Microsoft Security Bulletin (MS00-067)

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Patch Available for "Windows 2000 Telnet Client NTLM Authentication"
Vulnerability
Originally posted: September 14, 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:
  http://www.microsoft.com/Downloads/Release.asp?ReleaseID=24319

Note: The above URL may not be accessible.
If this is the case, please download the patch from the following
URL:
(this URL may be wrapped)
http://download.microsoft.com/download/win2000platform/patch/q272743/n
t5/en-us/q272743_w2k_sp2_x86_en.exe

Note: This patch may be applied to both Windows 2000 (pre SP1) and
Windows 2000 Service Pack 1 systems.

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.

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