[CVE-2026-72417][MODERATE 7.0] netfilter: flowtable: Validate iph->ihl in nf_flow_ip4_tunnel_proto() [ Upstream

[CVE-2026-72417][MODERATE 7.0] netfilter: flowtable: Validate iph->ihl in nf_flow_ip4_tunnel_proto() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72417
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: flowtable: Validate iph->ihl in nf_flow_ip4_tunnel_proto() [ Upstream
Commit: 025a41e76b51fbc7b8eaa5bacbaa9621d00e6aa7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=025a41e76b51fbc7b8eaa5bacbaa9621d00e6aa7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72417
Analysis date: Sat, 15 Aug 2026 20:14:20 -0400
ActionableScore: 6
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Malformed IPv4/IPIP tunnel packets can reach `nf_flow_ip4_tunnel_proto()` with an invalid `iph->ihl` value, causing incorrect header-size and offset handling that may lead to out-of-bounds access and remote DoS when the affected netfilter flowtable path is enabled.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72417 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72417

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-2026-72417	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';*CWE-20;CWE-125;CWE-754;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.2';DESCR 'Malformed IPv4 tunnel packets can reach nf_flow_ip4_tunnel_proto with an invalid iph ihl value. The value was used to derive the IPv4 header size before checking the minimum header length, which can move parsing to an invalid offset and cause out of bounds access in the netfilter flowtable IPIP receive path. For the CVSS the PR:N is used because a remote sender does not need local privileges on the target when the vulnerable flowtable path is enabled and reachable. The issue is network reachable through crafted packet traffic rather than a local control interface. Impact is primarily denial of service via kernel crash. A limited confidentiality impact is kept only in the paranoid score because the bug class indicates out of bounds read, while no write primitive or direct privilege escalation path is shown.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-72417 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: ab427db17885814069bae891834f20842f0ac3a4 YES  NO NO unknown 	MAYBE	REMOTE OOB SIMPLEFIX NETFILTER KPANIC  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The vulnerable path is in netfilter flowtable packet parsing and can be reached by malformed IPv4/IPIP tunnel traffic when the affected flowtable path is enabled.
* Reliable kernel crash / strong DoS: +1. The commit explicitly says the missing `iph->ihl` validation avoids out-of-bounds access with malformed IP headers, which is a credible kernel crash or packet-processing DoS condition.
* Common networking core path or packet-processing path: +1. This is netfilter flowtable packet handling, not a purely local ioctl or admin-only control path.

Paranoid additional signals:

* Memory corruption, weak/indirect corruption candidate: +1. The primitive is an out-of-bounds access caused by malformed header length and invalid offset computation, but no OOB write, UAF, type confusion, or attacker-controlled overwrite is shown.
* Confidentiality impact plausible: +1. Only for the paranoid score, limited C impact is defensible because this is an OOB read style parser bug, but there is no demonstrated disclosure path.

Not awarded:

* No privilege escalation plausible signal. The patch does not show write-after-read, UAF reclaim, callback control, refcount abuse, or arbitrary write.
* No strong generic memory corruption signal. This is not an OOB write or heap overwrite.
* No broad/default exposure bonus. The affected path depends on netfilter flowtable and IPIP acceleration being configured.

## 3. Reachability analysis

A remote packet sender can plausibly trigger the bug if crafted malformed IPv4/IPIP tunnel packets reach a host or router with the affected netfilter flowtable path enabled. The initial setup of flowtable rules normally requires administrative privileges such as CAP_NET_ADMIN, but the triggering packet itself does not require local code execution or privileges on the victim. Namespaces mainly affect who can configure the path, not whether an already configured forwarding or receive path can be hit by traffic. Realistic exposure is configuration dependent and more likely on routers, firewalls, gateways, or internal network systems using flowtable acceleration than on default end-user hosts.

Call-site confidence: medium. The changed function and packet parser context are visible, but the full caller chain and exact skb bounds guarantees are not included in the supplied patch.

## 4. Severity interpretation

This behaves stronger than an ordinary Moderate because it is a network-reachable packet parser bug in netfilter with explicit out-of-bounds access. Realistic impact is primarily remote DoS via kernel crash or malformed-packet processing failure. Theoretical confidentiality impact is limited and only used for the paranoid score. There is no realistic evidence of privilege escalation or integrity compromise from the supplied patch.

## 5. One-sentence report phrase

Malformed IPv4/IPIP tunnel packets can reach `nf_flow_ip4_tunnel_proto()` with an invalid `iph->ihl` value, causing incorrect header-size and offset handling that may lead to out-of-bounds access and remote DoS when the affected netfilter flowtable path is enabled.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable netfilter packet parsing issue with an explicit out-of-bounds access condition, so it should not be auto-closed even though the demonstrated impact is mainly DoS and the higher C impact remains paranoid only.

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

Patch: netfilter: flowtable: Validate iph->ihl in nf_flow_ip4_tunnel_proto() [ Upstream
Commit: 025a41e76b51fbc7b8eaa5bacbaa9621d00e6aa7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=025a41e76b51fbc7b8eaa5bacbaa9621d00e6aa7

Changed files:
  net/netfilter/nf_flow_table_ip.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=025a41e76b51fbc7b8eaa5bacbaa9621d00e6aa7

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

The original Linux kernel CVE announcement for CVE-2026-72417 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72417

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:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 8.2

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: 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: 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).