Summary

• A vulnerability in the errdisable per VLAN feature of Cisco IOS XE Software could allow an unauthenticated, adjacent attacker to cause the device to crash, leading to a denial of service (DoS) condition.

  The vulnerability is due to a race condition that occurs when the VLAN and port enter an errdisabled state, resulting in an incorrect state in the software. An attacker could exploit this vulnerability by sending frames that trigger the errdisable condition. A successful exploit could allow the attacker to cause the affected device to crash, leading to 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-20180926-errdisable

  This advisory is part of the September 26, 2018, release of the Cisco IOS and IOS XE Software Security Advisory Bundled Publication, which includes 12 Cisco Security Advisories that describe 13 vulnerabilities. For a complete list of the advisories and links to them, see Cisco Event Response: September 2018 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects Cisco Catalyst 3650, 3850, and 4500E Series Switches that are running a vulnerable release of Cisco IOS XE Software and have the errdisable feature enabled for a feature at both the VLAN and port level.

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

  Determining Whether a Device Is Running an Affected Configuration

  To determine whether a device is running an affected configuration at both the VLAN and port level, perform both of the following steps:

  Step 1: VLAN Level

  There are two options for completing Step 1.

  Option A: show errdisable detect

  Issue the CLI command show errdisable detect | include vlan. For each VLAN mode feature that the device lists in the output for this command, perform the corresponding portion of Step 2. The following example shows a device that is configured for a VLAN mode errdisable detect for bpduguard (BPDU guard), psecure-violation (port security violation), and security-violation (802.1x security violation) on the VLAN:

  switch#show errdisable detect | include vlan
    bpduguard                    Enabled          vlan
    psecure-violation            Enabled          port/vlan
    security-violation           Enabled          vlan
    switch#

  Option B: show running-config

  To determine whether a device is running an affected configuration at the VLAN level, issue the show running-config command twice, using two different include statements:

  • First, issue the CLI command show running-config | include shutdown vlan. For each VLAN mode feature that the device lists in the output for this command, perform the corresponding portion of Step 2. The following example shows a device that is configured for a VLAN mode errdisable detect for security-violation (802.1x security violation) and bpduguard (BPDU guard) on the VLAN:

    switch#show running-config | include shutdown vlan
          errdisable detect cause security-violation shutdown vlan
          errdisable detect cause bpduguard shutdown vlan
          switch#

  • Second, issue the CLI command show running-config | include switchport port-security violation shutdown vlan. If the command returns output, perform the corresponding portion of Step 2. The following example shows a device that is configured for a VLAN mode errdisable detect for port security violation on the VLAN :

    switch#show running-config | include switchport port-security violation shutdown vlan
          switchport port-security violation shutdown vlan
          switch#

  Step 2: Port Level

  For each VLAN mode configured feature revealed in Step 1, do the following to determine whether the feature is also enabled at the port level:

  • BPDU guard: If errdisable per VLAN for detection cause bpduguard is enabled, confirm that the device also has BPDU guard configured. If the configuration contains either of the following, the device is vulnerable:

    spanning-tree  portfast bpduguard default
      interface  <interface-id>
      spanning-tree portfast

    or

    interface  <interface-id>
      spanning-tree bpduguard enable 

  • Port security violation: Errdisable per port/vlan for detection cause psecure-violation is enabled by default. Confirm that the device also has port security configured and violation vlan shutdown configured. If the output of show port-security contains a row with Shutdown Vlan (as shown in the following example), the device is vulnerable:

    switch#show port-security
    Secure Port  MaxSecureAddr  CurrentAddr  SecurityViolation  Security Action
                    (Count)       (Count)          (Count)
    ---------------------------------------------------------------------------
       Gi1/0/48              1            1                  5    Shutdown Vlan
    ---------------------------------------------------------------------------
    Total Addresses in System (excluding one mac per port)     : 0
    Max Addresses limit in System (excluding one mac per port) : 4096
    switch#

  • 802.1x security violation: If errdisable per VLAN for detection cause security-violation is enabled, confirm that the device also has 802.1x authentication configured and the voice VLAN is configured. If the configuration contains the following and does not contain authentication violation {protect|replace|restrict}, the device is vulnerable:

    interface  <interface-id>
      authentication port-control value
      switchport voice vlan value

    However, if the following content is also present in the CLI output, the device is not vulnerable:

    authentication violation {protect|replace|restrict}

  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.

Workarounds

• There are no workarounds that address this vulnerability.

  For customers who can remove errdisable per VLAN detection and use the corresponding errdisable per port detection, this action may be a suitable mitigation until switches that are affected by this vulnerability can be upgraded.

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 Software or Cisco 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.

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-20180926-errdisable

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	—	Final	2018-September-26

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