[CVE-2026-46086][MODERATE REGULAR] net: bridge: use a stable FDB dst snapshot in RCU readers [ Upstream

[CVE-2026-46086][MODERATE REGULAR] net: bridge: use a stable FDB dst snapshot in RCU readers [ Upstream

[AL-KERNEL]

CVE: CVE-2026-46086
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: bridge: use a stable FDB dst snapshot in RCU readers [ Upstream
Commit: 0b9e4bbfb7c949151e3acd44ed4aa33614d2e110
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0b9e4bbfb7c949151e3acd44ed4aa33614d2e110
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46086
Analysis date: Mon, 06 Jul 2026 20:06:47 -0400
ActionableScore: 3
ActionableScore lower bound: 1
Actionable bucket: Borderline manual review recommended / not Important based on current evidence
Manual review required: YES

Summary:
A Linux bridge FDB RCU race can let readers observe inconsistent `f->dst` values across a NULL check and later dereference, allowing a local actor with bridge state control to trigger a kernel NULL pointer dereference and DoS.

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

The original Linux kernel CVE announcement for CVE-2026-46086 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46086

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 REGULAR
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-2026-46086	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:L/I:L/A:H';CWE-362;**CWE-476;CWE-367;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.8';DESCR 'A race in Linux bridge FDB handling can let RCU readers observe different f dst values across a NULL check and a later dereference. A concurrent local FDB update can rewrite f dst to another port or NULL while the entry remains alive, which can make br_fdb_fillbuf or related bridge readers dereference a NULL port pointer and crash the kernel. For the CVSS the PR:L is used because reliable triggering generally requires a local process able to drive bridge or FDB state changes, for example through CAP_NET_ADMIN in a network namespace, rather than only remote packet delivery. The issue is not directly network reachable as a standalone remote attack. Impact is at least local denial of service via kernel crash. 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 Borderline manual review recommended / not Important based on current evidence (with actual score 2) 	YES	OOB NULLPTR HARDWARE TEST 	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / not Important based on current evidence**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. The commit explicitly describes a NULL pointer dereference in `br_fdb_fillbuf()` when `f->dst` changes after a NULL check and before dereference.
* Broad networking subsystem relevance: +1. Linux bridge FDB handling is a common networking subsystem, though not necessarily enabled on every host.
* Hard or unreliable race / special timing required: -1. Triggering requires a concurrent local FDB rewrite and RCU reader path timing.
* Requires CAP_NET_ADMIN in typical deployments: -2. Reliable triggering generally requires bridge or FDB state changes, which normally need CAP_NET_ADMIN.

Paranoid additional interpretation:

* Local lower privilege trigger possibility: +1. In environments with user namespaces, containers, or delegated CAP_NET_ADMIN, a reduced privilege actor may be able to manipulate bridge state.
* Common packet/control networking path: +1. The affected readers include bridge FDB dump/read paths and ARP/ND proxy suppression paths, so it deserves more scrutiny than a rare driver-only bug.

Not counted:

* No generic memory corruption bonus. The patch supports inconsistent pointer observation and NULL dereference, not UAF, double free, OOB write, controlled overwrite, or reclaim.
* No privilege escalation bonus. There is no demonstrated stale object reuse, write primitive, callback dispatch, type confusion, or refcount takeover.
* No confidentiality or integrity impact in the conservative score. The demonstrated primitive is crash-oriented.

Call-site confidence: high. The provided patch shows both writer side `fdb_delete_local()` and multiple affected RCU readers.

## 3. Reachability analysis

The most concrete trigger is local: one path updates a local FDB entry in place while another RCU reader inspects it through bridge FDB read or dump paths. In normal host deployments, bridge configuration and FDB mutation require CAP_NET_ADMIN, so this is not a plain unprivileged local bug by default. In containerized or user namespace configurations, CAP_NET_ADMIN may be delegated to less trusted actors, which justifies the paranoid score.

The issue is not directly remote-network-triggerable as a standalone attack. Packet traffic may exercise some bridge reader paths, but the required concurrent local FDB update is the key control element.

## 4. Severity interpretation

This behaves like a race-triggered local kernel DoS, not an Important-class memory corruption issue based on the supplied patch. The theoretical concern is that this is an RCU reader/writer consistency bug in networking code, but the concrete primitive shown is a NULL pointer dereference after an inconsistent `f->dst` read. There is no evidence of attacker-controlled reuse, stale freed object access, arbitrary write, or privilege escalation.

Paranoid handling is still reasonable because bridge namespaces and delegated network administration can make the trigger surface more practical than full host-root-only bugs.

## 5. One-sentence report phrase

A Linux bridge FDB RCU race can let readers observe inconsistent `f->dst` values across a NULL check and later dereference, allowing a local actor with bridge state control to trigger a kernel NULL pointer dereference and DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: this is not strong enough for Important based on current evidence, but it is an RCU race in a networking subsystem with a real kernel crash and possible reachability from delegated CAP_NET_ADMIN or namespace environments.

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

Patch: net: bridge: use a stable FDB dst snapshot in RCU readers [ Upstream
Commit: 0b9e4bbfb7c949151e3acd44ed4aa33614d2e110
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0b9e4bbfb7c949151e3acd44ed4aa33614d2e110

Changed files:
  net/bridge/br_arp_nd_proxy.c
  net/bridge/br_fdb.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=0b9e4bbfb7c949151e3acd44ed4aa33614d2e110

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

The original Linux kernel CVE announcement for CVE-2026-46086 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46086

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:L/I:L/A:H
  The Best / paranoid CVSS score: 5.8

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: 1
  Paranoid ActionableScore: 3

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 REGULAR

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).