[CVE-2026-72323][IMPORTANT] ipv4: igmp: Fix potential UAF in igmp_gq_start_timer() [ Upstream

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

CVE: CVE-2026-72323
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: ipv4: igmp: Fix potential UAF in igmp_gq_start_timer() [ Upstream
Commit: 40a1e998cb266ed4cb529a0bb4fee2b0ba732702
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40a1e998cb266ed4cb529a0bb4fee2b0ba732702
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72323
Analysis date: Sat, 15 Aug 2026 15:07:10 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race between IPv4 IGMP query processing and `in_device` teardown can rearm IGMP timers after the object refcount reaches zero, leading to a timer callback use-after-free and kernel crash, with adjacent-network triggering possible under teardown timing conditions.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72323 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72323

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-2026-72323	IMPORTANT	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:H/I:H/A:H';*CWE-416;CWE-362;*CWE-672;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A race in the IPv4 IGMP path can let igmp_gq_start_timer() or igmp_ifc_start_timer() rearm a timer for an in_device whose refcount already reached 0 during inetdev teardown. The object is still readable under RCU, but the later timer reference is unsafe because the memory is already scheduled to be freed after the RCU grace period. When the timer fires it can access freed memory and cause a kernel crash (DoS), with a paranoid worst case treating the Use-After-Free as possible memory corruption impact. For the CVSS the PR:N is used for the paranoid score because an attacker does not need privileges on the target host and may only need adjacent network reachability to send IGMP query traffic while the teardown race is present. The issue is not generally Internet reachable because IGMP traffic is normally constrained to a local L2 multicast domain. Impact is at least adjacent network denial of service and in the worst case may allow confidentiality or integrity impact due to kernel Use-After-Free, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	RACE UAF TIMER HARDWARE IPV6 KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Network-triggerable: +2. IGMP packets can be supplied from an adjacent L2 network, although this is not general Internet reachability.
* Memory corruption, weak primitive: +1. The patch describes a UAF, but the conservative view treats it mainly as stale timer access without demonstrated reclaim or controlled replacement.
* Real lifetime corruption: +1. The bug is explicitly a refcount/RCU lifetime issue where `in_device` can be freed while a timer remains armed.
* Reliable kernel crash / strong DoS: +1. The commit states that timer expiry can access freed memory and cause kernel panic.
* Common networking packet-processing path: +1. The affected path is IPv4 IGMP receive/timer handling.
* Hard race / special timing required: -1. Exploitation requires overlap between device teardown and IGMP query processing.

Paranoid added signal:

* Weak LPE concern: +1. The UAF involves refcount abuse from zero and a stale asynchronous timer callback, so manual review is justified even though no attacker-controlled reclaim or write primitive is shown.

## 3. Reachability analysis

An adjacent network attacker may be able to supply IGMP query traffic, but reliable triggering also needs a concurrent `inetdev_destroy()` teardown window. The path is therefore best treated as AV:A rather than Internet-remote AV:N. A local attacker with namespace or interface-management ability could potentially make teardown timing more controllable, but ordinary interface teardown normally requires `CAP_NET_ADMIN`. The affected functionality is in the IPv4 networking stack and IGMP is common on multicast-capable interfaces, but the race condition makes exploitation timing-dependent.

Call-site confidence: high. The patch shows the affected timer arming sites and the new `refcount_inc_not_zero()` guard.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup. The conservative interpretation is an Actionable Moderate issue because it is a real kernel lifetime bug in a network-adjacent packet path with a credible kernel panic outcome. The paranoid interpretation reaches Strong Important candidate because the primitive is a UAF involving RCU lifetime, refcount resurrection from zero, and a stale timer callback, which historically warrants manual review even when privilege escalation is not demonstrated.

Realistic demonstrated impact is DoS. Theoretical memory-corruption impact is plausible enough for review, but the patch does not show controlled reclaim, arbitrary write, type confusion, or a concrete LPE chain.

## 5. One-sentence report phrase

A race between IPv4 IGMP query processing and `in_device` teardown can rearm IGMP timers after the object refcount reaches zero, leading to a timer callback use-after-free and kernel crash, with adjacent-network triggering possible under teardown timing conditions.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: this is a race-based UAF in a networking receive/timer path with real lifetime corruption and possible network-adjacent influence, even though the confirmed impact is primarily kernel DoS.

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

Patch: ipv4: igmp: Fix potential UAF in igmp_gq_start_timer() [ Upstream
Commit: 40a1e998cb266ed4cb529a0bb4fee2b0ba732702
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=40a1e998cb266ed4cb529a0bb4fee2b0ba732702

Changed files:
  include/linux/inetdevice.h
  net/ipv4/igmp.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=40a1e998cb266ed4cb529a0bb4fee2b0ba732702

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

The original Linux kernel CVE announcement for CVE-2026-72323 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72323

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:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 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).