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 running on EX4k and QFX5k Series platforms, including models such as QFX5110, QFX5120, QFX5200, EX4100, EX4300, EX4400, and EX4650. The flaw occurs specifically in environments configured with EVPN-VXLAN Virtual Port-Link Aggregation Groups (VPLAG). When an unauthenticated attacker adjacent to the network induces a link flap on an interface within an EVPN-VXLAN LAG, the system improperly handles this exceptional condition. This leads to dropped traffic between VXLAN Network Identifiers (VNIs) when multiple load-balanced next-hop routes exist for the same destination, effectively causing a denial of service (DoS) by interrupting inter-VNI communication. The vulnerability affects all Junos OS versions before 21.4R3-S12, all 22.2 versions, and certain releases in 22.4, 23.2, 23.4, 24.2, and 24.4 branches prior to their respective patch releases. Service restoration requires a manual restart of the affected Flexible PIC Concentrator (FPC) slot using the 'request chassis fpc restart slot <slot-number>' command. The CVSS v3.1 base score is 6.5, reflecting a medium severity with network attack vector, no privileges required, no user interaction, and impact limited to availability. No known exploits have been reported in the wild, but the vulnerability presents a risk to network stability in environments using EVPN-VXLAN VPLAG on affected Juniper platforms.

For European organizations relying on Juniper EX4k and QFX5k Series switches configured with EVPN-VXLAN VPLAG, this vulnerability can cause significant network disruption. The denial of service results in dropped traffic between VXLAN Network Identifiers, potentially impacting data center interconnects, cloud environments, and enterprise networks that use VXLAN overlays for segmentation and scalability. This can degrade application performance, interrupt critical services, and cause operational downtime until manual intervention is performed. The need for manual FPC restart may delay recovery and increase operational overhead. Organizations with high availability and strict uptime requirements, such as financial institutions, telecommunications providers, and critical infrastructure operators, may face increased risk. Although no confidentiality or integrity impact is reported, availability degradation can have cascading effects on business continuity and service level agreements (SLAs).

1. Apply Juniper's official patches and updates for Junos OS as soon as they become available, specifically versions 21.4R3-S12 and later patched releases for affected branches. 2. Monitor network interfaces for unusual link flapping events and implement automated alerts to detect potential exploitation attempts. 3. Where possible, limit network adjacency and control access to management and forwarding planes to reduce exposure to unauthenticated attackers. 4. Review EVPN-VXLAN VPLAG configurations to assess if multiple load-balanced next-hop routes can be optimized or simplified to reduce risk. 5. Implement redundancy and failover mechanisms to minimize impact during FPC restarts. 6. Prepare operational procedures for rapid FPC slot restart to minimize downtime if the issue occurs. 7. Consider network segmentation and micro-segmentation to isolate critical VXLAN segments. 8. Engage with Juniper support for guidance on interim workarounds if patching is delayed.