Summary

• A vulnerability in the MPLS Operation, Administration, and Maintenance (OAM) feature of Cisco NX-OS Software could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition on an affected device.

  This vulnerability is due to improper input validation when an affected device is processing an MPLS echo-request or echo-reply packet. An attacker could exploit this vulnerability by sending malicious MPLS echo-request or echo-reply packets to an interface that is enabled for MPLS forwarding on the affected device. A successful exploit could allow the attacker to cause the MPLS OAM process to crash and restart multiple times, causing the affected device to reload and 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:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-nxos-mpls-oam-dos-sGO9x5GM

  This advisory is part of the August 2021 Cisco FXOS and NX-OS Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: August 2021 Cisco FXOS and NX-OS Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects the following Cisco products if they are running a vulnerable release of Cisco NX-OS Software and have the MPLS OAM feature enabled:

  • Nexus 3000 Series Switches (CSCvx66765)
  • Nexus 7000 Series Switches (CSCvx48078)
  • Nexus 9000 Series Switches in standalone NX-OS mode (CSCvx66765)

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

  Determine the Status of the MPLS OAM Feature

  The MPLS OAM feature is disabled by default on Nexus 3000 Series Switches and Nexus 9000 Series Switches. To determine whether a Nexus 3000 Series or 9000 Series Switch is configured with the MPLS OAM feature enabled, use the show feature | include mpls_oam command at the Cisco NX-OS CLI and verify that the feature is enabled. The following example shows the MPLS OAM feature enabled on a Nexus 3000 Series or 9000 Series Switch:

  nxos# show feature | include mpls_oam
  mpls_oam               1          enabled

  MPLS OAM is disabled by default on Nexus 7000 Series Switches, but it is automatically enabled if any MPLS feature is enabled on the device. To determine whether a Nexus 7000 Series Switch has an MPLS feature enabled, use the show running-config | include "feature mpls" command at the Cisco NX-OS CLI as shown in the following example:

  nxos# show running-config | include "feature mpls"
  feature mpls ldp
  

  MPLS OAM can be manually disabled on Nexus 7000 Series Switches that have an MPLS feature enabled by using the no mpls oam command at the Cisco NX-OS CLI in global configuration mode. The show running-config all | include "no mpls oam" command may be used at the Cisco NX-OS CLI to verify whether MPLS OAM has been disabled in the configuration of a Nexus 7000 Series Switch running Cisco NX-OS Software Release 8.3(2) or earlier. However, Nexus 7000 Series Switches running a vulnerable Cisco NX-OS Software release 8.4(1) or later will not display no mpls oam in the running configuration. Refer to defect CSCwc58564 for additional details about this configuration display issue. To confirm MPLS OAM has been disabled for these releases, use the ping mpls command at the Cisco NX-OS CLI and verify the MPLS Embedded Management Subsystem is not running, as shown in the following example:

  nxos# ping mpls
  
  % MPLS Embedded Management Subsystem is not running
  To enable use 'mpls oam' global config command.

  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:

  • Firepower 1000 Series
  • Firepower 2100 Series
  • Firepower 4100 Series
  • Firepower 9300 Security Appliances
  • MDS 9000 Series Multilayer Switches
  • Nexus 1000 Virtual Edge for VMware vSphere
  • Nexus 1000V Switch for Microsoft Hyper-V
  • Nexus 1000V Switch for VMware vSphere
  • Nexus 5500 Platform Switches
  • Nexus 5600 Platform Switches
  • Nexus 6000 Series Switches
  • Nexus 9000 Series Fabric Switches in Application Centric Infrastructure (ACI) mode
  • UCS 6200 Series Fabric Interconnects
  • UCS 6300 Series Fabric Interconnects
  • UCS 6400 Series Fabric Interconnects

Details

• The MPLS OAM feature provides diagnostic capabilities that may be used to identify data plane failures in an MPLS network. MPLS OAM provides two main functions: MPLS ping and MPLS traceroute. These functions rely on the exchange of MPLS echo-request and echo-reply packets. These are UDP packets that have a destination port of 3503, and their contents follow a specific format that is defined in RFC 4379.

  This vulnerability is due to improper input validation when an affected device processes these packet types. An attacker could exploit this vulnerability by generating malicious MPLS echo-request or echo-reply packets in a way that would allow them to reach an interface enabled for MPLS forwarding on the affected device. To achieve this, the attacker must have access to the same MPLS domain as the affected device or be able to get these types of UDP packets forwarded through the MPLS network under specific network conditions.

  To process these packets, the affected device must have the MPLS OAM feature enabled. This feature is disabled by default on Nexus 3000 Series Switches and Nexus 9000 Series Switches, while it is automatically enabled on Nexus 7000 Series Switches if any other MPLS feature is enabled on the device.

Workarounds

• There are no workarounds that address this vulnerability.

  However, disabling the MPLS OAM feature removes the exploit vector. Administrators can disable the MPLS OAM feature by using the Cisco NX-OS CLI global configuration mode command no feature mpls oam on Nexus 3000 Series Switches and Nexus 9000 Series Switches, or by using no mpls oam on Cisco Nexus 7000 Series Switches. This action may be a suitable mitigation until devices that are affected by this vulnerability can be upgraded.

  Note: Nexus 7000 Series Switches running a vulnerable Cisco NX-OS Software release 8.4(1) or later will not display no mpls oam in the running configuration after the command has been applied. Refer to defect CSCwc58564 for additional details about this configuration display issue. To confirm MPLS OAM has been disabled on these releases, use the ping mpls command at the Cisco NX-OS CLI and verify the MPLS Embedded Management Subsystem is not running, as shown in the following example:

  nxos# ping mpls
  
  % MPLS Embedded Management Subsystem is not running
  To enable use 'mpls oam' global config command.

  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 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 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 NX-OS Software

  To help customers determine their exposure to vulnerabilities in Cisco NX-OS Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific Cisco NX-OS 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 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, platform, 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 or one or more specific advisories.

  Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by choosing the Cisco NX-OS Software and platform and then entering a release-for example, 7.0(3)I7(5) for Cisco Nexus 3000 Series Switches or 14.0(1h) for Cisco NX-OS Software in ACI mode:

  Cisco NX-OS Software Cisco NX-OS Software in ACI Mode MDS 9000 Series Multilayer Switches Nexus 1000V Series Switches Nexus 3000 Series Switches Nexus 5000 Series Switches Nexus 6000 Series Switches Nexus 7000 Series Switches Nexus 9000 Series Switches

  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 and check the Medium check box in the drop-down list under Impact Rating when customizing a search.

  Cisco Nexus 3000 and 9000 Series Switches SMUs

  For Cisco Nexus 3000 and 9000 Series Switches, SMUs are available for Cisco NX-OS Software. Customers can download the following SMUs from the Software Center on Cisco.com:

  • Release 7.0(3)I7(9): nxos.CSCvx66765-n9k_ALL-1.0.0-7.0.3.I7.9.lib32_n9000.rpm
  • Release 9.3(7a): nxos.CSCvx66765-n9k_ALL-1.0.0-9.3.7a.lib32_n9000.rpm

  For details about downloading and installing SMUs in Cisco NX-OS Software for Cisco Nexus 3000 and 9000 Series Switches, see the Performing Software Maintenance Upgrades section of the appropriate Cisco NX-OS system management configuration guide:

  • Cisco Nexus 3000 Series Switches
  • Cisco Nexus 9000 Series Switches
    
    

  Additional Resources

  For help determining the best Cisco NX-OS Software release for a Cisco Nexus Switch, see the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.

  Cisco MDS Series Switches
  Cisco Nexus 1000V for VMware Switch
  Cisco Nexus 3000 Series Switches
  Cisco Nexus 5500 Platform Switches
  Cisco Nexus 5600 Platform Switches
  Cisco Nexus 6000 Series Switches
  Cisco Nexus 7000 Series Switches
  Cisco Nexus 9000 Series Switches
  Cisco Nexus 9000 Series ACI-Mode Switches

  To determine the best release for Cisco UCS Software, see the Recommended Releases documents in the release notes for the device.

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-nxos-mpls-oam-dos-sGO9x5GM

Revision History

• Version	Description	Section	Status	Date
  1.1	Added information about a configuration display problem for Nexus 7000 Series Switches running vulnerable Cisco NX-OS Software 8.4 releases.	Affected Products, Workarounds	Final	2022-SEP-02
  1.0	Initial public release.	-	Final	2021-AUG-25

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