W32.Sasser.F.Worm

1. Attempts to create a mutex named billgate and exits if the attempt fails. This ensures that no more than one instance of the worm can run on a computer at any time.
   
   
2. Copies itself as %Windir%\napatch.exe.
   
   

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   Note: %Windir% is a variable. The worm locates the Windows installation folder (by default, this is C:\Windows or C:\Winnt) and copies itself to that location.

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3. Adds the value:
   
   "napatch.exe"="%Windir%\napatch.exe"
   
   to the registry key:
   
   HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Run
   
   so that the worm runs when you start Windows.
   
   
4. Uses the AbortSystemShutdown API to hinder the attempts to shut down or restart the computer.
   
   
5. Starts an FTP server on TCP port 5554. This server is used to spread the worm to other hosts.
   
   
6. Retrieves the IP addresses of the infected computer, using the Windows API, gethostbyname.
   
   

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   Note: The worm will ignore any of the following IP addresses:
   
   • 127.0.0.1
   • 10.x.x.x
   • 172.16.x.x - 172.31.x.x (inclusive)
   • 192.168.x.x
   • 169.254.x.x

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7. Generates another IP address, based on one of the IP addresses retrieved from the infected computer.
   
   • 25% of the time, the last two octets of the IP address are changed to random numbers. For example, if A.B.C.D is the IP address retrieved in step 7, C and D will be random.
   • 23% of the time, the last three octets of the IP address are changed to random numbers. For example, if A.B.C.D is the IP address retrieved in step 7, B, C, and D will be random.
   • 52% of the time, the IP address is completely random.
     
     

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     Notes:
   • Because the worm creates completely random addresses 52% of the time, any IP address can be infected, including those ignored in step 6.
   • This process is made up of 128 threads, which demands a lot of CPU time. As a result, an infected computer may become so slow and barely usable.

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8. Connects to the generated IP address on TCP port 445 to determine whether a remote computer is online.
   
   
9. If a connection is made to a remote computer, the worm will send shell code to it, which may cause it to open a remote shell on TCP port 9996.
   
   
10. Uses the shell on the remote computer to reconnect to the infected computer's FTP server, running on TCP port 5554, and to retrieve a copy of the worm. This copy will have a name consisting of four or five digits, followed by _up.exe. For example, 74354_up.exe.
    
    
11. The Lsass.exe process will crash after the worm exploits the Windows LSASS vulnerability. Windows will display the alert and shut down the system in one minute.
    
    
12. Creates a file at C:\win2.log that contains the IP address of the computer that the worm most recently attempted to infect, as well as the number of infected computers.

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.