Backdoor.Coreflood

• A .exe file, which is the loader.
• A .dll file, which contains the primary code.

1. Extracts the .dll file from itself, and then copies itself and the .dll file to the %System% folder. The file names are arbitrary.
   
   Note: %System% is a variable. The Trojan locates the System folder and copies itself to that location. By default this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).
   
   
2. Next, the .exe file calls a function within the .dll file to begin executing the main code. The main code hooks the Explorer.exe process in a way so that all of its actions run under the process context of Explorer.exe.
   
   
3. Sets itself to run when Windows is started. It does this in various ways:
   • Older variants add the value:
     
     "<file name>"="%system%\<file name.ext>"
     
     to the registry key:
     
     HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows/CurrentVersion/Run
     
     The Trojan monitors this registry key and may re-add it if it is modified or removed.
     
     
   • Newer variants discovered in July 2004 create the following registry keys instead:
     
     HKEY_CURRENT_USER/CLSID/{<random clsid 1>}
     
     HKEY_CURRENT_USER/CLSID/{<random clsid 2>}
     
     HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/
     Explorer/ShellIconOverlayIdentifiers/<random name>/(Default) = {<random clsid 1>}
     
     HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/
     Explorer/Browser Helper Objects/{<random clsid 2>}
     
     They also add the value:
     
     (Default) = %System%\<random name>.dll
     
     to the registry keys:
     
     HKEY_CURRENT_USER/CLSID/{<random clsid 1>}/InprocServer32
     HKEY_CURRENT_USER/CLSID/{<random clsid 2>}/InprocServer32
     
     Note: {<random clsid X>} is a random value of the form, {########-####-####-####-############}, for example, {01234567-89ab-cdef-0123-456789abcdef}.
     
     
4. Connects to an IRC server and joins a predefined chat channel. It listens for commands to execute. These commands allow an attacker to gain access to the computer and potentially conduct a DoS attack against other computer systems.

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.