[CVE-2026-97940][MODERATE 7.0] ipv6: fix fib6 walker UAF on seq stop

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-97940
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: fix fib6 walker UAF on seq stop
Commit: 0eede1689610e9eeaf729ba86fd1321799eb91a0
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0eede1689610e9eeaf729ba86fd1321799eb91a0
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97940
Analysis date: Tue, 29 Sep 2026 17:16:40 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-97940 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97940

The original announcement does not normally provide a security severity
estimate, CVSS assessment, or enough information to determine whether the
reported kernel bug represents a practically relevant security issue.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin. It combines
an autonomous classifier with LLM-assisted technical analysis and a
separate ActionableScore mechanism.

The purpose of this report is to prioritize Linux kernel CVEs before
manual review, identify cases that require prompt investigation, and
support automatic closure of issues that are unlikely to have meaningful
security impact.

Published priority for this report: MODERATE 7.0
Manual review required: YES

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-97940	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97940 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in IPv6 route iterator cleanup can leave a fib6 walker linked in net ipv6 fib6_walkers after the seq private object containing it has been freed. A later route deletion traverses the dangling list entry and accesses the freed walker, creating a confirmed use after free condition that can crash the kernel and may corrupt reused heap memory. The proc net ipv6_route iterator provides a local trigger path, while the BPF ipv6_route iterator can widen the race window. For the CVSS the PR:L value is used because a local user can access the proc iterator and may obtain route control inside an unprivileged network namespace when that feature is enabled. AC:H is required because exploitation depends on winning a timing race with route deletion. The issue is not directly network reachable. Impact is at least a local denial of service via kernel crash and in the worst defensible case may allow confidentiality, integrity, and privilege escalation impact through use after free memory reuse. Reliable exploitation beyond denial of service is not established and requires manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) 	MAYBE	REMOTE INIT BPF RACE UAF NETWORK IPV6  KPANIC INCREASED_TO_MODERATE7_FROM_MODERATE_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap changed either score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. The seq private object can be freed while its embedded `fib6_walker` remains linked in `fib6_walkers`. A later route deletion traverses and uses that freed walker.
* +1 Reliable kernel crash or strong DoS. Traversal of the dangling list entry can dereference invalid or reused memory and fault the kernel.
* +1 Common networking routing path. The defect affects IPv6 route iteration and route deletion rather than an unusual hardware-specific facility.
* Total: 4.

Paranoid-only signal:

* +1 Structured stale-object exploitation leverage. The dangling object remains in an active kernel list, and later route deletion consumes walker state during nontrivial traversal rather than merely invoking a cleanup destructor. Route-table layout and iterator progress influence fields such as `node`, `root`, and `state`. This supports unresolved post-free structured processing, although controlled reclaim is not demonstrated.
* Total: 5.

Material signals not awarded:

* Local unprivileged trigger +1 was withheld. Reading `/proc/net/ipv6_route` is locally accessible, but the supplied evidence does not conclusively establish that the same below-host-admin principal can perform the required route deletion in the evaluated environment.
* No race penalty. The patch establishes a race but does not show that its timing is one-shot, exceptionally narrow, or externally uncontrollable.
* No strong exploitation leverage +2. Controlled reclaim, attacker-shaped replacement contents, callback control, arbitrary read/write, and demonstrated privilege escalation are not established.
* No confidentiality or integrity points. Reuse of the freed allocation could produce corruption, but no concrete disclosure or unauthorized modification primitive is shown.
* No extra Availability point. The kernel crash is already counted once.
* No remote-reachability point. Network packets alone are not shown to invoke the vulnerable iterator-stop and route-deletion sequence.
* No repeated release/refcount point. The patch does not show two lifetime-mutating put, free, or refcount operations on the same reference.
* The first relevant lifetime event frees the seq private object, but the later operation is active traversal and use of its embedded walker, not a second release.
* Paranoid repeated-refcount leverage was withheld because no second refcount or lifetime mutation exists.
* REAL UAF FLOOR 5 was not applied mechanically. The conservative arithmetic remains 4, while the paranoid score reaches 5 through the separately identified active structured traversal.

3. Reachability analysis

The first stage is reachable through `/proc/net/ipv6_route`; the same stop helper is also used by the BPF IPv6 route iterator. The BPF path is privilege-sensitive and is not needed for the procfs path.

Completing the trigger requires an iterator to stop in the incorrect state and a later IPv6 route deletion to traverse `fib6_walkers`. Route modification normally requires `CAP_NET_ADMIN`. Such control may be obtainable inside an attacker-created user and network namespace on systems that permit unprivileged user namespaces, but the supplied evidence does not prove that this complete namespace-based sequence is available in the evaluated product.

Therefore, host-admin-equivalent privilege is not positively established, but below-host-admin reachability for the complete sequence is also not conclusively established. Both `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`.

No prior BPF setup, special hardware, boot option, or firmware is required for the procfs path. The race may be repeatable by repeatedly opening and stopping the iterator while changing routes, but repeatability by one below-host-admin principal remains environment-dependent.

4. Primitive and severity interpretation

The exact primitive is an escaping list-based UAF. A `fib6_walker` embedded in a freed seq private object remains linked in a live per-network-namespace walker list. A subsequent route deletion discovers the dangling list entry and processes the freed walker.

This is an active structured object, not fixed stale storage. The stale use occurs after cleanup but in a separate active route-deletion operation, so it is broader than an immediate destructor-only fault.

There is no concrete repeated release or refcount mutation. The object is freed once, and the later operation traverses and uses its embedded state rather than releasing it again.

The defect is CLASS_B because a genuine kernel lifetime violation and active stale-object traversal are established, but controlled replacement, arbitrary read/write, callback influence, and privilege escalation are not. The demonstrated consequence is kernel crash or DoS. Memory corruption through reuse of the seq allocation is technically plausible but remains unresolved.

`ImportantCandidate=YES` because the dangling object escapes into an active global list and is later consumed by nontrivial kernel routing logic. This is stronger than a shallow cleanup dereference and needs Important-vs-Moderate exploitation review. `AutoDowngradeAllowed=NO` because automatic closure would discard unresolved leverage from an active list-based UAF, even though Important severity is not independently proven.

5. One-sentence report phrase

A race in IPv6 route iterator cleanup can leave a freed fib6 walker linked in an active kernel list, allowing a later route deletion to traverse the dangling object and cause a kernel crash with unresolved post-free corruption potential.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The confirmed active-object UAF extends beyond immediate cleanup and reaches later routing traversal, while reclaim behavior, field mutations, and complete below-host-admin namespace reachability require source-level validation.

7. Call-site confidence

Call-site confidence: medium

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UPSTREAM PATCH SUMMARY
======================================================================

Patch: ipv6: fix fib6 walker UAF on seq stop
Commit: 0eede1689610e9eeaf729ba86fd1321799eb91a0
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0eede1689610e9eeaf729ba86fd1321799eb91a0

Commit description:

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.

Fixes: 8d2ca1d ("ipv6: avoid high order memory allocations for /proc/net/ipv6_route")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Co-developed-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Luxing Yin <root@tr0jan.top>
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/89699735763f6c297584d7c2ff106239cc1e8ce0.1788837093.git.zihanx@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/ip6_fib.c

Diff excerpt:

Not included in this email. See the upstream URL for the full patch.

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0eede1689610e9eeaf729ba86fd1321799eb91a0

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-97940 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97940

The original CVE announcement normally does not include a security-level
estimate. In particular, it may not contain a CVSS assessment, an impact
level, or enough information to determine whether the reported bug is a
practically relevant security issue. One purpose of this parallel CVE list
is to provide that missing technical and prioritization information.

The original goal of the AL-KERNEL project was to prioritize Linux kernel
CVE analysis automatically before manual review. The system can also help
identify non-security issues that may be suitable for automatic closure.

This report was generated by AL-KERNEL, an AI-assisted Linux kernel
vulnerability analysis system developed by Alexander Larkin.

The first analysis stage combines an autonomous classifier with additional
LLM-based analysis. The autonomous classifier runs locally on a CPU and is
based on a backpropagation neural network. Together, these mechanisms
produce a technical vulnerability description, identify likely weakness
types, estimate CVSS severity, and provide input for ActionableScore.

Two CVSS estimates are retained because incomplete kernel vulnerability
information often permits more than one defensible interpretation:

  Conservative CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

The conservative vector represents a lower-impact interpretation.
The Best/paranoid vector intentionally represents a plausible upper-bound
interpretation and should not automatically be treated as demonstrated
real-world impact.

CVSS may also need to be adjusted for a particular Linux deployment,
because actual reachability, privileges, enabled kernel configuration,
hardware, namespaces, exposed device nodes, and other environmental
conditions can differ significantly between systems.

A separate ActionableScore mechanism evaluates practical remediation
urgency. Its analysis may include reachability, attack prerequisites,
subsystem exposure, memory-corruption characteristics, denial-of-service
reliability, and possible confidentiality, integrity, or
privilege-escalation impact.

  Conservative ActionableScore: 4
  Paranoid ActionableScore: 5

The final base severity is taken directly from the second tab-separated
field of the AL-KERNEL classification result. ActionableScore does not
replace or independently override that final AL-KERNEL decision, and
if ActionableScore adjusted impact level of ALKERNEL, then you would see
self-readable flags above like INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7.

For an AL-KERNEL result of MODERATE, this report uses the following
additional presentation split:

  ActionableScore below 5   -> MODERATE REGULAR
  ActionableScore 5 or more -> MODERATE 7.0

The distinction between MODERATE REGULAR and MODERATE 7.0 makes it
possible to identify Moderate issues that should receive manual analysis
and fixes before lower-priority MODERATE REGULAR issues. In many cases,
MODERATE REGULAR fixes may wait for a later rebase or routine update.

There is one override in which MODERATE REGULAR becomes MODERATE 7.0
even when the ActionableScore is below 5. When the AL-KERNEL result
contains the KPANIC flag, a MODERATE result is always presented as
MODERATE 7.0. The KPANIC flag selected with few regexps without
usage of AI at all, so it helps to detect cases when Kernel Crash happens
and similar (to filter False-Negative results from the LLM usage).

KPANIC indicates that a reliable kernel crash, kernel panic, or similarly
serious kernel availability impact was identified by the classification
workflow.

AL-KERNEL base severity for this report: MODERATE
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): MODERATE 7.0

These results are intended to support engineering triage. They are
machine-generated estimates, and cases marked for manual review should
be validated by a human security engineer before final disposition.
For more info read docs linked from here: https://kernelcve.org/
(and you can submit you own patch there to generate such a report
for non-existant CVE-id yet).

Note that in many cases this AI tool selects higher severity, than
real is (means you can expect Importants instead of Moderate 7.0 or
Moderates 7.0 instead of regular Moderates). If you see such cases,
please use reply email interface to add additional manual analyses
info to this particular CVE.
And please, please, let me know when you see Lows instead of Importants
or Important instead of Low (because particular for such cases I
need to tune this AI tool to make it better for this one and next similar).
My contact email for such notifications is alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).