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Title: Flaw in SMB Signing Could Enable Group Policy to be
Modified (309376)
Released: 11 December 2002
Revised: 22 January 2003 (version 2.0)
Software: Microsoft Windows 2000
Microsoft Windows XP
Impact: Modify group policy.
Max Risk: Moderate

Bulletin: MS02-070

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

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Reason for Revision:

Subsequent to releasing this bulletin it was determined that the
fix was not included in Microsoft Windows XP Service Pack 1. The
bulletin has been updated to reflect this, and the patch had been
updated so that it installs on Windows XP Service Pack 1 systems.
Customers who are currently running XP Service Pack 1 should apply
the patch.

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:

• 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

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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