Cisco Security Advisory
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CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N/E:X/RL:X/RC:X
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A vulnerability in the endpoint group (EPG) contract functionality of Cisco Nexus 9000 Series Fabric Switches in ACI Mode could allow an unauthenticated, remote attacker to bypass configured EPG contracts.
This vulnerability is due to an improper control with EPG contracts. An attacker could exploit this vulnerability by sending IPv4 or IPv6 packets using UDP source and destination ports that are assigned to DHCP traffic through an affected device. A successful exploit could allow the attacker to bypass EPG contracts on the affected device.
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-aci-epgcbp-SfDU7NLfThis advisory is part of a group of advisories. For a complete list of the advisories and links to them, see Cisco Advance Notification for Publication of October 7, 2026, Security Advisories. In addition, for further documentation of improvements and fixes in Cisco NX-OS Software, see Cisco NX-OS Software Security Hardening Release: October 2026.
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Vulnerable Products
At the time of publication, this vulnerability affected Cisco Nexus 9000 Series Fabric Switches in ACI Mode, regardless of device configuration.
For information about which Cisco software releases are vulnerable, see the Fixed Software section of this advisory.
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
- Firepower 4100 Series
- Firepower 9300 Security Appliances
- MDS 9000 Series Multilayer Switches
- Nexus 3000 Series Switches
- Nexus 7000 Series Switches
- Nexus 9000 Series Switches in standalone NX-OS mode
- Secure Firewall 200 Series
- Secure Firewall 1200 Series
- Secure Firewall 3100 Series
- Secure Firewall 4200 Series
- Secure Firewall 6100 Series
- UCS 6300 Series Fabric Interconnects
- UCS 6400 Series Fabric Interconnects
- UCS 6500 Series Fabric Interconnects
- UCS 6600 Series Fabric Interconnects
- UCS X-Series Direct Fabric Interconnect 9108 100G
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The default configuration of the software for Cisco Nexus 9000 Series Fabric Switches in ACI Mode programs implicit rules for DHCPv4 and DHCPv6 traffic during switch initialization. These rules always permit DHCP-related traffic and take higher priority than user-defined contract rules. As a result, contracts that are configured for this traffic or the generic implicit deny will not be enforced. The implicit permit rules for DHCP always take precedence.
The fix for this vulnerability is a configuration knob that will allow administrators to disable (unprogram) or re-enable DHCP implicit rules through the Cisco APIC REST API. When implicit rules are disabled, user-defined contract permit, deny, and redirect actions for DHCP traffic are honored. The default action of the software for Cisco Nexus 9000 Series Fabric Switches in ACI Mode is that implicit rules are enabled.
For more information regarding this new configuration knob, see Cisco APIC Basic Configuration Guide, Release 6.2(x).
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There are no workarounds that address this vulnerability.
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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 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.
Fixed Releases
The fix for this vulnerability requires taking all of the following steps:
- Upgrade Cisco APIC to Release 6.0(9h), Release 6.1(6g), Release 6.2(3g), or later.
- Upgrade Cisco Nexus 9000 Series Fabric Switches in ACI Mode to Release 16.0(9h), Release 16.1(6g), Release 16.2(3g), or later.
- Apply the configuration knob that is described in the Details section of this advisory. For more information, see Cisco APIC Basic Configuration Guide, Release 6.2(x).
The Cisco Product Security Incident Response Team (PSIRT) validates only the affected and fixed release information that is documented in this advisory.
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The Cisco PSIRT is not aware of any public announcements or malicious use of the vulnerability that is described in this advisory.
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Cisco would like to thank Wilhelm Putz of System Integration Network Solutions for CANCOM Austria AG for 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.
For additional information on Cisco's vulnerability management, disclosure cadence, and software patching strategy, see Cisco's Transition to a Risk-Based Vulnerability Disclosure Model.
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Show LessVersion Description Section Status Date 1.0 Initial public release. — Final 2026-OCT-07
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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.
LEGAL DISCLAIMER DETAILS
CISCO DOES NOT MAKE ANY EXPRESS OR IMPLIED GUARANTEES OR WARRANTIES OF ANY KIND, INCLUDING THE WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR USE. WITHOUT LIMITING THE GENERALITY OF THE FOREGOING, CISCO DOES NOT GUARANTEE THE ACCURACY OR COMPLETENESS OF THIS INFORMATION. THIS DOCUMENT IS PROVIDED ON AN "AS IS" BASIS. YOUR USE OF THE INFORMATION ON THE DOCUMENT OR MATERIALS LINKED FROM THE DOCUMENT IS AT YOUR OWN RISK. CISCO RESERVES THE RIGHT TO CHANGE OR UPDATE THIS DOCUMENT AT ANY TIME.
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.