Multiple Cisco Operating Systems Unidirectional Link Detection Denial of Service Vulnerability

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Updated:September 22, 2021
Document ID:1632329673130518

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Cisco Security Advisory

Multiple Cisco Operating Systems Unidirectional Link Detection Denial of Service Vulnerability

Medium
Advisory ID:
cisco-sa-ios-nxos-xr-udld-dos-W5hGHgtQ
First Published:
2021 September 22 16:00 GMT
Version 1.0:
Final
Workarounds:
No workarounds available
CVE-2021-34714
CWE-20
CVSS Score:
Base 7.4Click Icon to Copy Verbose Score
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
CVE-2021-34714
CWE-20

Summary

  • A vulnerability in the Unidirectional Link Detection (UDLD) feature of Cisco FXOS Software, Cisco IOS Software, Cisco IOS XE Software, Cisco IOS XR Software, and Cisco NX-OS Software could allow an unauthenticated, adjacent attacker to cause an affected device to reload.

    This vulnerability is due to improper input validation of the UDLD packets. An attacker could exploit this vulnerability by sending specifically crafted UDLD packets to an affected device. A successful exploit could allow the attacker to cause the affected device to reload, resulting in a denial of service (DoS) condition.

    Note: The UDLD feature is disabled by default, and the conditions to exploit this vulnerability are strict. An attacker must have full control of a directly connected device. On Cisco IOS XR devices, the impact is limited to the reload of the UDLD process.

    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-ios-nxos-xr-udld-dos-W5hGHgtQ

    This advisory is part of the September 2021 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 2021 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

  • Vulnerable Products

    At the time of publication, this vulnerability affected Cisco devices if they were running a vulnerable release of the following Cisco software and had the UDLD feature enabled:

    At the time of publication, this vulnerability also affected the following Cisco products if they were running a vulnerable release of Cisco FXOS or NX-OS Software and had the UDLD feature enabled:

    1. Firepower 4100 and 9300 products do not officially support UDLD; however, the CLI includes commands to enable it. These products could be vulnerable only if UDLD has been enabled in error. In such cases, administrators are advised to disable UDLD to fully eliminate exposure to this vulnerability.

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

    Determine the Status of the UDLD Feature

    To determine whether the UDLD feature is enabled on a device, follow the product-specific instructions below:

    Cisco FXOS Software and UCS Fabric Interconnects

    Use the scope org command followed by the show udld-link-policy command at the device CLI. The command output will display either Enabled or Disabled under Admin State for the default and for any manually configured UDLD link policy. The following example shows the command output on a device that has UDLD disabled:

    fxos# scope org
    fxos# show udld-link-policy

    UDLD link policy:
    Name       Admin State UDLD mode
    ---------- ----------- ---------
    default       Disabled    Normal

    Cisco IOS and IOS XE Software

    Use the show udld | include "state: Enabled" command at the device CLI. If the command returns output, the UDLD feature is configured on some interfaces of the device. The following example shows the command output on a device that has UDLD enabled:

    Router# show udld | include "state: Enabled"
    Port enable operational state: Enabled / in aggressive mode

    Cisco IOS XR Software

    Use the show ethernet udld interfaces brief command at the device CLI. If the command returns output, the UDLD feature is configured on some interfaces of the device. The following example shows the command output on a device that has UDLD enabled:

    RP/0/RSP0/CPU0:router# show ethernet udld interfaces brief 
    
Port           State          Neighbor Device                 N’bor port
      -------------  -------------- ------------------------------- -------------
      Gi0/1/0/1      Bidirectional  london-xr22.cisco.com           Gi3/12/0/24
      Gi0/1/0/2      Bidirectional  [2 neighbors]                   -
      Gi0/1/0/3      Unknown        -                               -
      Gi0/1/0/4      Unidirectional sj-ios25.cisco.com              Gi3/5
      Te0/12/0/10    Admin Down     -                               -
      Te0/12/0/11    N’bor Mismatch long-device.cisco.com           LongPortNam>>

    Cisco NX-OS Software

    Use the show running-config | include "feature udld" command at the device CLI. If the command returns output, the UDLD feature is configured on the device. The following example shows the command output on a device that has UDLD enabled:

    nxos# show running-config | include "feature udld"
    feature udld

     

    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:

    • Firepower 1000 Series
    • Firepower 2100 Series
    • Meraki products
    • Nexus 1000 Virtual Edge for VMware vSphere
    • Nexus 1000V Switch for Microsoft Hyper-V
    • Nexus 1000V Switch for VMware vSphere
    • Nexus 9000 Series Fabric Switches in Application Centric Infrastructure (ACI) mode

Workarounds

  • There are no workarounds that address this vulnerability.

Fixed Software

  • 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.

    Cisco FXOS Software

    At the time of publication, the release information in the following table(s) was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

    The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability described in this advisory and which release included the fix for this vulnerability.

    Firepower 4100 Series and Firepower 9300 Security Appliances

    Cisco FXOS Software Release First Fixed Release for this Vulnerability
    2.2 2.2.2.1481
    2.3 2.3.1.2161
    2.4 2.4.1.2731
    2.6 2.6.1.2241
    2.7 2.7.1.1431
    2.8 2.8.1.143 1
    2.9 2.9.1.135 1
    2.10 Not vulnerable
    1. Firepower 4100and 9300 products do not officially support UDLD; however, the CLI includes commands to enable it. These products could be vulnerable only if UDLD has been enabled in error. In such cases, administrators are advised to disable UDLD to fully eliminate exposure to this vulnerability.

    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 to identify 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 the Cisco Software Checker to search advisories in the following ways:

    • Choose the software and one or more releases
    • Upload a .txt file that includes a list of specific releases
    • Enter the output of the show version command

    After initiating a search, customers can customize the search to include all Cisco Security Advisories, a specific advisory, or all advisories in the most recent bundled publication.

    Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by entering a Cisco IOS or IOS XE Software release—for example, 15.1(4)M2 or 3.13.8S:

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

    Cisco IOS XR Software

    At the time of publication, the release information in the following table(s) was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information, including SMU availability.

    The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability described in this advisory and which release included the fix for this vulnerability.

    Cisco IOS XR Software Release First Fixed Release for this Vulnerability
    6.6 Vulnerable; migrate to a fixed release.
    6.7 6.7.4
    6.8 6.8.1
    7.2 7.2.2
    7.3 7.3.1, 7.3.15
    7.4 7.4.1

     

    Cisco NX-OS Software

    To help customers determine their exposure to vulnerabilities in Cisco NX-OS Software, Cisco provides the Cisco Software Checker to identify any Cisco Security Advisories that impact a specific Cisco NX-OS 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 described in all the advisories identified (“Combined First Fixed”).

    Customers can use the Cisco Software Checker to search advisories in the following ways:

    • Choose the software, platform, and one or more releases
    • Upload a .txt file that includes a list of specific releases
    • Enter the output of the show version command

    After initiating a search, customers can customize the search to include all Cisco Security Advisories or one or more specific advisories.

    Customers can also use the following form to determine whether a release is affected by any Cisco Security Advisory by choosing the Cisco NX-OS Software and platform and then entering a release—for example, 7.0(3)I7(5) for Cisco Nexus 3000 Series Switches or 14.0(1h) for Cisco NX-OS Software in ACI mode:

     

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

    Cisco UCS Software

    At the time of publication, the release information in the following table(s) was accurate. See the Details section in the bug ID(s) at the top of this advisory for the most complete and current information.

    The left column lists Cisco software releases, and the right column indicates whether a release was affected by the vulnerability described in this advisory and which release included the fix for this vulnerability.

    UCS 6200, 6300 and 6400 Series Fabric Interconnects

    Cisco UCS Software Release First Fixed Release for This Vulnerability
    Earlier than 4.0 Migrate to a fixed release.
    4.0 4.0(4l)
    4.1 4.1(2c)
    4.2 Not vulnerable.

     

    Additional Resources

    For help determining the best Cisco NX-OS Software release for a Cisco Nexus Switch, see the following Recommended Releases documents. If a security advisory recommends a later release, Cisco recommends following the advisory guidance.

    Cisco MDS Series Switches
    Cisco Nexus 1000V for VMware Switch
    Cisco Nexus 3000 Series Switches
    Cisco Nexus 5500 Platform Switches
    Cisco Nexus 5600 Platform Switches
    Cisco Nexus 6000 Series Switches
    Cisco Nexus 7000 Series Switches
    Cisco Nexus 9000 Series Switches
    Cisco Nexus 9000 Series ACI-Mode Switches

    To determine the best release for Cisco UCS Software, see the Recommended Releases documents in the release notes for the device.

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 by Marco Cassini of Cisco during internal security testing.

Cisco Security Vulnerability Policy

  • 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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Revision History

  • Version Description Section Status Date
    1.0 Initial public release. — Final 2021-SEP-22
    Show Less

Cisco Security Vulnerability Policy

  • 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.

Subscribe to Cisco Security Notifications

Related to This Advisory