Cisco Security Advisory: Vulnerabilities in Kerberos 5 Implementation

From: Cisco Systems Product Security Incident Response Team (
Date: 08/31/04

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    Date: Tue, 31 Aug 2004 15:13:00 -0400 

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    Cisco Security Advisory: Vulnerabilities in Kerberos 5 Implementation

    Revision 1.0

    For Public Release 2004 August 31 1830 UTC (GMT)

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


        Affected Products
        Software Versions and Fixes
        Obtaining Fixed Software
        Exploitation and Public Announcements
        Status of This Notice: FINAL
        Revision History
        Cisco Security Procedures

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


    Two vulnerabilities in the Massachusetts Institute of Technology
    (MIT) Kerberos 5 implementation that affect Cisco VPN 3000 Series
    Concentrators have been announced by the MIT Kerberos Team.

    Cisco VPN 3000 Series Concentrators authenticating users against a
    Kerberos Key Distribution Center (KDC) may be vulnerable to remote code
    execution and to Denial of Service (DoS) attacks. Cisco has made free
    software available to address these problems.

    Cisco VPN 3000 Series Concentrators not authenticating users against a
    Kerberos Key Distribution Center (KDC) are not impacted.

    No exploitations of these vulnerabilities have been reported.

    This advisory is available at

    Affected Products

    Vulnerable Products
    - -------------------

    The following products have their Kerberos 5 implementation based on MIT
    Kerberos code and are affected by these vulnerabilities:

      * Cisco VPN 3000 Series Concentrators. All 4.0.x software versions prior
        to 4.0.5.B and all 4.1.x software versions prior to 4.1.5.B are
        vulnerable. Versions prior to 4.0.x are not vulnerable since they do
        not support Kerberos authentication.

    Note that vulnerable products are impacted only if they are configured
    to authenticate users against a Kerberos KDC.

    Products Confirmed Not Vulnerable
    - ---------------------------------

    The following products have Kerberos 5 support, but their implementation
    is not based on MIT Kerberos, and therefore are not affected by the
    vulnerabilities discussed in this advisory:

      * Cisco IOS (Kerberos support available in release 11.2 or later)
      * Cisco CatOS

    The following products do not have Kerberos 5 support and therefore are
    not affected by these vulnerabilities:

      * Cisco PIX Firewall
      * Cisco Firewall Services Module (FWSM) for the Cisco Catalyst 6500
        Series and Cisco 7600 Series routers

    No other Cisco products are currently known to be affected by these


    Kerberos is a secret-key network authentication protocol developed at
    the Massachusetts Institute of Technology (MIT) that uses the Data
    Encryption Standard (DES) cryptographic algorithm for encryption and
    authentication. Kerberos was designed to authenticate requests for
    network resources. Kerberos, like other secret-key systems, is based on
    the concept of a trusted third party that performs secure verification
    of users and services. In the Kerberos protocol, this trusted third
    party is called the Key Distribution Center (KDC).

    The primary use of Kerberos is to verify that users and the network
    services they use are really who and what they claim to be. To
    accomplish this, a trusted Kerberos server issues tickets to users.
    These tickets, which have a limited lifespan, are stored in a
    user's credential cache and can be used in place of the standard
    username-and-password authentication mechanism.

    The Kerberos credential scheme embodies a concept called "single logon."
    This process requires authenticating a user once, and then allows secure
    authentication (without encrypting another password) wherever that
    user's credential is accepted.

    Vulnerable Cisco devices using versions of Kerberos based on the
    MIT implementation to authenticate users are affected by two
    vulnerabilities. The first vulnerability consists of a double-free error
    that can happen under certain error conditions, and that can potentially
    allow a remote attacker to execute arbitrary code.

    The second vulnerability consists of an infinite loop in the Abstract
    Syntax Notation (ASN) 1 decoder that can be entered upon receipt of an
    ASN.1 SEQUENCE type with invalid Basic Encoding Rules (BER) encoding.
    This vulnerability can be exploited by an attacker impersonating a
    legitimate Kerberos KDC or application server to cause a client program
    to hang inside an infinite loop, and thus creating a Denial of Service
    condition. This vulnerability can also be exploited to cause a KDC or
    application server to hang inside an infinite loop.

    More information about these MIT
    Kerberos vulnerabilities is available at
    The information in these links is provided by MIT.

    The two vulnerabilities described above are documented for the Cisco VPN
    3000 Series Concentrators in Bug IDs CSCef24692 and CSCef24900.


    An exploitation of the double-free vulnerability could potentially give
    an attacker control of the Cisco device and potentially compromise an
    entire Kerberos realm.

    An exploitation of the "infinite loop in the ASN.1 decoder"
    vulnerability could potentially take out of service an affected product.
    The vulnerability could potentially be repeatedly exploited to keep the
    product out of service until an upgrade can be performed.

    Software Versions and Fixes

    The vulnerabilities described in this advisory are fixed in software
    versions 4.0.5.B and later and 4.1.5.B and later of the Cisco VPN 3000
    Series Concentrators. If you are currently running the identified
    vulnerable software, you should obtain fixed software, as detailed

    Obtaining Fixed Software

    Customers with Service Contracts
    - --------------------------------

    Customers with Cisco service contracts should obtain upgraded software
    through their regular update channels. For most customers, this means
    that upgrades should be obtained through the Software Center on Cisco's
    worldwide website at

    Customers using Third-party Support Organizations
    - -------------------------------------------------

    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 without Service Contracts
    - -----------------------------------

    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:

    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 "" or
    "" for software upgrades.


    There is no workaround available to mitigate the effects of this
    vulnerability Affected users should upgrade to a fixed version of the
    affected software.

    Exploitation and Public Announcements

    The Cisco PSIRT is not aware of any malicious exploitation of the
    vulnerabilities described in this advisory.

    These vulnerabilities were reported by the MIT Kerberos Team in concert
    with the CERT Coordination Center.

    These vulnerabilities may impact other products that are not provided
    by Cisco. CERT/CC is coordinating the public disclosure of the impact
    these vulnerabilities may have on other, non-Cisco products. This Cisco
    Security Advisory is being published in coordination with CERT/CC.

    The MIT Kerberos Team advisories for these vulnerabilities can be found at
    (MITKRB5-SA-2004-002 and MITKRB5-SA-2004-003).

    The CERT/CC advisories for these vulnerabilities can be found at

    Status of This Notice: FINAL



    This advisory will be posted on Cisco's worldwide website 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.

      * (includes CERT/CC)

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

    Revision History

    | Revision | 2004-August-31 | Initial public |
    | 1.0 | | 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 Cisco's worldwide website at
    This includes instructions for press inquiries regarding Cisco
    security notices. All Cisco security advisories are available at

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