[NEWS] Cisco Vulnerable to SSH Malformed Packet Vulnerabilities

From: support@securiteam.com
Date: 12/26/02

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To: list@securiteam.com
Date: 26 Dec 2002 23:53:03 +0200

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      Cisco Vulnerable to SSH Malformed Packet Vulnerabilities
    ------------------------------------------------------------------------

    SUMMARY

    Certain Cisco products containing support for the Secure Shell (SSH)
    server are vulnerable to a Denial of Service (DoS) if the SSH server is
    enabled on the device. A malformed SSH packet directed at the affected
    device can cause a reload of the device. No authentication is necessary
    for the packet to be received by the affected device. The SSH server in
    Cisco IOS is disabled by default.

    Cisco will be making free software available to correct the problem as
    soon as possible.

    The malformed packets can be generated using the SSHredder test suite from
    Rapid7, Inc. Workarounds are available. The Cisco PSIRT is not aware of
    any malicious exploitation of this vulnerability.

    DETAILS

    Affected Products:
    Multiple Cisco products which contain support for an SSH server are
    vulnerable if the SSH server is enabled. Cisco routers and Catalyst
    switches running the affected versions of Cisco IOS shown in the Software
    Version and Fixes section below have been confirmed to be vulnerable.

    Cisco products which contain SSH server functionality that are confirmed
    not to be vulnerable include:

     * Cisco Catalyst Switches running Cisco CatOS
     * Cisco VPN3000 series concentrators
     * Cisco PIX Firewall
     * Cisco Secure Intrusion Detection System (NetRanger) appliance
     * Cisco Secure Intrusion Detection System Catalyst Module
     * Cisco SN5400 Series Storage Routers
     * CiscoWorks 1105 Wireless LAN Solution Engine (WLSE)
     * CiscoWorks 1105 Hosting Solution Engine (WLSE)

    Details:
    A suite of crafted packets has been developed to test implementations of
    the Secure Shell (SSH) protocol. If the SSH server has been enabled,
    several of the test cases cause a forced reload of the device before the
    authentication process is called. Each time an SSH connection attempt is
    made to a Cisco device running Cisco IOS with one of the crafted packets,
    and the SSH server is enabled on the device, the device reboots.

    The SSH server feature is available in the following Cisco IOS release
    trains: 12.0S, 12.0ST, 12.1T, 12.1E, 12.2, 12.2T, 12.2S. All releases
    which have the SSH server feature are vulnerable when the SSH server is
    enabled by issuing the command "crypto key generate rsa" in configuration
    mode.

    All products running vulnerable versions of Cisco IOS except the Cisco
    3550 will automatically reload and resume service following the crash. The
    Cisco 3550 will not reload, and will require manual intervention to resume
    normal processing.

    This Cisco IOS defect is documented in DDTS CSCdz60229.

    Impact:
    The vulnerability can be exploited to make an affected product unavailable
    for several minutes while the device reloads. Once it has resumed normal
    processing, the device is still vulnerable and can be forced to reload
    repeatedly.

    Software Versions and Fixes:
    A table listing all the versions being affected, and their available fixes
    can be found here:
     
    <http://www.cisco.com/warp/public/707/ssh-packet-suite-vuln.shtml#Software> http://www.cisco.com/warp/public/707/ssh-packet-suite-vuln.shtml#Software

    Obtaining Fixed Software:
    Customers with 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 <http://www.cisco.com/tacpage/sw-center/>
    http://www.cisco.com/tacpage/sw-center/.

    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 obtaining the free software
    upgrade(s).

    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)
     * email: tac@cisco.com .

    See <http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml>
    http://www.cisco.com/warp/public/687/Directory/DirTAC.shtml for additional
    TAC contact information, including special localized telephone numbers and
    instructions and e-mail addresses for use in various languages.

    Please have your product serial number available and give the URL of this
    advisory 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.

    Workarounds:
    Workarounds consist of disabling the SSH server, removing SSH as a remote
    access method, permitting only trusted hosts to connect to the server, and
    blocking SSH traffic to the device completely via external mechanisms.

    Caution: The following workaround will have undesirable side effects for
    IPSEC sessions that terminate on the device that use RSA key pairs for
    device authentication, or that use certificates based on those RSA key
    pairs. IPSEC sessions using other authentication methods will not be
    affected.

    For Cisco IOS the SSH server can be disabled by applying the command
    "crypto key zeroize rsa" while in configuration mode. The SSH server is
    enabled automatically upon generating an RSA key pair. Zeroing the RSA
    keys is the only way to completely disable the SSH server.

    Access to the SSH server on Cisco IOS may also be disabled via removing
    SSH as a valid transport protocol. This can be done by reapplying the
    "transport input" command with 'ssh' removed from the list of permitted
    transports on VTY lines while in configuration mode. For example:

    line vty 0 4
     transport input telnet
    end

    If SSH server functionality is desired, access to the server can be
    restricted to specific source IP addresses or blocked entirely through the
    use of Access Control Lists (ACLs) on the VTY lines as shown in the
    following URL:
     
    <http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/1219ea1/scg/swacl.htm#xtocid14> http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/1219ea1/scg/swacl.htm#xtocid14

    More information on configuring ACLs can be found on Cisco's public
    website:
     <http://www.cisco.com/warp/public/707/confaccesslists.html>
    http://www.cisco.com/warp/public/707/confaccesslists.html

    An example of a VTY access-list can be found here:

    access-list 2 permit 10.1.1.0 0.0.0.255
    access-list 2 deny any

    line vty 0 4
    access-class 2 in
    end

    You may also block inbound SSH connections for your device with an
    external packet filtering device such as a firewall or a router that
    blocks traffic to TCP port 22.

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:psirt@cisco.com> Cisco
    Systems Product Security Incident Response Team.

    ========================================

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