Backdoor.IRC.Critical

Normally, this backdoor Trojan horse is distributed as one large installation executable. Once run, this executable pretends to install a benign antivirus program, while in the background it installs the backdoor files in various places on the system.

The inserted files are:

• C:\Windows\DskLoad.exe
• C:\Windows\Prun.exe
• C:\Windows\Sys.exe
• C:\Program Files\Accessories\Backup\System\Critical\Cons1.dll
• C:\Program Files\Accessories\Backup\System\Critical\Deg326.dll
• C:\Program Files\Accessories\Backup\System\Critical\Expl32.exe
• C:\Program Files\Accessories\Backup\System\Critical\Explorer2.exe
• C:\Program Files\Accessories\Backup\System\Critical\ins.dll
• C:\Program Files\Accessories\Backup\System\Critical\Mir436.dll
• C:\Program Files\Accessories\Backup\System\Critical\Mirc.ini
• C:\Program Files\Accessories\Backup\System\Critical\Moo.dll
• C:\Program Files\Accessories\Backup\System\Critical\Mstg1.dll
• C:\Program Files\Accessories\Backup\System\Critical\Scan31.dll
• C:\Program Files\Accessories\Backup\System\Critical\Updatex1.dll
• C:\Program Files\Accessories\Backup\System\Critical\Win32x.dll
• C:\Program Files\Accessories\Backup\System\Critical\Winexp32.dll
• C:\Program Files\Accessories\Backup\System\Critical\Winvar32.dll

NOTE: Most of the .dll files in this list are not dynamic link libraries but are IRC script files and other data files.

All the files in the C:\Program Files\Accessories\Backup\System\Critical folder are created with "Hidden" attributes, in an attempt to escape being noticed. In order to gain control of the compromised system, this Trojan horse does the following:

• It modifies the run= line of the C:\Windows\Win.ini file so that it becomes:
  
  run=C:\Windows\DskLoad.exe
  
• It makes the following modifications to the registry:
  
  • In the key
    
    HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall\mIRC
    
    it adds or modifies the value
    
    DisplayName mIRC
    
  • In the key
    
    HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Uninstall\mIRC
    
    it adds or modifies the value
    
    UninstallString c:\program files\accessories\backup\system\critical\expl32.exe" -uninstall"
    
  • In the key
    
    HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Run
    
    it adds the value
    
    StubPath C:\Windows\DskLoad.exe
    
  • In the key
    
    HKEY_LOCAL_MACHINE\Software\Microsoft\Active Setup\Installed Components\KeyName
    
    it adds or modifies the value
    
    MSSysDisk C:\Windows\DskLoad.exe
    
  • Finally, IRC file extensions are hooked in HKEY_LOCAL_MACHINE\Software\Classes that call "c:\program files\accessories\backup\system\critical\expl32.exe" when chat files are opened.

The inserted file C:\Windows\Sys.exe is waiting for the user or a program to call the "sys" command. In DOS-based operating systems, the file Sys.com is used to make a data medium DOS bootable. On Windows computers, this file would reside in C:\Windows\Command\sys.com. However, when running "sys" from the command prompt, the viral C:\Windows\Sys.exe is run instead of the proper C:\Windows\Command\sys.com.

This viral sys.exe installs Backdoor.Subseven.22 on the system allowing for even more control over the compromised system.

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