[CVE-2026-72249][MODERATE 7.0] netfilter: flowtable: use dst in this direction when pushing IPIP header

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

CVE: CVE-2026-72249
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: flowtable: use dst in this direction when pushing IPIP header
Commit: ecb78fbb03d3f86e2e93767875e273308086a424
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ecb78fbb03d3f86e2e93767875e273308086a424
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72249
Analysis date: Sat, 15 Aug 2026 11:04:23 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A netfilter flowtable IPIP/IP6IP6 tunnel acceleration bug used the route `dst` from the opposite direction when pushing the outer tunnel header, allowing matching network traffic on configured systems to trigger incorrect skb headroom or fragmentation handling and cause forwarding disruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72249 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72249

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-2026-72249	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:L/A:H';CWE-682;CWE-670;CWE-754;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'netfilter flowtable IPIP and IP6IP6 tunnel transmit acceleration used the route dst from the opposite direction when pushing the outer tunnel header. This can make the kernel calculate skb headroom and frag_off from the wrong route, which may cause packet drops, broken encapsulation, MTU related failures, or degraded forwarding for affected flows. For the CVSS the PR:N is used because after the vulnerable flowtable and tunnel path is configured, the attacker does not need local code execution and only needs the ability to send matching traffic through that path. The issue is network reachable in the sense of packet processing, but exposure is usually limited to systems using nf_tables flowtable offload with IPIP or IP6IP6 software tunnel acceleration on internal or controlled networks. Impact is primarily denial of service or traffic integrity degradation, with no direct evidence of a UAF, OOB write, arbitrary write, information leak, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-72249 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: d30301ba4b07ac92eb38353a111833b009003170 YES  NO NO unknown 	MAYBE	REMOTE SIMPLEFIX NETFILTER SKB IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **6**
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Remote reachable / network-triggerable: +2**
  After nf_tables flowtable IPIP/IP6IP6 acceleration is configured, matching packets can exercise this path without local code execution on the target.

* **Memory layout invariant restoration: +1**
  The patch fixes use of the wrong `dst` when calculating tunnel-header headroom and `frag_off` behavior in skb tunnel header construction.

* **Common networking core path or packet-processing path: +1**
  The affected code is in the netfilter flowtable packet forwarding path, although limited to IPIP/IP6IP6 tunnel software acceleration.

* **Availability impact realistic: +1**
  Wrong route-derived headroom or fragmentation state can cause packet drops, broken encapsulation, MTU failures, or forwarding disruption for affected flows.

Paranoid additional signal:

* **Integrity impact plausible: +1**
  Incorrect tunnel encapsulation or fragmentation metadata can plausibly corrupt packet forwarding semantics or produce traffic integrity degradation, but this is not evidence of kernel memory corruption.

Not awarded:

* **Generic memory corruption: +0**
  The patch does not show UAF, OOB write, double free, type confusion, arbitrary write, or attacker-controlled overwrite.

* **Privilege escalation plausible: +0**
  No LPE primitive is supported by the diff or commit message.

* **Reliable kernel crash / strong DoS: +0**
  The commit describes wrong headroom and `frag_off`, not a confirmed panic, Oops, or host-wide crash.

## 3. Reachability analysis

The bug is reachable from packet processing when nf_tables flowtable offload with IPIP or IP6IP6 tunnel software acceleration is configured. Configuration normally requires administrative privileges, but once configured, an attacker only needs to send traffic that matches the accelerated flow path. For the practical attack model this is closer to adjacent or controlled-network exposure than arbitrary Internet exposure. Containers or namespaces matter mainly if a tenant can influence traffic through a preconfigured host flowtable or has delegated network administration, but ordinary unprivileged users normally cannot configure the vulnerable path themselves.

Call-site confidence: **high**, because the patch includes the affected callers in `nf_flow_offload_ip_hook()` and `nf_flow_offload_ipv6_hook()`.

## 4. Severity interpretation

This behaves like an **Actionable Moderate** issue rather than a clear Important vulnerability. The reason for manual relevance is that it sits in a network packet-processing path and affects skb tunnel-header construction, including headroom and fragmentation-related state. The realistic impact appears to be forwarding failure, packet drops, or traffic degradation. The theoretical concern is limited traffic integrity impact from incorrect encapsulation behavior, but there is no demonstrated memory corruption or privilege escalation primitive.

## 5. One-sentence report phrase

A netfilter flowtable IPIP/IP6IP6 tunnel acceleration bug used the route `dst` from the opposite direction when pushing the outer tunnel header, allowing matching network traffic on configured systems to trigger incorrect skb headroom or fragmentation handling and cause forwarding disruption.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This should not be auto-closed because it is network-triggerable in a packet-processing path and affects skb headroom handling, even though the patch does not currently support scoring it as memory corruption or privilege escalation.

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

Patch: netfilter: flowtable: use dst in this direction when pushing IPIP header
Commit: ecb78fbb03d3f86e2e93767875e273308086a424
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ecb78fbb03d3f86e2e93767875e273308086a424

Commit description:

When pushing the IPIP header, the route of the other direction is used
to calculate the headroom, use the route in this direction. Accessing
the other tuple to set the IP source and destination is fine because
this tuple does not provide such information to avoid storing redundant
information. However, this tuple already provides the dst for this
direction, this went unnoticed because this bug affects headroom and
iph->frag_off only at this stage.

Fixes: d30301b ("netfilter: flowtable: Add IPIP tx sw acceleration")
Fixes: 93cf357 ("netfilter: flowtable: Add IP6IP6 tx sw acceleration")
Cc: stable@vger.kernel.org
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=ecb78fbb03d3f86e2e93767875e273308086a424

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

The original Linux kernel CVE announcement for CVE-2026-72249 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72249

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:L/A:H
  The Best / paranoid CVSS score: 5.9

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: 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).