ESB-2018.0909 - [Cisco] Cisco IOS XE: Denial of service - Existing account 2018-03-29

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             AUSCERT External Security Bulletin Redistribution

         Cisco IOS XE Software Simple Network Management Protocol
                Double-Free Denial of Service Vulnerability
                               29 March 2018


        AusCERT Security Bulletin Summary

Product:           Cisco IOS XE
Publisher:         Cisco Systems
Operating System:  Cisco
Impact/Access:     Denial of Service -- Existing Account
Resolution:        Patch/Upgrade
CVE Names:         CVE-2018-0160  

Original Bulletin:

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Cisco Security Advisory

Cisco IOS XE Software Simple Network Management Protocol Double-Free Denial of
Service Vulnerability


Advisory ID:     cisco-sa-20180328-snmp-dos

First Published: 2018 March 28 16:00 GMT

Version 1.0:     Final

Workarounds:     Yes

Cisco Bug IDs:



CVSS Score:


  o A vulnerability in Simple Network Management Protocol (SNMP) subsystem of
    Cisco IOS XE Software could allow an authenticated, remote attacker to
    cause a denial of service (DoS) condition.

    The vulnerability is due to improper management of memory resources,
    referred to as a double free. An attacker could exploit this vulnerability
    by sending crafted SNMP packets to an affected device. A successful exploit
    could allow the attacker to cause the affected device to reload, resulting
    in a DoS condition.

    To exploit this vulnerability via SNMP Version 2c or earlier, the attacker
    must know the SNMP read-only community string for an affected system. To
    exploit this vulnerability via SNMP Version 3, the attacker must know the
    user credentials for the affected system.

    Cisco has released software updates that address this vulnerability. There
    are workarounds that address this vulnerability.

    This advisory is available at the following link:

    This advisory is part of the March 28, 2018, release of the Cisco IOS and
    IOS XE Software Security Advisory Bundled Publication, which includes 20
    Cisco Security Advisories that describe 22 vulnerabilities. For a complete
    list of the advisories and links to them, see Cisco Event Response: March
    2018 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled

Affected Products

  o Vulnerable Products

    This vulnerability affects Cisco devices that are running a vulnerable
    release of Cisco IOS XE Software, have been configured to be queried over
    SNMP, and have Network Address Translation (NAT) enabled.

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

    Assessing the SNMP Configuration

    To determine whether a device is configured for SNMP, administrators can
    use the show run | include snmp command in the CLI. The following example
    shows the output of the show run | include snmp command on a device that is
    configured with both read-only and read-write community strings:

        Router# show run | include snmp

        snmp-server community public RO
        snmp-server community write RW

    Determining the Cisco IOS XE Software Release

    To determine which Cisco IOS XE Software release is running on a device,
    administrators can log in to the device, use the show version command in
    the CLI, and then refer to the system banner that appears. If the device is
    running Cisco IOS XE Software, the system banner displays Cisco IOS
    Software, Cisco IOS XE Software, or similar text.

    The following example shows the output of the command for a device that is
    running Cisco IOS XE Software Release 16.2.1 and has an installed image
    name of CAT3K_CAA-UNIVERSALK9-M:

        ios-xe-device# show version

        Cisco IOS Software, Catalyst L3 Switch Software (CAT3K_CAA-UNIVERSALK9-M), Version Denali 16.2.1, RELEASE SOFTWARE (fc1)
        Technical Support:
        Copyright (c) 1986-2016 by Cisco Systems, Inc.
        Compiled Sun 27-Mar-16 21:47 by mcpre

    For information about the naming and numbering conventions for Cisco IOS XE
    Software releases, see the Cisco IOS and NX-OS Software Reference Guide.

    Products Confirmed Not Vulnerable

    No other Cisco products are currently known to be affected by this

    Cisco has confirmed that this vulnerability does not affect Cisco IOS
    Software, Cisco IOS XR Software, or Cisco NX-OS Software.


  o An attacker could exploit this vulnerability by sending a crafted SNMP
    packet to an affected device via IP Version 4 or IP Version 6. Only traffic
    that is directed to an affected system can be used to exploit this

    To exploit this vulnerability via SNMP Version 2c or earlier, the attacker
    must know the SNMP read-only community string for an affected system. A
    community string is a password that is applied to a device to restrict both
    read-only and read-write access to SNMP data on a device. These community
    strings, as with all passwords, should be chosen carefully to ensure that
    they are not trivial. They should also be changed at regular intervals and
    in accordance with network security policies. For example, the strings
    should be changed when a network administrator changes roles or leaves the

    To exploit this vulnerability via SNMP Version 3, the attacker must know
    the user credentials for an affected system.

Indicators of Compromise

  o A successful exploit of this vulnerability will cause an affected device to
    reload and generate a crashinfo file.

    A successful exploit of this vulnerability may be confirmed by decoding the
    stack trace for the device and determining whether the stack trace
    correlates with this vulnerability.

    Error messages that may be found in the device logs or the crashinfo file
    when the SNMP process crashes may be similar to the following example:

        Exception to IOS Thread:
        Frame pointer 0x7F48DA5F5300, PC = 0x7F4B736A40EB
        UNIX-EXT-SIGNAL: Segmentation fault(11), Process = SNMP ENGINE

    Contact the Cisco Technical Assistance Center (TAC) to review the crashinfo
    file and determine whether the device has been compromised by exploitation
    of this vulnerability.


  o Administrators can mitigate this vulnerability by disabling the affected
    Management Information Base (MIB), in this case the NAT-MIB. To create or
    update a view entry and disable the NAT-MIB, administrators can log in to
    the device and use the snmp-server view global configuration command in the
    CLI, as shown in the following example:

        !Standard VIEW and Security Exclusions
        snmp-server view NO_BAD_SNMP iso included
        snmp-server view NO_BAD_SNMP internet included
        snmp-server view NO_BAD_SNMP snmpUsmMIB excluded
        snmp-server view NO_BAD_SNMP snmpVacmMIB excluded
        snmp-server view NO_BAD_SNMP snmpCommunityMIB excluded
        snmp-server view NO_BAD_SNMP ciscoMgmt.252 excluded
        !End Standard View

        !Advisory Specific Mappings
        snmp-server view NO_BAD_SNMP mib-2.123 excluded

    To apply the configuration to a community string, administrators can use
    the following command:

        snmp-server community mycomm view NO_BAD_SNMP RO

    For SNMP Version 3, administrators can use the following command:

        snmp-server group v3group auth read NO_BAD_SNMP write NO_BAD_SNMP

    SNMP Access Control List

    Note: Because the vulnerable feature uses UDP as a transport, it is
    possible to spoof the sender's IP address, which may defeat access control
    lists (ACLs) that permit communication to these ports from trusted IP
    addresses. Unicast Reverse Path Forwarding (RPF) should be considered in
    conjunction with ACLs to offer a better mitigation solution. The following
    is an example of an ACL that allows only hosts at to query the
    device with SNMP requests:

        access-list 1 permit
        snmp-server community example RO 1

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

    Cisco IOS and IOS XE Software

    To help customers determine their exposure to vulnerabilities in Cisco IOS
    and IOS XE Software, Cisco provides a tool, the Cisco IOS Software Checker,
    that identifies 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 this tool to perform the following tasks:

       Initiate a search by choosing one or more releases from a drop-down
        list or uploading a file from a local system for the tool to parse
       Enter the output of the show version command for the tool to parse
       Create a custom search by including all previously published Cisco
        Security Advisories, a specific advisory, or all advisories in the most
        recent bundled publication

    To determine whether a release is affected by any published Cisco Security
    Advisory, use the Cisco IOS Software Checker on or enter a
    Cisco IOS Software or Cisco IOS XE Software release--for example, 15.1(4)M2
    or 3.13.8S--in the following field:

    [                    ] [Check]

    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.


  o This vulnerability was found during the resolution of a 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.

Subscribe to Cisco Security Notifications

  o Subscribe

Related to This Advisory

  o Cisco Event Response: March 2018 Semiannual Cisco IOS and IOS XE Software
    Security Advisory Bundled Publication
    Cisco Security Blog: Cisco IOS and IOS XE Software Bundled Publication
    March 2018



Revision History

    | Version  |        Description         | Section  | Status  |      Date      |
    | 1.0      | Initial public release.    | --       | Final   | 2018-March-28  |

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