[CVE-2026-98487][MODERATE 7.0] net: bridge: mdb: restart port group walk after deletion

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

CVE: CVE-2026-98487
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: bridge: mdb: restart port group walk after deletion
Commit: 46ec55855c972504258c371677ac039ba20e5d12
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46ec55855c972504258c371677ac039ba20e5d12
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98487
Analysis date: Mon, 19 Oct 2026 06:44:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A reentrant bridge MDB port-group deletion can leave the active mp ports list pointing into freed memory, allowing a later local RTM_GETMDB dump to trigger a kernel use-after-free and crash while further exploitation leverage remains unresolved.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98487 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98487

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-2026-98487	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98487 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-416;*CWE-672;CWE-825;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7.8';DESCR 'A use after free in the bridge MDB control path can occur because br_mdb_flush_pgs keeps a pointer to pointer cursor while br_multicast_del_pg can reenter the same MDB entry and remove additional port groups. If the cursor points into one of the deleted groups, the next iteration dereferences freed memory and mp ports may retain a dangling list pointer. A following local RTM_GETMDB request can reach the stale pointer in br_mdb_dump and cause a kernel crash. For the CVSS the PR:L reflects that triggering requires local MDB management access, commonly through CAP_NET_ADMIN in a network namespace or a delegated container context. The issue is not directly reachable through ordinary remote bridge traffic because the demonstrated trigger uses the local netlink control plane. Impact is at least denial of service through a kernel crash. The confirmed use after free and possible list corruption may also permit confidentiality or integrity impact if freed memory is reused in a controllable way, so privilege escalation cannot be excluded and requires manual review. Automated impact decision path. The original SimpleNet classifier selected MODERATE. The primary LLM estimated paranoid CVSS 7.8 and the production ActionableScore returned 5 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because FD false positive review allowed a downgrade. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 3.6 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	READ DANGER UAF LINUS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap changed either score.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. Reentrant deletion can free a port group containing the pointer-to-pointer cursor, after which the cursor is dereferenced.
* +1 Reliable kernel crash. KASAN confirms a slab-use-after-free read in `br_mdb_dump` after `RTM_GETMDB`.
* +1 Attacker-influenced active-object state and lifetime manipulation. The MDB topology and bulk deletion determine which groups are recursively unlinked and whether the cursor becomes stale.

Paranoid-only signal:

* +1 Structured stale-object leverage. The defect can leave the active `mp->ports` list head pointing at freed memory, and a separate later netlink dump traverses that corrupted state. This is stronger than an immediate cleanup-only fault, although controlled reclaim is not demonstrated.

Important non-awarded signals:

* No remote-reachability point. Ordinary bridge traffic is not shown to reach the vulnerable control-plane operation.
* No local-unprivileged point. Complete below-host-admin access to MDB bulk deletion is not established by the supplied source.
* No strong-corruption or strong-LPE points. Controlled reclaim, attacker-shaped replacement contents, arbitrary write, callback control, and privilege escalation are not demonstrated.
* No confidentiality or integrity impact points. The confirmed operation is an eight-byte stale read used for pointer traversal, not a demonstrated disclosure or attacker-directed write.
* No additional availability point. The KASAN fault and potential kernel crash are the same consequence.
* No race penalty. This is deterministic reentrancy during deletion, not a narrow timing race.
* No repeated-release/refcount point. The evidence does not show two lifetime-mutating releases on the exact same dangling reference.
* B5-R does not apply. No retry-accumulated heap OOB write exists.
* B3-PENDING does not apply. No shared-backing or file-backed write exists.
* B4-PENDING does not apply. No authorization decision changes from DENY to ALLOW.

V33 active-state assessment:

* Active-state corruption: established. `mp->ports` can retain a pointer to a freed port-group object.
* Write mechanism: recursive group removal mutates the active list while the outer walk retains a cursor into a removed group. A controlled write into a chosen reclaimed object is not established.
* Downstream consumer: `br_mdb_dump` later traverses `mp->ports` for `RTM_GETMDB`.
* Trust boundary: a network-namespace or delegated `CAP_NET_ADMIN` boundary is credible but unresolved from the supplied call sites.
* Consequence: UAF read and crash are demonstrated. Disclosure, directed corruption, and privilege escalation remain unproven.

3. Reachability analysis

The trigger requires an MDB configuration in which `br_multicast_del_pg()` reenters the same entry and removes another group containing the current cursor. An MDB bulk-deletion operation then creates the stale cursor, and a later `RTM_GETMDB` consumes the dangling list state.

Bridge MDB modification normally uses the rtnetlink control plane and is expected to require `CAP_NET_ADMIN`. The supplied patch does not show the exact capability check or prove whether every required operation is available in an attacker-created user and network namespace. Therefore, `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`; no privilege penalty or unprivileged bonus is applied.

A bridge and suitable MDB port-group topology must already exist or be created by the actor. No special hardware, boot option, or timing race is required. The demonstrated path is local netlink control-plane activity, not ordinary remote multicast traffic.

4. Primitive and severity interpretation

The exact primitive is an active structured-object UAF. A pointer-to-pointer cursor survives deletion of the port group containing it, and the active MDB list may retain a dangling pointer. A later, separate dump operation traverses that state and reads freed slab memory.

This is broader than a shallow destructor-only stale dereference because corrupted list state escapes the deletion operation and is consumed by `br_mdb_dump`. It remains `CLASS_B`, not `CLASS_A`, because the supplied evidence does not establish controlled reclaim, attacker-controlled post-free contents, an arbitrary read or write, callback influence, or a completed privilege-escalation chain.

The demonstrated consequence is a KASAN-detected UAF read and likely kernel crash. The unresolved consequence is whether reclaim of the freed port-group storage can turn subsequent list traversal or mutation into a useful corruption or disclosure primitive.

`ImportantCandidate=YES` follows from the persistent active-list corruption, later privileged consumer, and unresolved namespace or delegated-management boundary. `AutoDowngradeAllowed=NO` preserves this V33 candidate for human analysis without claiming that Important severity or privilege escalation has been proven.

5. One-sentence report phrase

A reentrant bridge MDB port-group deletion can leave the active mp ports list pointing into freed memory, allowing a later local RTM_GETMDB dump to trigger a kernel use-after-free and crash while further exploitation leverage remains unresolved.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The confirmed UAF survives into active list state and is consumed by a separate kernel operation, while namespace reachability, reclaim behavior, and non-crash integrity consequences require call-chain review.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: bridge: mdb: restart port group walk after deletion
Commit: 46ec55855c972504258c371677ac039ba20e5d12
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=46ec55855c972504258c371677ac039ba20e5d12

Commit description:

br_mdb_flush_pgs() keeps a pointer-to-pointer cursor while walking
mp->ports. br_multicast_del_pg() can re-enter the same MDB entry through
br_multicast_sg_del_exclude_ports() and unlink other port groups. If the
cursor points into one of those groups, the next iteration dereferences a
stale cursor and can leave mp->ports pointing at freed memory.

A following RTM_GETMDB exposes the dangling pointer:

  BUG: KASAN: slab-use-after-free in br_mdb_dump
  Read of size 8
    br_mdb_dump
    rtnl_mdb_dump
    rtnl_dumpit
    netlink_dump

Reset the cursor to mp->ports after every deletion. The deletion removes at
least the selected group, so the restarted walk always makes progress.

Fixes: a6acb53 ("bridge: mdb: Add MDB bulk deletion support")
Cc: stable@vger.kernel.org
Signed-off-by: Fourie Zhang <fouriezhang@tencent.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260920110852.60293-1-fouriezhang@tencent.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/br_mdb.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=46ec55855c972504258c371677ac039ba20e5d12

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

The original Linux kernel CVE announcement for CVE-2026-98487 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98487

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:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.8

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: 4
  Paranoid ActionableScore: 5

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).