[CVE-2026-74695][MODERATE 7.0] netfilter: nf_flow_table: drop existing skb dst before skb_dst_set_noref() [ Upstream

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

CVE: CVE-2026-74695
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nf_flow_table: drop existing skb dst before skb_dst_set_noref() [ Upstream
Commit: 12afa450a6a6c0cce2c42b7545a9958f62d8a00c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12afa450a6a6c0cce2c42b7545a9958f62d8a00c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74695
Analysis date: Sat, 22 Aug 2026 17:35:28 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-triggerable dst_entry reference leak in netfilter flowtable and XFRM offload paths can be caused when skb_dst_set_noref() overwrites an existing refcounted skb destination, allowing sustained traffic to exhaust kernel networking resources and cause DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74695 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74695

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-2026-74695	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'nf_flow_table can overwrite an existing refcounted skb dst_entry with skb_dst_set_noref without first dropping the old reference. This leaks dst_entry references on packets that enter the flowtable offload or XFRM offload path with an already attached dst, and sustained traffic may exhaust kernel networking resources. For the CVSS the PR:N is used because an attacker does not need a local account when they can send traffic through a configured flowtable path. The issue is network reachable only on systems where nf_flow_table or the relevant offload path is enabled and reachable by attacker controlled traffic. Impact is denial of service through resource exhaustion. There is no clear evidence of memory corruption, information disclosure, or privilege escalation from this patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 7)	YES	REMOTE SIMPLEFIX NETFILTER LEAK SKB IPV6 SYZBOT  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	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
  Traffic crossing a configured nf_flow_table or XFRM offload path can trigger the bug without local code execution on the target.

* Reliable kernel crash / strong DoS: +1
  The primitive is a dst_entry reference leak. Sustained traffic can plausibly exhaust kernel networking resources.

* Availability impact realistic: +1
  The leak can accumulate across packets and may become system-wide resource exhaustion rather than a single harmless warning.

* Common networking core path or packet-processing path: +1
  The affected code is in netfilter flowtable packet forwarding and XFRM/offload transmit handling.

Paranoid additional signal:

* Broad/default/common subsystem or broadly deployed exposure: +1
  Netfilter is broadly deployed, and flowtable offload is a realistic router/firewall configuration even if not enabled everywhere.

Not awarded:

* No generic memory corruption bonus. The patch shows a reference leak, not UAF, OOB write, double free, or type confusion.
* No privilege escalation bonus. There is no supported LPE primitive.
* No confidentiality or integrity impact. The visible consequence is resource exhaustion only.
* No admin privilege penalty for the attacker. Administrative setup is required to enable the feature, but packet sender privileges are not required once the path is exposed.

## 3. Reachability analysis

The bug is triggerable by packets entering a configured netfilter flowtable offload path or the related XFRM offload path while the skb already carries a refcounted dst_entry. The attacker does not need a local account if they can send traffic through the affected forwarding path, so PR:N is reasonable for network-exposed router or firewall deployments. The path is configuration-dependent because nf_flow_table or the relevant offload path must be enabled, but this is still a realistic networking deployment and should not be treated as an obscure debug-only feature. Containers and namespaces mainly matter for who can configure the flowtable, not for a remote packet sender once forwarding is active.

Call-site confidence: high. The patch directly shows the affected call sites before skb_dst_set_noref() in IPv4, IPv6, and XFRM-related flowtable paths.

## 4. Severity interpretation

This behaves like an actionable Moderate networking DoS rather than an Important memory-corruption issue. The practical primitive is a reference-count leak of dst_entry objects, which can cause resource exhaustion under sustained traffic. Theoretical escalation beyond DoS is not supported by the patch because the old dst reference is leaked, not freed too early or reused through a stale pointer. The paranoid score is higher only because the trigger can be network-facing in real packet-forwarding deployments and the leak may be cumulative.

## 5. One-sentence report phrase

A network-triggerable dst_entry reference leak in netfilter flowtable and XFRM offload paths can be caused when skb_dst_set_noref() overwrites an existing refcounted skb destination, allowing sustained traffic to exhaust kernel networking resources and cause DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not memory corruption, but it is in a packet-processing path and can be remotely triggered in configured flowtable deployments, so it should not be auto-closed as an ordinary low-risk resource leak.

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

Patch: netfilter: nf_flow_table: drop existing skb dst before skb_dst_set_noref() [ Upstream
Commit: 12afa450a6a6c0cce2c42b7545a9958f62d8a00c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=12afa450a6a6c0cce2c42b7545a9958f62d8a00c

Changed files:
  include/linux/skbuff.h
  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=12afa450a6a6c0cce2c42b7545a9958f62d8a00c

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

The original Linux kernel CVE announcement for CVE-2026-74695 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74695

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:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/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).