CVE-2026-98568 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
vxlan: use one headroom snapshot for neighbour replies
vxlan_na_create() samples LL_RESERVED_SPACE() to size the reply skb and then samples it again to reserve headroom. A concurrent vxlan_changelink() can update needed_headroom between the two reads, creating a TOCTOU race. The second value can exceed the allocation and make the Ethernet header write out of bounds.
The race is reproducible on the unpatched kernel. It occurred when vxlan_na_create() generated a neighbour reply while vxlan_changelink() changed the link headroom. KASAN caught a four-byte write two bytes beyond a 704-byte skbuff_small_head allocation.
Snapshot the headroom once and use that value for both allocation and reservation.
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Analysis
by VulDB Data Team • 10/19/2026
The vulnerability identified in the Linux kernel involves a time-of-check to time-of-use race condition within the Virtual Extensible LAN implementation, specifically affecting how network buffer headers are allocated and reserved during neighbor advertisement processing. The core issue resides in the vxlan_na_create function, which is responsible for generating neighbor replies. In its original implementation, this function performs two separate operations involving headroom calculation: first, it samples LL_RESERVED_SPACE to determine the size of the socket buffer required for the reply packet, and subsequently, it calls a second sampling operation to reserve that calculated amount of space at the beginning of the buffer. This sequential approach assumes that the network interface configuration remains static between these two steps. However, this assumption is flawed because concurrent modifications to the VXLAN link settings can alter the required headroom value during the narrow window between the initial size calculation and the actual reservation.
A concurrent invocation of vxlan_changelink allows an attacker or a misconfigured system process to modify the network interface parameters while the neighbor reply creation is in progress. When this occurs, the second read for headroom requirements may return a larger value than what was originally allocated based on the first snapshot. This discrepancy leads directly to a buffer overflow scenario where subsequent writes intended to populate the Ethernet header exceed the bounds of the pre-allocated socket buffer memory. The severity of this flaw is underscored by real-world reproduction cases captured using Kernel Address Sanitizer, which detected out-of-bounds writes extending beyond the allocated skbuff_small_head space. In observed instances, a four-byte write occurred two bytes past the end of a 704-byte allocation, indicating that the kernel was attempting to access memory it did not own, potentially leading to data corruption or arbitrary code execution depending on the surrounding memory layout and exploitation context.
From a technical classification perspective, this vulnerability aligns with CWE-362, which describes concurrent execution using shared resources with improper synchronization resulting in race conditions. The specific mechanism of reading a value for allocation and then re-reading it for reservation without ensuring consistency is a classic example of this class of defect. Furthermore, the potential impact on system stability and security maps to MITRE ATT&CK techniques related to memory corruption and privilege escalation if an attacker can leverage the out-of-bounds write to overwrite critical kernel structures or control flow pointers. The lack of atomicity in checking and setting resource requirements creates a window that is difficult to mitigate through application-level controls, necessitating changes within the kernel source code itself.
The resolution implemented by the Linux maintainers addresses this flaw by enforcing strict consistency between allocation and reservation steps. Instead of performing two separate reads for headroom requirements, the patched version snapshots the required headroom value once at the beginning of the operation. This single snapshot is then used consistently for both determining the socket buffer size and reserving the header space within that buffer. By eliminating the second read opportunity where configuration changes could take effect, the race condition is effectively closed. This change ensures that the memory allocated matches exactly what is reserved, preventing any possibility of out-of-bounds writes resulting from dynamic interface reconfiguration during packet generation.
To mitigate this vulnerability in environments running unpatched kernels, administrators should prioritize applying the latest security updates provided by their distribution vendors as soon as they become available. Since this issue involves kernel-level memory management and concurrent execution paths that are difficult to restrict via user-space policies, patching is the primary defense mechanism. Additionally, minimizing unnecessary changes to network interface configurations while high-frequency neighbor discovery or VXLAN traffic generation is active can reduce the likelihood of triggering the race condition in legacy systems where immediate patching is not feasible. Monitoring for kernel panics or memory corruption errors reported by tools like KASAN or similar debugging utilities may also help identify affected instances before they are exploited maliciously.