CVE-2026-21910 is a vulnerability classified under CWE-754 (Improper Check for Unusual or Exceptional Conditions) found in the packet forwarding engine (PFE) of Juniper Networks Junos OS on EX4k and QFX5k Series platforms. These platforms include models such as QFX5110, QFX5120, QFX5200, EX4100, EX4300, EX4400, and EX4650 that support EVPN-VXLAN Virtual Port-Link Aggregation Groups (VPLAG). The vulnerability is triggered when an unauthenticated network-adjacent attacker induces a link flap on an interface configured with EVPN-VXLAN LAG. This causes the PFE to improperly handle the exceptional condition, leading to dropped traffic between VXLAN Network Identifiers (VNIs) when multiple load-balanced next-hop routes exist for the same destination. The root cause is an improper check in the PFE logic that fails to maintain traffic forwarding under these conditions. The impact is a denial of service (DoS) affecting availability but not confidentiality or integrity. Recovery requires a manual restart of the affected Flexible PIC Concentrator (FPC) using the 'request chassis fpc restart slot <slot-number>' command. The vulnerability affects all Junos OS versions before 21.4R3-S12, all 22.2 versions, and certain releases up to 24.4R2. No public exploits or active exploitation have been reported. The CVSS v3.1 score is 6.5 (AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H), indicating medium severity with network-adjacent attack vector, low attack complexity, no privileges or user interaction required, and high impact on availability.

For European organizations, especially those operating data centers, cloud infrastructure, or enterprise networks using Juniper EX4k and QFX5k Series switches with EVPN-VXLAN VPLAG configurations, this vulnerability poses a risk of network disruption and service outages. The denial of service can interrupt critical inter-VNI traffic, potentially impacting multi-tenant environments, data center fabrics, and service provider networks. This can degrade business continuity, affect customer-facing services, and cause operational downtime. Since the attack requires network adjacency but no authentication or user interaction, it could be exploited by malicious insiders or attackers who gain access to the local network segment. The need for manual intervention to restore service increases operational overhead and response time. Although confidentiality and integrity are not impacted, availability degradation in core network infrastructure can have cascading effects on dependent services and applications. Organizations relying on these Juniper platforms for high availability and scalable VXLAN deployments should prioritize remediation to avoid potential service disruptions.

1. Upgrade Junos OS to the latest patched versions starting from 21.4R3-S12 or later releases that address this vulnerability, ensuring all affected devices are updated promptly. 2. Implement network segmentation and strict access controls to limit network adjacency exposure, reducing the attack surface for unauthenticated attackers. 3. Monitor interface status and link flap events closely using network management tools and SNMP traps to detect abnormal interface behavior indicative of exploitation attempts. 4. Automate alerting and response workflows to quickly identify and remediate link flapping issues before they cause traffic disruption. 5. Prepare operational procedures for rapid FPC restart to restore service if a DoS occurs, minimizing downtime. 6. Review and validate EVPN-VXLAN LAG configurations to ensure they follow best practices and avoid unnecessary complexity that could exacerbate failure conditions. 7. Engage with Juniper support for additional guidance and consider deploying redundant paths or failover mechanisms to maintain availability during incidents. 8. Conduct regular vulnerability assessments and penetration testing focusing on network infrastructure to identify and mitigate similar risks proactively.