Backdoor.DarkSky.C

SUMMARY

Backdoor.DarkSky.C is a Trojan that is used to gain unauthorized access to an infected computer. It copies itself to the %windir% and %system% folders.

Protection

• Virus Definitions (LiveUpdate™ Weekly) October 16, 2002
• Virus Definitions (Intelligent Updater) October 14, 2002

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
• Compromises Security Settings: Can allow unauthorized access to the infected system

Distribution

• Distribution Level: Low

TECHNICAL DETAILS

When the Trojan is executed, it first runs as a service and then copies itself as:

• %system%Msinter.exe
• %system%Notepd.exe
• %windir%Mobbs.exe
• %windir%Nuscr.exe

NOTES:

• %windir% is a variable. The Trojan locates the Windows installation folder (by default this is C:Windows or C:Winnt) and copies itself to that location.
• %system% is a variable. The Trojan locates the System folder and copies itself to that location. By default this is C:WindowsSystem (Windows 95/98/Me), C:WinntSystem32 (Windows NT/2000), or C:WindowsSystem32 (Windows XP).

This Trojan then adds the value

TaskMonitor %windir%\%system%Msinter.exe

to the registry key

HKEY_LOCAL_MACHINESoftwareMicrosoftWindowsCurrentVersionRun

so that the Trojan runs each time that you start Windows.

In the registry key

HKEY_LOCAL_MACHINESoftwareClassesexefileshellopencommand

it changes the (Default) value to

%windir%\%system%Notepd.exe "%1" %*

so that the Trojan runs each time that you run an .exe file.

In the registry key:

HKEY_CLASSES_ROOT xtfileshellopencommand

it changes the (Default) value to

%windir%\%system%Notepd.exe "%1"

so that the Trojan runs each time that you open a .txt file.


In the registry key:

HKEY_CLASSES_ROOTscrfileshellopencommand

it changes the (Default) value to

%windir%Nuscr.exe "%1"

so that the Trojan runs each time that you run a .scr file.


In the registry key:

HKEY_CLASSES_ROOTchm.fileshellopencommand

it changes the (Default) value to

%windir%Nuscr.exe "%1"

so that the Trojan runs each time that you open .chm (Help) file.

The Trojan also adds the line run=%windir%Mobbs.exe in the Win.ini file. This will run the Trojan each time that you start Windows on Windows 95/98/Me computers.

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

NOTES:

• These instructions are for all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.
• If the Trojan has run, you may not be able to start most programs, including Symantec antivirus software or LiveUpdate. In this case, first follow the instructions in the section To edit the registry which follows, and then start at the beginning of the removal instructions.

1. Update the virus definitions.
2. Run a full system scan, and delete all files that are detected as Backdoor.DarkSky.C.
3. Edit the registry and reverse any changes that were made by the Trojan.
4. (Windows 95/98/Me only.) Remove the line that it added to the Win.ini file.
   

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 Backdoor.DarkSky.C, click Delete.

To edit the registry:
If the Trojan has run and changed the registry, you first must make a copy of the Registry Editor (Regedit.exe) as Regedit.com, and then use Regedit.com to make the necessary changes.

To copy Regedit.exe to Regedit.com:
If the Trojan successfully modified the registry so that you cannot run .exe files, you must first make a copy of the Registry Editor as a file with the .com extension, and then run that file.

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.
   

1. Proceed to the next section, "To edit the registry and remove keys and changes made by the worm," only after you have performed the previous steps.


NOTES:

• The Registry Editor will open in front of the DOS window. After you finish editing the registry and have closed the Registry Editor, then close the DOS window, as well.
• (Optional) After the Trojan has been successfully removed, then you may delete the Regedit.com file.

To reverse the changes that the Trojan made:

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. Navigate to and select the following key:
   
   HKEY_LOCAL_MACHINESoftwareClassesexefileshellopencommand
   
   CAUTION: The HKEY_LOCAL_MACHINESoftwareClasses key contains many subkey entries that refer to other file extensions. One of these file extensions is .exe. Changing this extension can prevent any files ending with an .exe extension from running. Make sure that you browse all the way along this path until you reach the command subkey.
   
   Modify the HKEY_LOCAL_MACHINESoftwareClassesexefileshellopencommand subkey that is shown in the following figure:
   
   <<=== NOTE: Modify this key.
   
2. In the right pane, double-click the (Default) value.
3. Delete the current value data, and then type: "%1" %* (That is, type the following characters: quote-percent-one-quote-space-percent-asterisk.)
   
   NOTES:
   • Under Windows 95/98/Me/NT, the Registry Editor automatically encloses the value within quotation marks. When you click OK, the (Default) value should look exactly like this:
     
     ""%1" %*"  
     
   • Under Windows 2000/XP, the additional quotation marks will not appear. 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 before you type the correct data. If you leave a space 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, start over at the beginning of this document, and make sure that you completely remove the current value data
     
4. Navigate to and select the key
   
   HKEY_CLASSES_ROOT xtfileshellopencommand
   
5. In the right pane, double-click the (Default) value.
6. Delete the current value data, and replace it with the correct value for your version of Windows.
   
   NOTE: This value varies with the version of Windows and, on some systems, the installation path. You may need to look at that same key on a working computer that has the same version of Windows and configuration to determine this. Two common values are:
   
   • Windows 98: C:WindowsNotepad.exe %
   • Windows 2000: %SystemRoot%system32NOTEPAD.EXE %1
     
7. Navigate to and select the key
   
   HKEY_CLASSES_ROOTscrfileshellopencommand
   
8. In the right pane, double-click the (Default) value.
9. Delete the current value data, and replace it with:
   
   %1
   
10. Navigate to and select the key
    
    HKEY_CLASSES_ROOTchm.fileshellopencommand
    
11. In the right pane, double-click the (Default) value.
12. Delete the current value data, and replace it with:
    
    hh.exe
    
13. Navigate to the key
    
    HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionRun
    
14. In the right pane, delete the value
    
    TaskMonitor %windir%\%system%Msinter.exe
    
15. Exit the Registry Editor.

To edit the Win.ini file:
This is necessary only for Windows 95/98/Me 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:WindowsRecent 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:WindowsRecent, 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:windowswin.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=%windir%Mobbs.exe
   
4. If the line exists, select the entire line. Be sure that you do not select any other text, and then press Delete.
5. Click File, and click Save.
6. Click File, and click Exit.
7. Restart the computer.