[VulnWatch] Cisco Security Advisory: Denial-of-Service of TCP-based Services in CatOS

From: Cisco Systems Product Security Incident Response Team (psirt_at_cisco.com)
Date: 07/09/03

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    Cisco Security Advisory: Denial-of-Service of TCP-based Services in CatOS

    Revision 1.0

    For Public Release 2003 July 09 at 16:00 UTC (GMT)

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

    After receiving eight TCP connection attempts using a non-standard TCP
    flags combination, a Catalyst switch will stop responding to further TCP
    connections to that particular service. In order to re-establish
    functionality of that service, the switch must be rebooted. There is no
    workaround. This vulnerability affects only CatOS. No other Cisco
    products are affected.

    This advisory is available at

    Affected Products
    The CatOS for the following Catalyst models are affected:

      * Catalyst 4000 Series including models 2948G and 2980G/2980G-A
      * Catalyst 5000 Series including models 2901, 2902 and 2926
      * Catalyst 6000
    No other Cisco products are affected.

    After receiving eight connection attempts on any TCP service, the switch
    will stop responding to any further connection attempts to that service.
    These attempts must use a non-standard combination of TCP flags. The
    switch will continue to pass other switched traffic normally and the
    console is also not affected. Only the service to which connections were
    made will become unresponsive. Standard TCP services include HTTP,
    Telnet, and SSH.

    This vulnerability is documented as Cisco Bug ID CSCdw52219 (registered
    customers only) .

    By exploiting this vulnerability, an attacker can prevent further use of
    the specified TCP-based service. Depending on the configuration of the
    device, if SSH or Telnet are enabled and exploited, the availability of
    those services could be affected, possibly resulting in a loss of
    management capability using those same services. However, UDP-based
    services such as Simple Network Management Protocol (SNMP) would still be
    available and unaffected.

    Software Versions and Fixes
    The vulnerability is fixed in the following releases.

    | | Description | |
    | Train | of Image or | Availability of Fixed Releases* |
    | | Platform | |
    | | | Rebuild | Interim | Maintenance |
    | | | | ** | |
    | | Catalyst | | 5.5 | |
    | 5.5 | 4000, 5000, | | (13.5) | 5.5(14) |
    | | 6000 | | | |
    | | Catalyst | | 6.3 | |
    | 6.3 | 4000, 5000, | | (5.10) | 6.3(6) |
    | | 6000 | | | |
    | | Catalyst | |
    | 6.4 | 4000, 5000, | Not affected |
    | | 6000 | |
    | 7.2 | Catalyst | | 7.2 | 7.2(1) |
    | | 4000, 6000 | | (0.65) | |
    | | Catalyst | | 7.4 | |
    | 7.4 | 4000, 6000 | | (0.2) | 7.4(1) |
    | | | | CLR | |
    | 8.1 | Catalyst | Not affected |
    | | 6000 | |

    * All dates are estimates and subject to change.

    ** Interim releases are subjected to less rigorous testing than regular
    maintenance releases, and may have serious bugs.

    Obtaining Fixed Software
    Cisco is offering free software upgrades to address these vulnerabilities
    for all affected customers. Customers may only install and expect support
    for the feature sets they have purchased. By installing, downloading,
    accessing or otherwise using such software upgrades, Customers agree to
    be bound by the terms of Cisco software license terms found at
    http://www.cisco.com/public/sw-license-agreement.html, or as otherwise set
    forth at the Cisco Connection Online Software Center at

    Customers with service contracts should contact their regular update
    channels to obtain the free software upgrade identified via this
    advisory. For most customers with service contracts, this means that
    upgrades should be obtained through the Software Center on the Cisco
    worldwide website at http://www.cisco.com/tacpage/sw-center/sw-lan.shtml.
    To access the software download URL, you must be a registered user and
    you must be logged in.

    Customers whose Cisco products are provided or maintained through prior
    or existing agreement with third-party support organizations such as
    Cisco Partners, authorized resellers, or service providers should contact
    that support organization for assistance with the upgrade, which should
    be free of charge.

    Customers who purchase direct from Cisco but who do not hold a Cisco
    service contract and customers who purchase through third-party vendors
    but are unsuccessful at obtaining fixed software through their point of
    sale should get their upgrades by contacting the Cisco Technical
    Assistance Center (TAC). TAC contacts are as follows.

      * +1 800 553 2447 (toll free from within North America)
      * +1 408 526 7209 (toll call from anywhere in the world)
      * e-mail: tac@cisco.com
    Please have your product serial number available and give the URL of this
    notice as evidence of your entitlement to a free upgrade. Free upgrades
    for non-contract customers must be requested through the TAC.

    Please do not contact either "psirt@cisco.com" or
    "security-alert@cisco.com" for software upgrades.

    There is no workaround. In order to continue using an affected TCP
    service, the switch must be rebooted.

    It is possible to mitigate the exposure by configuring VLAN Access
    Control Lists (VACLs) on the switch (where they are supported) that will
    allow only legitimate hosts to connect to the desired services. This must
    be combined with Unicast Reverse Path Forwarding (uRPF), or some other
    anti-spoofing technique, on the network edge to protect against spoofed
    packets from the outside of the network.

    Exploitation and Public Announcements
    This vulnerability has been reported to Cisco by a customer. The Cisco
    PSIRT has received no reports of malicious exploitation of this
    vulnerability and we are not aware of any public discussion.

    Status of This Notice: FINAL
    This is a final advisory. Although Cisco cannot guarantee the accuracy of
    all statements in this advisory, all of the facts have been checked to
    the best of our ability. Cisco does not anticipate issuing updated
    versions of this advisory unless there is some material change in the
    facts. Should there be a significant change in the facts, Cisco will
    update this advisory.

    A stand-alone copy or paraphrase of the text of this security advisory
    that omits the distribution URL in the following section is an
    uncontrolled copy, and may lack important information or contain factual

    This advisory will be posted on the Cisco Worldwide Web site at

    In addition to Worldwide Web posting, a text version of this notice is
    clear-signed with the Cisco PSIRT PGP key and is posted to the following
    e-mail and Usenet news recipients.

        * cust-security-announce@cisco.com

        * bugtraq@securityfocus.com

        * full-disclosure@lists.netsys.com

        * first-teams@first.org (includes CERT/CC)

        * cisco@spot.colorado.edu

        * cisco-nsp@puck.nether.net

        * comp.dcom.sys.cisco

        * Various internal Cisco mailing lists

    Future updates of this advisory, if any, will be placed on the Cisco
    Worldwide Web server, but may or may not be actively announced on mailing
    lists or newsgroups. Users concerned about this problem are encouraged to
    check the URL given above for any updates.

    Revision History
    | Revision | 2003-July-09 | Initial |
    | 1.0 | 16:00 UTC | public |
    | | (GMT) | release |

    Cisco Security Procedures

    Complete information on reporting security vulnerabilities in Cisco
    products, obtaining assistance with security incidents, and registering
    to receive security information from Cisco, is available on the Cisco
    Worldwide Web site at
    This includes instructions for press inquiries regarding Cisco
    security notices.

    All Cisco Security Advisories are available at

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

    This notice is Copyright 2003 by Cisco Systems, Inc. This notice may be
    redistributed freely after the release date given at the top of the text,
    provided that redistributed copies are complete and unmodified, and
    include all date and version information.

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