[CVE-2026-45913][LOW] net: bridge: mcast: always update mdb_n_entries for vlan contexts [ Upstream

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

CVE: CVE-2026-45913
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: net: bridge: mcast: always update mdb_n_entries for vlan contexts [ Upstream
Commit: d0fdad1bdd21a358cc2c85da3681ae27b86ce6ce
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0fdad1bdd21a358cc2c85da3681ae27b86ce6ce
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45913
Analysis date: Wed, 27 May 2026 21:45:06 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
A bridge multicast VLAN accounting bug can leave `mdb_n_entries` inconsistent and let a privileged or delegated local network administrator trigger a WARN during MDB flush, causing possible DoS under `panic_on_warn`.

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

The original Linux kernel CVE announcement for CVE-2026-45913 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45913

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

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

CVE-2026-45913	LOW	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-682;CWE-191;CWE-617;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A bridge multicast accounting bug can leave mdb_n_entries out of sync for VLAN multicast contexts, allowing an MDB flush to decrement a counter that was never incremented. A local actor with bridge and MDB configuration rights can trigger a WARN by combining VLAN multicast snooping state changes with MDB add and flush operations. For the CVSS the PR:L is used in the paranoid score because delegated network administration in containers or service accounts can expose these netlink operations below full host root, while typical host triggering requires CAP_NET_ADMIN. The issue is not network reachable and is triggered through control plane configuration rather than packet traffic. Impact is denial of service through WARN or panic_on_warn behavior. No supported evidence indicates information disclosure, integrity impact, kernel memory corruption, or privilege escalation.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 1) 	MAYBE	LOCK DANGER INIT HARDWARE SYZBOT  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	-	-	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**::

## 2. Signal breakdown

* Availability impact realistic: +1
  The demonstrated impact is a WARN from unmatched MDB counter decrement. It can become stronger only if `panic_on_warn` is enabled.

* Local delegated network administration trigger: +1 in paranoid scoring
  Bridge and MDB operations are normally `CAP_NET_ADMIN` gated, but some container or service setups may delegate network administration.

* Broad networking control-plane subsystem: +1 in paranoid scoring
  Linux bridge is common, but this is configuration/control-plane accounting rather than packet-processing reachability.

* Requires CAP_NET_ADMIN in typical deployments: -2 conservative
  Creating bridges, configuring VLAN multicast snooping, adding MDB entries, and flushing MDB state normally require network administration privileges.

* No memory corruption: +0
  The patch shows counter accounting mismatch, not UAF, OOB write, arbitrary write, or object lifetime corruption.

* No remote trigger: +0
  The trigger is netlink configuration and MDB flush, not incoming network traffic.

## 3. Reachability analysis

A local actor with bridge and MDB configuration rights can trigger the bug using VLAN multicast snooping state changes and MDB add or flush operations. In normal host deployments this requires `CAP_NET_ADMIN`. Namespaces or containers may reduce practical privilege if network administration is delegated to tenants or service accounts.

The issue is not network reachable. Packet traffic is not the triggering primitive. Realistic exploitation is limited to WARN or `panic_on_warn` DoS.

## 4. Severity interpretation

This behaves like ordinary Moderate or Low-like DoS. It is an accounting bug in bridge multicast state, not memory corruption or privilege escalation. Theoretical impact is availability through WARN or panic-on-warning behavior, while realistic evidence does not show data exposure, policy bypass, or kernel memory corruption.

## 5. One-sentence report phrase

A bridge multicast VLAN accounting bug can leave `mdb_n_entries` inconsistent and let a privileged or delegated local network administrator trigger a WARN during MDB flush, causing possible DoS under `panic_on_warn`.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This is a local `CAP_NET_ADMIN` control-plane accounting issue with WARN-only impact and no evidence of memory corruption, information disclosure, privilege escalation, or remote reachability.

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

Patch: net: bridge: mcast: always update mdb_n_entries for vlan contexts [ Upstream
Commit: d0fdad1bdd21a358cc2c85da3681ae27b86ce6ce
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d0fdad1bdd21a358cc2c85da3681ae27b86ce6ce

Changed files:
  net/bridge/br_multicast.c
  net/bridge/br_mdb.c
  net/core/rtnetlink.c
  net/netlink/af_netlink.c
  net/socket.c
  arch/x86/entry/syscall_64.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=d0fdad1bdd21a358cc2c85da3681ae27b86ce6ce

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

The original Linux kernel CVE announcement for CVE-2026-45913 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-45913

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:H/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.5

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: 0
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).