[CVE-2026-74480][MODERATE 7.0] net: bridge: stop fast-leave after deleting a port group

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

CVE: CVE-2026-74480
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: bridge: stop fast-leave after deleting a port group
Commit: 159ad90cb929c033308bb39a2c5f8fbf393b77aa
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=159ad90cb929c033308bb39a2c5f8fbf393b77aa
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74480
Analysis date: Sat, 15 Aug 2026 13:21:47 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A bridge multicast fast-leave path can continue iterating after deleting a port group, leaving stale multicast list state and creating a network-adjacent UAF candidate that can cause kernel DoS and requires manual review for possible stronger impact.

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

The original Linux kernel CVE announcement for CVE-2026-74480 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74480

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-2026-74480	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A stale pointer and use after free candidate exists in the bridge multicast fast leave path. br_multicast_leave_group deletes a matching port group with br_multicast_del_pg but the loop can continue through the removed entry when multiple port groups exist for the same port and multicast group after multicast to unicast state changes. A network adjacent host may be able to trigger the path by sending multicast leave traffic on an affected bridge port if bridge multicast snooping and fast leave are enabled and the required port group state exists. For the CVSS the PR:N is used because the trigger can be packet based once the bridge is configured, although the setup conditions make AC:H appropriate. Impact is at least denial of service via kernel crash. In the paranoid view the stale pointer can be treated as a UAF candidate with possible confidentiality or 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 7)	YES	SIMPLEFIX LINUS  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Remote reachable / network-triggerable: +2**
  Trigger is plausibly packet based from an adjacent L2 host via bridge multicast leave behavior, not a local syscall-only path.

* **Memory corruption, weak/indirect corruption candidate: +1**
  The patch describes iteration through a deleted port-group entry and a stale pointer left in `mp->ports`. This is stronger than pure logic DoS, but no attacker-controlled reclaim or overwrite primitive is shown.

* **Real lifetime corruption: +1**
  The deleted port group can remain reachable through multicast list state, making this a stale-pointer / UAF-lifetime issue.

* **Reliable kernel crash / strong DoS: +1**
  Later traversal of a deleted bridge multicast port group can plausibly crash the kernel or corrupt bridge multicast state.

* **Common networking / packet-processing path: +1**
  Linux bridge multicast handling is a networking data-plane path, even though the exact trigger requires bridge multicast features.

* **Hard or special triggering conditions: -1**
  Requires a specific bridge multicast state: fast leave, previous multicast-to-unicast behavior, multiple port groups for the same port/group, and then a matching leave.

Paranoid additional interpretation:

* **Memory corruption, strong corruption primitive: +2 instead of +1**
  In the paranoid view, leaving `mp->ports` pointing at a deleted port group is treated as a stronger UAF candidate because later list traversal may access freed kernel memory.

* **Weak LPE concern: +1**
  No practical LPE primitive is demonstrated, but stale list pointers in a network subsystem can sometimes become exploitable beyond DoS after allocator reuse. This is a manual-review concern, not a proven escalation path.

## 3. Reachability analysis

A network-adjacent attacker may be able to trigger the vulnerable path by sending multicast leave traffic on an affected bridge port, assuming bridge multicast snooping and fast-leave behavior are enabled and the necessary multicast-to-unicast state was created earlier. This is not Internet-routable in the normal sense, so the exposure is best viewed as adjacent-network rather than broad AV:N remote.

No local privileges are required once the bridge is configured and reachable by the attacker. However, creating the vulnerable bridge configuration itself usually requires administrator or orchestration control. Containers and namespaces matter if an untrusted workload can attach to or influence a host bridge, but ordinary isolated containers without bridge control may not be able to shape the required state.

Call-site confidence: **high**. The patch and commit message directly identify the affected function, deletion call, stale iteration behavior, and resulting stale `mp->ports` condition.

## 4. Severity interpretation

This is more serious than an ordinary Moderate logic bug because the commit explicitly describes continuing through a deleted entry and leaving a core bridge multicast list pointing at a deleted object. The realistic impact is likely kernel crash or bridge multicast state corruption under special network conditions.

Theoretical memory-corruption potential exists because this is a stale pointer / UAF candidate. Realistic privilege escalation is not demonstrated, and there is no shown reclaim, controlled overwrite, callback control, or type confusion primitive. Therefore the conservative score is **Actionable Moderate**, while the paranoid score reaches **Strong Important candidate** for manual triage.

## 5. One-sentence report phrase

A bridge multicast fast-leave path can continue iterating after deleting a port group, leaving stale multicast list state and creating a network-adjacent UAF candidate that can cause kernel DoS and requires manual review for possible stronger impact.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: the patch explicitly describes a deleted object remaining reachable through bridge multicast list state. This is a stale-pointer / UAF candidate in a packet-triggerable networking path, so it should not be auto-closed even though exploitation beyond DoS is not proven.

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

Patch: net: bridge: stop fast-leave after deleting a port group
Commit: 159ad90cb929c033308bb39a2c5f8fbf393b77aa
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=159ad90cb929c033308bb39a2c5f8fbf393b77aa

Commit description:

br_multicast_leave_group() iterates mp->ports with pp = &p->next in
its fast-leave path. After br_multicast_del_pg() removes p,
continuing the loop advances pp through the deleted entry.

If multicast-to-unicast was enabled, the bridge can hold multiple port
groups for the same port and group with different source MAC
addresses. Once multicast-to-unicast is disabled,
br_port_group_equal() matches those entries by port only. A fast leave
can then delete one entry and continue from its stale next pointer,
leaving mp->ports pointing at a deleted port group.

Fast leave only needs to remove one matching port group. Break after
br_multicast_del_pg() so the loop stops before dereferencing the
removed entry.

Fixes: 6db6f0e ("bridge: multicast to unicast")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/1cf0898872ef7c72d5f4c0304414a192c6dac591.1784707712.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/br_multicast.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=159ad90cb929c033308bb39a2c5f8fbf393b77aa

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

The original Linux kernel CVE announcement for CVE-2026-74480 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74480

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7.5

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: 5
  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).