10 Geographical distribution: Medium Threat containment: Moderate Removal: Moderate

Threat Metrics Wild: Low Damage: Medium Distribution: Medium

Damage

• Payload:
  • Modifies files: system files and registry.
  • Compromises security settings: Allows malicious users access to the infected system.

Distribution

• Ports: Any port (chosen randomly, in most cases).
• Target of infection: Microsoft Windows 9x and NT computers.

When executed, the Trojan horse drops a copy of itself into the \Windows folder. The copy is usually named with random alphanumeric characters, although the name is sometimes fixed (for example, Rundll16.exe). The dropped copy provides potentially malicious users with access to the infected computer.

The Trojan horse sometimes drops an application into the \Windows or \Windows\System folders. This application file can be an .exe or a .dll, and it is between 10 KB and 15 KB in size.

The .exe variation of this application is generally named using a random set of alphanumeric characters. The file name of the .dll variation is fixed, for example, Watching.dll.

The application serves as an executable file launcher, similar to Explorer.exe. While launching an executable, this application may also launch the Trojan. To force the executable files in the system to be launched only by this application, the Trojan modifies specific locations in the Windows registry. If the launcher is the .exe application, it inserts the application's name into one of the values, usually [Default], in the HKEY_CLASSES_ROOT\exefile\shell\open\command registry key. If the launcher is the .dll file type, it creates the HKEY_CLASSES_ROOT\.dl registry key, and sets the [Default] value to .exe file, and sets the content value to application/x-msdownload.

The Trojan horse generally creates or uses existing HKEY_LOCAL_MACHINE\Hardware\Data, HKEY_LOCAL_MACHINE\Hardware\Enum or HKEY_LOCAL_MACHINE\Software\Microsoft\DirectXMedia registry keys to store data. It creates randomly named variables and stores data in them. The variable names and the data usually appear as junk.

The Trojan horse uses any combination of the following methods to enable itself at startup:

• It adds the name of the dropped copy to the "load=" and/or the "run=" lines of the Win.ini file.
• It adds the name of the dropped copy to the "shell=explorer.exe" line of the System.ini file.
• It creates some value (usually "WinLoader") in the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Run" and/or HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\RunServices registry keys, and then sets that value to the name of its dropped copy.

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.

This Trojan can be difficult to remove because it does not make the same changes to all systems. To remove this Trojan, you need to:

• Remove any references to the Trojan from the following registry keys:
  HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Run
  HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\RunServices
• Delete every occurrence of the Trojan's name in the Win.ini and System.ini files.
• Shut down and turn off the computer to remove the Trojan from memory, or use a utility capable of stopping the process, such as Dllview.exe.
• After restarting, delete all instances of the Trojan.
• Examine the following registry keys:
  HKEY_CLASSES_ROOT\exefile\shell\open\command
  HKEY_CLASSES_ROOT\.dl
  Remove any value containing a reference to the name of the .exe launcher that the Trojan dropped, or delete the HKEY_CLASSES_ROOT\.dl registry key if it exists.
• Delete all randomly named variables from the following keys:
  HKEY_LOCAL_MACHINE\Hardware\Data
  HKEY_LOCAL_MACHINE\Hardware\Enum
  HKEY_LOCAL_MACHINE\Software\Microsoft\DirectXMedia
• Run a full system scan.

1. Click Start, and click Run. The Run dialog box appears.
2. Type regedit and then click OK. The Registry Editor opens.
   
   NOTE: If the Registry Editor does not open, and you are sure that you have typed the command as shown, the Trojan has modified the registry so that .exe files will not run. In this case, first make a copy of the file Regedit.exe as Regedit.com, and then use the copy to edit the registry. For instructions on how to do this, read the section To copy Regedit.exe to Regedit.com at the very end of these removal instructions.
   
3. Browse to and select the following subkey:
   
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Run
   
4. In the right pane, look for values that do not refer to programs that should run at startup. In most cases, this value will be Winloader.
5. Write down the name of the file that the value is loading, for example:
   
   Winloader C:\Windows\Pl9k09t.exe
   
6. Delete the suspect value.
   
   NOTE: This does not delete the file itself; it only prevents the file from loading when Windows starts. If you remove a value that loads a program that you want to start when Windows starts, you will need to re-enable the loading of the program.
   
7. Browse to and select the following subkey:
   
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\RunServices
   
8. Repeats steps 4 through 6.
9. Exit the Registry Editor.

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.
   
   CAUTION: The steps that follow instruct you to remove text from the load= and run= lines of the Win.ini file. If you are using older programs, they may be loading at startup from one of these lines. The Trojan adds lines such as run=<random file name>. If you are sure that the text contained in these lines are for programs that you normally use, we suggest you do not remove them. If you are not sure, you can prevent the lines from loading by placing a semicolon in front of the line, for example:
   
   ; run=accounts.exe
   
3. Locate the load= line within the [windows] section of the Win.ini file; it is usually located near the top of the file.
4. Position the cursor immediately to the right of the equal sign.
5. Press Shift+End to select all of the text to the right of the equal sign, and then press Delete.
6. Repeat steps 5 and 6 for the run= line, which is usually beneath the load= line.
7. Click File, click Exit, and then click Yes when you are prompted to save the changes.
8. Click Start, and click Run.
9. Type the following, and then click OK:
   
   edit c:\windows\system.ini
   
10. Locate the line that begins with shell=
11. Position the cursor immediately to the right of the equal sign.
12. Press Shift+End to select all of the text to the right of the equal sign, and then press Delete.
13. Type the following text:
    
    explorer.exe
    
    The line should now read
    
    shell=explorer.exe
    
    NOTE: Some computers may have an entry other than Explorer.exe after shell=. If this is the case and you are running an alternative Windows shell, then change this line to shell=explorer.exe for now. You can change it back to your preferred shell after you have finished this procedure.
    
14. Click File, click Exit, and then click Yes when you are prompted to save the changes.
15. Exit the System Configuration Editor.

1. Click Start, point to Find, and click Files or Folders.
2. Make sure that "Look in" is set to (C:) and that "Include subfolders" is checked.
3. In the Named box, type the file names that you removed from Win.ini, System.ini, or the registry.
4. Click Find Now. Windows will find the files and display them in the lower pane of the Find dialog box.
5. Delete the displayed files.
6. Right-click the Recycle Bin icon on your desktop, and then click Empty Recycle Bin.
7. Close the Find Files window.

1. Click Start, and click Run. The Run dialog box appears.
2. Type regedit and click OK. The Registry Editor opens.
3. Look for the following subkey:
   
   HKEY_CLASSES_ROOT\.d
   
4. If the key exists, delete it.
5. Navigate to and open the following key:
   
   HKEY_CLASSES_ROOT\exefile\shell\open\command
   
   
   CAUTION: Do not inadvertently modify the HKEY_CLASSES_ROOT\.exe subkey. Changes made to that key can prevent .exe files (program files) from running. Be sure to navigate past this all the way down to the HKEY_CLASSES_ROOT\exefile\shell\open\command subkey as shown in the following figure.
   
   
   <- This is the subkey that you will edit.
   
   
6. Double-click the (Default) value in the right pane.
7. Delete the current value data, and then type: "%1" %* (That is, type the following characters: quote-percent-one-quote-space-percent-asterisk.)
   
   NOTE: The Registry Editor will automatically enclose the value within quotation marks. When you click OK, the (Default) value should look exactly like this: ""%1" %*"
   
   Make sure that you completely delete all value data in the command key prior to typing the correct data. If a space is left accidentally at the beginning of the entry, any attempt to run program files will result in the error message, "Windows cannot find .exe." If this happens to you, then start over at the beginning of this section, making sure to completely remove the current value data.
   
8. Locate the following registry keys. Remove any randomly named values that have been added:
   
   HKEY_LOCAL_MACHINE\Hardware\Data
   HKEY_LOCAL_MACHINE\Hardware\Enum
   HKEY_LOCAL_MACHINE\Software\Microsoft\DirectXMedia
   
9. Exit the Registry Editor.

1. Do one of the following, depending on which version of Windows you are running:
   • Windows 95/98 users: Click Start, point to Programs, and click MS-DOS Prompt. A DOS window opens at the C:\Windows prompt. Go on to step 2 of this section.
   • Windows Me users: Click Start, point to Programs, point to Accessories, and then click MS-DOS Prompt. A DOS window opens at the C:\Windows prompt. Go on to step 2 of this section.
   • Windows NT/2000 users:
     1. Click Start, and click Run.
     2. Type the following and then press Enter:
        
        command
        
        A DOS window opens.
        
     3. Type the following, and then press Enter:
        
        cd \winnt
        
     4. Go on to step 2 of this section.
   • Windows XP:
     1. Click Start, and click Run.
     2. Type the following, and then press Enter:
        
        command
        
        A DOS window opens.
        
     3. Type the following, and then press Enter after typing each one:
        
        cd\
        cd \windows
        
     4. Proceed to step 2 of this section.
        
2. Type the following, and then press Enter:
   
   copy regedit.exe regedit.com
   
3. Type the following, and then press Enter:
   
   start regedit.com
   
   The Registry Editor will open in front of the DOS window. After you finish editing the registry, exit the Registry Editor, and then exit the DOS window, as well.
   

Additional information:

Norton Internet Security/Norton Internet Protection users
If you are using either of these Symantec firewall programs, the name that is used by the Trojan Block rule to prevent the Trojan from being downloaded to your computer is different from the name that is used by Norton AntiVirus to detect the same threat if it were actually run on your computer or received in email.

Norton Internet Security/Norton Internet Protection will block Backdoor.SubSeven22 from being downloaded to your computer using the Block Rule Backdoor/SubSeven.

Write-up by: Dmitry Reyder