Summary

• 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:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-alg-dos-KU9Z8kFX

  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 Publication.

Affected Products

• 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 200-X
  • 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 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, 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 configuration:

  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

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 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 environment.

Fixed Software

• 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:
  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.

  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 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. 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.

  
  

  Only this advisory All Critical and High advisories All advisories

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-KU9Z8kFX

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2022-SEP-28

  Show Less