CVE-2026-77191 in EOS
Summary
by MITRE • 09/15/2026
An authenticated supplicant on an adjacent network may bypass intended network authorization policy and send unrestricted traffic during a brief window (milliseconds to seconds) between the completion of the authentication phase and the full enforcement of its assigned ACL.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability described represents a critical race condition within the 802.1X port-based Network Access Control framework, specifically affecting the transition period from authentication success to policy enforcement. In standard IEEE 802.1X implementations, when an authenticated supplicant completes its Extensible Authentication Protocol exchange with the RADIUS server and receives authorization attributes such as VLAN assignments or ACLs, there is a finite time interval before these policies are fully applied at the data plane level of the network switch or wireless access point. During this brief window, which can last from milliseconds to several seconds depending on hardware processing speeds and configuration latency, the port remains in an open state where traffic forwarding rules have not yet been restricted by the newly assigned security policies. This gap allows any authenticated device connected to that specific port to transmit unrestricted network traffic before the enforcement mechanisms take effect.
From a technical perspective, this flaw exploits the asynchronous nature of control plane and data plane operations within networking infrastructure. The authentication server validates credentials and returns policy directives via RADIUS or Diameter protocols. However, the Network Access Device must parse these attributes, update its internal forwarding tables, and apply access lists to the specific interface associated with the supplicant. If this process is not atomic or if there is no mechanism to hold packets until enforcement is confirmed, an attacker can exploit the timing discrepancy. This is particularly dangerous in environments where dynamic VLAN assignment or role-based ACLs are used, as it allows a newly authenticated device to potentially access restricted subnets, exfiltrate sensitive data, or launch lateral movement attacks against other network segments before being isolated by its assigned security profile.
The operational impact of this vulnerability is significant for organizations relying on strict segmentation and zero-trust architectures. An adversary who gains initial foothold through compromised credentials or rogue device connection can leverage this window to perform reconnaissance, scan internal services, or establish command-and-control channels that bypass perimeter defenses designed to block unauthorized traffic immediately upon authentication success. This undermines the fundamental assumption of 802.1X that access is granted only after full policy compliance and enforcement. The risk is exacerbated in high-throughput environments where processing delays are more pronounced, increasing the size of the attack window available for exploitation.
This vulnerability aligns with CWE-367, Time-of-check to time-of-use (TOCTOU) race condition, as it involves a discrepancy between when access rights are verified and when they are actually enforced in the data path. It also relates to CWE-284, Improper Access Control, specifically regarding the failure to enforce authorization policies during state transitions. In terms of MITRE ATT&CK framework tactics, this behavior facilitates Initial Access by allowing unauthorized lateral movement or privilege escalation immediately after credential validation, bypassing intended network segmentation controls that are supposed to limit post-compromise impact.
Mitigation strategies must focus on closing this temporal gap through architectural and configuration changes. Network administrators should implement port security features such as MACsec for encryption and integrity protection at the link layer, which can help mitigate some aspects of unauthorized traffic flow although it does not directly solve the ACL enforcement delay. More critically, switches and access points should be configured to use strict 802.1X modes that drop all non-authorized traffic immediately upon authentication completion or utilize features like dynamic port blocking until policy application is confirmed. Some vendors offer specific patches or firmware updates that address this race condition by synchronizing the data plane enforcement with the control plane authorization event. Additionally, implementing network segmentation at higher layers using micro-segmentation tools can provide defense-in-depth, ensuring that even if a device bypasses initial access controls, it cannot freely traverse internal networks without further verification. Regular auditing of authentication logs and traffic flows during onboarding processes can also help detect anomalies indicative of this exploitation attempt.