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Discovered: March 04, 2005
Updated: February 13, 2007 12:34:41 PM
Type: Trojan Horse
Systems Affected: Windows

Trojan.Flush.A is a Trojan horse program that modifies DNS settings on the compromised computer.

Antivirus Protection Dates

  • Initial Rapid Release version March 04, 2005
  • Latest Rapid Release version April 12, 2018 revision 039
  • Initial Daily Certified version March 04, 2005 revision 003
  • Latest Daily Certified version April 13, 2018 revision 003
  • Initial Weekly Certified release date March 06, 2005

Click here for a more detailed description of Rapid Release and Daily Certified virus definitions.

Writeup By: John Canavan

Discovered: March 04, 2005
Updated: February 13, 2007 12:34:41 PM
Type: Trojan Horse
Systems Affected: Windows

When Trojan.Flush.A runs, it does the following:

  1. Queries "Ipconfig" for the value of each adapter found at the following location:


    Note: The aforementioned value typically holds a pointer to a corresponding entry at:


  2. Attempts to add the following value at that location:

    "NameServer" = ","

    to the registry subkey:


  3. Checks for the presence of the following file:


    Note: %CSIDL_COMMON_APPDATA% is a variable that refers to the folder containing application data for all users. By default this is C:\Documents and Settings\All Users\Application Data (Windows NT/2000/XP).

  4. Adds the following entries to the aforementioned file, if that file is found:

    • "IpDnsAddress="
    • "IpDns2Address="
    • "IpNameAssign=2"

  5. Adds the value:

    "NameServer" = ","

    to the following registry key:


  6. Enumerates the entries found at the following location:


    attempts to add the following hex value to each found entry:

    "IP" = "02,00,00,00,00,00,00,00,c4,b0,32,45,25,b0,e1,c3,00,00,00,00,00,00,00,00,00,00,00,00"

  7. Executes the following commands on the computer:

    • ipconfig /dnsflush
    • ipconfig /registerdns
    • ipconfig /renew
    • ipconfig /renew_all


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: John Canavan

Discovered: March 04, 2005
Updated: February 13, 2007 12:34:41 PM
Type: Trojan Horse
Systems Affected: Windows

The following instructions pertain to all current and recent Symantec antivirus products, including the Symantec AntiVirus and Norton AntiVirus product lines.

  1. Disable System Restore (Windows Me/XP).
  2. Update the virus definitions.
  3. Run a full system scan and delete all the files detected as Trojan.Flush.A.
  4. Delete the value that was added to the registry.
  5. Delete the entries that was added to the RAS phone-book file.
For specific details on each of these steps, read the following instructions.

1. To disable System Restore (Windows Me/XP)
If you are running Windows Me or Windows XP, we recommend that you temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.

Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file on your computer, even after you have cleaned the infected files from all the other locations.

Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.

For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
When you are completely finished with the removal procedure and are satisfied that the threat has been removed, re-enable System Restore by following the instructions in the aforementioned documents.

For additional information, and an alternative to disabling Windows Me System Restore, see the Microsoft Knowledge Base article, "Antivirus Tools Cannot Clean Infected Files in the _Restore Folder ," Article ID: Q263455.

2. To update the virus definitions
Symantec Security Response fully tests all the virus definitions for quality assurance before they are posted to our servers. There are two ways to obtain the most recent virus definitions:
  • Running LiveUpdate, which is the easiest way to obtain virus definitions: These virus definitions are posted to the LiveUpdate servers once each week (usually on Wednesdays), unless there is a major virus outbreak. To determine whether definitions for this threat are available by LiveUpdate, refer to the Virus Definitions (LiveUpdate).
  • Downloading the definitions using the Intelligent Updater: The Intelligent Updater virus definitions are posted daily. You should download the definitions from the Symantec Security Response Web site and manually install them. To determine whether definitions for this threat are available by the Intelligent Updater, refer to the Virus Definitions (Intelligent Updater).

    The Intelligent Updater virus definitions are available: Read "How to update virus definition files using the Intelligent Updater" for detailed instructions.

3. To scan for and delete the infected files
  1. Start your Symantec antivirus program and make sure that it is configured to scan all the files.
  2. Run a full system scan.
  3. If any files are detected as infected with Trojan.Flush.A, click Delete.

    If your Symantec antivirus product reports that it cannot delete an infected file, Windows may be using the file. To fix this, run the scan in Safe mode. For instructions, read the document, "How to start the computer in Safe Mode." Once you have restarted in Safe mode, run the scan again.

    After the files are deleted, restart the computer in Normal mode and proceed with section 4.
4. To delete the value from the registry
Important: Symantec strongly recommends that you back up the registry before making any changes to it. Incorrect changes to the registry can result in permanent data loss or corrupted files. Modify the specified subkeys only. Read the document, "How to make a backup of the Windows registry ," for instructions.
  1. Click Start > Run.
  2. Type regedit

    Then click OK.

  3. Navigate to the subkey:


  4. In the right pane, delete the value:

    "NameServer" = ","

  5. Navigate to the subkey:


  6. In the right pane, delete the value:

    "IP" = "02,00,00,00,00,00,00,00,c4,b0,32,45,25,b0,e1,c3,00,00,00,00,00,00,00,00,00,00,00,00"

  7. Exit the Registry Editor.

5. To delete the added entries from the RAS phone-book file

Note: The location of the RAS phone-book file rasphone.pbk may vary and some computers may not have this file. For example, if the file exists in Windows XP, it is usually located in the C:\Documents and Settings\All Users\Application Data\Microsoft\Network\Connections\Pbk folder.

Follow the instructions for your operating system:
  • Windows 95/98/Me/NT/2000
    1. Click Start, point to Find or Search, and then click Files or Folders.
    2. Make sure that "Look in" is set to (C:) and that "Include subfolders" is checked.
    3. In the "Named" or "Search for..." box, type:

    4. Click Find Now or Search Now.
    5. If you find rasphone.pbk, right-click the file, and then click "Open With."
    6. Deselect the "Always use this program to open this program" check box.
    7. Scroll through the list of programs and double-click Notepad.
    8. When the file opens, delete the entries below:


    9. Close Notepad and save your changes when prompted.

  • Windows XP
    1. Click Start, and then click Search.
    2. Click All files and folders.
    3. In the "All or part of the file name" box, type:

    4. Verify that "Look in" is set to "Local Hard Drives" or to (C:).
    5. Click "More advanced options."
    6. Check "Search system folders."
    7. Check "Search subfolders."
    8. Click Search.
    9. Click Find Now or Search Now.
    10. If you find rasphone.pbk file, right-click the file, and then click "Open With."
    11. Deselect the "Always use this program to open this program" check box.
    12. Scroll through the list of programs and double-click Notepad.
    13. When the file opens, delete the entries below:


    14. Close Notepad and save your changes when prompted.

Writeup By: John Canavan