1. Symantec/
  2. Security Response/
  3. Trojan.Flush.G


Risk Level 1: Very Low

May 2, 2006
February 13, 2007 12:53:20 PM
Trojan Horse
Systems Affected:

When Trojan.Flush.G is first installed, it performs the following actions:
  1. Copies itself as one of the following files:

    • %System%\hgqhp.exe
    • %System%\yaemu.exe

      Note: %System% is a variable that refers to the System folder. By default this is C:\Windows\System (Windows 95/98/Me), C:\Winnt\System32 (Windows NT/2000), or C:\Windows\System32 (Windows XP).

  2. Deletes the file that originally arrived on the compromised computer.

  3. Adds one of the following values:

    "hgqhp.exe" = "%System%\hgqhp.exe"
    "hgqhp.exe" = "%System%\yaemu.exe"

    to the registry subkey:


    so that it runs every time Windows starts.

  4. Queries all the entries named Ipconfig in the following registry subkey to get the CLSID references for the network adapters installed on the compromised computer:


  5. Adds the following value:

    "NameServer" = ","

    the the registry subkey:


    For each adapter found to change the DNS server settings.

    Note: [CLSID] represents the CLSID for one of the network adapters installed on the compromised computer.

  6. Checks for the presence of the following file:

    %UserProfile%\Application Data\Microsoft\Network\Connections\Pbk\rasphone.pbk

    Note: %UserProfile% is a variable that refers to the current user's profile folder. By default, this is C:\Documents and Settings\[CURRENT USER] (Windows NT/2000/XP).

  7. Adds the following lines to it, if that file is found:

    IpDnsAddress =
    IpDns2Address =
    IpNameAssign = 2

  8. Executes the following Windows commands to update the changes made to the configuration of the compromised computer:

    • ipconfig.exe/flushdns
    • ipconfig.exe/registerdns
    • ipconfig.exe/dnsflush
    • ipconfig.exe/renew
    • ipconfig.exe/renew_all

  9. Starts Internet Explorer as a hidden window and regularly sends PING echo requests to the following server:

  10. May cause the compromised computer to lose its Internet connectivity.


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.
Writeup By: Nicolas Falliere
Summary| Technical Details| Removal

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