Trojan.Bookmarker.C

Trojan.Bookmarker.C is distributed as an executable installer through Web browser exploits or downloaders.

When the installer program is executed, it does the following:

1. Creates the following files:
   • %System%\Mtwirl32.dll
   • %System%\Mtwcnl32.dll, which is actually a text file that controls the Trojan's behavior.
     
     

     ---

     Note: %System% is a variable. By default, this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).

     ---

     
     
2. Depending on the operating system, the Trojan does the following:
   
   • Windows NT\2000\XP:
     
     1. Adds the values:
        
        (Default) = '%System%\mtwirl32.dll'
        ThreadingModel = 'Apartment'
        
        to the registry key:
        
        HKEY_CLASSES_ROOT\CLSID\{3F143C3A-1457-6CCA-03A7-7AA23B61E40F}\InProcServer32
        
        
     2. Adds the value:
        
        {3F143C3A-1457-6CCA-03A7-7AA23B61E40F} = 'DDE Control Module'
        
        to the registry key:
        
        HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\SharedTaskScheduler
        
        so that the Trojan file, Mtwirl32.dll, is loaded each time you open Windows Explorer.
        
        
   • Windows 95\98\Me
     
     1. Adds the values:
        
        (Default) = '%System%\mtwirl32.dll'
        ThreadingModel = 'Apartment'
        
        to the registry key:
        
        HKEY_CURRENT_USER\SOFTWARE\Classes\CLSID\{3F143C3A-1457-6CCA-03A7-7AA23B61E40F}\InProcServer32
        
        
     2. Adds the value:
        
        {3F143C3A-1457-6CCA-03A7-7AA23B61E40F} = 'DDE Control Module'
        
        to the registry key:
        
        HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\ShellServiceObjectDelayLoad
        
        so that the Trojan file, Mtwirl32.dll, is loaded each time you open Windows Explorer.
        
        
3. Invokes the Mtwirl32.dll file by executing this command:
   
   rundll32.exe %System%\mtwirl32.dll,load

When the mtwirl32.dll file is loaded (either by Rundll32.exe or automatically loaded with Windows Explorer), it does the following:

1. Attempts to delete the following files, which are likely to be the Trojan installer:
   • C:\Y.exe
   • C:\Z.exe
     
     
2. Reads the text file, %System%\Mtwcnl32.dll.
   
   
3. Depending on the text in the Mtwcnl32.dll file, the Trojan could modify any of the following registry subkeys/values:
   
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\"Start Page"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\"Search Page"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Search\"SearchAssistant"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Search\"CustomizeSearch"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Main\"Search Page"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Main\"Default_Search_URL"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Main\"Start Page"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\"SearchURL"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\"Search Bar"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\"Default_Search_URL"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\"Default_Page_URL"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Search\"SearchAssistant"
   HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Search\"CustomizeSearch"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\URL\"DefaultPrefix"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\URL\Prefixes\"home"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\URL\Prefixes\"www"
   HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\URL\Prefixes\"mosaic"
   
   
4. Adds the following registry values:
   
   "Use Search Assistant" = "yes"
   
   to the registry key:
   
   HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Main\
   
   
5. May add the line:
   
   <ip address> auto.search.msn.com
   
   to the file, %System%\drivers\etc\hosts
   
   which redirects Internet Explorer's default search page.

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.

Summary