Cisco Security Advisory
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CVSS:3.1/AV:N/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 DHCP snooping feature of Cisco IOS XE Software could allow an unauthenticated, remote attacker to cause BOOTP packets to be forwarded between VLANs, resulting in a denial of service (DoS) condition.
This vulnerability is due to improper handling of BOOTP packets on Cisco Catalyst 9000 Series Switches. An attacker could exploit this vulnerability by sending BOOTP request packets to an affected device. A successful exploit could allow an attacker to forward BOOTP packets from one VLAN to another, resulting in BOOTP VLAN leakage and potentially leading to high CPU utilization. This makes the device unreachable (either through console or remote management) and unable to forward traffic, resulting in a DoS condition.
Note: This vulnerability can be exploited with either unicast or broadcast BOOTP packets.
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-bootp-WuBhNBxAThis advisory is part of the March 2026 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: March 2026 Semiannual Cisco IOS and IOS XE Software Security Advisory Bundled Publication.
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Vulnerable Products
This vulnerability affects Cisco Catalyst 9000 Series Switches if they are running a vulnerable release of Cisco IOS XE Software and have the following configuration conditions:
- IP DHCP snooping is enabled
- An ip helper-address is configured on a Switched Virtual Interface (SVI)
- The next hop of the ip helper-address is a sub-interface
- One of the sub-interfaces has the native VLAN configured
If those four conditions are true, then BOOTP packets will be forwarded from the source interface to the sub-interface where the native VLAN is configured. In addition, if the native VLAN is part of the IP DHCP snooping range, CPU utilization will increase, resulting in a DoS condition.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
Determine the Device Configuration
Use the following methods to determine whether a device has a vulnerable configuration.
Determine Whether IP DHCP Snooping Is Enabled on a Device
To determine whether a device has IP DHCP snooping enabled, use Administrator privileges to connect to the device CLI and use the show running-config | include ip dhcp snooping exec command. If output is returned, the device has IP DHCP snooping enabled. The following example shows a device that has IP DHCP snooping enabled on VLANs 16, 32, and 64:
Router# show running-config | include ip dhcp snooping
ip dhcp snooping vlan 16, 32, 64
ip dhcp snooping
Router#If no output is returned, the device is not affected.
Determine Whether ip helper-address Is Configured on the SVI on a Device
To determine whether a device has an ip helper-address configured on an SVI, use Administrator privileges to connect to the device CLI and use the show running-config | section interface Vlan exec command. If the returned output contains a line starting with ip helper-address <ip_address>, then an IP helper address is configured under that SVI. The following example shows a device that has an IP helper address configured under SVI Vlan64:
Router# show running-config | section interface Vlan
!
interface Vlan16
ip address 10.101.16.1 255.255.255.0
no ip redirects
ip ospf 1 area 0
!
interface Vlan32
ip address 10.101.32.1 255.255.255.0
ip ospf 2 area 0
!
interface Vlan64
ip address 10.101.64.1 255.255.255.0
ip helper-address 10.100.128.1
no ip redirects
ip ospf 3 area 0
!
Router#If no output is returned or the output does not contain an ip helper-address command, the device is not affected.
Determine Whether Native VLAN Is Configured on One of the Sub Interfaces
To determine whether a device has a native VLAN configured on a sub-interface, use Administrator privileges to connect to the device CLI and use the show running-config | section ^interface exec command. If the output contains encapsulation dot1q <value> native under a sub-interface, then the native VLAN is configured on a sub-interface. The following example shows a device that has the native VLAN configured under sub-interface twentyFiveGigE 1/0/2.2:
Router#show running-config | section ^interface
!
interface twentyFiveGigE 1/0/2
no switchport
no ip address
ip ospf network point-to-point
ip ospf 4 area 0
!
interface twentyFiveGigE 1/0/2.1
encapsulation dot1Q 16
ip address 10.100.16.1 255.255.255.252
no ip redirects
ip ospf network point-to-point
ip ospf 4 area 0
!
interface twentyFiveGigE 1/0/2.2
encapsulation dot1Q 32 native
ip address 10.100.32.1 255.255.255.252
no ip redirects
ip ospf network point-to-point
ip ospf 5 area 0
!
Router#If the output does not contain a native parameter on the encapsulation dot1Q <value>, the device is not affected.
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 XR Software
- NX-OS Software
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There is a workaround that addresses this vulnerability.
For environments that do not need to handle BOOTP traffic, configure ip dhcp relay bootp ignore on the affected device.
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.
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Cisco considers any workarounds and mitigations (if applicable) to be temporary solutions until an upgrade to a fixed software release is available. To fully remediate this vulnerability and avoid future exposure as described in this advisory, Cisco strongly recommends that customers upgrade to the fixed software indicated in this advisory.
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:
- Choose which advisories the tool will search—only this advisory, only advisories with a Critical or High Security Impact Rating (SIR), or all advisories.
- Enter a release number—for example, 15.9(3)M2 or 17.3.3.
- Click Check.
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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.
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This vulnerability was found during the resolution of a Cisco Technical Assistance Center (TAC) support case.
Cisco would like to thank Matthijs van der Wal for assisting with troubleshooting and reporting this vulnerability.
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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 2026-MAR-25
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SOFTWARE DOWNLOADS AND TECHNICAL SUPPORT
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. Please note that customers may download only software that was procured from Cisco directly or through a Cisco authorized reseller or partner and for which the license is still valid.
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 Technical Assistance Center (TAC). 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.
When considering software upgrades, customers are advised to regularly consult the advisories for the relevant Cisco products 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.
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Copies or summaries of the information contained in this Security Advisory may lack important information or contain factual errors. Customers are advised to visit the Cisco Security Advisories page for the most recent version of this Security Advisory. The Cisco Product Security Incident Response Team (PSIRT) assesses only the affected and fixed release information that is documented in this advisory. See the Cisco Security Vulnerability Policy for more information.