CVE-2026-97940 in Linux
Summary
by MITRE • 09/25/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv6: fix fib6 walker UAF on seq stop
ipv6_route_iter_active() treats a walker in FWS_U at the table root as already unlinked. fib6_del_route() can move a still-linked walker into that same state when the current leaf is the last route at the root, so ipv6_route_native_seq_stop() skips fib6_walker_unlink(). The seq private object can then be freed while it remains on net->ipv6.fib6_walkers. A later route deletion walks the dangling list and uses the freed walker.
Use the list head as membership state and reinitialize it when unlinking. Keep the existing w->node check so a never-started iterator with a zeroed private object is not treated as linked.
The same stop helper is used by /proc/net/ipv6_route and by the BPF ipv6_route iterator. The BPF show path only widens the race.
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Analysis
by VulDB Data Team • 09/25/2026
This vulnerability represents a use-after-free condition within the IPv6 forwarding information base implementation of the Linux kernel, specifically affecting the route iteration mechanism used for network diagnostics and eBPF programs. The core issue stems from an incorrect assumption regarding the state management of walker objects during sequence file operations. In the ipv6_route_iter_active function, a walker in the FWS_U state at the table root is incorrectly treated as already unlinked from the global list of active walkers. This logic flaw creates a race condition where the kernel fails to properly clean up references when routes are deleted concurrently with iteration processes.
The operational trigger for this vulnerability occurs during route deletion operations handled by fib6_del_route. When the current leaf being processed is the last route at a specific root level, the function moves a still-linked walker into the FWS_U state. Subsequently, when ipv6_route_native_seq_stop is invoked to terminate the sequence iteration, it observes this FWS_U state and erroneously skips calling fib6_walker_unlink. Because unlinking was skipped, the private object associated with the sequence file remains on the net->ipv6.fib6_walkers list despite being logically considered inactive by other parts of the codebase. This discrepancy leaves a dangling pointer to memory that is scheduled for deallocation but has not yet been reclaimed or removed from the tracking structure.
The security impact arises when subsequent route deletion operations attempt to walk the fib6_walkers list. Since the walker object was never properly unlinked, it remains present in the list even after its underlying memory has been freed by other kernel subsystems. When the iteration logic accesses this dangling pointer during a new traversal, it results in a use-after-free condition. This can lead to unpredictable behavior including kernel panics, data corruption, or potentially arbitrary code execution if an attacker can control the contents of the freed memory region and trigger specific allocation patterns that overwrite critical structures with malicious payloads.
This vulnerability affects multiple interfaces within the Linux networking stack because the same stop helper function is utilized by both /proc/net/ipv6_route for standard user-space diagnostics and by BPF ipv6_route iterators used in eBPF programs. The exposure through BPF significantly widens the attack surface, as eBPF programs can be loaded with varying privileges depending on system configuration. An attacker with access to create or modify network routes could exploit this race condition to destabilize the kernel or escalate privileges by manipulating walker states during high-frequency route changes.
Mitigation strategies primarily involve applying vendor-provided kernel patches that address the state management logic in fib6_del_route and ipv6_route_native_seq_stop. The fix involves using the list head as a definitive indicator of membership state rather than relying solely on the FWS_U flag, ensuring that walkers are correctly unlinked before their memory is freed. Additionally, reinitializing the list head during unlinking prevents stale references from persisting in global lists. System administrators should ensure all systems running affected kernel versions are updated to include these fixes and monitor for unusual network configuration changes or eBPF program loads that might trigger the race condition under load.
From a classification perspective, this vulnerability aligns with CWE-416 Use After Free, as it involves accessing memory after it has been freed due to improper lifecycle management of kernel objects. In terms of adversarial tactics, this flaw could be leveraged within ATT&CK technique T1059 Command and Scripting Interpreter via eBPF abuse or T1078 Valid Accounts if an attacker gains initial access through network configuration privileges. The vulnerability highlights the critical importance of maintaining strict consistency between logical state flags and physical list membership in concurrent kernel data structures to prevent race conditions that lead to memory safety violations.