[CVE-2026-98320][IMPORTANT] netfilter: flowtable: hold reference on ct until flow is released [ Upstream

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

CVE: CVE-2026-98320
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: netfilter: flowtable: hold reference on ct until flow is released [ Upstream
Commit: 93ff1594be1aad4da364462dd8db050ebcfda2de
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=93ff1594be1aad4da364462dd8db050ebcfda2de
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98320
Analysis date: Sat, 10 Oct 2026 00:08:31 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98320 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98320

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: IMPORTANT
Manual review required: YES

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

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

CVE-2026-98320	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98320 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT TEMP_KPANIC_MERGE=IMPORTANT_HARD_VETO PRE_MERGE_LEGACY=IMPORTANT (independent Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A race in the netfilter flowtable lifetime handling can release the conntrack reference before all RCU readers have finished using the associated flow entry. nf_ct_put can immediately release the conntrack extension area while the flowtable datapath still holds flow.ct. SLAB_TYPESAFE_BY_RCU semantics can also allow the stale reference to resolve to a different reused conntrack object. A remote peer may exercise the vulnerable datapath by creating and terminating accelerated connections when nf_flowtable is configured, although reliable triggering depends on winning the RCU lifetime race. For the CVSS the PR:N value is used because packet processing does not require an authenticated local user or administrative access once an affected flowtable is active and reachable. Impact is at least denial of service through a kernel crash. In the worst still defensible case the stale conntrack and freed extension access may affect confidentiality and integrity or support kernel code execution, although such exploitation is not established.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	REMOTE INIT NETFILTER LEAK UAF LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS TEMP_KPANIC_MERGE_IMPORTANT_VETO KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Applied conservative floor: 4 -> 5 because a real RCU UAF affects an active structured conntrack object, and the stale reference remains usable by the flowtable datapath rather than being limited to cleanup.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF and lifetime violation. `nf_ct_put(flow->ct)` can release `ct->ext` while RCU readers can still access the conntrack through the RCU-visible flow.
* +1 Concrete RCU referenced-object lifetime mismatch. The flow remains visible during its grace period, but its referenced conntrack was released before that period ended. The fix moves `nf_ct_put()` into the RCU callback.
* +1 Common packet-processing path. The stale conntrack is consumed from the netfilter flowtable datapath.
* No negative points. Host-admin privilege, rare hardware, narrow one-shot timing, and harmless containment are not established.

Paranoid additions:

* +1 Attacker-influenced repeatable lifetime manipulation. Once flow offload is configured, a network peer can repeatedly create and terminate matching connections, influencing flow and conntrack lifecycle transitions.
* +1 Structured stale-object exploitation leverage. The datapath may consume a released `ct->ext` area or interpret a reused `SLAB_TYPESAFE_BY_RCU` object as the original conntrack. This is distinct from merely faulting on a fixed stale address.

Not awarded:

* Direct remote-reachability +2 was withheld because nf_flowtable policy requires prior privileged configuration.
* Reliable-crash +1 was withheld because the commit establishes stale consumption and wrong-object reference but does not document a specific reproducible panic.
* Strong exploitation leverage +2 was withheld because controlled reclaim, attacker-shaped replacement contents, arbitrary read or write, callback control, and LPE are not demonstrated.
* Confidentiality and integrity points were not awarded because no disclosure or unauthorized modification consequence is established.
* No additional availability point was awarded.
* No repeated release or double-refcount signal exists. The fix extends the conntrack lifetime rather than preventing a second put on the same dangling reference.
* B5-R does not apply because no retry-accumulated heap OOB write exists.
* B3-PENDING does not apply because no shared-backing write exists.
* B4-PENDING does not apply because no authorization decision changes from DENY to ALLOW.

3. Reachability analysis

The vulnerable reader is in the nf_flowtable packet datapath. A host administrator must initially configure flowtable policy, but an external peer does not need administrative credentials or host UID 0 to send traffic through an already active configuration. Therefore, `HostAdminRequired=NO`, while prior privileged setup remains a deployment prerequisite.

No attacker capability is required for subsequent packet processing. Whether all configuration stages can be performed with namespace-scoped `CAP_NET_ADMIN` is not established, so `NamespaceScopedPrivilege=UNKNOWN`.

No special hardware, boot mode, or unusual processor feature is required. The race is RCU-lifetime dependent, but the supplied evidence does not establish narrow or one-shot timing. Repeated matching connections may provide repeated attempts.

4. Primitive and severity interpretation

The exact primitive is premature release of a conntrack referenced by an RCU-visible `flow_offload`. `nf_ct_put()` may release `ct->ext` before existing readers finish, and `SLAB_TYPESAFE_BY_RCU` reuse may cause the stale reference to identify a different conntrack.

This is an active structured-object UAF, not fixed stale storage and not cleanup-only behavior. The stale object participates in live packet processing. There is no concrete second release or refcount mutation through the dangling reference.

The primitive is CLASS_B rather than CLASS_A because controlled replacement, directed corruption, arbitrary kernel access, and privilege escalation are not established. The demonstrated mechanism supports stale extension access and wrong-conntrack consumption. Kernel crash and incorrect flow processing are plausible, while code execution and privilege escalation remain unproven.

`ImportantCandidate=YES` because the broken cross-object RCU lifetime invariant reaches active datapath processing and allows semantically valid reuse as the wrong conntrack. `AutoDowngradeAllowed=NO` because automatically treating this as an ordinary crash-only race would discard unresolved exploitation leverage from active structured-object reuse. The likely operational severity is Actionable Moderate, with Important-class review required.

5. One-sentence report phrase

A premature conntrack reference release allows the netfilter flowtable datapath to consume freed extension state or a reused wrong conntrack during the RCU grace period, creating an active structured UAF that requires mandatory exploitation review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The bug establishes a network-exercisable active-object RCU UAF with possible wrong-object consumption, while the extent of post-reuse memory and flow-state influence is unresolved.

7. Call-site confidence

Call-site confidence: medium

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

Patch: netfilter: flowtable: hold reference on ct until flow is released [ Upstream
Commit: 93ff1594be1aad4da364462dd8db050ebcfda2de
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=93ff1594be1aad4da364462dd8db050ebcfda2de

Changed files:
  net/netfilter/nf_flow_table_core.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=93ff1594be1aad4da364462dd8db050ebcfda2de

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

The original Linux kernel CVE announcement for CVE-2026-98320 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98320

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:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 5
  Paranoid ActionableScore: 6

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

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).