[CVE-2026-64410][MODERATE 7.0] netfilter: flowtable: IPIP tunnel hardware offload is not yet support

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

CVE: CVE-2026-64410
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: flowtable: IPIP tunnel hardware offload is not yet support
Commit: 9efe838c13133acb70c78d04c49e8362fe533566
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9efe838c13133acb70c78d04c49e8362fe533566
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64410
Analysis date: Sat, 25 Jul 2026 08:43:12 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
Unsupported IPIP or IP6IP6 tunnel flows could repeatedly enter the netfilter flowtable hardware offload refresh path in deployments with flowtable offload enabled, causing unnecessary worker activity and potential denial of service without evidence of memory corruption or privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-64410 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64410

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: NO

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-2026-64410	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-400;CWE-770;CWE-755;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.3';DESCR 'Unsupported IPIP or IP6IP6 tunnel flows could enter the netfilter flowtable hardware offload path even though no driver supports this offload mode. Repeated flow refreshes may enqueue or retry offload work for entries that cannot be programmed in hardware, creating unnecessary worker activity and possible resource consumption. For the CVSS the PR:N is used because a remote peer does not need local privileges on the target once the affected router or gateway is already configured with flowtable hardware offload and IPIP tunnel handling. The issue is network reachable only in deployments where attacker controlled traffic can traverse the affected tunnel flow path, usually inside routing, storage, or data center networks rather than the public Internet. Impact is denial of service only through CPU or workqueue pressure, with no evidence of confidentiality or integrity impact from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-64410 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: d98103575dcdd3a730e0901ab457791a9ac6930c YES  NO NO unknown 	MAYBE	REMOTE NETFILTER HARDWARE TEST  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Attacker-controlled traffic can trigger the affected path if it traverses a router or gateway configured with netfilter flowtable hardware offload and IPIP/IP6IP6 tunnel flows.

* Common networking core path or packet-processing path: +1
  The bug is in netfilter flowtable offload/refresh handling, which is part of packet-processing infrastructure.

* Availability impact realistic: +1
  Unsupported tunnel flows could repeatedly enqueue or retry hardware offload work on refresh, causing worker/CPU resource pressure.

Paranoid additional interpretation:

* Reliable kernel crash / strong DoS: +1
  Not a crash primitive, but in the worst defensible case sustained traffic could cause significant workqueue or CPU pressure if refresh retries are frequent enough.

Not counted:

* No memory corruption signal.
  The patch shows unsupported offload filtering and flag movement, not UAF, OOB, double-free, type confusion, or arbitrary write.

* No privilege escalation signal.
  There is no concrete corruption or security-boundary bypass primitive.

* No confidentiality or integrity signal.
  The effect is resource consumption only.

## 3. Reachability analysis

The bug is triggerable by traffic only when the target is already configured as a router/gateway using netfilter flowtable hardware offload with IPIP or IP6IP6 tunnel acceleration. A remote peer does not need local privileges on the target in that deployment, but the path is not a generic Internet-exposed service by default. Namespaces or containers matter mainly for configuration rights: creating flowtables and tunnel/offload setup usually requires CAP_NET_ADMIN, but once configured, packet traffic can exercise the affected refresh path. Realistic exploitation requires access to the relevant routed/tunneled traffic path, commonly an internal, datacenter, or provider network rather than arbitrary public Internet reachability.

## 4. Severity interpretation

This behaves like an actionable Moderate resource-consumption issue, not an Important-class memory corruption vulnerability. The patch context supports repeated unsupported hardware-offload work and possible DoS, but not kernel memory corruption, privilege escalation, information disclosure, or integrity impact. Conservative handling is borderline/manual-review because it is network-triggerable in specific netfilter offload deployments. Paranoid handling reaches Actionable Moderate due to packet-path exposure plus possible sustained worker/CPU pressure.

## 5. One-sentence report phrase

Unsupported IPIP or IP6IP6 tunnel flows could repeatedly enter the netfilter flowtable hardware offload refresh path in deployments with flowtable offload enabled, causing unnecessary worker activity and potential denial of service without evidence of memory corruption or privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue is network-triggerable in specific netfilter flowtable hardware-offload tunnel deployments, but it should not be treated as Important without evidence of memory corruption, privilege escalation, or a reliable system-wide DoS reproducer.

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

Patch: netfilter: flowtable: IPIP tunnel hardware offload is not yet support
Commit: 9efe838c13133acb70c78d04c49e8362fe533566
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9efe838c13133acb70c78d04c49e8362fe533566

Commit description:

No driver supports for IPIP tunnels yet, give up early on setting up the
hardware offload for this scenario.

This patch adds a stub that can be enhanced to add more configuration
that are currently not supported. As of now, the offload work is
enqueued to the worker, then ignored if the hardware offload
configuration is not supported.

Check the NF_FLOW_HW flag to know if this entry was already tried once
to be offloaded so this is not retried on refresh when unsupported. Move
NF_FLOW_HW flag check to nf_flow_offload_add(). If this NF_FLOW_HW flag
is unset the _del and _stats variants are never called.

This can be updated later on to skip hardware offload work to be queued
in case hardware offload does not support it.

Fixes: d981035 ("netfilter: flowtable: Add IP6IP6 rx sw acceleration")
Fixes: ab427db ("netfilter: flowtable: Add IPIP rx sw acceleration")
Cc: stable@vger.kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Reported-by: Zhengyang Chen <chzhengyang2023@lzu.edu.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/netfilter/nf_flow_table.h
  net/netfilter/nf_flow_table_core.c
  net/netfilter/nf_flow_table_offload.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=9efe838c13133acb70c78d04c49e8362fe533566

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

The original Linux kernel CVE announcement for CVE-2026-64410 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64410

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.3

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).