Backdoor.SubSeven22

When executed, the Trojan horse drops a copy of itself into the \Windows folder. The copy is usually named with random alphanumeric characters, although the name is sometimes fixed (for example, Rundll16.exe). The dropped copy provides potentially malicious users with access to the infected computer.

The Trojan horse sometimes drops an application into the \Windows or \Windows\System folders. This application file can be an .exe or a .dll, and it is between 10 KB and 15 KB in size.

The .exe variation of this application is generally named using a random set of alphanumeric characters. The file name of the .dll variation is fixed, for example, Watching.dll.

The application serves as an executable file launcher, similar to Explorer.exe. While launching an executable, this application may also launch the Trojan. To force the executable files in the system to be launched only by this application, the Trojan modifies specific locations in the Windows registry. If the launcher is the .exe application, it inserts the application's name into one of the values, usually [Default], in the HKEY_CLASSES_ROOT\exefile\shell\open\command registry key. If the launcher is the .dll file type, it creates the HKEY_CLASSES_ROOT\.dl registry key, and sets the [Default] value to .exe file, and sets the content value to application/x-msdownload.

The Trojan horse generally creates or uses existing HKEY_LOCAL_MACHINE\Hardware\Data, HKEY_LOCAL_MACHINE\Hardware\Enum or HKEY_LOCAL_MACHINE\Software\Microsoft\DirectXMedia registry keys to store data. It creates randomly named variables and stores data in them. The variable names and the data usually appear as junk.

The Trojan horse uses any combination of the following methods to enable itself at startup:

• It adds the name of the dropped copy to the "load=" and/or the "run=" lines of the Win.ini file.
• It adds the name of the dropped copy to the "shell=explorer.exe" line of the System.ini file.
• It creates some value (usually "WinLoader") in the HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\Run" and/or HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\Current Version\RunServices registry keys, and then sets that value to the name of its dropped copy.

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