[CVE-2026-74746][IMPORTANT] netfilter: flowtable: publish GC-visible tuple last [ Upstream

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

CVE: CVE-2026-74746
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: flowtable: publish GC-visible tuple last [ Upstream
Commit: 0a00254585827f1695aa2700114af622ea754cfa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0a00254585827f1695aa2700114af622ea754cfa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74746
Analysis date: Wed, 26 Aug 2026 10:56:00 -0400
ActionableScore: 9
ActionableScore lower bound: 8
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in netfilter flowtable insertion can let GC free a partially installed flow while `flow_offload_add()` is still publishing tuple nodes, producing KASAN-confirmed slab-use-after-free reads and writes in a network-reachable packet-processing path when flowtable offload is enabled.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74746 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74746

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74746	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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A race in netfilter flowtable insertion can expose an original direction tuple to GC before the reply direction tuple is installed. Because nf_flow_table_iterate treats original direction tuple nodes as owning entries, GC can observe a partially installed flow and free it while flow_offload_add is still inserting the second tuple. KASAN reports slab use-after-free reads and writes in the flowtable/rhashtable path, so this is a memory corruption class issue and not just a warning. For the CVSS the PR:N is selected because remote packets can trigger flow creation on an already configured vulnerable flowtable without an attacker account on the target. The issue is network reachable when nftables flowtable offload is enabled and attacker controlled traffic can create matching flows. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to UAF read/write on kernel heap objects.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE NETFILTER UAF KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=9
ActionableScoreLower=8

## 1. ActionableScore

* Conservative score: 8
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  Attacker controlled traffic can plausibly create matching flowtable entries when nftables flowtable offload is already configured.

* Generic memory corruption, strong corruption primitive: +2
  The commit explicitly reports KASAN slab-use-after-free read and write reports in the flowtable/rhashtable path.

* Real lifetime corruption: +1
  GC can observe a partially inserted flow and free it while `flow_offload_add()` is still inserting the second tuple.

* Reliable kernel crash / strong DoS: +1
  UAF read/write in rhashtable, GC, lookup, and delete paths is a realistic kernel crash condition.

* Common networking core path or packet-processing path: +1
  The affected code is in netfilter flowtable packet-processing/offload infrastructure.

* Confidentiality impact plausible: +1
  Conservative low confidentiality concern is reasonable because this is a kernel heap UAF read path, although no direct leak primitive is shown.

* Integrity impact plausible: +1
  Conservative low integrity concern is reasonable because KASAN reports include UAF writes, although no controlled overwrite primitive is demonstrated.

* Hard or unreliable race / special timing required: -1
  The bug depends on GC racing with partially completed flow insertion.

Paranoid additional signal:

* Weak LPE / stronger exploitation concern: +1
  Not because LPE is demonstrated, but because a network-reachable kernel heap UAF with both read and write reports deserves manual review for exploitability beyond DoS.

## 3. Reachability analysis

The likely trigger is attacker-controlled network traffic that causes flow creation while nftables flowtable offload is enabled. The attacker does not need a local account on the target in that configured state, but an administrator must have already configured the vulnerable flowtable path. This is not a default exposure on every Linux host, but it is part of netfilter networking infrastructure and should not be treated as a rare driver-only or admin-only control-plane bug.

Namespaces do not materially reduce the conservative concern for systems where the affected host flowtable is exposed to untrusted traffic. The main reliability constraint is the race with GC during flow insertion.

Call-site confidence: high. The commit describes the exact ownership rule used by `nf_flow_table_iterate()` and the changed insertion order in `flow_offload_add()`.

## 4. Severity interpretation

This behaves more like a Strong Important candidate than an ordinary Moderate issue. The realistic demonstrated impact is kernel memory corruption leading to DoS, with KASAN confirming slab-use-after-free reads and writes. A full privilege escalation or remote code execution primitive is not demonstrated, so the conservative interpretation should not assume arbitrary code execution. However, because the corruption is in a network-reachable netfilter data path when flowtable offload is configured, auto-closing as ordinary Moderate would be unsafe.

## 5. One-sentence report phrase

A race in netfilter flowtable insertion can let GC free a partially installed flow while `flow_offload_add()` is still publishing tuple nodes, producing KASAN-confirmed slab-use-after-free reads and writes in a network-reachable packet-processing path when flowtable offload is enabled.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable kernel heap UAF with both read and write reports in netfilter flowtable/rhashtable paths. The most likely practical impact is DoS, but the memory corruption class and packet-triggered reachability justify manual exploitability review.

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

Patch: netfilter: flowtable: publish GC-visible tuple last [ Upstream
Commit: 0a00254585827f1695aa2700114af622ea754cfa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0a00254585827f1695aa2700114af622ea754cfa

Changed files:
  net/netfilter/nf_flow_table_core.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=0a00254585827f1695aa2700114af622ea754cfa

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

The original Linux kernel CVE announcement for CVE-2026-74746 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74746

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 8
  Paranoid ActionableScore: 9

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).