Summary

• A vulnerability in the Easy Virtual Switching System (VSS) of Cisco IOS XE Software on Catalyst 4500 Series Switches could allow an unauthenticated, adjacent attacker to cause the switches to reload.

  The vulnerability is due to incomplete error handling when processing Cisco Discovery Protocol (CDP) packets used with the Easy Virtual Switching System. An attacker could exploit this vulnerability by sending a specially crafted CDP packet. An exploit could allow the attacker to cause the device to reload, resulting in a denial of service (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-20190327-evss

  This advisory is part of the March 27, 2019, release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication, which includes 17 Cisco Security Advisories that describe 19 vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: March 2019 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco Catalyst 4500/4500X Series Switches that are running a vulnerable release of Cisco IOS XE Software and have CDP enabled. 

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

  There are two different configurations where the switch is vulnerable:

  1. Cisco Catalyst 4500/4500X Series device has CDP enabled with CDP Application TLVs.
  2. When an admin is converting the switches' configuration from standalone mode to virtual switch mode using easy virtual switching. 

  Determining if CDP Is Enabled

  To determine whether use of the Cisco Discovery Protocol is enabled for a device, administrators can use the show cdp command in the device CLI. The output of the command displays global Cisco Discovery Protocol information or, if the protocol is disabled, indicates that use of the protocol is not enabled. Examples of both are shown as follows:

  Switch#show cdp
  Global CDP information:
  	Sending CDP packets every 60 seconds
  	Sending a holdtime value of 180 seconds
  	Sending CDPv2 advertisements is  enabled
  Switch#
  Switch#show cdp
  % CDP is not enabled
  Switch#
  

  Determining if CDP Application TLVs Are Enabled

  By default, CDP Application TLV processing is enabled. To determine if CDP Application TLVs are disabled, administrators can issue the show running-config | include no cdp tlv app CLI command. If the output returns nothing, then processing of CDP Application TLVs is enabled. If it returns output, it will indicate which interfaces have the CDP Application TLVs processing disabled, as shown in the following example:

  Switch#show running-config | include no cdp tlv app
  no cdp tlv app
   no cdp tlv app
   no cdp tlv app
  Switch# 

  Determining if Admin Is Converting the Switches’ Configuration From Standalone Mode to Virtual Switch Mode Using Easy Virtual Switching

  When an admin issues the switch convert mode easy-virtual-switch exec command on a device, it commences configuring the switches to operate as a VSS. Once the configuration has completed, the switches will reload. Part of this process is that UDP port 5500 is opened until the switches reload. During this period, it is possible to remotely exploit this vulnerability. Once the system reloads as a VSS, only the adjacent vector is available.

  Determining the Cisco IOS XE Software Release

  To determine which Cisco IOS XE Software release is running on a device, administrators can log in to the device, use the show version command in the CLI, and then refer to the system banner that appears. If the device is running Cisco IOS XE Software, the system banner displays Cisco IOS Software, Cisco IOS XE Software, or similar text.

  The following example shows the output of the command for a device that is running Cisco IOS XE Software Release 16.2.1 and has an installed image name of CAT3K_CAA-UNIVERSALK9-M:

  ios-xe-device# show version
  
  Cisco IOS Software, Catalyst L3 Switch Software (CAT3K_CAA-UNIVERSALK9-M), Version Denali 16.2.1, RELEASE SOFTWARE (fc1)
  Technical Support: http://www.cisco.com/techsupport
  Copyright (c) 1986-2016 by Cisco Systems, Inc.
  Compiled Sun 27-Mar-16 21:47 by mcpre
  .
  .
  .

  For information about the naming and numbering conventions for Cisco IOS XE Software releases, see the Cisco IOS and NX-OS Software Reference Guide.

  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 Cisco IOS Software, Cisco IOS XR Software, or Cisco NX-OS Software.

Details

• While the attack vector is network during the admin converting the switches configuration from standalone mode to virtual switch mode using easy virtual switching, the attack complexity also increases to high, resulting in a lower CVSSv3 score than would be assigned with the attack vector being adjacent and the attack complexity being low.

Indicators of Compromise

• The traceback generated with the crash will indicate that the system crashed in the process vss bringup, as shown in the following example:
  

  IOSD-EXT-SIGNAL: Segmentation fault(11), Process = vss bringup

  Customers experiencing a crash with this traceback should raise a case with their support organization to confirm they are hitting this vulnerability.

Workarounds

• Disabling CDP Application Type, Length, Value (TLV)

  By default, CDP Application TLVs are enabled. Disabling CDP Application TLVs will mitigate this vulnerability. To disable use of CDP Application TLVs globally for a device, use the no cdp tlv app command in the global configuration CLI. To disable use of CDP Application TLVs for a specific interface of a device, use the no cdp tlv app command in the interface configuration CLI.

  To see if CDP Application TLVs are in use on the device before disabling them, use the command show cdp tlv app. If the command returns output, then it is advisable not to disable the CDP TLV App globally.

  Disabling CDP

  If CDP is not required, administrators may disable use of the Cisco Discovery Protocol by a device. To disable use of the protocol globally for a device, use the no cdp run command in the global configuration CLI. To disable use of the protocol for a specific interface of a device, use the no cdp enable command in the interface configuration CLI.

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 and Alerts 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 a tool, the Cisco IOS Software Checker, that identifies any Cisco Security Advisories that impact a specific software release and the earliest release that fixes the vulnerabilities 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 this tool to perform the following tasks:

  • Initiate a search by choosing one or more releases from a drop-down list or uploading a file from a local system for the tool to parse
  • Enter the output of the show version command for the tool to parse
  • Create a custom search by including all previously published Cisco Security Advisories, a specific advisory, or all advisories in the most recent bundled publication

  To determine whether a release is affected by any published Cisco Security Advisory, use the Cisco IOS Software Checker on Cisco.com or enter a Cisco IOS or IOS XE Software release—for example, 15.1(4)M2 or 3.13.8S—in the following field:

  By default, the Cisco IOS Software Checker includes results only for vulnerabilities that have a Critical or High Security Impact Rating (SIR). To include results for Medium SIR vulnerabilities, use the Cisco IOS Software Checker on Cisco.com and check the Medium check box in the Impact Rating drop-down list.

  For a mapping of Cisco IOS XE Software releases to Cisco IOS Software releases, refer to the Cisco IOS XE 2 Release Notes, Cisco IOS XE 3S Release Notes, or Cisco IOS XE 3SG Release Notes, depending on the Cisco IOS XE Software release.

  Note: Starting with Cisco IOS XE Software Release 16.9.1, any upgrade will require Smart Licensing. Customers who plan to upgrade Cisco IOS XE to Release 16.9.1 or later are advised to consider the Smart Licensing requirement. The following documentation provides additional information: Smart Licensing.

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-20190327-evss

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2019-March-27

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