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Title: Web Client NTLM Authentication Vulnerability
Date: January 11, 2001
Software: Office 2000, Windows 2000, and Windows Me
Impact: NTLM Credentials sent regardless of prompt setting
Bulletin: MS01-001

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-001.asp.

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Issue:

The Web Extender Client (WEC) is a component that ships as part of
Office 2000, Windows 2000, and Windows Me. WEC allows IE to view and
publish files via web folders, similar to viewing and adding files in
a directory through Windows Explorer. Due to an implementation flaw,
WEC does not respect the IE Security settings regarding when NTLM
authentication will be performed - instead, WEC will perform NTLM
authentication with any server that requests it. If a user
established a session with a malicious user's web site - either by
browsing to the site or by opening an HTML mail that initiated a
session with it - an application on the site could capture the user's
NTLM credentials. 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.

The 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 or to gain access to resources to
which that user was authorized access. In order to leverage the NTLM
credentials (or a 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.

Mitigating Factors:

• The client would need to be coerced into visiting a malicious web
  site
  or read malicious e-mail.

Patch Availability:

• A patch is available to fix this vulnerability. Please read the
  Security Bulletin
  http://www.microsoft.com/technet/security/bulletin/ms01-001.asp
  for information on obtaining this patch.

Acknowledgment:

• David Litchfield (http://www.atstake.com)

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