F-Secure Virus Descriptions : Krei
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Krei is a backdoor that provides access to the compromised computer for
remote attackers. Krei affects Windows 2000 and XP systems only.
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:
'HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\vmm32421'
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.
Removal
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'
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
[Analysis: Gergely Erdelyi; F-Secure Corp, February 3rd, 2003]
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