CVE-2021-47109 is a vulnerability in the Linux kernel's neighbor subsystem, specifically related to the handling of neighbor table entries with the NUD_NOARP state. The neighbor table is a critical data structure used by the kernel to manage network layer to link layer address mappings, such as IPv6 addresses on point-to-point interfaces. The vulnerability arises because NUD_NOARP entries, which are used by IFF_POINTOPOINT interfaces for IPv6, can be forced to remain in the neighbor table without timely garbage collection. After the kernel commit 58956317c8de ("neighbor: Improve garbage collection"), the garbage collection mechanism was changed to prevent removal of entries unless they are marked NUD_FAILED or older than 5 seconds. This change inadvertently allows an attacker to fill the neighbor table with many NUD_NOARP entries, causing the table to overflow and preventing valid connections from being established. This can lead to denial of service conditions where legitimate network traffic is dropped or delayed due to neighbor table exhaustion. The vulnerability does not require user interaction or authentication, as it can be triggered by sending crafted network packets to the affected system. There are no known exploits in the wild at this time, and no CVSS score has been assigned yet. The issue affects Linux kernel versions containing the specified commit and likely subsequent versions until patched.

For European organizations, this vulnerability poses a risk primarily to network infrastructure devices and servers running Linux kernels with the affected neighbor table garbage collection logic. The exhaustion of the neighbor table can cause denial of service by disrupting IPv6 connectivity on point-to-point interfaces, which may be used in VPNs, WAN links, or other critical network segments. This can impact availability of services, internal communications, and remote access solutions. Organizations relying on Linux-based routers, firewalls, or network appliances are particularly at risk. Given the increasing adoption of IPv6 in Europe and the widespread use of Linux in enterprise and governmental infrastructure, the potential for disruption is significant. While no data confidentiality or integrity compromise is directly indicated, the denial of service impact can affect business continuity and operational resilience. The lack of known exploits suggests the threat is currently theoretical, but the ease of triggering neighbor table exhaustion means attackers with network access could exploit this vulnerability to degrade or disrupt services.

To mitigate this vulnerability, European organizations should: 1) Identify and inventory Linux systems running kernels with the affected commit (58956317c8de) or versions released after it. 2) Apply vendor-provided patches or kernel updates that address the neighbor table garbage collection issue as soon as they become available. 3) Implement network segmentation and access controls to limit exposure of vulnerable systems to untrusted networks, reducing the attack surface. 4) Monitor neighbor table usage and network interface statistics for abnormal growth in NUD_NOARP entries, which may indicate exploitation attempts. 5) Consider deploying rate limiting or filtering on network traffic that can generate excessive neighbor entries, especially on point-to-point IPv6 interfaces. 6) Engage with Linux distribution security advisories and maintain up-to-date kernel versions to benefit from ongoing security improvements. 7) For critical infrastructure, plan for fallback or redundancy to maintain availability in case of neighbor table exhaustion attacks.