CVE-2026-90105 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
vxlan: fix reading neigh ha
Currently arp/neigh_reduce read neigh ha directly which can lead to partial reads while the neigh is being updated. Use neigh_ha_snapshot to take a stable snapshot of the address similar to route_shortcircuit which already does the right thing.
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Analysis
by VulDB Data Team • 09/17/2026
The Linux kernel vulnerability identified in the VXLAN subsystem involves a race condition during neighbor resolution that can result in partial reads of hardware addresses, commonly referred to as MAC addresses or link-layer addresses. This issue stems from how the arp_neigh_reduce function interacts with the network device's neighbor cache structures. Specifically, the code was directly accessing and reading the neigh_ha field within the neighbour entry structure without employing proper synchronization mechanisms. In a concurrent environment where multiple threads or interrupt handlers may be updating this data simultaneously, such direct access creates a window for inconsistent state observation. This is particularly critical in high-throughput network environments typical of VXLAN deployments, where neighbor resolution occurs frequently to maintain connectivity across virtual networks encapsulated within UDP packets.
The technical flaw lies in the lack of atomicity when retrieving the hardware address from the neighbour cache entry. When one process or kernel thread reads the neigh_ha field while another is in the middle of updating it due to a change in network topology, ARP reply processing, or link state changes, the reader may capture only part of the new value and part of the old value. This results in a corrupted MAC address being used for subsequent packet forwarding decisions. Such corruption can lead to packets being sent to incorrect destinations, causing communication failures within the VXLAN overlay network. In severe cases, this could facilitate traffic interception or denial of service conditions if an attacker can influence neighbor resolution timing to trigger these race conditions repeatedly.
From a security and operational impact perspective, this vulnerability affects the integrity of layer 2 forwarding decisions in virtualized environments relying on Linux kernel networking features. VXLAN is widely used in data center virtualization and cloud infrastructure to create scalable overlay networks. If an attacker can manipulate ARP traffic or exploit timing windows during neighbor updates, they might induce packet misdirection. While a direct remote code execution vector is unlikely given the nature of this memory consistency issue, the potential for network disruption and service degradation remains significant. The vulnerability aligns with CWE-362, which describes concurrent execution race conditions that lead to unintended side effects or inconsistent states. It also relates to improper synchronization mechanisms often cited in systems programming vulnerabilities where shared data structures are accessed without adequate locking or atomic operations.
The mitigation implemented by the Linux kernel maintainers involves replacing direct field access with the neigh_ha_snapshot function. This helper function is designed specifically to handle the complexities of reading hardware addresses safely across different architectures and under concurrent modification scenarios. It ensures that a consistent, stable snapshot of the MAC address is obtained before it is used for any further processing. This approach mirrors best practices already established in other parts of the kernel networking stack, such as route_shortcircuit logic, which correctly handles similar concurrency issues by using standardized snapshots rather than raw field access. By adopting this pattern, the code eliminates the race window and ensures that forwarding decisions are based on complete and accurate neighbor information.
To address this issue effectively, system administrators should ensure their Linux kernels are updated to versions containing this fix. For environments running older kernel versions where immediate patching is not feasible, mitigating factors include monitoring for unusual network behavior indicative of packet loss or misrouting in VXLAN segments. Additionally, reducing the frequency of neighbor resolution events through appropriate ARP timeout tuning can minimize the window of opportunity for such race conditions to manifest, although this does not eliminate the underlying flaw. The fix represents a critical hardening measure for virtualized networking stacks, reinforcing the importance of using kernel-provided synchronization primitives when accessing shared data structures in high-concurrency contexts like network packet processing paths.