Summary

• A vulnerability in the DHCP snooping security feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause a full interface queue wedge, which could result in a denial of service (DoS) condition.

  This vulnerability is due to improper handling of DHCP request packets. An attacker could exploit this vulnerability by sending DHCP request packets to an affected device. A successful exploit could allow the attacker to cause packets to wedge in the queue, creating a DoS condition for downstream devices of the affected system and requiring that the system restart to drain the queue.

  Note: This vulnerability can be exploited with either unicast or broadcast DHCP packets on a VLAN that does not have DHCP snooping enabled.

  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-iosxe-dhcpsn-dos-xBn8Mtks

  This advisory is part of the May 2025 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: May 2025 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.

Affected Products

• Vulnerable Products

  This vulnerability affects the following Cisco products if they are running a vulnerable release of Cisco IOS XE Software and have DHCP snooping enabled for at least one, but not all, VLANs that are configured on the device:

  • 1100 Series Integrated Service Routers (ISRs)
  • Catalyst 8200 Series Edge Platforms
  • Catalyst 8300 Series Edge Platforms

  Note: If the packet is received on a VLAN that has DHCP snooping enabled, the packet will not be held. If the packet is received on a VLAN that does not have DHCP snooping enabled, the packet will be held.

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

  Determine the Device Configuration

  To determine whether DHCP snooping is enabled for some, but not all, VLANs on a device, run the show ip dhcp snooping command, as shown in the following example:

  Router#show ip dhcp snooping
  Switch DHCP snooping is enabled
  Switch DHCP gleaning is disabled
  DHCP snooping is configured on following VLANs:
  2
  DHCP snooping is operational on following VLANs:
  2
  Proxy bridge is configured on following VLANs:
  none 
  Proxy bridge is operational on following VLANs:
  none
  DHCP snooping is configured on the following L3 Interfaces:
  
  Insertion of option 82 is enabled
     circuit-id default format: vlan-mod-port
     remote-id: 4c77.6d2e.3bf1 (MAC)
  
  Option 82 on untrusted port is not allowed
  Verification of hwaddr field is enabled
  Verification of giaddr field is enabled
  
  DHCP snooping trust/rate is configured on the following Interfaces:
  
  Interface                  Trusted    Allow option    Rate limit (pps)
  -----------------------    -------    ------------    ----------------
  Router#

  To view all active VLANs, use the show vlan command, as shown in the following example:

  Router#show vlan
  
  VLAN Name                             Status    Ports
  ---- -------------------------------- --------- -------------------------------
  1    default                          active    Gi0/1/0, Gi0/1/1, Gi0/1/2
                                                  Gi0/1/3
  2    VLAN0002                         active
  1002 fddi-default                     act/unsup
  1003 token-ring-default               act/unsup
  1004 fddinet-default                  act/unsup
  1005 trnet-default                    act/unsup

  
  

  Products Confirmed Not Vulnerable

  Cisco has confirmed that this vulnerability does not affect the following Cisco products:

  • IOS Software
  • IOS XR Software
  • ISR 1000 Series Routers
  • Meraki products
  • NX-OS Software

Details

• Use a Temporary Measure to Allow Traffic to Pass Until the Device Can Be Reloaded

  If this vulnerability has been exploited and the attack has stopped, administrators can set a maximum hold-queue value that is greater than the currently configured value for the affected interface to allow traffic to pass until the device can be reloaded. The following example shows how to set the value to 350 by using the hold-queue in interface configuration command:

  Router# configure terminal
  Router(config)# interface gigabitEthernet 1
  Router(config-if)# hold-queue 350 in

  Monitor and Detect Queue Wedges with Cisco IOS Embedded Event Manager

  To monitor and detect interface queue wedges that are caused by this vulnerability, administrators can use a Cisco IOS Embedded Event Manager (EEM) policy that is based on Tool Command Language (Tcl). This policy would allow administrators to monitor interfaces for devices that are running Cisco IOS XE Software and detect when input queues are full. When Cisco IOS EEM detects potential exploitation of this vulnerability, the policy sends an alert to the network administrator, who can then decide to implement an upgrade, implement suitable mitigations, or reload the device to clear the input queue.

  The Tcl script is available for download at Interface Input Queue Monitor.

Indicators of Compromise

• To identify an affected interface, run the following commands:

  Router#show interfaces | include line|Input
  GigabitEthernet1 is up, line protocol is up
    Input queue: 376/375/0/0 (size/max/drops/flushes); Total output drops: 0

  An input queue is wedged when the size value is greater than the max value.

Workarounds

• There is a workaround that addresses this vulnerability. Administrators can enable DHCP snooping on all VLANs by using the ip dhcp snooping vlan ? command, as shown in the following example:

  Device-1(config)#ip dhcp snooping vlan ?
   WORD DHCP Snooping vlan first number or vlan range, example: 1,3-5,7,9-11

  While this workaround 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.

  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. This tool identifies any Cisco security advisories that impact a specific 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 that are described in all the advisories that the Software Checker identifies (“Combined First Fixed”).

  To use the tool, go to the Cisco Software Checker page and follow the instructions. Alternatively, use the following form to determine whether a release is affected by any Cisco Security Advisory. To use the form, follow these steps:

  1. Choose which advisories the tool will search-only this advisory, only advisories with a Critical or High Security Impact Rating (SIR), or all advisories.
  2. Enter a release number-for example, 15.9(3)M2 or 17.3.3.
  3. Click Check.

  
  

  Only this advisory All Critical and High advisories All advisories

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-iosxe-dhcpsn-dos-xBn8Mtks

Revision History

• Version	Description	Section	Status	Date
  1.0	Initial public release.	-	Final	2025-MAY-07

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