CVE-2026-20038 in NX-OS System Software
Summary
by MITRE • 10/07/2026
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.
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Analysis
by VulDB Data Team • 10/07/2026
The Cisco Nexus 9000 Series Fabric Switches operating in Application Centric Infrastructure mode rely heavily on Endpoint Group, or EPG, contracts to enforce micro-segmentation and policy-based access control within data center environments. These contracts define which endpoints are permitted to communicate with each other based on layer four protocol criteria such as TCP, UDP, ICMP, and others. The identified vulnerability resides in the processing logic governing these EPG contract evaluations, specifically concerning how traffic matching specific port assignments is handled by the switch hardware or software forwarding plane. This flaw represents a critical failure in access control mechanisms where the system fails to correctly apply security policies under certain conditions, allowing unauthorized network traversal that would otherwise be blocked by configured rules.
The technical root cause of this vulnerability stems from an improper implementation of input validation and state management within the EPG contract processing engine. Specifically, when IPv4 or IPv6 packets are transmitted using UDP source and destination ports that correspond to those assigned for Dynamic Host Configuration Protocol traffic, the switch incorrectly processes these packets in relation to existing security policies. DHCP is a fundamental protocol used by devices to obtain IP addresses and other network configuration parameters from a server on an IP network. By leveraging this specific port assignment, an unauthenticated remote attacker can craft malicious UDP packets that trigger the flawed logic path within the Nexus 9000 switch. The device fails to properly associate these incoming packets with the restrictive EPG contracts intended for them, effectively treating them as if no restrictions apply or misidentifying their policy context entirely.
From a threat actor perspective, this vulnerability allows an unauthenticated remote attacker to bypass configured security boundaries without needing valid credentials on the affected system. This capability significantly degrades the effectiveness of network segmentation strategies that organizations implement to contain lateral movement and limit blast radius in the event of a compromise. An attacker positioned within or able to reach the affected subnet could exploit this flaw to communicate with endpoints that should be isolated from them according to organizational security policies. For instance, if an EPG contract is designed to prevent web servers from initiating connections to database servers using specific UDP ports associated with DHCP services, an attacker could utilize this vulnerability to establish such unauthorized communications. This bypass undermines the principle of least privilege and exposes sensitive internal resources to potential exploitation by malicious actors who have gained initial foothold or network access within the environment.
The operational impact extends beyond simple policy violation; it compromises the integrity of the entire ACI security model which depends on strict adherence to EPG contracts for zero-trust enforcement. If attackers can freely traverse between segments that are supposed to be isolated, they may escalate privileges, exfiltrate data, or deploy malware across previously protected zones. This vulnerability is particularly dangerous because it does not require authentication, making it susceptible to automated scanning and exploitation by opportunistic adversaries. Furthermore, since DHCP traffic is often permitted through firewalls and security devices due to its necessity for network operation, attackers may use this vector to blend malicious activity with legitimate-looking traffic patterns, complicating detection efforts for intrusion prevention systems that rely on standard signature-based or behavioral analysis techniques.
To mitigate the risks associated with this vulnerability, organizations must apply the latest software updates provided by Cisco for their Nexus 9000 Series switches running in ACI mode. These patches address the underlying logic errors in EPG contract processing and restore proper enforcement of security policies regardless of UDP port assignments. In addition to patching, network administrators should review current EPG contracts to ensure that no critical services rely solely on DHCP-related ports for essential communications where strict isolation is required. Implementing additional layers of defense such as host-based firewalls or advanced threat protection solutions can provide compensating controls while the infrastructure is being updated. Regular audits of ACI policies and continuous monitoring for anomalous traffic patterns involving UDP packets with unusual port usage are recommended to detect any potential exploitation attempts that may occur before patches are fully deployed across all affected devices.
This vulnerability aligns with CWE-284 Improper Access Control, as it involves a failure in the system's ability to restrict access to resources based on defined policies. It also relates to CWE-755 Improper Handling of Exceptional Conditions if the flaw arises from unhandled edge cases during packet processing. From an offensive security perspective, this exploit technique maps to MITRE ATT&CK techniques involving Lateral Movement and potentially Command and Control if used for covert communication channels. Understanding these mappings helps in categorizing the risk within broader cybersecurity frameworks and ensures that remediation efforts address both the immediate technical flaw and the strategic implications for network defense posture.