WinNuke.Trojan

WinNuke.Trojan uses an known exploit to crash remote computers. If the remote computer does not have the appropriate security patches, WinNuke.Trojan sends OOB (Out of Band) strings and causes the compromised computer to crash.

The attack uses port 139 on Win 95/98/Me-based computers, and port 135 on Windows NT/2000/XP-based computers. The target computer is selected using the Trojan's user interface.

When WinNuke.Trojan runs, it copies itself to several files; all of these files are configured with the hidden attribute. The files are:

• %windir%\Taskmon.exe
• %windir%\scanregw.exe
• %windir%\Rundll32*.exe
  
  NOTE: Each time the Trojan runs, it creates and references a new copy of Rundll32.exe, adding spaces after the 32, so that successive iterations will be Rundll32 .exe, Rundll32  .exe, Rundll32   .exe, and so on. Registry values are adjusted to reflect the most current file name.
  
• %windir%\system\internet.exe

It also creates (or overwrites):
%windir%\system\systemdllx.vxd
%windir%\system\tdllcope.vxd

Next, it creates the UI portion of the Trojan as %windir%\system\<original file name>. The UI portion of the Trojan is detected as Hacktool.Nuker.

NOTE: %windir% is a variable. The Trojan locates the Windows main installation folder (by default this is C:\Windows or C:\Winnt) and copies itself to that location.

This Trojan modifies the system registry and subverts existing registry keys. The subverted keys involve reference to scanregw.exe and taskmon.exe. Since the Trojan drops hidden versions of these files, the malicious code is run at startup. After these hidden copies are removed, the "legitimate" files are run as designed. Be sure that the removed files are 319 KB in length.

Finally, in the registry key

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run

the Trojan creates or modifies the value

LoadPowerProfile

with the Value Data

Rundll32*.exe

NOTE: * denotes 0 or more spaces, depending on the number of times Trojan has run. This value changes each time a new Rundll32*.exe is created.

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