CVE-2026-99071 in Linux
Summary
by MITRE • 10/23/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv4: do not warn on route notification size race
Commit 680aea08e78c ("net: ipv4: Emit notification when fib hardware flags are changed") added asynchronous route notifications for hardware flag changes.
fib_alias_hw_flags_set() sizes the skb with fib_nlmsg_size() and later fills it with fib_dump_info() while holding only RCU. With nexthop compatibility mode enabled, a concurrent replacement can grow the group between these independent snapshots. fib_dump_info() can then legitimately return -EMSGSIZE, so the warning does not prove a sizing bug.
Remove the warning. The existing error path still frees the skb and reports the error to listeners.
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Analysis
by VulDB Data Team • 10/23/2026
The Linux kernel vulnerability addressed in commit 680aea08e78c involves an incorrect assumption regarding network route notification size consistency within the IPv4 forwarding information base subsystem. Specifically, the function fib_alias_hw_flags_set is responsible for preparing asynchronous notifications when hardware flags associated with a routing entry are modified. This process begins by calculating the required buffer size using fib_nlmsg_size and allocating a socket buffer skb accordingly. The data is then populated into this buffer via fib_dump_info while holding only Read-Copy-Update RCU locks rather than exclusive write locks that would prevent concurrent modifications to the underlying route structures.
The core technical flaw arises from a race condition inherent in this locking strategy when nexthop compatibility mode is enabled. Nexthop groups can dynamically change size due to concurrent replacement operations performed by other kernel threads or network stack components. Because fib_nlmsg_size captures the group size at one point in time and fib_dump_info accesses it later, there exists a window where the actual required buffer size may exceed the initially allocated size if the nexthop group grows during that interval. Consequently, fib_dump_info legitimately returns an -EMSGSIZE error indicating message size mismatch rather than signaling a critical sizing bug or memory corruption issue as previously assumed by developers who added warning logs for this condition.
This race condition does not represent a security vulnerability in terms of exploitation potential such as buffer overflow privilege escalation or remote code execution because the kernel correctly handles the oversized payload scenario through its existing error handling path. The socket buffer is properly freed and the error is reported to listeners, ensuring system stability remains intact. However, the presence of spurious warning messages generated by this race condition can lead to unnecessary log noise which may obscure genuine security alerts or performance issues in production environments where high-frequency route updates occur under nexthop compatibility mode.
From a classification perspective, this issue aligns with CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization Race Conditions although it is benign due to proper error handling rather than exploitable memory corruption. In the context of MITRE ATT&CK techniques related to Linux systems, this relates to T1078 Valid Accounts and specifically how legitimate system processes interact with network subsystems where improper synchronization can lead to operational anomalies even if they do not directly facilitate unauthorized access or persistence mechanisms.
The mitigation implemented by removing the warning message is appropriate given that the underlying race condition cannot be easily eliminated without significant performance penalties from switching to exclusive locking which would serialize route updates and degrade throughput in high-performance networking scenarios. Administrators should ensure their kernel versions include this fix to prevent log pollution while maintaining the current level of concurrency for network routing operations. No additional configuration changes are required as the error handling path already ensures safe cleanup of resources regardless of whether the notification succeeds or fails due to size mismatches during concurrent modifications.