[NEWS] Cisco IOS DHCP Blocked Interface DoS

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

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    Date: 11 Nov 2004 18:33:31 +0200

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      Cisco IOS DHCP Blocked Interface DoS


    Cisco IOS devices running several branches of Cisco IOS that have Dynamic
    Host Configuration Protocol (DHCP) server or relay agent enabled, even if
    not configured, are vulnerable to a denial of service where the input
    queue becomes blocked when receiving specifically crafted DHCP packets.


    Vulnerable Versions:
     * Any Cisco device running IOS versions 12.2(18)EW, 12.2(18)EWA,
    12.2(14)SZ, 12.2(18)S, 12.2(18)SE, 12.2(18)SV, 12.2(18)SW

    Vulnerable Systems:
     * Cisco 7200, 7300, 7500 platforms
     * Cisco 2650, 2651, 2650XM, 2651XM Multiservice platform
     * Cisco ONS15530, ONS15540
     * Cisco Catalyst 4000, Sup2plus, Sup3, Sup4 and Sup5 modules
     * Cisco Catalyst 4500, Sup2Plus TS
     * Cisco Catalyst 4948, 2970, 3560, and 3750
     * Cisco Catalyst 6000, Sup2/MSFC2 and Sup720/MSFC3
     * Cisco 7600 Sup2/MSFC2 and Sup720/MSFC3

    Immune Systems:
     * 700 series dialup routers (750, 760, and 770 series) are not affected.
     * WAN switching products such as the IGX, BPX and MGX lines are not
     * No host-based software is affected.
     * The Cisco PIX Firewall is not affected
     * The Cisco LocalDirector is not affected.
     * The Cisco Content Engine and ACNS is not affected.
     * The Catalyst 2901/2902, 2948G, 2980G, 4000, 5000, and 6000 switches
    running CatOS.
     * Cisco Network Registrar is not affected.
     * Cisco VPN 3000 series is not affected
     * Cisco IOS-XR platform is not affected.
     * Cisco MDS 9000 family is not affected.

    Note: Any other Cisco device running IOS of a version not listed above in
    the affected versions section or Cisco products running an affected
    version with the option 'no service dhcp'.

    DHCP services allow devices to request and receive basic host
    configuration information from the DHCP server via the network. Cisco
    routers can be configured to both provide dynamic host configuration
    information (termed DHCP server function), and forward DHCP and BootP
    packets across separate broadcast domains (termed DHCP relay agent
    function). Cisco routers are configured to process and accept DHCP packets
    by default, therefore the command "service dhcp" does not appear in the
    running configuration display, and only the command for the disabled
    feature, no service dhcp, will appear in the running configuration display
    when the feature is disabled.

    The vulnerability is present, regardless if the DHCP server or relay agent
    configurations are present on an affected product. The only required
    configuration for this vulnerability in affected versions is the lack of
    the no service dhcp command. Certain crafted DHCP packets may be
    undeliverable, but will remain in the queue instead of being dropped. If a
    number of packets are sent that equal the size of the input queue, no more
    traffic will be accepted on that interface.

    On a blocked Ethernet interface, Address Resolution Protocol (ARP) times
    out after a default time of four hours, and no inbound or outbound traffic
    can be processed, including both IP and non-IP traffic such as IPX. The
    device must be rebooted to clear the input queue on the interface, and
    will not reload without user intervention.

    The attack may be repeated on all interfaces, causing the router to be
    remotely inaccessible, excluding the console port where DHCP packets are
    not processed by default and which can be used for out-of-band management
    and configured for remote access. Workarounds are available, and are
    documented in the Workarounds section below. Other types of interfaces,
    including but not limited to ATM, Serial and POS interfaces, are affected,
    but ARP is not a factor.

    To identify a blocked input interface, use the show interfaces command and
    look for the Input Queue line. The size of the input queue may keep
    increasing. If the current size (in this case, 76) is larger than the
    maximum size (75), the input queue is blocked. Example:
       Router#show interface ethernet 0/0
        Ethernet0/0 is up, line protocol is up
          Hardware is AmdP2, address is 0050.500e.f1e0 (bia 0050.500e.f1e0)
          Internet address is
          MTU 1500 bytes, BW 10000 Kbit, DLY 1000 usec, rely 255/255, load
          Encapsulation ARPA, loopback not set, keepalive set (10 sec)
          ARP type: ARPA, ARP Timeout 04:00:00
          Last input 00:00:41, output 00:00:07, output hang never
          Last clearing of "show interface" counters 00:07:18
          Input queue: 76/75/1091/0 (size/max/drops/flushes); Total output
    drops: 0

    Note: The 76/75 queue count means the queue is blocked.

    A device receiving these specially crafted DHCP packets will force the
    inbound interface to stop processing traffic. The device may stop
    processing packets destined to the router, including routing protocol
    packets and ARP packets. No alarms will be triggered, nor will the router
    reload to correct itself. This vulnerability may be exercised repeatedly
    resulting in loss of availability until a workaround has been applied or
    the device has been upgraded to a fixed version of code.

    The effects of this vulnerability can be monitored by the same methods
    outlined in the white paper entitled Uses of Network Management for
    Monitoring the "IP Packet Blocks Input Queue" PSIRT Advisory which details
    methods to identify impacted devices via SNMP, RMON, and Network
    Management products.

    Vendor Status:
    Cisco has released software patches for the vulnerability and is available
    for download from the software center at:

    For software installation and upgrade procedures consult:

    Registered Cisco users can review all repaired images from:

    There are three possible workarounds:
     * Disabling the DHCP service
     * Control Plane Policing
     * Two versions of Access Control Lists

    Disabling the DHCP Service
    This vulnerability can be mitigated by utilizing the command:
        no service dhcp

    However, this workaround will disable all DHCP processing on the device,
    including the DHCP helper functionality that may be necessary in some
    network configurations.

    Control Plane Policing Feature
    The Control Plane Policy feature may be used to mitigate this
    vulnerability, as in the following example:
        access-list 140 deny udp host any eq bootps
        access-list 140 deny udp any host eq bootps
        access-list 140 deny udp any host eq bootps
        access-list 140 permit udp any any eq bootps

        class-map match-all bootps-class
          match access-group 140

        policy-map control-plane-policy
          class bootps-class

             police 8000 1500 1500 conform-action drop exceed-action drop

         service-policy input control-plane-policy

    Note: For this example is a legitimate DHCP server.

    Additional information on the configuration and use of the CPP feature can
    be found at this link:
    <http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1838/products_feature_guide09186a00801afad4.html> http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1838/products_feature_guide09186a00801afad4.html

    This workaround is only applicable to Cisco IOS 12.2S, as this feature is
    only available in Cisco IOS versions 12.2S and 12.3T. Cisco IOS 12.3T is
    not impacted by this advisory.

    Access Lists - Two Methods
    Access lists can be applied to block DHCP/BootP traffic destined to any
    router interface addresses, as in the following example. In this example,
    the IP address represents a legitimate DHCP server, the
    addresses and represent router interface
    addresses, and represents a loopback interface on the router.
    Any bootp/dhcp packets destined to the router interface addresses are

        access-list 100 remark permit bootps from the DHCP server
        access-list 100 permit udp host any eq bootps
        access-list 100 remark deny bootps from any to router f1/0
        access-list 100 deny udp any host eq bootps
        access-list 100 remark deny bootps from any to router f0/0
        access-list 100 deny udp any host eq bootps
        access-list 100 remark deny bootps from any to router loopback1
        access-list 100 deny udp any host eq bootps
        access-list 100 remark permit all other traffic
        access-list 100 permit ip any any

    Access-list 100 is applied to f0/0 and f1/0 physical interfaces.

        interface FastEthernet0/0
         ip address
         ip access-group 100 in
        interface FastEthernet1/0
         ip address
         ip access-group 100 in
         ip helper-address

    An alternate configuration for the interface access-list workaround. This
    example would also need to be applied to all physical interfaces, but deny
    statements for all of the IP addresses configured on the router are not
    necessary in this approach. In this example, the address
    represents a legitimate DHCP server.

        access-list 100 permit udp host any eq bootps
        access-list 100 permit udp any host eq bootps
        access-list 100 permit udp any host eq bootps
        access-list 100 deny udp any any eq bootps

        interface FastEthernet0/0
         ip address
         ip access-group 100 in
        interface FastEthernet1/0
         ip address
         ip access-group 100 in
         ip helper-address

    Note: These workarounds will not prevent spoofed IP packets with the
    source IP address set to that of the DHCP server.

    For more information on anti-spoofing refer to:
    <http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080120f 48.shtml#sec_ip> http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080120f 48.shtml#sec_ip
     <http://www.ietf.org/rfc/rfc2827.txt> http://www.ietf.org/rfc/rfc2827.txt

    The Unicast Reverse Path Forwarding (Unicast RPF) feature helps to
    mitigate problems that are caused by spoofed IP source addresses. For
    further details, please refer to:
    <http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fsecur_c/fothersf/scfrpf.htm> http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fsecur_c/fothersf/scfrpf.htm


    The information has been provided by <mailto:psirt@cisco.com> Cisco
    Systems Product Security Incident Response Team.
    The original article can be found at:


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