[CVE-2026-72389][MODERATE 7.0] bridge: stp: Fix a potential use-after-free when deleting a bridge [ Upstream

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

CVE: CVE-2026-72389
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: bridge: stp: Fix a potential use-after-free when deleting a bridge [ Upstream
Commit: c86579b0a2d201792bcb59316629f4ba4758cfc8
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c86579b0a2d201792bcb59316629f4ba4758cfc8
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72389
Analysis date: Sat, 15 Aug 2026 23:50:08 -0400
ActionableScore: 7
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A bridge STP timer use-after-free can occur when `br_topology_change_detection()` arms `topology_change_timer` after the bridge is administratively down and the bridge is then deleted, causing a local kernel crash and potential memory corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72389 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72389

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-2026-72389	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-362;*CWE-672;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A timer use-after-free can occur in the Linux bridge STP code because br_topology_change_detection could arm the topology_change_timer even when the bridge device was administratively down. If the bridge is then deleted while that timer remains active, the timer callback may run after the net_bridge object has been freed, leading to a kernel crash and potential memory corruption. For the CVSS the PR:L is used because reliable triggering requires local ability to configure or delete bridge devices, typically CAP_NET_ADMIN or an equivalent delegated network administration context. The issue is not directly network reachable as a standalone remote attack and is mainly triggered through local bridge control operations. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, the timer UAF may allow confidentiality and integrity impact because callback execution on a freed kernel object can become a memory corruption primitive. For the paranoid score, choose the highest still-defensible interpretation supported by the bug class and patch context, with preference for manual-review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 6)	YES	INIT UAF TIMER HARDWARE KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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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: +2. The commit explicitly describes a use-after-free involving an active `timer_list` after bridge deletion.
* Real lifetime corruption: +1. This is an async timer lifetime bug where a timer can survive beyond the lifetime of the `net_bridge` object.
* Reliable kernel crash / strong DoS: +1. The reported ODEBUG warning shows `free active` timer state, and the timer callback can run after object teardown.
* Weak LPE concern: +1. Timer callback execution after free is more concerning than a plain stale dereference, but no reclaim or controlled replacement primitive is demonstrated.
* Broad/default/common subsystem exposure: +1. Linux bridge is a common networking subsystem.
* Hard or unreliable race / special timing required: -1. The issue requires engineering a timing window where the topology change timer is armed while the bridge is administratively down and then deleted.
* Requires CAP_NET_ADMIN in typical deployments: -2. Normal bridge creation, STP control, and bridge deletion usually require network administration privileges.

Paranoid additional interpretation:

* Local unprivileged trigger: +1. On systems with unprivileged user namespaces and network namespaces enabled, a local user may gain CAP_NET_ADMIN inside a namespace and exercise bridge code.
* Cross-boundary isolation/policy bypass: +1. If reachable from an unprivileged or containerized netns context, crashing or corrupting the host kernel crosses the intended namespace isolation boundary.

## 3. Reachability analysis

The bug is locally triggerable through bridge and STP control operations. In typical host deployments, this requires CAP_NET_ADMIN, so the conservative score applies a privilege penalty. In deployments where unprivileged user namespaces are enabled, or where containers receive delegated network administration rights, the practical privilege requirement may be lower. The issue is not directly network reachable as a standalone remote attack. Network topology events may help create STP activity, but the critical operation is local bridge lifecycle manipulation. Call-site confidence: high, because the patch shows both the arming-side guard in `br_topology_change_detection()` and the teardown-side `timer_shutdown_sync()` calls in `br_dev_delete()`.

## 4. Severity interpretation

This is not an ordinary Moderate resource bug. It is a real kernel lifetime bug involving an active timer after object deletion. The demonstrated impact is at least local DoS through warning, crash, or callback execution after free. The realistic exploitation evidence for privilege escalation is limited because no attacker-controlled reclaim or object replacement is shown. However, timer UAFs deserve manual review because callback execution after free can sometimes become a memory corruption primitive. Conservative handling is borderline manual-review Moderate. Paranoid handling is a Strong Important candidate because namespace reachability plus timer UAF semantics can raise practical risk.

## 5. One-sentence report phrase

A bridge STP timer use-after-free can occur when `br_topology_change_detection()` arms `topology_change_timer` after the bridge is administratively down and the bridge is then deleted, causing a local kernel crash and potential memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is a real timer use-after-free in a common networking subsystem, with possible namespace-lowered reachability and potential memory corruption beyond simple DoS.

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UPSTREAM PATCH SUMMARY
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Patch: bridge: stp: Fix a potential use-after-free when deleting a bridge [ Upstream
Commit: c86579b0a2d201792bcb59316629f4ba4758cfc8
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c86579b0a2d201792bcb59316629f4ba4758cfc8

Changed files:
  net/bridge/br_stp_timer.c
  lib/debugobjects.c
  net/bridge/br_if.c
  net/bridge/br_stp.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=c86579b0a2d201792bcb59316629f4ba4758cfc8

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

The original Linux kernel CVE announcement for CVE-2026-72389 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72389

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: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).