[CVE-2026-53349][MODERATE 7.0] netfilter: nf_conntrack: destroy stale expectfn expectations on unregister [ Upstream

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

CVE: CVE-2026-53349
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nf_conntrack: destroy stale expectfn expectations on unregister [ Upstream
Commit: fbfde85308b99938a6092c48753214d190ece48d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fbfde85308b99938a6092c48753214d190ece48d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53349
Analysis date: Wed, 01 Jul 2026 10:28:24 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale `expectfn` callback in netfilter conntrack expectations can survive NAT helper module unload and later be invoked from packet processing, causing a use-after-free of module text and at least a kernel crash with worst-case control-flow corruption concerns.

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

The original Linux kernel CVE announcement for CVE-2026-53349 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53349

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-53349	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;CWE-362;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A stale function pointer use-after-free can occur in netfilter connection tracking expectations when a NAT helper module is unloaded while live expectations still reference its expectfn callback in module text. When a later packet or connection matches the pending expectation, init_conntrack can invoke exp expectfn after the helper code has been unloaded, causing a jump into freed module text and a kernel crash. For the CVSS the PR:H is selected because reaching the dangling state requires CAP_SYS_MODULE in the initial user namespace to unload a NAT helper while expectations are live. The final trigger may be network traffic matching the expectation, but unauthenticated network traffic alone is not sufficient to create the vulnerable state. Impact is at least denial of service. For the paranoid score, stale function pointer execution into freed module text is treated as a memory corruption candidate with possible confidentiality and integrity impact, so manual review is warranted.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 4)	YES	REMOTE INIT NETFILTER UAF  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption / strong corruption primitive: +2
  `exp->expectfn` is a stale function pointer into unloaded module text and is later invoked from `init_conntrack()`.

* Real lifetime corruption: +1
  The expectation object can outlive the module code that its callback points to.

* Reliable kernel crash / strong DoS: +1
  The commit includes a reproduced Oops on KASAN after unloading `nf_nat_h323` and triggering the expected connection.

* Common packet-processing path: +1
  The stale callback is reached from netfilter conntrack packet processing.

* Requires admin/root/CAP_SYS_MODULE in typical deployments: -2
  Creating the dangling state requires unloading a NAT helper module in the initial user namespace.

Conservative total: 3

Paranoid additions:

* Confidentiality impact plausible: +1
  A stale function pointer into freed module text is a control-flow lifetime bug, so non-DoS impact cannot be fully dismissed.

* Integrity impact plausible: +1
  In a worst-case interpretation, reuse of freed module memory could turn this into broader kernel control-flow corruption.

Paranoid total: 5

## 3. Reachability analysis

The vulnerable state requires a local privileged actor with `CAP_SYS_MODULE` in the initial user namespace to unload a NAT helper module while live conntrack expectations still reference its `expectfn`. The final trigger can be a packet or connection matching the pending expectation, but unauthenticated network traffic alone cannot create the dangling callback state.

Namespaces do not significantly reduce the conservative privilege requirement because module unload is not normally delegated to containers. The affected runtime configuration also requires NAT helpers such as H.323 or SIP and live expectations, so this is not a default unauthenticated remote exposure. Call-site confidence: high, because the commit provides the callback invocation path through `init_conntrack()` and the patch explicitly destroys matching expectations during helper teardown.

## 4. Severity interpretation

This is not an ordinary DoS-only cleanup because the stale pointer is a callback into unloaded module text. Realistically, the demonstrated impact is a privileged local kernel crash. However, the primitive is a function-pointer lifetime violation, so it deserves manual review despite the high privilege requirement.

This behaves as Actionable Moderate at minimum. It is a Strong Important candidate only under the paranoid interpretation where freed module text reuse could make the stale callback more than a crash primitive.

## 5. One-sentence report phrase

A stale `expectfn` callback in netfilter conntrack expectations can survive NAT helper module unload and later be invoked from packet processing, causing a use-after-free of module text and at least a kernel crash with worst-case control-flow corruption concerns.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a stale function pointer / UAF into unloaded module text with callback dispatch, even though the dangling state requires `CAP_SYS_MODULE` and the demonstrated impact is DoS.

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

Patch: netfilter: nf_conntrack: destroy stale expectfn expectations on unregister [ Upstream
Commit: fbfde85308b99938a6092c48753214d190ece48d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fbfde85308b99938a6092c48753214d190ece48d

Changed files:
  net/netfilter/nf_conntrack_core.c
  net/netfilter/nf_conntrack_proto.c
  net/netfilter/core.c
  net/ipv4/ip_output.c
  net/ipv4/tcp_output.c
  net/ipv4/tcp_ipv4.c
  net/socket.c
  include/net/netfilter/nf_conntrack_helper.h
  net/ipv4/netfilter/nf_nat_h323.c
  net/netfilter/nf_conntrack_helper.c
  net/netfilter/nf_nat_core.c
  net/netfilter/nf_nat_sip.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=fbfde85308b99938a6092c48753214d190ece48d

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

The original Linux kernel CVE announcement for CVE-2026-53349 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53349

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:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:H/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 6.4

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: 3
  Paranoid ActionableScore: 5

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: NO
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).