[CVE-2026-97959][LOW] net/sched: cls_route: free emptied bucket on filter move [ Upstream

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

CVE: CVE-2026-97959
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net/sched: cls_route: free emptied bucket on filter move [ Upstream
Commit: 2e2f130951d34871b0643bf700164d97cff4b427
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e2f130951d34871b0643bf700164d97cff4b427
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97959
Analysis date: Sat, 26 Sep 2026 16:41:04 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-97959 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97959

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: LOW
Manual review required: NO

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

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-97959	LOW	https://www.kernelcve.org/list/?q=CVE-2026-97959 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A resource leak in the cls_route route4 filter classifier can occur when an existing filter is moved to a different top level bucket. The old bucket can become empty but remain linked in head table, causing route4_delete to keep the otherwise unused tcf_proto alive. Repeated local tc filter add change and delete operations can accumulate leaked classifier objects and may eventually exhaust kernel memory or related networking resources. For the CVSS the PR:L reflects that triggering requires local control of traffic control netlink operations and CAP_NET_ADMIN, which can be available to a container or network namespace administrator without full host root privileges. The issue is not directly network reachable because packet traffic alone does not perform the required classifier configuration changes. Impact is denial of service through resource exhaustion. No memory corruption, information disclosure, integrity impact, or privilege escalation primitive is indicated by the affected code path.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	REMOTE INIT LEAK UAF NETWORK QDISC  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=1
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=NO
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Concrete persistent kernel resource/reference leak. A route4 bucket can become empty after an existing filter is moved to another top level bucket, but the empty bucket remains linked in head->table. This prevents route4_delete from recognizing that the tcf_proto is empty and allows the tcf_proto and related objects to remain pinned.
* No additional resource exhaustion point awarded. Repetition is shown by the reproducer and could accumulate leaked objects, but the supplied evidence does not establish a bypass of memory accounting, host wide exhaustion from a constrained actor, or immediate system wide failure.
* No memory corruption signal awarded. The patch establishes missing reclamation of an empty bucket, not an access to freed memory, stale dereference, double free, invalid refcount mutation, OOB access, or type confusion.
* No UAF signal awarded. The new refcount protects bucket reclamation after the last filter is removed. It does not fix an existing post free use of the bucket.
* No remote reachability point awarded. Packet traffic does not reach the vulnerable path. Triggering requires rtnetlink traffic control configuration operations.
* No Local unprivileged trigger point awarded. The supplied reproducer requires tc qdisc and tc filter operations, but the complete below host admin namespace capability path is not explicitly established in the supplied evidence.
* No privilege penalty awarded. Host admin equivalent privilege is not positively established. The exact ability of an attacker to create the relevant network namespace, obtain CAP_NET_ADMIN there, and complete all cls_route operations is unresolved.
* No Confidentiality or Integrity point awarded. The leak does not provide a disclosure, unauthorized modification, authorization bypass, or ownership violation.
* No strong DoS point awarded. The demonstrated issue is a persistent resource leak. A kernel crash, persistent host outage, or reliable host memory exhaustion is not directly established.

3. Reachability analysis

The trigger is a local traffic control control plane sequence. The attacker creates a clsact qdisc, adds a route4 filter, changes the filter so that its recomputed handle selects a different top level bucket, and then deletes the filter.

The relevant interface is rtnetlink traffic control configuration, which normally requires CAP_NET_ADMIN relative to the target network namespace. The supplied patch and reproducer do not prove that the complete sequence is available to an ordinary unprivileged host user. They also do not prove that host init namespace CAP_NET_ADMIN is mandatory.

NamespaceScopedPrivilege is therefore UNKNOWN. The loopback device and qdisc objects may be namespace local, but the supplied evidence does not establish the full attacker reachable namespace creation and capability chain for the evaluated environment. No special hardware, boot option, timing race, or firmware interaction is involved. The required kernel configuration includes CONFIG_NET_CLS_ROUTE4, CONFIG_NET_SCH_INGRESS, and CONFIG_NET_CLS_ACT.

4. Primitive and severity interpretation

The exact primitive is a persistent lifecycle and bookkeeping leak. A route4_filter is removed from its previous route4_bucket after a filter move, but an emptied bucket is not removed from head->table. The stale empty bucket makes route4_delete report that filters remain, pinning the tcf_proto rather than freeing it.

This is not an active structured object UAF. The bucket remains allocated and linked. There is no free followed by stale use, no second lifetime mutation through a dangling reference, and no controlled reclaim or active object replacement primitive.

PrimitiveClass is NONE because the patch establishes a resource leak but no corruption, authorization, ownership, or stale lifetime primitive. The practical consequence is resource consumption under repeated local configuration operations. The paranoid interpretation remains the same because sustained repetition can increase resource consumption, but it does not create a different vulnerability class or establish a host wide exhaustion or corruption primitive.

ImportantCandidate is NO. AutoDowngradeAllowed is YES because no unresolved meaningful memory corruption, security boundary bypass, or lifetime exploitation primitive remains. Operationally, this is a low priority local resource leak, subject to normal maintenance remediation rather than urgent security response.

5. One-sentence report phrase

A local traffic control configuration sequence can leave empty cls_route buckets linked after route4 filter moves, pinning unused tcf_proto objects and causing a repeatable kernel resource leak.

6. Manual review recommendation

NO MANUAL CHECK NEEDED

The patch demonstrates a bounded resource lifecycle leak without memory corruption, UAF, authorization bypass, direct network reachability, or an established host wide denial of service condition.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: cls_route: free emptied bucket on filter move [ Upstream
Commit: 2e2f130951d34871b0643bf700164d97cff4b427
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2e2f130951d34871b0643bf700164d97cff4b427

Changed files:
  net/sched/cls_route.c
  net/dst.h
  net/route.h

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=2e2f130951d34871b0643bf700164d97cff4b427

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

The original Linux kernel CVE announcement for CVE-2026-97959 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97959

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:L/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: unknown
  Paranoid ActionableScore: 1

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).