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HtmlHelp - .CHM File Heap Overflow
From: "Brett Moore" <brett.moore () security-assessment com>
Date: Wed, 14 Jul 2004 17:36:48 +1200

========================================================================
= HtmlHelp - .CHM File Heap Overflow
=
= MS Bulletin posted: 
= http://www.microsoft.com/technet/security/bulletin/MS04-023.mspx
=
= Affected Software:
=       Microsoft Windows 98, 98SE, ME
=       Microsoft Windows NT 4.0
=       Microsoft Windows 2000 Service Pack 4
=       Microsoft Windows XP, Microsoft Windows XP Service Pack 1 
=       Microsoft Windows Server 2003 
=
= Public disclosure on July 14, 2004
========================================================================

When thinking about buffer overflow vulnerabilities, a file can sometimes
be as harmful as a packet. Even though past security issues have taught
us that it is unwise to use an unvalidated value from a file/packet as 
a text length parameter, that is what happened here.

The HtmlHelp application (hh.exe) will read a value from a .CHM file
and use this as the 'length' parameter in a REPZ MOVSD operation. 
By setting this to a large value, it is possible to overwrite sections
of the heap with attacker supplied values. 

This results in a typical win32 heap overflow landing either on the
common mov [ecx],eax / mov [eax+4],ecx pair, or on a call [eax+4]. In
either case the registers are under the control of the attacker
leading to code execution.

== Description ==

When the corrupt file is opened an exception error will first occur at

0x78010044 REPZ MOVSD

The error has occurred because the destination address has reached the end
of its allocated space. After clicking OK on the popup error box, execution
will continue until it eventually reaches.

0x77fcc663 mov [ecx],eax
0x77fcc665 mov [eax+4],ecx

At this time the EAX and ECX values have been filled with the data used to 
overwrite the heap, allowing an attacker to write an arbitrary value to 
a known place.

The corrupt file must be constructed in such a way to jump through some 
hoops first. It must pass some checks reliant on a value in the file that
sets ESI.

* This value must be valid memory
* [ESI+1c] must be non null
* [ESI+24] must be null. 

This value is simple to achieve resulting in a reliable heap exploit using
any of the multiple methods now known to exploit heap overflows.

== Exploitation ==

Remote exploitation through Internet Explorer can be obtained through the
use of the window.showhelp() function. Either using a public UNC share or
through a 'coupled' browser exploit that saves the file to a known location
before opening it. There may of course also be other ways of having a
corrupt .CHM file loaded without requiring a user to download and run it,
although a compiled help file may be easily accepted by a user anyway.

Automatic exploitation of browser based bugs, does not rely on an attacker
sending a link, requiring the target user to click on it. Links, references
and other objects can easily be opened through script code. And I am told
that this can also be achieved without script code.

== Solutions ==

- Install the vendor supplied patch.

== Credit ==

Discovered and advised to Microsoft January 12, 2004 by Brett Moore of
Security-Assessment.com

%-) Well Ruxcon rocked, so gotta say thanks to all that were there. They
%-) known who they are. Now for vegas....

== About Security-Assessment.com ==

Security-Assessment.com is a leader in intrusion testing and security
code review, and leads the world with SA-ISO, online ISO17799 compliance
management solution. Security-Assessment.com is committed to security
research and development, and its team have previously identified a
number of vulnerabilities in public and private software vendors products.


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