Summary

• A vulnerability in the segment routing feature for the Intermediate System-to-Intermediate System (IS-IS) protocol of Cisco IOS XR Software could allow an unauthenticated, adjacent attacker to cause a denial of service (DoS) condition on an affected device.

  This vulnerability is due to insufficient input validation of ingress IS-IS packets. An attacker could exploit this vulnerability by sending specific IS-IS packets to an affected device after forming an adjacency. A successful exploit could allow the attacker to cause the IS-IS process on all affected devices that are participating in the Flexible Algorithm to crash and restart, resulting in a DoS condition.

  Note: The IS-IS protocol is a routing protocol. To exploit this vulnerability, an attacker must be Layer 2-adjacent to the affected device and must have formed an adjacency. This vulnerability affects segment routing for IS-IS over IPv4 and IPv6 control planes as well as devices that are configured as level 1, level 2, or multi-level routing IS-IS type.

  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-isis-xehpbVNe

  This advisory is part of the September 2024 release of the Cisco IOS XR Software Security Advisory Bundled Publication. For a complete list of the advisories and links to them, see Cisco Event Response: September 2024 Semiannual Cisco IOS XR Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco platforms if they are running a vulnerable release of Cisco IOS XR Software and have the IS-IS Segment Routing Flexible Algorithm enabled with either of the following features also enabled:

  • Segment Routing Microloop Avoidance
  • Topology Independent Loop-Free Alternate (TI-LFA) for Flexible Algorithm

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

  Determine Whether the Device Has a Vulnerable Configuration

  To determine whether a device has a vulnerable configuration, complete the following steps. If the device has IS-IS Segment Routing Flexible Algorithm enabled but not Microloop Avoidance or TI-LFA, it is not affected by this vulnerability.

  1. Determine the IS-IS Segment Routing Flexible Algorithm Status

  To determine whether a device has the IS-IS Segment Routing Flexible Algorithm enabled, use the show running-config router isis | include flex-algo EXEC command in the CLI. If the device is configured for IS-IS Segment Routing Flexible Algorithm, this command will return output. The following example shows the partial output on a device that is configured for IS-IS Segment Routing Flexible Algorithm:

  RP/0/RSP0/CPU0:XR#show running-config router isis | include flex-algo
  Wed Sept 11 16:00:00.000 UTC
  flex-algo 200
  RP/0/RSP0/CPU0:XR#

  If the device returns output, it may be affected by this vulnerability. Continue to Step 2. 

  If the device returns no output, it is not affected by this vulnerability. It is not necessary to complete Step 2 or Step 3.

  2. Determine the IS-IS Segment Routing Microloop Avoidance Status

  To determine whether a device has IS-IS Segment Routing Microloop Avoidance enabled, use the show running-config router isis | include microloop EXEC command in the CLI. If the device is configured for IS-IS Segment Routing Microloop Avoidance, this command will return output. The following example shows the partial output on a device that is configured for IS-IS Segment Routing Microloop Avoidance:

  RP/0/RSP0/CPU0:XR#show running-config router isis | include microloop
  Wed Sept 11 16:00:00.000 UTC
    microloop avoidance segment-routing
  RP/0/RSP0/CPU0:XR#

  If the device returns output and IS-IS Segment Routing Flexible Algorithm is enabled, it is affected by this vulnerability.

  If the device returns no output, it does not have IS-IS Segment Routing Microloop Avoidance enabled. Continue to Step 3.

  3. Determine the TI-LFA for Flexible Algorithm Status

  To determine whether a device has TI-LFA for Flexible Algorithm enabled, use the show running-config router isis | include ti-lfa EXEC command in the CLI. If the device is configured for IS-IS TI-LFA, this command will return output. The following example shows the partial output on a device that is configured for IS-IS TI-LFA for Flexible Algorithm.

  RP/0/RSP0/CPU0:XR#show running-config router isis | include ti-lfa
  Wed Sept 11 16:00:00.000 UTC
     fast-reroute per-prefix ti-lfa
  RP/0/RSP0/CPU0:XR#

  If the device returns output and IS-IS Segment Routing Flexible Algorithm is enabled, it is affected by this vulnerability.

  If the device returns no output, it does not have IS-IS TI-LFA for Flexible Algorithm enabled and it is not affected by this vulnerability.

  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 XE Software
  • NX-OS Software

Workarounds

• There are no workarounds that address this vulnerability. However, there is a mitigation.

  As a best practice, configure IS-IS area authentication so attackers would first have to pass the authentication to trigger this vulnerability. For more information on IS-IS authentication, see Configuring IS-IS Authentication.

  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.

  Fixed Releases

  In the following table, the left column lists Cisco software releases or trains. The right column indicates whether a release (train) is affected by the vulnerability that is described in this advisory and the first release that includes the fix for this vulnerability.

  Cisco IOS XR Software Release	First Fixed Release
  6.7.4 and earlier	Not affected.
  6.8	Migrate to a fixed release.
  6.9	Migrate to a fixed release.
  7.0 through 7.3	Not affected.
  7.4 through 7.10	Migrate to a fixed release.
  7.11	7.11.2
  24.1 and later	Not affected.

  Cisco has released the following SMUs to address this vulnerability. Customers who require SMUs for platforms or releases that are not listed are advised to contact their support organization.

  Cisco IOS XR Software Release	Platform	SMU Name
  7.5.2	8000 Series ASR9K-X64 NCS540 NCS540L NCS5500 NCS560	8000-7.5.2.CSCwi39542 asr9k-x64-7.5.2.CSCwi39542 ncs540-7.5.2.CSCwi39542 ncs540l-7.5.2.CSCwi39542 ncs5500-7.5.2.CSCwi39542 ncs560-7.5.2.CSCwi39542
  7.7.2	8000 Series ASR9K-X64	8000-7.7.2.CSCwi39542 asr9k-x64-7.7.2.CSCwi39542
  7.9.2	8000 Series ASR9K-X64 NCS5500	8000-7.9.2.CSCwi39542 asr9k-x64-7.9.2.CSCwi39542 ncs5500-7.9.2.CSCwi39542
  7.9.21	ASR9K-X64	asr9k-x64-7.9.21.CSCwi39542
  7.10.2	8000 Series ASR9K-X64	8000-7.10.2.CSCwi39542 asr9k-x64-7.10.2.CSCwi39542

  The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.

Exploitation and Public Announcements

• The Cisco 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 by Prathap Raju Hongere Devaraju of Cisco during internal security testing.

URL

• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-isis-xehpbVNe

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2024-SEP-11

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