Threat Description

Backdoor: W32/PcClient.YW


Category: Malware
Type: Backdoor
Platform: W32
Date Discovered: March 08, 2007
Aliases: Backdoor:W32/PcClient.YW


A remote administration utility which bypasses normal security mechanisms to secretly control a program, computer or network.


Automatic action

Once detected, the F-Secure security product will automatically disinfect the suspect file by either deleting it or renaming it.

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Technical Details

Backdoor:W32/PcClient.YW attempts to hide processes, files, and registry data. It allows the attacker to perform arbitrary actions on the infected machine. Backdoor:W32/PcClient.YW also has a rootkit functionality and steals sensitive information from an infected computer.


Once the Backdoor:W32/PcClient.YW had beenÃÂ?ÂÂ?ÃÂ? executed, it will drop its components in the following path and filename:

  • %programfiles%\internet explorer\connection wizard\zhyrikwo.dll - backdoor
  • %programfiles%\internet explorer\connection wizard\zhyrikwo.drv - keylogger

Note: the file size of zhyrikwo.dll might vary due to garbage code appended at the end of the file.It will also drop the following driver that will communicate with the .dll files in order to hide the malware processes, registry entries and files:

  • %programfiles%\internet explorer\connection wizard\zhyrikwo.sys - rootkit

Part of its payload is that it logs all the keystrokes made by the user and sends this file to a remote hacker.Another part of the payload is that it has a backdoor component. The backdoor routine is injected into svchost.exe, which is capable of doing the following:

  • updating itself
  • remote execution

This malware connects to the following site:


RegistryIt modifies the following known registry entry as its autostart technique: Data before:

  • [HKLM\SYSTEM\CurrentControlSet\Services\sens\Parameters] ServiceDll = %sysdir%\sens.dll

Data after:

  • [HKLM\SYSTEM\CurrentControlSet\Services\sens\Parameters] ServiceDll = %programfiles%\internet explorer\connection wizard\zhyrikwo.dll

The file zhyrikwo.dll will intercept any access to the original file, sens.dll. as a stealth mechanism, and after executing its malicious routines, will transfer the correct parameters to sens.dll. It also adds the following autostart registry entry for the driver:

  • [HKLM\System\ControlSet001\Services\zhyrikwo] ImagePath= %programfiles%\internet explorer\connection wizard\zhyrikwo.sys


F-Secure Anti-Virus detects this malware with the following updates:

Detection Type: PC
Database: 2007-03-07_10


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