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Aliases :

Krei, BackDoor-ALI, IERK, IERK8243


Krei is a backdoor that provides access to the compromised computer for remote attackers. Krei affects Windows 2000 and XP systems only.


After removing these and restarting the system the backdoor and the kernel module can be deleted from the system.

Starting the computer in Safe Mode:

  • 1, Restart the computer
  • 2, Press F8 when the Windows loading screen appears
  • 3, Choose Safe Mode from the menu

A False Positive is when a file is incorrectly detected as harmful, usually because its code or behavior resembles known harmful programs. A False Positive will usually be fixed in a subsequent database update without any action needed on your part. If you wish, you may also:

  • Check for the latest database updates

    First check if your F-Secure security program is using the latest updates, then try scanning the file again.

  • Submit a sample

    After checking, if you still believe the file is incorrectly detected, you can submit a sample of it for re-analysis.

    Note: If the file was moved to quarantine, you need to collect the file from quarantine before you can submit it.

  • Exclude a file from further scanning

    If you are certain that the file is safe and want to continue using it, you can exclude it from further scanning by the F-Secure security product.

    Note: You need administrative rights to change the settings.

Technical Details

When first run it installs itself as a service using Service Control Manager. This makes sure that the backdoor will be run when the computer is started.

When Krei is active it listens on TCP port 961 waiting for the remote client to connect.

After Krei had been installed on the system the following key appears in the Registry:


Kernel component

The Krei trojan makes use of an external component that helps to hide it from all the conventional tools. This kernel component resides in a file called 'ierk8243.sys'. When this driver is loaded it creates a special device that can be used to control the hiding feature. This module hooks several low-level Windows API functions to make sure that the hidden processes, registry keys are not visible when this driver is active.


Since the kernel module hides both itself and the backdoor the removal requires the computer to be started in Safe Mode. In Safe Mode the following Registry keys have to be removed:

'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\vmm32421' 'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\ierk8243'