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             AUSCERT External Security Bulletin Redistribution

         Cisco IOS XR Software Intermediate System-to-Intermediate
                  System Denial of Service Vulnerability
                               8 August 2019


        AusCERT Security Bulletin Summary

Product:           Cisco IOS XR
Publisher:         Cisco Systems
Operating System:  Cisco
Impact/Access:     Denial of Service -- Remote/Unauthenticated
Resolution:        Patch/Upgrade
CVE Names:         CVE-2019-1918  

Original Bulletin: 

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

Cisco Security Advisory

Cisco IOS XR Software Intermediate System-to-Intermediate System Denial of
Service Vulnerability


Advisory ID:		cisco-sa-20190807-iosxr-isis-dos-1918
First Published:	2019 August 7 16:00 GMT
Version 1.0:		Final
Workarounds:		No workarounds available
Cisco Bug IDs:		CSCvp90854
CVSS Score:		Base 7.4



  o A vulnerability in the implementation of Intermediate
    System-to-Intermediate System (IS-IS) routing protocol functionality in
    Cisco IOS XR Software could allow an unauthenticated attacker who is in the
    same IS-IS area to cause a denial of service (DoS) condition.

    The vulnerability is due to incorrect processing of IS-IS link-state
    protocol data units (PDUs). An attacker could exploit this vulnerability by
    sending specific link-state PDUs to an affected system to be processed. A
    successful exploit could allow the attacker to cause incorrect calculations
    used in the weighted remote shared risk link groups (SRLG) or in the IGP
    Flexible Algorithm. It could also cause tracebacks to the logs or
    potentially cause the receiving device to crash the IS-IS process,
    resulting in a DoS condition.

    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:

    A companion advisory is available at the following link:

Affected Products

  o Vulnerable Products

    This vulnerability affects Cisco devices if they are running Cisco IOS XR
    Software releases later than 6.5.1 and earlier than 6.6.3.

    Cisco has confirmed that this vulnerability affects both Cisco IOS XR
    32-bit Software and Cisco IOS XR 64-bit Software.

    Determining Whether the Device Has a Vulnerable Configuration

    To determine if the device is configured for IS-IS routing, administrators
    can use the show running-config router isis EXEC command in the CLI. If the
    router is configured with IS-IS routing, it will return output. The
    following example shows the output of the show running-config router isis
    command on a device that is configured as a level 1 IS-IS router.

    The device is affected if it is configured as one of the following: is-type
    level-1, level-1-2 or level-2-only.

        RP/0/RSP1/CPU0:IOSXR#show running-config router isis
        Sun May 26 05:20:23.088 UTC
        router isis core
        is-type level-1
        net 49.0001.1921.6825.5091.00
        address-family ipv4 unicast
        interface TenGigE0/0/0/1
          address-family ipv4 unicast

    For the device to be exploitable, it would need at least one formed
    IS-IS adjacency.

        RP/0/RSP1/CPU0:IOSXR#show isis adjacency

        IS-IS p Level-1 adjacencies:
        System Id      Interface        SNPA           State Hold     Changed  NSF      BFD
        12a4           PO0/1/0/1        *PtoP*         Up    23       00:00:06 Capable  Init
        12a4           TenGigE0/0/0/1   0004.2893.f2f6 Up    56       00:04:01 Capable  Up

        Total adjacency count: 2

    Products Confirmed Not Vulnerable

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


  o Cisco recommends that customers ensure the authentication of routers within
    the IS-IS area for adjacency establishment (Hello packets) and for
    link-state PDUs when implementing IS-IS. If this best practice is followed,
    then only an authenticated adjacency within the same IS-IS area that sends
    the specific IS-IS link-state PDU could exploit this vulnerability.

    Cisco also recommends that customers enable Non-Stop Forwarding and/or
    Non-Stop Routing.


  o There are no workarounds that address this vulnerability. However,
    following Cisco best practices and ensuring that IS-IS authentication is
    enabled for adjacencies (Hello packets) and link-state PDUs would restrict
    this vulnerability to only authenticated link-state PDUs. For example:

        router isis core
        lsp-password hmac-md5 encrypted 13061E010803
        interface TenGigE0/0/0/6
          hello-password hmac-md5 encrypted 094F471A1A0A

    For more information on IS-IS Authentication, see the IS-IS Authentication
    chapter of Implementing IS-IS on Cisco IOS XR Software.

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:

    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 and Alerts 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

    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:

    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

    Fixed Releases

    Cisco fixed this vulnerability in Cisco IOS XR Software Release 6.6.3.

    Cisco has also released the following software maintenance upgrades (SMUs)
    that address this vulnerability:

    IOS XR Release      SMU ID    Platform    SMU Name
    6.5.3               AA15778   ASR9000     asr9k-px-6.5.3.CSCvq45118^1
    6.6.1               AA15881   NC56K       ncs6k-6.6.1.CSCvq45118.tar^1

^1The SMUs contain the fixes for the vulnerabilities described in
    cisco-sa-20190807-iosxr-isis-dos-1910 and
    cisco-sa-20190807-iosxr-isis-dos-1918 advisories.

    Customers can download the SMUs from the Software Center on Cisco.com by
    doing the following:

     1. Click Browse All.
     2. Choose IOS and NX-OS Software > IOS XR > IOS XR Software > Optical
        Networking or Routers.
     3. Choose the appropriate product and model.
     4. Access releases by using the left pane of the appropriate product page.

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.


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

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.


  o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/

Revision History

    | Version  |        Description        | Section  | Status  |      Date       |
    | 1.0      | Initial public release.   | --        | Final   | 2019-August-07  |

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