W32.HLLW.Nebiwo

When the worm is executed, it does the following:

1. Drops and runs Trojan Horses, such as Backdoor.Sdbot, Backdoor.Litmus (2), and Trojan.KillAV.
   
   
2. If the operating system is Windows 95/98/Me, it will register itself as a service process to hide itself from the task list and to continue to run after you log off. In this case, the worm will close only when the system is shut down.
   
   
3. Launches 100 instances of the network replication thread.
   
   
4. Enters an infinite loop of adding the value:
   
   "NAV Live Update"="<path to worm>"
   
   to the registry key:
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
   
   so that the worm runs when you start Windows.
   

Network replication thread
The network replication thread will enumerate IP addresses that it will use for the attack. The enumeration loop starts from the base IP address. To increase the chances of generating the valid IP addresses, the worm retrieves the local host IP address and uses it to produce the base IP address.

If the first two octets of the local host IP address are 192 and 168, the worm will accept 192.168.0.1 as the base IP address. If the first octet is 10, then the worm accepts 10.0.0.1 as the base IP address. For all other IP addresses, the local host IP address will be accepted as the base IP address.

The following pseudo-code (IPx stands for the x-th octet of the IP address) describes the enumeration loop:

inc (IP4)
if IP4 <= 240 then
   attack %generated_IP_address%
else
   IP4 = 1
   inc (IP3)
   if IP3 <= 240 then
      attack %generated_IP_address%
   else
      IP3 = 0
      inc (IP2)
      if IP2 <= 240 then
         attack %generated_IP_address%
      else
         IP2 = 0
         inc (IP1)
         if IP1 <= 240 then
            attack %generated_IP_address%
         else
            IP1 = 10    
         endif
      endif
   endif
endif


NOTE: The octets, IP1.IP2.IP3.IP4, represent %generated_IP_address%.

The worm will probe a socket connection to %generated_IP_address% via port 139. If it succeeds, the attack routine will attempt to connect to the following network resources:

%generated_IP_address%\C

and:

%generated_IP_address%\C$


The connection has the following characteristics:

• It uses a disk type of resource to connect.
• A local device to map is not specified.
• A password is not used.
• The following user names are specified to establish the connection:
  
  Administrator
  Guest
  Owner
  [default user name, provided by the user context for the process]

If the connection is successfully established, the worm attempts to copy itself under the following filenames:

• %connected_resource%\Winnt\Profiles\All Users\Start Menu\Programs\Startup\%worm_filename%
• %connected_resource%\Windows\Start Menu\Programs\Startup\%worm_filename%
• %connected_resource%\Documents and Settings\All Users\Start Menu\Programs\Startup\%worm_filename%

This process indicates that any user logging on to the system thereafter will start W32.HLLW.Nebiwo during logon.

NOTE: %connected_resource% is one of the following network resources:

• %generated_IP_address%\C
• %generated_IP_address%\C$

NOTE: %worm_filename% is the name of the executed W32.HLLW.Nebiwo file. For example, Security Response has received samples, some of which had the following filenames:

• oocfwm.exe
• results.exe
• ~2.EXE
• lknq.exe
• oocfwm.exe
• gwigsb.exe

To run the sample that the worm has copied to an unrestricted share, W32.HLLW.Nebiwo uses network management schedule functions. For example, if the worm successfully copies itself as:

%generated_IP_address%\C$\Windows\Start Menu\Programs\Startup\%worm_filename%

it attempts to submit a job to %generated_IP_address% to run the following job:

C:\Windows\Start Menu\Programs\Startup\%worm_filename%

The time specified for this job depends on the time-of-day information retrieved from a compromised computer. This functionality requires that the schedule service be started at a compromised computer to which the job is submitted.

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