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TUCoPS :: Windows :: a6084.htm

NT Service Killer



22th Mar 2003 [SBWID-6084]
COMMAND

	NT Service Killer

SYSTEMS AFFECTED

	Windows NT4, 2000, XP

PROBLEM

	tomotocigare [tomotocigare@securiteinfo.com] says :
	
	 http://www.securiteinfo.com
	
	Picture yourself as a win32 programmer, you  were  provided  with  local
	administrator  rights.  You  are  in  charge  of  developing  NT  system
	services, i.e. applications that  do  not  need  opened  session  to  be
	running. During the  debugging  phase,  you  might  need  to  stop  your
	service prototype. Trying to kill it  using  the  kill  command  or  the
	Windows™ NT task manager simply won't work.  In  addition  to  that  the
	Stop event cannot be reached because of any  bug  in  the  core  of  the
	executable.
	
	Imagine  you  are  a  privileged  Windows™  NT  user,  with  full  local
	administrator rights. A  virus  worm  could  be  implemented  as  an  NT
	service that your mail client will  set  up.  Such  a  service  will  be
	running in quite a malicious way. You cannot  stop  it  using  the  kill
	command nor the task manager. Moreover, the virus  programmers  "forgot"
	to handle the stop event so that  you  cannot  stop  this  very  service
	using the net stop command.
	
	You need a new tool. Such a tool is also an  NT  service  that  you  can
	register provided you have sufficient rights.  It  allows  stopping  any
	service running on your machine. It was actually validated  on  Windows™
	2000. It is supposed to work on NT 4.0 and XP.
	
	 Development 
	 ===========
	
	You may download the proof of concept from our site
	
	 http://www.securiteinfo.com/download/ntskiller.zip
	
	This tool is very easy to handle. It consists of  a  single  executable.
	First of all the service killer has to be installed  using  the  command
	line 'skill -i'. Secondly the presented  service  needs  to  be  started
	using the command line 'net start skill'. Enter the PID of  the  service
	that is to be halted in the field. You can reiterate this operation,  as
	many times as required, if you needed to  kill  several  services.  Then
	you may stop the service killer by typing 'net stop skill'.
	
	How does it work?
	
	On a Windows™ NT-based workstation, two users use the CPU.
	
	 - The currently logged on user
	 - The local system (that handles the operating system subroutines)
	
	The logged user has no impact on the local system,  even  if  this  very
	user  is  granted  with  the  administrator  rights.  This  is  a  major
	difference comparing to UNIX-based systems where the root  user  can  do
	everything. By default, a system service is  launched  under  the  local
	system account. Therefore, it can handle this account's processes.  This
	is the mean by which one can stop easily any  services,  even  if  those
	services are armed against the stop event.
	
	You can program a pesky NT service, which won't stop. To do so, you  can
	use Visual C++, create  a  new  COM  project.  Check  the  service  .exe
	option. Alter the Stop event to get the following:
	
	void CServiceApp :: Stop() {
	
	       // removed to refrain the service from stopping: if( m_hStop )
	       // removed to refrain the service from stopping
	              //::SetEvent(m_hStop);
	       ::AfxMessageBox("I refuse to stop!",MB_OK,NULL);
	} 
	
	Because of the fact that the SetEvent method is not called then  service
	is not stopped by the OS, nor the associated process.
	
	 Conclusion 
	 ==========
	
	This is a proof a concept of killing presumably protected  local  system
	services. This also highlights a system security  bias.  The  Microsoft™
	developers seem to have design a boundary between the  core  system  and
	the users' workspace in order to protect the  running  system.  This  is
	why there are always two distinct users whereas on the UNIX systems  the
	root user might ruin the system since the running OS uses the same  root
	account. However, a  bias  exists  so  that  a  programmer  can  find  a
	workaround to this designed protection.

SOLUTION

	None


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