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Microsoft Security Bulletin (MS00-067)
- -------------------------------------

Re-release: Patch Available for "Windows 2000 Telnet Client NTLM
Authentication" Vulnerability

Originally posted: September 14, 2000
Re-Released: September 21, 2000

Summary
=======
On September 14, 2000, Microsoft released the original version of
this bulletin, which was revised the following day to advise of a
problem with the patch. On September 21, 2000, a new version of the
patch was released, and the bulletin was updated to advise of its
availability. Microsoft recommends that all customers, including
those who applied the original version of the patch, apply the new
version.

The patch eliminates a security vulnerability in the telnet client
that ships with Microsoft(r) Windows 2000. The vulnerability could,
under certain circumstances, allow a malicious user to obtain
cryptographically protected logon credentials from another user.

Frequently asked questions regarding this vulnerability and
the patch can be found at
http://www.microsoft.com/technet/security/bulletin/fq00-067.asp

Issue
=====
Windows 2000 includes a telnet client capable of using NTLM
authentication when connecting to a remote NTLM enabled telnet
server. A vulnerability exists because the client will, by default,
perform NTLM authentication when connecting to the remote telnet
server. This could allow a malicious user to obtain another user's
NTLM authentication credentials without the user's knowledge.

A malicious user could exploit this behavior by creating a
carefully-crafted HTML document that, when opened, could attempt to
initiate a Telnet session to a rogue telnet server - automatically
passing NTLM authentication credentials to the malicious server's
owner. The malicious user could then use an offline brute force
attack to derive the password or, with specialized tools, could
submit a variant of these credentials in an attempt to access
protected resources.

This vulnerability would only provide the malicious user with the
cryptographically protected NTLM authentication credentials of
another user. It would not, by itself, allow a malicious user to gain
control of another user's computer. In order to leverage the NTLM
credentials (or subsequently cracked password), the malicious user
would have to be able to remotely logon to the target system.
However, best practices dictate that remote logon services be blocked
at border devices, and if these practices were followed, they would
prevent an attacker from using the credentials to logon to the target
system. Best practices also strongly recommend that Windows 2000
users logon to their hosts with User level credentials, and if these
practices were followed, they would prevent a malicious user from
obtaining Administrator level NTLM credentials.

Affected Software Versions
==========================
 - Microsoft Windows 2000

Patch Availability
==================
 - Microsoft Windows 2000:
   http://www.microsoft.com/Downloads/Release.asp?ReleaseID=24399

Note: Customers who applied the original version of the patch should
consider applying the current version.  The original version
eliminated the vulnerability; however, if a malicious user attempted
to exploit the vulnerability, the patch caused the Telnet client to
fail. The current version of the patch eliminates the vulnerability
without interfering with Telnet connections.

Note: This patch will also be included in the next Service Pack for
Windows 2000. It can be applied to computers with or without Service
Pack 1.

Note Additional security patches are available at the Microsoft
Download Center

More Information
================
Please see the following references for more information related to
this issue.
 - Frequently Asked Questions: Microsoft Security Bulletin MS00-067,
   http://www.microsoft.com/technet/security/bulletin/fq00-067.asp
 - Microsoft Knowledge Base (KB) article Q272743,
   http://www.microsoft.com/technet/support/kb.asp?ID=272743
 - Microsoft TechNet Security web site,
   http://www.microsoft.com/technet/security/default.asp

Obtaining Support on this Issue
===============================
This is a fully supported patch. Information on contacting Microsoft
Product Support Services is available at
http://support.microsoft.com/support/contact/default.asp.

Acknowledgments
===============
Microsoft thanks  DilDog of @Stake Inc. (www.atstake.com) for
reporting this issue to us and working with us to protect customers.

Revisions
=========
 - September 14, 2000: Bulletin Created.
 - September 15, 2000: Bulletin re-released to advise of problem with
patch.
 - September 21, 2000: Bulletin re-released to advise of availability
of new patch.

- -----------------------------------------------------------

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THE FOREGOING LIMITATION MAY NOT APPLY.

September 21, 2000

(c) 2000 Microsoft Corporation. All rights reserved. Terms of use.


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