Re-Release of Microsoft Security Bulletin (MS00-067)

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Microsoft is re-releasing this Bulletin to update important
information in the Patch Availability section.
- --------------------------------------------------------

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

Patch Available for "Windows 2000 Telnet Client NTLM Authentication"
Vulnerability
Originally posted: September 14, 2000
Re-Released: September 15, 2000

Summary
=======
Microsoft has released a patch that 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:

Due to continuing operational issues with the Microsoft.com download
servers, the final patch for this issue was not uploaded to the
download servers. Instead, a beta version of the patch was made
available. This patch has subsequently been removed.

Those who have downloaded and applied the beta patch are protected
from the vulnerability discussed in this Bulletin. The beta patch
will prompt users before passing NTLM credentials to the remote
server and
will only present NTLM credentials if approved by the user, as
discussed in the FAQ.

A bug exists in the beta patch wherein the telnet client may crash
while requesting an NTLM authentication session with a non-Windows
2000 NTLM enabled telnet server. At no time will NTLM credentials be
passed to the remote server if the user does not specifically choose
to send the credentials.

The correct version of the patch will uploaded to the download center
shortly. Users who have installed the beta patch can overwrite that
version with the soon to be released final version.

Users without the beta patch can perform the following workaround
until such time as the final patch can be made available.

To disable NTLM authentication, perform the following steps:

 - Type 'telnet' at the command prompt.
 - Type 'unset ntlm' and hit Enter.
 - Type 'quit' to exit telnet and save your preferences.

To determine what form of authentication you are currently using,
perform the following steps:
 - Type 'telnet' at a command prompt.
 - Type 'display' at the telnet prompt.
   - A value of 'Will Auth (NTLM Authentication)' means telnet will
     use NTLM authentication by default.
   - A value of 'Not Auth (NTLM Authentication)' means telnet will
     not use NTLM authentication.

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 article Q272743 discusses this issue and
   will be available soon.
 - 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 update Patch
Availability.

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

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


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