Cisco Security Advisory
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CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:N/A:H/E:X/RL:X/RC:X
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A vulnerability in the handling of specific Ethernet frames by Cisco IOS XR Software for various Cisco Network Convergence System (NCS) platforms could allow an unauthenticated, adjacent attacker to cause critical priority packets to be dropped, resulting in a denial of service (DoS) condition.
This vulnerability is due to incorrect classification of certain types of Ethernet frames that are received on an interface. An attacker could exploit this vulnerability by sending specific types of Ethernet frames to or through the affected device. A successful exploit could allow the attacker to cause control plane protocol relationships to fail, resulting in a DoS condition. For more information, see the Details section of this advisory.
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-l2services-2mvHdNuCThis 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.
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Vulnerable Products
This vulnerability affects the following Cisco NCS devices if they are running a vulnerable release of Cisco IOS XR Software and if they are configured with Layer 2 Ethernet services:
- NCS 55A1 Fixed Chassis
- NCS 55A2 Fixed Chassis
- NCS 540 Series Routers (except N540-24Q8L2DD-SYS)
- NCS 560 Series Routers
- NCS 5500 Series Modular Chassis1
- NCS 5501 Fixed Chassis
- NCS 5502 Fixed Chassis
1. All product IDs except those starting with NCS57 are affected by this vulnerability. For more information, see the Cisco Network Convergence System 5500 Series Modular Chassis data sheet.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Determine Whether the Device is Configured with Layer 2 Ethernet Services
To determine whether Layer 2 services are configured on the device, use the command show running-config | include l2transport. If output is returned, the device is affected, as shown in the following example:
RP/0/RP0/CPU0:NCS5501#show running-config | include l2transport
Tue Jan 16 01:14:07.977 UTC
Building configuration...
interface GigabitEthernet0/0/0/1.200 l2transport
RP/0/RP0/CPU0:NCS5501#If the output returns no information, the device 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
- IOS XR Platforms that are not listed in the Vulnerable Products section of this advisory
- NX-OS Software
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When specific frames are received on the Layer 2 interface, they are incorrectly classified as traffic class 7 virtual output queue (TC7 VOQ). If enough of these frames are sent at sustained rates, the VOQ will start to drop packets in TC7. The affected port is the interface by which the Layer 2 traffic receives egresses from the device. In most cases, this is the core interface of a service provider.
Because control plane traffic is also handled within this queue, this vulnerability could cause keepalive or hello packets to be dropped, resulting in a DoS condition for protocols such as OSPF, Intermediate System-to-Intermediate System (ISIS), Bidirectional Forwarding Detection (BFD), Border Gateway Protocol (BGP), and Label Distribution Protocol (LDP).
To monitor the state of the VOQs, use the show controllers npu stats voq ingress interface command. The following example shows command output of a TC_7 VOQ that is being exhausted:
RP/0/RP0/CPU0:NCS-5501#show controllers npu stats voq ingress interface tenGigE 0/0/0/2 instance 0 location 0/0/CPU0
Wed Mar 13 16:00:00.000 UTC
Interface Name = Te0/0/0/2
Interface Handle = 118
Location = 0/0/CPU0
Asic Instance = 0
VOQ Base = 1088
Port Speed(kbps) = 10000000
Local Port = local
ReceivedPkts ReceivedBytes DroppedPkts DroppedBytes
-------------------------------------------------------------------
Core-0:
TC_0 = 0 0 0 0
TC_1 = 47708616 37410160369 0 0
TC_2 = 0 0 0 0
TC_3 = 0 0 0 0
TC_4 = 0 0 0 0
TC_5 = 0 0 0 0
TC_6 = 0 0 0 0
TC_7 = 11033161168 1108832775990 6905125762 791656829609
Core-1:
TC_0 = 0 0 0 0
TC_1 = 0 0 0 0
TC_2 = 0 0 0 0
RP/0/RP0/CPU0:NCS-5501#
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There are no workarounds that address this vulnerability.
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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.htmlAdditionally, 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 7.6 and earlier Not affected. 7.7 Migrate to a fixed release. 7.8 Migrate to a fixed release. 7.9 Migrate to a fixed release. 7.10 7.10.2 7.11 7.11.1 24.1 24.1.1 Cisco has released the following SMUs to address this vulnerability. Customers who require SMUs for releases that are not listed in the following table are advised to contact their support organization.
Cisco IOS XR Software Release Platform SMU Name 7.8.2 NCS540 ncs540-7.8.2.CSCwh30122 7.9.2 IOSXRWBD
NCS5500iosxrwbd-7.9.2.CSCwh30122
ncs5500-7.9.2.CSCwh30122The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.
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The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.
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This vulnerability was found during the resolution of a Cisco TAC support case.
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To learn about Cisco security vulnerability disclosure policies and publications, see the Security Vulnerability Policy. This document also contains instructions for obtaining fixed software and receiving security vulnerability information from Cisco.
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Show LessVersion Description Section Status Date 1.0 Initial public release. — Final 2024-SEP-11
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