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HyperTerminal - Buffer Overflow In .ht File
From: "Brett Moore" <brett.moore () security-assessment com>
Date: Wed, 15 Dec 2004 11:59:45 +1300

========================================================================
= HyperTerminal - Buffer Overflow In .ht File
=
= MS Bulletin posted: 
= http://www.microsoft.com/technet/security/bulletin/MS04-043.mspx
=
= Affected Software:
=      Microsoft Windows NT Server 4.0 SP 6a 
=      Microsoft Windows NT Server 4.0 Terminal Server Edition SP6
=      Microsoft Windows 2000 SP4
=      Microsoft Windows XP SP2 
=      Microsoft Windows XP 64-Bit Edition SP1
=      Microsoft Windows XP 64-Bit Edition Version 2003 
=      Microsoft Windows Server 2003 
=      Microsoft Windows Server 2003 64-Bit Edition
=
= Public disclosure on December 15, 2004
========================================================================

== Overview ==

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 a string from a file/packet without first
checking its length, this is what happened here.

HyperTerminal will save sessions as files with the extension of .ht which
will contain the connection info for the current session. It is then
possible to have the connection restarted by loading or executing the
saved session file.

Through the creation of a corrupt .ht file, it is possible to gain 
control of EIP and execute arbitrary code.

== Exploitation ==

It appears that a section of the heap, that is overwritten with the 
corrupt file, contains a lookup table that is later used through a
CALL [ECX+374] instruction.

This allows for exploitation even on systems like XP SP2, as the
stack/heap protection does not come into play.

Basic exploitation can be achieved through sending the target user the
corrupt file. Once the file is opened, and HyperTerminal is closed any
arbitrary code will be executed.

Remote exploitation through Internet Explorer can be obtained through the
use of an iframe or other similar object to open a file from a public
UNC share or through a 'coupled' browser exploit that saves the file to
a known location before opening it. If HyperTerminal is the current
default telnet handler, Internet Explorer will automatically open the 
corrupt file, leading to exploitation. 

There did appear to be some URL manipulation that caused the \ character
to be altered, preventing the use of the UNC share, but this filtering 
could be prevented by the use of another valid URL character.

== Solutions ==

- Install the vendor supplied patch.
- Remove the HyperTerminal application.

== Credit ==

Discovered and advised to Microsoft July 15, 2004 by Brett Moore of
Security-Assessment.com

%-) Merry Xmas guys, Ceaser, Ollie, R and the unconventional, Dave,
%-) K2 and the convers crew, DarkSpyrit and the whole eEye team.

== 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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