[CVE-2026-64032][MODERATE 7.0] bridge: mcast: Fix a possible use-after-free when removing a bridge port [ Upstream

[CVE-2026-64032][MODERATE 7.0] bridge: mcast: Fix a possible use-after-free when removing a bridge port [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64032
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bridge: mcast: Fix a possible use-after-free when removing a bridge port [ Upstream
Commit: ddefd1b8e5eb58933a697ab38334f0fd82e7fb8b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ddefd1b8e5eb58933a697ab38334f0fd82e7fb8b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64032
Analysis date: Sun, 19 Jul 2026 21:26:46 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A bridge multicast state mismatch can leave a per-port multicast timer active after bridge port teardown, causing a use-after-free that is locally triggerable through bridge configuration and may lead to kernel crash, with privilege escalation requiring further manual assessment.

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

The original Linux kernel CVE announcement for CVE-2026-64032 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64032

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-2026-64032	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;CWE-703;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use-after-free can occur in the bridge multicast code when per-VLAN multicast snooping and global multicast snooping transitions leave both per-port and per-port/VLAN multicast contexts enabled on the same bridge port. When the port is later removed, the per-port multicast context can remain active and its timer can survive past object teardown, leading to a timer callback on freed bridge port related memory. For the CVSS the PR:L is selected because reliable triggering requires a local actor able to change bridge multicast settings, commonly CAP_NET_ADMIN or an equivalent delegated network administration context rather than full host root. The issue is not directly network reachable through packet traffic and is triggered through local netlink bridge configuration. Impact is at least local denial of service via kernel warning or crash. In the paranoid interpretation, the active timer use-after-free is a memory corruption primitive that may justify confidentiality and integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	UAF HARDWARE SYZBOT KPANIC 	YES	NO	checked

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

ActionableScore=7
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Generic memory corruption, strong primitive: +2. The patch and report describe a real use-after-free where an active `timer_list` remains associated with a bridge port context after the port is freed.
* Real lifetime corruption: +1. The bug is specifically a stale lifetime issue involving multicast context teardown and a timer surviving object destruction.
* Reliable kernel crash / strong DoS: +1. ODEBUG reports freeing an active timer object, and this class can lead to warning, crash, or later timer callback on freed memory.
* Broad/default/common subsystem: +1. Linux bridge multicast is a common networking subsystem, though this is a control-plane configuration path rather than a packet receive path.
* Requires CAP_NET_ADMIN in typical deployments: -2. Reliable triggering requires creating/configuring a bridge, toggling multicast options, and removing a bridge port.

Paranoid additional signals:

* Local unprivileged trigger: +1. In environments with unprivileged user namespaces and network namespace delegation, a local user may obtain CAP_NET_ADMIN inside a netns and exercise bridge configuration paths.
* Weak LPE concern: +1. The stale active timer UAF is more concerning than a NULL dereference because it involves asynchronous callback execution after object teardown, although no controlled reclaim or overwrite primitive is demonstrated.

No remote/network-triggerable score is added because packet traffic alone does not trigger the bug. The trigger is local netlink bridge configuration.

## 3. Reachability analysis

The realistic trigger is a local actor able to configure bridge multicast settings and bridge ports. In the initial host network namespace this normally requires CAP_NET_ADMIN, so the conservative score applies a privilege penalty.

Namespaces matter. If unprivileged user namespaces are enabled and bridge/dummy device creation is allowed inside a user-created network namespace, the practical privilege requirement can drop to a local unprivileged user obtaining delegated CAP_NET_ADMIN in that namespace. That supports the paranoid score.

The affected path is not directly exposed to remote packets. It is a bridge control-plane state transition involving `mcast_snooping`, `mcast_vlan_snooping`, VLAN filtering, and port removal. Exploitation conditions are realistic for a local attacker with network administration capability, but there is no demonstrated remote trigger or controlled object reclaim.

Call-site confidence: high. The provided commit message describes the exact state transition, the teardown behavior, and the active timer UAF evidence.

## 4. Severity interpretation

This is not an ordinary low-risk cleanup bug. The evidence supports a real UAF involving an active kernel timer that can survive bridge port teardown.

Realistically, the demonstrated impact is local DoS through warning, crash, or unsafe timer callback after free. Theoretical privilege escalation is plausible enough for manual review because asynchronous timer UAFs can sometimes become exploitable with heap reuse, but the patch does not show attacker-controlled reclaim, arbitrary write, type confusion, or function pointer control.

Conservative handling is Borderline / manual review recommended. Paranoid handling is Strong Important candidate / Actionable Moderate at minimum because it is a concrete lifetime corruption bug in a common networking subsystem with possible namespace-lowered reachability.

## 5. One-sentence report phrase

A bridge multicast state mismatch can leave a per-port multicast timer active after bridge port teardown, causing a use-after-free that is locally triggerable through bridge configuration and may lead to kernel crash, with privilege escalation requiring further manual assessment.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a concrete UAF with an active timer object and possible namespace-mediated reachability, so it should not be auto-closed as an ordinary Moderate even though the demonstrated impact is primarily DoS.

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

Patch: bridge: mcast: Fix a possible use-after-free when removing a bridge port [ Upstream
Commit: ddefd1b8e5eb58933a697ab38334f0fd82e7fb8b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ddefd1b8e5eb58933a697ab38334f0fd82e7fb8b

Changed files:
  net/bridge/br_multicast.c
  lib/debugobjects.c
  mm/slub.c
  lib/kobject.c
  kernel/rcu/tree.c
  kernel/softirq.c
  arch/x86/kernel/apic/apic.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=ddefd1b8e5eb58933a697ab38334f0fd82e7fb8b

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

The original Linux kernel CVE announcement for CVE-2026-64032 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64032

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:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: 4
  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: 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).