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  Windows 2000 SMB signing protection bypass

  Microsoft Security Bulletin MS02-070: Flaw in SMB Signing Could Enable Group Policy to be Modified (309376)

From:MICROSOFT <secure_(at)_microsoft.com>
Date:14.12.2002
Subject:Microsoft Security Bulletin MS02-070: Flaw in SMB Signing Could Enable Group Policy to be Modified (309376)

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- ----------------------------------------------------------------------
Title:      Flaw in SMB Signing Could Enable Group Policy to be
           Modified (309376)
Date:       11 December 2002
Software:   Windows 2000, Windows XP
Impact:     Modify group policy
Max Risk:   Moderate
Bulletin:   MS02-070

Microsoft encourages customers to review the Security Bulletins at:
http://www.microsoft.com/technet/security/bulletin/MS02-070.asp
http://www.microsoft.com/security/security_bulletins/ms02-070.asp.
- ----------------------------------------------------------------------

Issue:
======
Server Message Block (SMB) is a protocol natively supported by all
versions of Windows. Although nominally a file-sharing
protocol, it is used for other purposes as well, the most important
of which is disseminating group policy information from
domain controllers to newly logged on systems. Beginning with Windows
2000, it is possible to improve the integrity of SMB
sessions by digitally signing all packets in a session. Windows 2000
and Windows XP can be configured to always sign, never
sign, or sign only if the other party requires it.

A flaw in the implementation of SMB Signing in Windows 2000 and
Windows XP could enable an attacker to silently downgrade the
SMB Signing settings on an affected system. To do this, the attacker
would need access to the session negotiation data as it
was exchanged between a client and server, and would need to modify
the data in a way that exploits the flaw. This would
cause either or both systems to send unsigned data regardless of the
signing policy the administrator had set. After having
downgraded the signing setting, the attacker could continue to
monitor the session and change data within it; the lack of
signing would prevent the communicants from detecting the changes.

Although this vulnerability could be exploited to expose any SMB
session to tampering, the most serious case would involve
changing group policy information as it was being disseminated from a
Windows 2000 domain controller to a newly logged-on
network client. By doing this, the attacker could take actions such
as adding users to the local Administrators group or
installing and running code of his or her choice on the system.

Mitigating Factors:
====================
- A fix for this issue is already included in Windows
  XP Service Pack 1.
- Exploiting the vulnerability would require the attacker
  to have significant network access already. In most
  cases, the attacker would need to be located on the
  same network segment as one of the two participants
  in the SMB session.
- The attacker would need to exploit the vulnerability
  separately for each SMB session he or she wanted to
  interfere with.
- The vulnerability would not enable the attacker to
  change group policy on the domain controller, only
  to change it as it flowed to the client.
- SMB Signing is disabled by default on Windows 2000
  and Windows XP because of the performance penalty
  it exacts. On networks where SMB Signing has not been
  enabled, the vulnerability would pose no additional
  risk - because SMB data would already be vulnerable
  to modification.

Risk Rating:
============
- Windows 2000: Moderate
- Windows XP: Low
- Windows XP Service Pack 1: None

Patch Availability:
===================
- A patch is available to fix this vulnerability. Please read the
  Security Bulletin at
  http://www.microsoft.com/technet/security/bulletin/ms02-070.asp
  for information on obtaining this patch.

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

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PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS
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WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS
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INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR
SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE
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LIMITATION MAY NOT APPLY.

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