Summary

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

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

  The vulnerability can be exploited only by traffic that is sent through an affected device via IPv4 packets. The vulnerability cannot be exploited via 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:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-alg-dos-hbBS7SZE

  This advisory is part of the March 2021 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: March 2021 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects 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 feature enabled. The DNS ALG feature is enabled as soon as NAT is configured on the device.

  Assess the NAT Configuration

  1. Determine whether a device is configured to perform NAT

  Administrators can determine whether NAT is active on the device (preferred) or whether NAT commands are present in the device configuration.

  To determine whether NAT is active on a device, administrators can log in to the device and issue 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 interface.

  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:
    GigabitEthernet0/0/3
  Inside interfaces: 
    GigabitEthernet0/0/1
  
  

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

  Alternatively, administrators can determine whether NAT is active on a device by issuing the show running-config command in the CLI and assessing 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.

  2. Determine whether DNS ALG is disabled in the NAT configuration
  

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

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

  If both no ip nat service dns tcp and no ip nat service dns udp do 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:

  • IOS Software
  • IOS XR Software
  • NX-OS Software

Workarounds

• 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 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 and no ip nat service dns udp 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 environment.

Fixed Software

• 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:

  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

• 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

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

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-alg-dos-hbBS7SZE

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2021-MAR-24

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