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             AUSCERT External Security Bulletin Redistribution

                               ESB-2020.3302
         Cisco IOS XE Software for Cisco cBR-8 Converged Broadband
               Routers DHCP Denial of Service Vulnerability
                             25 September 2020

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        AusCERT Security Bulletin Summary
        ---------------------------------

Product:           Cisco IOS XE Software
Publisher:         Cisco Systems
Operating System:  Cisco
Impact/Access:     Denial of Service -- Remote/Unauthenticated
Resolution:        Patch/Upgrade
CVE Names:         CVE-2020-3509  

Original Bulletin: 
   https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-iosxe-dhcp-dos-JSCKX43h

- --------------------------BEGIN INCLUDED TEXT--------------------

Cisco IOS XE Software for Cisco cBR-8 Converged Broadband Routers DHCP Denial
of Service Vulnerability

Priority:        High

Advisory ID:     cisco-sa-iosxe-dhcp-dos-JSCKX43h

First Published: 2020 September 24 16:00 GMT

Version 1.0:     Final

Workarounds:     No workarounds availableCisco Bug IDs:   CSCvr70940

CVE-2020-3509    

CWE-388

CVSS Score:
8.6  AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X

Summary

  o A vulnerability in the DHCP message handler of Cisco IOS XE Software for
    Cisco cBR-8 Converged Broadband Routers could allow an unauthenticated,
    remote attacker to cause the supervisor to crash, which could result in a
    denial of service (DoS) condition.

    The vulnerability is due to insufficient error handling when DHCP version 4
    (DHCPv4) messages are parsed. An attacker could exploit this vulnerability
    by sending a malicious DHCPv4 message to or through a WAN interface of an
    affected device. A successful exploit could allow the attacker to cause a
    reload of the affected device.

    Note: On Cisco cBR-8 Converged Broadband Routers, all of the following are
    considered WAN interfaces:

       10 Gbps Ethernet interfaces
       100 Gbps Ethernet interfaces
       Port channel interfaces that include multiple 10 and/or 100 Gbps
        Ethernet interfaces

    Cisco has released software updates that address this vulnerability. There
    are no workarounds that address this vulnerability.

    This advisory is available at the following link:
    https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
    cisco-sa-iosxe-dhcp-dos-JSCKX43h

    This advisory is part of the September 24, 2020, release of the Cisco IOS
    and IOS XE Software Security Advisory Bundled Publication, which includes
    25 Cisco Security Advisories that describe 34 vulnerabilities. For a
    complete list of the advisories and links to them, see Cisco Event
    Response: September 2020 Semiannual Cisco IOS and IOS XE Software Security
    Advisory Bundled Publication .

Affected Products

  o Vulnerable Products

    This vulnerability affects Cisco cBR-8 Converged Broadband Routers if they
    are running a vulnerable release of Cisco IOS XE Software and have a WAN
    interface connected. On these devices, the DHCP relay feature is enabled by
    default on all WAN interfaces and cannot be disabled.

    For information about which Cisco software releases are vulnerable, see the
    Fixed Software section of this advisory.

    Products Confirmed Not Vulnerable

    Only products listed in the Vulnerable Products section of this advisory
    are known to be affected by this vulnerability.

    Cisco has confirmed that this vulnerability does not affect the following
    Cisco products:

       IOS Software
       IOS XR Software
       NX-OS Software

Workarounds

  o There are no workarounds that address this vulnerability.

    To reduce the attack surface, customers can apply infrastructure access
    control lists (iACLs) that permit DHCP messages from only trusted DHCP
    servers to all WAN interfaces. Using iACLs is a network security best
    practice and a long-term addition to good network security, as well as a
    partial mitigation for this vulnerability. To help protect all devices with
    IP addresses in the infrastructure IP address range, customers are advised
    to include the following iACL example as part of the deployed iACL:

        !---
        !--- Define trusted DHCP servers
        !---

        object-group network Trusted_Dhcp_Servers
         host TRUSTED_DHCP_SERVER_1
         host TRUSTED_DHCP_SERVER_2

        !---
        !--- Define infrastructure addresses
        !---

        object-group network Infrastructure_Addresses
          NETWORK_1 WILDCARD_1
          NETWORK_2 WILDCARD_2

        !---
        !--- Permit traffic from trusted DHCP servers
        !---

        ip access-list extended WAN-IN
         10 permit udp object-group Trusted_Dhcp_Servers eq bootps object-group Infrastructure_Addresses eq bootpc

        !---
        !--- Deny traffic to DHCP clients from all other sources
        !---

        ip access-list extended WAN-IN
         20 deny udp any any eq bootpc

        !---
        !--- Permit/deny all other Layer 3 and Layer 4 traffic in
        !--- accordance with existing security policies and
        !--- configurations. Permit all other traffic to transit the
        !--- device.
        !---

        ip access-list extended WAN-IN
         30 permit ip any object-group Infrastructure_Addresses

        !---
        !--- Apply access-list to all WAN interfaces (only one example
        !--- shown)
        !---

        interface Port-channel1
         ip access-group WAN-IN in

    For further guidelines and recommendations for deployment techniques for
    iACLs, see the white paper Protecting Your Core: Infrastructure Protection
    Access Control Lists and the Cisco Guide to Harden Cisco IOS Devices .

Fixed Software

  o Cisco has released free software updates that address the vulnerability
    described in this advisory. Customers may only install and expect support
    for software versions and feature sets for which they have purchased a
    license. By installing, downloading, accessing, or otherwise using such
    software upgrades, customers agree to follow the terms of the Cisco
    software license:
    https://www.cisco.com/c/en/us/products/end-user-license-agreement.html

    Additionally, customers may only download software for which they have a
    valid license, procured from Cisco directly, or through a Cisco authorized
    reseller or partner. In most cases this will be a maintenance upgrade to
    software that was previously purchased. Free security software updates do
    not entitle customers to a new software license, additional software
    feature sets, or major revision upgrades.

    When considering software upgrades , customers are advised to regularly
    consult the advisories for Cisco products, which are available from the
    Cisco Security Advisories page , to determine exposure and a complete
    upgrade solution.

    In all cases, customers should ensure that the devices to be upgraded
    contain sufficient memory and confirm that current hardware and software
    configurations will continue to be supported properly by the new release.
    If the information is not clear, customers are advised to contact the Cisco
    Technical Assistance Center (TAC) or their contracted maintenance
    providers.

    Customers Without Service Contracts

    Customers who purchase directly from Cisco but do not hold a Cisco service
    contract and customers who make purchases through third-party vendors but
    are unsuccessful in obtaining fixed software through their point of sale
    should obtain upgrades by contacting the Cisco TAC: https://www.cisco.com/c
    /en/us/support/web/tsd-cisco-worldwide-contacts.html

    Customers should have the product serial number available and be prepared
    to provide the URL of this advisory as evidence of entitlement to a free
    upgrade.

    Cisco IOS and IOS XE Software

    To help customers determine their exposure to vulnerabilities in Cisco IOS
    and IOS XE Software, Cisco provides the Cisco Software Checker to identify
    any Cisco Security Advisories that impact a specific software release and
    the earliest release that fixes the vulnerabilities described in each
    advisory ("First Fixed"). If applicable, the tool also returns the earliest
    release that fixes all the vulnerabilities described in all the advisories
    identified ("Combined First Fixed").

    Customers can use the Cisco Software Checker to search advisories in the
    following ways:

       Choose the software and one or more releases
       Upload a .txt file that includes a list of specific releases
       Enter the output of the show version command

    After initiating a search, customers can customize the search to include
    all Cisco Security Advisories, a specific advisory, or all advisories in
    the most recent bundled publication.

    Customers can also use the following form to determine whether a release is
    affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE
    Software release-for example, 15.1(4)M2 or 3.13.8S :

    [                    ] [Check]

    By default, the Cisco Software Checker includes results only for
    vulnerabilities that have a Critical or High Security Impact Rating (SIR).
    To include results for Medium SIR vulnerabilities, customers can use the
    Cisco Software Checker on Cisco.com and check the Medium check box in the
    drop-down list under Impact Rating when customizing a search.

    For a mapping of Cisco IOS XE Software releases to Cisco IOS Software
    releases, refer to the Cisco IOS XE 2 Release Notes , Cisco IOS XE 3S
    Release Notes , or Cisco IOS XE 3SG Release Notes , depending on the Cisco
    IOS XE Software release.

Exploitation and Public Announcements

  o The Cisco Product Security Incident Response Team (PSIRT) is not aware of
    any public announcements or malicious use of the vulnerability that is
    described in this advisory.

Source

  o This vulnerability was found during the resolution of a Cisco TAC support
    case.

Cisco Security Vulnerability Policy

  o To learn about Cisco security vulnerability disclosure policies and
    publications, see the Security Vulnerability Policy . This document also
    contains instructions for obtaining fixed software and receiving security
    vulnerability information from Cisco.

Related to This Advisory

  o Cisco Event Response: September 2020 Semiannual Cisco IOS and IOS XE
    Software Security Advisory Bundled Publication

URL

  o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/
    cisco-sa-iosxe-dhcp-dos-JSCKX43h

Revision History

  o +----------+---------------------------+----------+--------+--------------+
    | Version  |        Description        | Section  | Status |     Date     |
    +----------+---------------------------+----------+--------+--------------+
    | 1.0      | Initial public release.   | -        | Final  | 2020-SEP-24  |
    +----------+---------------------------+----------+--------+--------------+

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