[NEWS] Cisco Secure Access Control Server EAP-TLS Authentication Vulnerability

From: SecuriTeam (support_at_securiteam.com)
Date: 11/03/04

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Date: 3 Nov 2004 18:20:13 +0200

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      Cisco Secure Access Control Server EAP-TLS Authentication Vulnerability
    ------------------------------------------------------------------------

    SUMMARY

    Cisco Secure Access Control Server provides centralized authentication,
    authorization, and accounting (AAA) services to network devices that
    function as AAA clients, such as a network access servers, PIX firewalls,
    routers and switches. With Cisco Secure ACS, network administrators can
    quickly administer accounts and globally change levels of service
    offerings for entire groups of users.

    A Cisco Secure Access Control Server (ACS) that is configured to use
    Extensible Authentication Protocol-Transport Layer Security (EAP-TLS) to
    authenticate users to the network will allow access to any user that uses
    a cryptographically correct certificate as long as the user name is valid.
    Cryptographically correct means that the certificate is in the appropriate
    format and contains valid fields. The certificate can be expired, or come
    from an untrusted Certificate Authority (CA) and still be
    cryptographically correct.

    DETAILS

    Vulnerable Systems:
     * Cisco Secure ACS for Windows and Cisco Secure ACS Solution Engine
    version 3.3.1

    Immune Systems:
     * Cisco Secure ACS for UNIX and Cisco Secure ACS Solution Engine versions
    prior to 3.3.1 and 3.3.2 and above

    EAP is a general protocol for authentication that supports multiple
    authentication methods, such as token cards, Kerberos, one-time passwords,
    certificates, public key authentication and smart cards. TLS is a protocol
    that provides privacy and data integrity between client/ server
    applications communicating over an insecure network such
    as the Internet.

    EAP and TLS are both IETF RFC standards. The EAP protocol carries initial
    authentication information, specifically EAPOL (the encapsulation of EAP
    over LANs as established by IEEE 802.1X). TLS uses certificates both for
    user authentication and for dynamic ephemeral session key generation. The
    EAP-TLS authentication protocol uses the
    certificates of Cisco Secure ACS and of the end-user client, enforcing
    mutual authentication of the client and of Cisco Secure ACS. More detailed
    information on EAP, TLS, and EAP-TLS can be found in the following IETF
    RFCs: RFC 2284 (PPP Extensible Authentication Protocol), RFC 2246 (The TLS
    Protocol), and RFC 2716 (PPP EAP TLS Authentication Protocol).

    The vulnerability described in this document affects user authentication
    in the following way: when the EAP-TLS protocol is enabled in version
    3.3.1 of Cisco Secure ACS for Windows or Cisco Secure ACS Solution Engine,
    and network devices and services are configured to authenticate users via
    the ACS, access will be granted to any user that uses a certificate that
    is cryptographically correct as long as the user name is valid and
    regardless of whether the certificate is from a trusted Certificate
    Authority or whether the certificate has expired. Cryptographically
    correct means that the certificate is in the appropriate format and
    contains valid fields.

    If EAP-TLS is configured (through the ACS global authentication page) to
    perform binary comparison of user certificates as the only user
    certificate comparison method, user authentication is not affected by this
    vulnerability, as long as the user entry in LDAP/AD contains only valid
    certificates. The reason user authentication is not affected
    under this scenario is that when using the binary comparison method, the
    certificate that is sent by the user's machine during the EAP-TLS
    conversation is also compared to the user certificate that is stored in
    the user entry in the LDAP/AD.

    Note: This vulnerability has no effect, that is, user authentication is
    not impacted, if EAP-TLS is configured in the Cisco Secure ACS with binary
    comparison of user certificates as the only comparison method and if the
    user entry in Lightweight Directory Access Protocol/Active Directory
    (LDAP/AD) contains only valid certificates.

    Impact
    Successful exploitation of this vulnerability could allow unauthorized
    access to the entire network, provided that the Cisco Secure ACS is being
    used to control network access.

    Workaround
    If the user account resides in an LDAP/AD server and the user certificate
    is stored in the user object in LDAP/AD, binary comparison of user
    certificates can be configured in the ACS Global Authentication page as
    the only allowed comparison method. This will work around the
    vulnerability described in this document provided that only valid
    certificates are stored in the user entry in LDAP/AD.

    Please note that for this workaround to work, no other certificate
    comparison methods can be enabled, that is, SAN and CN certificate
    comparison must be disabled in the Global Authentication page.

    Vendor Status:
    The vulnerability described in this advisory is fixed in version 3.3.2 of
    the Cisco Secure ACS for Windows software and of the Cisco Secure ACS
    Solution Engine. If you are currently running the identified vulnerable
    software and are using EAP-TLS, you should obtain fixed software, as
    detailed below.

    If you are running Cisco Secure ACS for Windows you can either upgrade to
    version 3.3.2 or just replace the current CSCRL.dll Windows Dynamic Link
    Library (DLL) in the Windows System32 folder with a fixed DLL and restart
    Cisco Secure ACS for Windows. Replacing the DLL fixes the problem and does
    not require a full upgrade.

    The DLL fix can be downloaded from
    <http://www.cisco.com/pcgi-bin/tablebuild.pl/cs-acs-win>
    http://www.cisco.com/pcgi-bin/tablebuild.pl/cs-acs-win. The file name is
    CSCef62913-fix-ACSWIN-v3.3.1.16.zip. The accompanying Readme file
    (available from the same location) contains detailed installation
    instructions.

    If you are using the Cisco Secure ACS Solution Engine you can also upgrade
    to version 3.3.2 or run an upgrade package to replace the affected DLL (an
    upgrade package is needed because there is no access to the System32
    directory when using the ACS Solution Engine.)

    The upgrade package for the DLL fix can be downloaded from
    <http://www.cisco.com/pcgi-bin/tablebuild.pl/acs-soleng-3des>
    http://www.cisco.com/pcgi-bin/tablebuild.pl/acs-soleng-3des. The file name
    is CSCef62913-fix-ACSSE-v3.3.1.16.zip. The accompanying Readme file
    (available from the same location) contains detailed installation
    instructions.

    Either upgrade method, a full upgrade to version 3.3.2, or just an upgrade
    of the affected DLL, is provided free of charge.

    ADDITIONAL INFORMATION

    The information has been provided by <psirt@cisco.com> Cisco Systems
    Product Security Incident Response Team.
    The original article can be found at:
    <http://www.cisco.com/warp/public/707/cisco-sa-20041102-acs-eap-tls.shtml>
    Cisco Bug ID CSCef62913

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