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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. 
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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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