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29 September 2022

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             AUSCERT External Security Bulletin Redistribution

         Cisco IOS XE Software DNS NAT Protocol Application Layer
                  Gateway Denial of Service Vulnerability
                             29 September 2022


        AusCERT Security Bulletin Summary

Product:           ASR 1000 Series
                   Catalyst 8500 Series
Publisher:         Cisco Systems
Operating System:  Cisco
Resolution:        Patch/Upgrade
CVE Names:         CVE-2022-20837  

Original Bulletin: 

Comment: CVSS (Max):  8.6 CVE-2022-20837 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H)
         CVSS Source: Cisco Systems
         Calculator:  https://www.first.org/cvss/calculator/3.1#CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H

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Cisco IOS XE Software DNS NAT Protocol Application Layer Gateway Denial of
Service Vulnerability

Priority:        High
Advisory ID:     cisco-sa-alg-dos-KU9Z8kFX
First Published: 2022 September 28 16:00 GMT
Version 1.0:     Final
Workarounds:     No workarounds available
Cisco Bug IDs:   CSCwa78096
CVE Names:       CVE-2022-20837
CWEs:            CWE-754


  o A vulnerability in the DNS application layer gateway (ALG) functionality
    that is used by Network Address Translation (NAT) in Cisco IOS XE Software
    could allow an unauthenticated, remote attacker to cause an affected device
    to reload.

    This vulnerability is due to a logic error that occurs when an affected
    device inspects certain TCP DNS packets. An attacker could exploit this
    vulnerability by sending crafted DNS packets through the affected device
    that is performing NAT for DNS packets. A successful exploit could allow
    the attacker to cause the device to reload, resulting in a denial of
    service (DoS) condition on the affected device.

    Note: This vulnerability can be exploited only by sending IPv4 TCP packets
    through an affected device. This vulnerability cannot be exploited by
    sending IPv6 traffic.
    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:

    This advisory is part of the September 2022 release of the Cisco IOS and
    IOS XE Software Security Advisory Bundled Publication. For a complete list
    of the advisories and links to them, see Cisco Event Response: September
    2022 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled

Affected Products

  o Vulnerable Products

    This vulnerability affects the following Cisco devices if they are running
    a vulnerable release of Cisco IOS XE Software that is configured for NAT
    operation and has the DNS ALG for TCP feature enabled. The DNS ALG feature
    is enabled as soon as NAT is configured on the device.

       ASR 1000 Series Embedded Services Processors models ESP 100-X and ESP
       Catalyst 8500 Series Edge Platforms models C8500-12X4QC and C8500-12X

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

    Determine the Device Configuration

    Determine Whether a Device is Configured to Perform NAT

    To determine whether NAT is active on a device (preferred method),
    administrators can log in to the device and use the show ip nat statistics 
    command in the CLI. If NAT is active, the Outside interfaces and Inside
    interfaces sections of the command output will include at least one

    The following example shows the output of the show ip nat statistics 
    command for a device where NAT is active:

        Router# show ip nat statistics

        Total active translations: 1 (0 static, 1 dynamic; 0 extended)
        Outside interfaces:
        Inside interfaces:

    If the output of the show ip nat statistics command does not list any
    interfaces, NAT is not active on the device.

    Alternatively, issue the show running-config command in the CLI to assess
    whether NAT commands are present in the device configuration. If NAT is
    active on the device, the output of the show running-config command will
    include the ip nat inside and ip nat outside interface commands. In the
    case of the NAT Virtual Interface, the ip nat enable interface command will
    be present.

    Determine Whether DNS ALG for TCP is Disabled in the NAT Configuration

    To determine whether the DNS ALG for TCP is disabled in the NAT
    configuration, use the show running-config | include ip nat service dns 
    privileged EXEC command. The presence of no ip nat service dns tcp in the
    output of show running-config | include ip nat service dns indicates that
    the DNS ALG for TCP is disabled in the NAT configuration.

    The following is the output of show running-config | include ip nat in
    Cisco IOS XE Software that has the DNS ALG disabled in the NAT

        Router#show running-config | include ip nat service
         no ip nat service dns tcp

    If no ip nat service dns tcp does not appear in the output of show
    running-config | include ip nat service dns , and the device runs an
    affected version of Cisco IOS XE Software with NAT enabled, that
    configuration is vulnerable.

    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:

       ASR 1000 Series Embedded Services Processors models ESP 100 and ESP
        200, and models earlier than these
       Catalyst 8500 Series Edge Platforms model C8500L-8S4X
       IOS Software
       IOS XR Software
       NX-OS Software


  o There are no workarounds that address this vulnerability. However, a
    mitigation is available.

    Administrators may mitigate this vulnerability by disabling the NAT ALG for
    DNS TCP packets. However, this action may negatively impact normal
    operation of any device that sends or receives traffic through the affected
    device and, consequently, may disrupt normal network operations.

    Administrators should verify that their network environment does not
    require use of a NAT ALG for DNS packets before they disable this
    functionality. To disable use of the NAT ALG for DNS packets,
    administrators can use the no ip nat service dns tcp command in global
    configuration mode.

    While this mitigation has been deployed and was proven successful in a test
    environment, customers should determine the applicability and effectiveness
    in their own environment and under their own use conditions. Customers
    should be aware that any workaround or mitigation that is implemented may
    negatively impact the functionality or performance of their network based
    on intrinsic customer deployment scenarios and limitations. Customers
    should not deploy any workarounds or mitigations before first evaluating
    the applicability to their own environment and any impact to such

Fixed Software

  o Cisco has released free software updates that address the vulnerability
    described in this advisory. Customers with service contracts that entitle
    them to regular software updates should obtain security fixes through their
    usual update channels.

    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.

    The Cisco Support and Downloads page on Cisco.com provides information
    about licensing and downloads. This page can also display customer device
    support coverage for customers who use the My Devices tool.

    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

    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

    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 the Cisco Software Checker . This tool
    identifies any Cisco security advisories that impact a specific software
    release and the earliest release that fixes the vulnerabilities that are
    described in each advisory ("First Fixed"). If applicable, the tool also
    returns the earliest release that fixes all the vulnerabilities that are
    described in all the advisories that the Software Checker identifies
    ("Combined First Fixed").

    To use the tool, go to the Cisco Software Checker page and follow the
    instructions. Alternatively, use the following form to determine whether a
    release is affected by any Cisco Security Advisory. To use the form, follow
    these steps:

     1. Choose which advisories the tool will search-only this advisory, only
        advisories with a Critical or High Security Impact Rating (SIR) , or
        all advisories.
     2. Enter a release number-for example, 15.9(3)M2 or 17.3.3.
     3. Click Check 

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.

Related to This Advisory

  o Cisco Event Response: September 2022 Semiannual Cisco IOS and IOS XE
    Software Security Advisory Bundled Publication


  o https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-alg-dos-KU9Z8kFX

Revision History

  o +----------+---------------------------+----------+--------+--------------+
    | Version  |        Description        | Section  | Status |     Date     |
    | 1.0      | Initial public release.   | -        | Final  | 2022-SEP-28  |

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