[CVE-2026-72248][IMPORTANT] netfilter: flowtable: support IPIP tunnel with direct xmit

[CVE-2026-72248][IMPORTANT] netfilter: flowtable: support IPIP tunnel with direct xmit

[AL-KERNEL]

CVE: CVE-2026-72248
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: flowtable: support IPIP tunnel with direct xmit
Commit: 0880c4ed122d0cddc9f29a2b28f055d1f24f0fca
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0880c4ed122d0cddc9f29a2b28f055d1f24f0fca
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72248
Analysis date: Sun, 16 Aug 2026 05:20:56 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A netfilter flowtable IPIP direct xmit path can transmit tunnel traffic without valid dst_entry state, allowing reachable packet traffic in the affected configuration to trigger invalid dst usage and crash the kernel.

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

The original Linux kernel CVE announcement for CVE-2026-72248 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72248

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-2026-72248	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';**CWE-476;*CWE-664;CWE-825;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'IPIP tunnel traffic through netfilter flowtable direct xmit can reach the tunnel transmit path without valid dst_entry state. The tunnel path needs dst state to check skb headroom and to update IP fragmentation fields, and missing or invalid dst state can crash the kernel. For the CVSS the PR:N is used because an attacker does not need a local account when a vulnerable flowtable and IPIP tunnel configuration is already deployed and reachable by packets. AC:H is used because the bug depends on a specific non-default combination of flowtable offload, IPIP tunnel metadata, and direct xmit such as bridge based forwarding. Impact is at least remote or network adjacent denial of service via kernel crash. For the paranoid score, limited confidentiality and integrity impact are kept as a manual review signal because the fix changes dst_entry lifetime and sharing, but the patch does not show a concrete arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-72248 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: d30301ba4b07ac92eb38353a111833b009003170 YES  NO NO unknown 	MAYBE	REMOTE DANGER SIMPLEFIX NETFILTER SKB HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6	-	-	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative score signals:

* Remote reachable / network-triggerable: +2
  Once a vulnerable IPIP plus flowtable direct xmit configuration exists, the crash can plausibly be triggered by packet traffic rather than by local code execution.

* Memory corruption, weak/indirect corruption candidate: +1
  The issue is described as invalid dst usage in the tunnel transmit path. The patch does not show an arbitrary write or controlled object replacement, so this is weak pointer/lifetime state corruption rather than a strong corruption primitive.

* Real lifetime corruption: +1
  The fix changes dst_cache and dst_cookie lifetime handling, avoids premature dst_release for tunnel direct xmit, and moves release to the common dst release path.

* Reliable kernel crash / strong DoS: +1
  The commit explicitly says invalid dst usage can trigger a crash in the tunnel transmit path.

* Common networking core path or packet-processing path: +1
  This is netfilter flowtable packet forwarding with tunnel xmit handling, not a purely local control-plane-only issue.

Paranoid additional signal:

* Availability impact realistic: +1
  In the vulnerable deployed configuration, crafted or matching traffic can drive a host kernel crash, making the DoS system-wide and operationally relevant.

Not awarded:

* No strong LPE plausibility. The patch does not show attacker-controlled reclaim, arbitrary write, type confusion, callback control, or refcount takeover.
* No confidentiality or integrity impact in the conservative score. The dst lifetime issue justifies manual review, but the patch does not demonstrate an info leak or write primitive.
* No admin privilege subtraction. Admin privileges are needed to create the vulnerable network configuration, but the practical trigger can be packet driven after deployment.
* No rare subsystem penalty. The configuration is specific, but netfilter and packet forwarding paths are not treated as rare-only code for triage.

## 3. Reachability analysis

The bug is reachable when netfilter flowtable offload is used with IPIP tunnel metadata and FLOW_OFFLOAD_XMIT_DIRECT, for example in bridge-like direct transmit forwarding. This is not a default Internet-exposed service by itself, and it requires a specific network setup created by an administrator. However, after that setup exists, the triggering condition is packet-path behavior, so an attacker may not need a local account on the target.

Namespaces and containers mainly matter for who can create the configuration. Creating flowtable, bridge, tunnel, or netfilter state normally requires CAP_NET_ADMIN, but a reduced-privilege container or delegated network namespace could make configuration control less than full host-root in some deployments. For the packet trigger itself, PR:N is defensible if the affected forwarding path is reachable.

Realistic exploitation is mainly DoS. The patch supports a dst_entry lifetime/state bug and a crash, but not a controlled corruption primitive.

Call-site confidence: medium. The patch shows route fill, dst release, and dst validation changes, and the commit identifies the tunnel transmit crash path, but the full tunnel xmit call chain is not included in the supplied diff.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because it is packet-path reachable after a plausible netfilter/IPIP deployment and involves dst_entry lifetime/state handling in kernel networking. It should be treated as Actionable Moderate at minimum, with Strong Important candidate handling for triage because a remotely triggerable kernel crash in a packet-processing path should not be auto-closed.

The realistic impact is denial of service through kernel crash. The theoretical concern is invalid dst lifetime/state use, but the evidence does not justify assuming privilege escalation, arbitrary code execution, or reliable confidentiality/integrity compromise.

## 5. One-sentence report phrase

A netfilter flowtable IPIP direct xmit path can transmit tunnel traffic without valid dst_entry state, allowing reachable packet traffic in the affected configuration to trigger invalid dst usage and crash the kernel.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is packet-path reachable in a common kernel networking subsystem and the fix changes dst_entry lifetime handling, so it should receive manual review even though the demonstrated impact is DoS rather than proven privilege escalation.

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

Patch: netfilter: flowtable: support IPIP tunnel with direct xmit
Commit: 0880c4ed122d0cddc9f29a2b28f055d1f24f0fca
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0880c4ed122d0cddc9f29a2b28f055d1f24f0fca

Commit description:

The combination of IPIP tunnel with direct xmit, eg. bridge device,
breaks because no dst_entry is provided to check the skb headroom and to
set the iph->frag_off field. This leads to invalid dst usage and can
trigger a crash in the tunnel transmit path.

Fix this by moving dst_cache and dst_cookie out of the runtime union so
that they can be shared by neighbour, xfrm, and direct tunnel flows.
For FLOW_OFFLOAD_XMIT_DIRECT tuples carrying tunnel metadata, preserve
route state in these shared fields and release it through the common
dst release path.

Since dst_entry is now available to the three supported xmit modes and
dst_release() already deals with NULL dst, remove the xmit type check
in nft_flow_dst_release(). Moreover, skip the check if the dst entry
is NULL in nf_flow_dst_check() which is now the case for the direct
xmit case.

Based on patch from Rein Wei <n05ec@lzu.edu.cn>.

Fixes: d30301b ("netfilter: flowtable: Add IPIP tx 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>
Reported-by: Ren Wei <n05ec@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_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=0880c4ed122d0cddc9f29a2b28f055d1f24f0fca

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

The original Linux kernel CVE announcement for CVE-2026-72248 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72248

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:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 7

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: 6
  Paranoid ActionableScore: 7

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: YES
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).