CVE-2026-21908 is a Use After Free vulnerability identified in the dot1xd daemon responsible for 802.1X authentication in Juniper Networks Junos OS and Junos OS Evolved. The flaw exists in the handling of Change of Authorization (CoA) messages when a port bounce occurs. Specifically, the code frees a pointer but later references it within the same execution path, leading to a use-after-free condition. This can cause the dot1xd process to crash, resulting in a denial of service (DoS). More critically, due to the process running with root privileges, an attacker may exploit this flaw to execute arbitrary code with elevated privileges. The attacker must be authenticated and network-adjacent, meaning they have some level of access to the network but do not require direct user interaction. Exploitation is complicated by the need for precise timing between freeing and referencing the pointer, making successful exploitation less straightforward but still feasible. The vulnerability affects multiple versions of Junos OS and Junos OS Evolved from 23.2R2-S1 up to versions before the respective fixed releases (e.g., 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, 25.2R1-S2). The vulnerability is tracked under CWE-416 (Use After Free) and has a CVSS v3.1 score of 7.1, reflecting high severity with attack vector as adjacent network, high complexity, low privileges required, no user interaction, and impacts on confidentiality, integrity, and availability. No public exploits have been reported yet, but the potential for root-level code execution makes this a critical issue for affected environments. Systems with 802.1X port-based network access control enabled are vulnerable, which is common in enterprise and service provider networks for controlling network access. Juniper has published patches in the indicated fixed versions, and organizations should apply these promptly to mitigate risk.

For European organizations, the impact of CVE-2026-21908 can be significant, especially for those relying on Juniper Networks infrastructure with 802.1X authentication enabled. The vulnerability can lead to denial of service by crashing the dot1xd process, disrupting network access control and potentially causing network outages or degraded service availability. More severely, the possibility of arbitrary code execution with root privileges could allow attackers to compromise network devices, leading to unauthorized access, interception or manipulation of network traffic, and lateral movement within the network. This could impact confidentiality, integrity, and availability of critical network infrastructure. Sectors such as telecommunications, finance, government, and critical infrastructure in Europe that deploy Juniper devices extensively are at higher risk. The requirement for the attacker to be authenticated and network-adjacent limits the attack surface but does not eliminate risk, especially in environments with large user bases or guest networks. The disruption or compromise of network access control mechanisms can have cascading effects on organizational security posture and compliance with regulations such as GDPR. Given the high severity and potential for root-level compromise, the threat poses a substantial risk to operational continuity and data security in European enterprises and service providers.

European organizations should immediately assess their Juniper Networks Junos OS and Junos OS Evolved deployments to identify affected versions running 802.1X authentication. The primary mitigation is to apply the vendor-provided patches available in the fixed releases (e.g., 23.2R2-S5, 23.4R2-S6, 24.2R2-S3, 24.4R2-S1, 25.2R1-S2 and their EVO equivalents). Until patches are applied, organizations should consider the following additional measures: 1) Restrict network adjacency by segmenting and isolating management and authentication traffic to trusted zones to reduce attacker access. 2) Monitor network logs and dot1xd process behavior for crashes or anomalies indicating exploitation attempts. 3) Implement strict access controls and authentication mechanisms to limit the number of authenticated users who can trigger port bounces. 4) Employ network intrusion detection systems (NIDS) tuned to detect unusual CoA or port bounce activity. 5) Conduct regular security audits and vulnerability scans focusing on network access control components. 6) Coordinate with Juniper support for guidance and verify firmware versions during maintenance windows. These steps, combined with timely patching, will reduce exposure and mitigate the risk of exploitation.