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IIS .HTR Remote Buffer Overflow
13th Jun 2002 [SBWID-5429]

	IIS .HTR Remote Buffer Overflow


	 Microsoft Windows NT 4.0 Internet Information Services 4.0

	 Microsoft Windows 2000 Internet Information Services 5.0



	In eEye Digital Security Advisory AD20020612 [http://www.eEye.com]:

	A  vulnerability  in  transfer  chunking,  in   combination   with   the
	processing of HTR request sessions can be exploited to remotely  execute
	code of an attackers choice on the  vulnerable  machine.  By  sending  a
	carefully crafted session, an attacker can overwrite a  section  of  the
	heap. Data structures in the overwritten  heap  can  be  manipulated  to
	move attacker-supplied  data  to  attacker  supplied  memory  addresses,
	thereby altering  the  flow  of  execution  into  an  attacker  supplied

	This  is  a  very  serious  vulnerability   and   eEye   suggests   that
	administrators  install  the  Microsoft  supplied  patch  as   soon   as

	The  following  example  will  show  the   vulnerable   condition.   The
	dllhost.exe child process will silently die because the developers  have
	replaced the default exception filter. So if you want  to  examine  this
	closer, load a debugger up on the dllhost child process before you  send
	this example session over the wire.


	**************Begin Session****************

	POST /EEYE.htr HTTP/1.1

	Host: 0day.big5.com

	Transfer-Encoding: chunked







	**************End Session******************



	 Technical Description:


	The example  session  above  overwrites  a  section  of  the  heap  that
	contains data structures related to the  memory  management  system.  By
	manipulating the  content  of  these  structures  we  can  overwrite  an
	arbitrary 4 bytes of memory with an attacker supplied address.

	While many may believe that the risk for these types of  vulnerabilities
	is fairly low due to the fact  that  addressing  is  dynamic  and  brute
	force techniques would need to  be  use  in  an  attack,  eEye  strongly
	disagrees. This premise is false as successful exploitation can be  made
	with one attempt, across dll versions. An attacker can overwrite  static
	global  variables,  stored   function   pointers,   process   management
	structures, memory management structures, or any number  of  data  types
	that will allow him to gain control of the  target  application  in  one

	SecureIIS(tm) Application Firewall for Microsoft IIS

	It should be noted that clients using  any  version  of  SecureIIS  from
	eEye  Digital  Security  are  secure  from  this   vulnerability.   This
	vulnerability was discovered by  the  eEye  team  while  testing  a  new
	version of SecureIIS to  help  further  its  protection  abilities  from
	similar    classes    of     attack.     To     learn     more     visit

	 Vendor Status:

	Microsoft has released a security bulletin and patch:




	Beyond installing the Microsoft security patch it is also  recommend  to
	disable the  .htr  ISAPI  filter  if  you  have  not  already  done  so.
	Microsoft’s security advisory references more information on  the  steps
	of how to disable the .htr ISAPI filter.

	Credit: Riley Hassell

	Greetings: Caesar, K2, Dark Spyrit, Solar Designer, Joey, Halvar,  Gera,
	Scut, Ilfak Guilfanov. And last but not least, Kasia and Jenn ;) and  as
	always, www.securityfocus.com.

	 Copyright (c) 1998-2002 eEye Digital Security

	Permission is hereby  granted  for  the  redistribution  of  this  alert
	electronically. It is not to  be  edited  in  any  way  without  express
	consent of eEye. If you wish to reprint the whole or any  part  of  this
	alert in any other medium excluding  electronic  medium,  please  e-mail
	alert@eEye.com for permission.


	The information within this paper may  change  without  notice.  Use  of
	this information constitutes acceptance for use in an AS  IS  condition.
	There are NO warranties with regard to this  information.  In  no  event
	shall the author be liable for any damages whatsoever arising out of  or
	in connection with the use or spread of this  information.  Any  use  of
	this information is at the user\'s own risk.


	Please send suggestions, updates, and comments to:


	eEye Digital Security






	 Update (18 June 2002)



	Brett Moore adds :

	This following applies to the recent .asp and .htr Buffer Overflows  for
	IIS and possibly other similar vulnerabilities.

	From  the  eEye  Advisory:  \"By  manipulating  the  content  of   these
	structures we can overwrite an arbitrary  4  bytes  of  memory  with  an
	attacker supplied address.\"

	This statement is misleading and should read similar to:

	\"we can overwrite multiple  memory  addresses  with  attacker  supplied

	So what is the difference?

	From the Eeye Advisory:

	\"While  many  may  believe  that  the   risk   for   these   types   of
	vulnerabilities is fairly  low  due  to  the  fact  that  addressing  is
	dynamic and brute force techniques would need to be used in  an  attack,
	eEye  strongly  disagrees.  This  premise   is   false   as   successful
	exploitation can be made with one attempt, across dll versions.\"

	The recent .asp exploits that I have seen all work  in  a  similar  way.
	They overwrite the exception handler, which is a static  memory  address
	with the address of the payload. Then when the  execption  happens  code
	execution jumps to this address. This address is dynamic,  so  to  reach
	this address exploits can

	1) Hard code the address as in;

	 IIS5.0 .asp overrun remote exploit

	 Programmed by hsj  : 02.04.14

	 #define RET                 0x0045C560  /* our payload. ugh, direct


	2) Brute force the address as in;

	 IIS5.0 .asp overrun remote brute force exploit

	 by isno(isno@xfocus.org)

	 #define RET                 0x00450000  /* brute force addr */

	 #define STEP                2000  /* brute force step */


	3) Find a static address that has the required code  to  do  a  relative
	jump as in

	 * the address of our payload is at [esp+xx]

	 * we find a static location with the instuctions for jmp [esp+xx].


	While #3 is obviously the best way, it is not always  possible  to  find
	the instructions required.

	But because we can write to multiple addresses an exploit can work  like

	 * locate the static memory address for the exception handler

	 * locate another static memory address

	 * overwrite the exception handler with the second address

	 * overwrite the second address with the required instructions for our

	   relative jmp

	 * cause an exception


	The result?

	A security vulnerability is always a serious issue. The fact that  there
	are not always exploits  created  and  made  available  to  the  public,
	should in no way suggest that the risk is lower.

	Where did I find this?

	It was there inside my computer just waiting for me. And I could  almost
	say 100% that \"I\'m not the only one to know  this\".  But  it  is  not
	something that I have seen on any resource for IT Security and  as  such
	the general impression has been that vulnerabilities of  this  type  are
	\"Low Risk because of the dynamic address issue\". So  either  only  the
	true  underground  know  about  this  or  perhaps  the  professional  IT
	security industry is somewhat like the NSA,FBI,CIA and doesn\'t like  to
	share information.

	Microsoft Resources


	Title:      Heap Overrun in HTR Chunked Encoding Could Enable Web

	            Server Compromise (Q321599)

	Date:       12 June 2002

	Max Risk:   Moderate

	Bulletin:   MS02-028

	Mitigating Factors:


	 - On default installations of IIS 5.0, exploiting the

	   vulnerability to run code would grant the attacker the privileges

	   of the IWAM_computername account, which has only the privileges

	   commensurate with those of an interactively logged-on

	   unprivileged user.


	Title:      Unchecked Buffer in Remote Access Service Phonebook Could

	            Lead to Code Execution (Q318138)

	Date:       12 June 2002

	Impact:     Local Privilege Escalation

	Max Risk:   Critical

	Bulletin:   MS02-029

	Mitigating Factors:


	 - The vulnerability could only be exploited by an attacker who had

	   the appropriate credentials to log onto an affected system.

	 - Best practices suggests that unprivileged users not be allowed to

	   interactively log onto business-critical servers. If this

	   recommendation has been followed machines such as domain

	   controllers, ERP servers, print and file servers, database

	   servers, and others would not be at risk from this vulnerability.


	So put the together the two vulnerabilities that were  released  on  the
	same day, and we have a remote system level exploit  for  IIS?  Yet  the
	HTR Buffer Overflow only has a Max Risk:   Moderate.


	Apply Microsoft patch:



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