Printer Friendly Page

Discovered: August 16, 2000
Updated: February 13, 2007 11:52:49 AM
Also Known As: Backdoor.Trojan
Type: Trojan Horse

The Backdoor.BlaTrojan allows unauthorized access to your computer. It tries to mask the infection by displaying a program interface that is actually the interface of the backdoor client program.

Norton Internet Security/Norton Internet Protection users
If you are using either of these Symantec firewall programs, the name that is used by the Trojan Block rule to prevent the Trojan from being downloaded to your computer is different from the name that is used by Norton AntiVirus to detect the same threat if it were actually run on your computer or received in email.

Norton Internet Security/Norton Internet Protection will block Backdoor.Bla.Trojan from being downloaded to your computer using the Block Rule "Bla".

Antivirus Protection Dates

  • Initial Rapid Release version December 18, 2000
  • Latest Rapid Release version March 23, 2017 revision 037
  • Initial Daily Certified version December 18, 2000
  • Latest Daily Certified version March 23, 2017 revision 041

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

Technical Description

When Backdoor.Bla is executed, it does the following:

  1. It creates a new file in the \Windows\System folder. This file runs in the background until you attempt to shut down or restart the computer.
  2. When you command Windows to restart or shut down, the dropped file adds a value (which may vary from the examples that follow) such as



    IO System Debug

    to the registry key


    This causes the file to run when Windows starts.

NOTE: In most cases, you will be unable to restart Windows due to the large number of illegal operations performed by the Trojan. The infected computer usually displays the blue warning screens generated by Microsoft Windows.


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.


To remove this Trojan, delete files detected as Backdoor.Bla and remove the value that it added to the registry.

To remove the Trojan:

  1. Run LiveUpdate to make sure that you have the most recent virus definitions.
  2. Start Norton AntiVirus (NAV), and run a full system scan, making sure that NAV is set to scan all files.
  3. If any files are detected as Backdoor.Bla, first write down the file names, and then delete them.

    NOTE: If NAV displays a message that it cannot delete the file, you must first remove the value that it added to the registry, restart the computer, and run the scan again.

To edit the registry:
    CAUTION: We strongly recommend that you back up the system registry before making any changes. Incorrect changes to the registry could result in permanent data loss or corrupted files. Please make sure you modify only the keys specified. Please see the document How to back up the Windows registry before proceeding.
  1. Click Start, and click Run. The Run dialog box appears.
  2. Type regedit and then click OK. The Registry Editor opens.
  3. Navigate to the key

  4. In the right pane, in the Data column, look for a reference to the file name that you wrote down earlier (the file that NAV detected as infected). The entry in the Name column will vary, but the following have been reported:

    IO System Debug
  5. Delete the entry that refers to the Trojan file name.
  6. Click Registry, and then click Exit to save the change.