Summary

• A vulnerability in the Multicast Leaf Recycle Elimination (mLRE) feature of Cisco IOS XE Software for Cisco ASR 1000 Series Aggregation Services Routers could allow an unauthenticated, remote attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition. 

  This vulnerability is due to incorrect handling of certain IPv6 multicast packets when they are fanned out more than seven times on an affected device. An attacker could exploit this vulnerability by sending a specific IPv6 multicast or IPv6 multicast VPN (MVPNv6) packet through the affected device. A successful exploit could allow the attacker to cause a reload of the affected device, resulting in a DoS condition.

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

  This advisory is available at the following link:
  https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-mlre-H93FswRz

  This advisory is part of the September 2023 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 2023 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco ASR 1000 Series Aggregation Services Routers when all of the following conditions are met:

  • The device is running an affected release of Cisco IOS XE Software.
  • The device contains one of the following Embedded Services Processors (ESPs) or integrated ESPs:
    • ASR1000-ESP40
    • ASR1000-ESP100
    • ASR1000-ESP200
    • ASR1001-X
    • ASR1001-HX
    • ASR1002-X
    • ASR1002-HX
  • The device is configured with either IPv6 multicast or IPv6 multicast VPN and has ipv6 multicast-routing configured.
  • The device is configured so that IPv6 multicast packets fan out more than seven times. To determine how many times IPv6 multicast packets are configured to fan out on a device, use the show ipv6 mfib IPv6 group address CLI command. The following example shows the CLI output on a device that is configured to fan out IPv6 multicast packets eight times and is therefore affected:

    Router# show ipv6 mfib IPv6 group address
    (2001:DB8:FFFF:48::5:FFFF,FF3E::9800:200) Flags: HW
       SW Forwarding: 0/0/0/0, Other: 0/0/0
       HW Forwarding:   5/0/110/0, Other: 0/0/0
       GigabitEthernet2/1/1 Flags: A
       Port-channel1.8 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.7 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.6 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.5 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.4 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.3 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.2 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
       Port-channel1.1 Flags: F IC NS
         Pkts: 0/0/0    Rate: 0 pps
    8 F interfaces

  • The device has Cisco mLRE enabled.
    Note: mLRE is enabled by default on all affected platforms. If mLRE has been disabled, the output of the show running-config command will include platform multicast lre off. On Cisco IOS XE Software releases 17.1.1 and later, administrators can check the status of mLRE using the show platform hardware qfp active feature multicast lre command, as shown in the following example:

    Router# show platform hardware qfp active feature multicast lre
    === mcast lre config ===
      Platform v4mcast LRE config: On
      Platform v6mcast LRE config: On
    Router#

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

  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
  • Meraki products
  • NX-OS Software

  Cisco has also confirmed that this vulnerability does not affect Cisco ASR 1000 Series Aggregation Services Routers that are running the following ESP models:

  • ASR1000-ESP100-X
  • ASR1000-ESP200-X

Workarounds

• Administrators may disable mLRE on an affected device, as shown in the following example:

  Router#configure terminal
  Enter configuration commands, one per line.  End with CNTL/Z.
  Router(config)#platform multicast lre off
  Router(config)#end
  Router#show platform hardware qfp active feature multicast lre
  === mcast lre config ===  
  Platform v4mcast LRE config: Off  
  Platform v6mcast LRE config: Off
  Router#

  When the mLRE feature is disabled, packets are processed separately for the different interfaces that replicate multicast traffic. For more information on mLRE and the impact of disabling it, see mLRE Feature on the IOS-XE Router.

  While this workaround 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-mlre-H93FswRz

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2023-SEP-27

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