[NT] Flaw in SMB Signing Could Enable Group Policy to be Modified

From: support@securiteam.com
Date: 12/12/02

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    Date: 12 Dec 2002 17:44:42 +0200

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      Flaw in SMB Signing Could Enable Group Policy to be Modified


    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.


    Affected Software:
     * Microsoft Windows 2000
     * Microsoft Windows XP

    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.

    Patch availability:
    Download locations for this patch
    Microsoft Windows 2000:
     * All languages except NEC Japanese -
     * Japanese NEC -

    Microsoft Windows XP:
     * 32-bit Edition -
     * 64-bit Edition -

    What's the scope of the vulnerability?
    This vulnerability could provide a means by which a communications channel
    that is ostensibly cryptographically protected could in reality be
    unprotected. In the most serious case, exploiting the vulnerability could
    enable an attacker to change security policy information as it flowed from
    a Windows 2000 domain controller to a newly logged on system, thereby
    gaining the ability to take actions such as installing and running
    programs on the system.

    Exploiting the vulnerability would require the attacker to not only have
    access to the network's communications media, but to also have a favorable
    location within the network. In addition, the attacker would need to
    exploit the vulnerability separately for every communications session he
    or she wanted to modify.

    What causes the vulnerability?
    The vulnerability results because of an error in the Windows 2000 and
    Windows XP implementations of the SMB signing function that could cause
    signing to not be done even when it's configured to always be required.

    What is SMB?
    SMB (Server Message Block) is a file-sharing protocol that is natively
    supported in all versions of Windows. SMB (and its follow-on, Common
    Internet File System) allows users and computers to efficiently locate and
    access files on other systems, and work with the data in them without
    creating conflicts with other users.

    What is SMB signing?
    SMB signing is a feature available in Windows 2000 and Windows XP, through
    which all communications using the Server Message Block (SMB) protocol can
    be digitally signed at the packet level. Digitally signing the packets
    enables the recipient of the packets to confirm their point of origination
    and their authenticity - that is that they came from the expected location
    and haven't been tampered with in transit.

    Why would someone want to digitally sign SMB traffic?
    The most straightforward reason is that a user might want to ensure that
    any files he or she retrieves from, for instance, a file server haven't
    been altered in transit by an attacker on the network. But there's a more
    pressing reason. Windows domains use SMB to transfer some types of
    security related information. For instance, when a workstation logs onto a
    domain, the domain controller sends group policy information to the
    workstation via SMB. Signing provides a way to ensure that the workstation
    is receiving bona fide group policy.

    How is SMB Signing configured?
    SMB Signing is configured via local policy settings (specifically,
    Computer Configuration\Windows Settings\Security Settings\Local
    Policies\Security Options). There are four settings available: two govern
    how the system operates when acting as a client, and two govern how it
    operates when acting as a server. In each case, the system can be
    configured to allow, disallow, or require signing. The system defaults for
    both Windows 2000 and Windows XP enable signing but do not require it.

    What's wrong with the way SMB Signing is implemented?
    There's a flaw in the way Windows 2000 and Windows XP respond in the case
    where a system has been configured to require signing. By design, when a
    Windows 2000 or Windows XP system establishes an SMB session with another
    system, it carries out a negotiation to determine what the other system's
    signing requirements are. If the other system can't or won't meet its
    needs (for instance, if a Windows XP system were configured to require
    signing but the other system was configured never to sign SMB data), it
    should refuse to establish the communications channel.

    A security vulnerability results because, if the negotiation information
    were malformed in a particular way, a Windows 2000 or Windows XP system
    that had been configured to require signing could instead silently drop
    the requirement, and engage in unsigned communications despite the local
    policy settings.

    What could an attacker do via this vulnerability?
    An attacker could use this vulnerability to cause SMB signing on a Windows
    2000 or Windows XP system to be silently disabled for the duration of an
    SMB session, with the result that the subsequently transmitted data would
    be unsigned. This would enable anyone who could access the data to modify
    it as it transited the network.

    All SMB sessions, whether used to transfer files, group policy, or some
    other information, could be affected by the vulnerability. However, it
    would pose the greatest risk in the case of group policy - specifically,
    where a Windows 2000 domain controller had been configured to require SMB
    signing, as a way of ensuring the authenticity of the group policy
    settings it downloads to domain members. By exploiting the vulnerability
    and then subsequently changing the group policy data as it was sent from
    the domain controller to a client, the attacker could change the policy
    settings that the client received.

    What would be the effect of changing the group policy settings?
    It would depend on the specific changes. However, group policy is a
    powerful technology through which much of the behavior of Windows systems
    can be controlled. For instance, group policy can be used to change
    permissions on folders and files, download and run programs at system
    startup, and take other actions.

    Would the attacker be able to use the vulnerability to gain administrative
    privileges on the domain?
    No. Domain group memberships are held at the domain controller, not the
    client. Changing the group policy that's downloaded to the client wouldn't
    allow the attacker to change his or her domain group memberships. On the
    other hand, the attacker could use the vulnerability to change the group
    memberships on the local machine and, for instance, add users to the local
    Administrators group.

    Who could exploit the vulnerability?
    In order to exploit the vulnerability, the attacker would need to already
    have a significant degree of access to communications on the network. He
    or she would need to be able to monitor and modify the communications
    between the two systems in real-time. This would typically require the
    attacker to not only have physical access to the network media, but a
    favorable location within the network as well.

    What do you mean "a favorable location within the network"?
    It wouldn't be enough for the attacker to have access to the network
    media. He or she would also have to be located along the path taken by the
    data as it passed between the client and the server. The vulnerability
    provides no way for the attacker to force the communications to take a
    particular path so, in most cases, he or she would need to be located on
    the same network segment as one of the two communicants.

    Could the attacker change group policy on a system that was already logged
    onto the domain?
    No. The opportunity to impose new policy would occur when a new system
    logged onto the domain. If the attacker missed the opportunity, he or she
    would need to wait until the next time the system logged on.

    Would exploiting the vulnerability once permanently disable SMB signing?
    No. The attacker would need to exploit the vulnerability separately for
    each communications session that he or she wanted to interfere in.

    Could the attacker change the group policy on the Windows 2000 domain
    No. The vulnerability would only allow the attacker to change the data
    received by the client. It would not provide any way to change the data as
    it resides on the server.

    Why have you discussed the domain controller scenario only in the context
    of Windows 2000?
    Windows XP cannot be used as a domain controller. As a result, this
    scenario - which is the highest-risk scenario associated with the
    vulnerability - doesn't apply to Windows XP.

    I heard that Windows XP clients can inadvertently trigger the
    vulnerability. Is this true?
    Yes. Windows XP Service Pack 1 contained a regression error that adds
    information to the negotiation information it sends. This information can
    trigger the vulnerability, and cause systems running Windows XP Gold or
    Windows 2000 to drop SMB signing. A fix is available to eliminate this
    regression error.

    It is important to understand two critical points regarding the regression
    error in Windows XP Service Pack 1:
     * The regression error poses no security threat to Windows XP Service
    Pack 1 systems. Instead, it poses an availability risk. Consider a
    scenario where both a Windows XP SP1 system and a Windows 2000 domain
    controller were configured to require signing. The regression could cause
    the Windows 2000 system to downgrade its signing, which the Windows XP SP1
    system would then reject. The result would be that the communications
    couldn't occur.

     * Installing the fix for the regression error in Windows XP doesn't
    eliminate the vulnerability. The vulnerability lies in Windows XP Gold and
    Windows 2000; the regression error simply triggers it in some cases. If
    you apply the security patch to all Windows XP Gold and Windows 2000
    systems that require signing, it doesn't matter whether you apply the
    regression fix to your Windows XP SP1 systems - if no systems on the
    network have the vulnerability, it doesn't matter whether there are
    systems that could inadvertently trigger it.

    I don't understand. You said in the preceding question that Windows XP
    Service Pack 1 has a regression error associated with the vulnerability,
    but in the "Additional Information About This Patch" section you say that
    it doesn't need the patch. Why is this?
    Windows XP Service Pack 1 does not have the vulnerability. That is, no
    matter what negotiation information is sent to it, Windows XP Service Pack
    1 will not silently drop SMB Signing. However, it does have an error that
    can cause it to send negotiation information that exploits the
    vulnerability in other systems.

    Should I apply the patch to every Windows 2000 or Windows XP system?
    You can, but it only makes sense to install it on systems that are
    configured to require SMB Signing, or that communicate with systems that

    Is SMB signing required by default?
    No. There is a performance penalty associated with SMB signing, and as a
    result it is not required in default configurations of Windows 2000 or
    Windows XP.

    SMB signing isn't enabled on my systems. Am I at any risk from this
    If SMB signing isn't enabled, your network is at no increased risk because
    of this vulnerability. That is, if signing is disabled, an attacker could
    already modify the SMB data stream.

    How does the patch address the vulnerability?
    The patch causes Windows 2000 and Windows XP to correctly handle
    negotiation sessions that contain the type of malformed information
    discussed above.


    The information has been provided by
    <mailto:0_42043_E51E4D7D-DECD-43AE-9A29-36080E8D4C3C_US@Newsletters.Microsoft.com> Microsoft Product Security.


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