[CVE-2026-64537][LOW] bridge: cfm: reject invalid CCM interval at configuration time [ Upstream

[CVE-2026-64537][LOW] bridge: cfm: reject invalid CCM interval at configuration time [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64537
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: bridge: cfm: reject invalid CCM interval at configuration time [ Upstream
Commit: 2870056a78961e0fecd652362ee9d3fcfd24a8a6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2870056a78961e0fecd652362ee9d3fcfd24a8a6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64537
Analysis date: Mon, 27 Jul 2026 22:18:58 -0400
ActionableScore: 2
ActionableScore lower bound: 1
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
An invalid bridge CFM CCM interval can make ccm_tx_work_expired requeue immediately and allocate skbs in a tight loop until OOM, causing local denial of service for users with bridge CFM configuration privileges.

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

The original Linux kernel CVE announcement for CVE-2026-64537 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64537

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.

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

CVE-2026-64537	LOW	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-20;**CWE-400;*CWE-835;CWE-770;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.5';DESCR 'An invalid bridge CFM CCM interval can make ccm_tx_work_expired requeue itself with zero delay through queue_delayed_work. This creates a tight local worker loop that keeps allocating skbs and can drive the host into OOM, causing denial of service. For the CVSS the PR:L is used for the paranoid score because some deployments delegate CAP_NET_ADMIN to containers, network namespaces, or service accounts that are not equivalent to full host root in practical risk terms. The issue is not directly network reachable and is triggered through local netlink bridge CFM configuration rather than by receiving packets. Impact is availability loss only because the patch context shows resource exhaustion and does not show UAF, OOB access, information disclosure, or an arbitrary write primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)  SKIP CVE-2026-64537 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB SIMPLEFIX  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=1

## 1. ActionableScore

* Conservative score: 1
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

## 2. Signal breakdown

Conservative signals:

* Reliable kernel crash / strong DoS: +1. Invalid CCM interval can cause immediate requeue of ccm_tx_work_expired(), repeatedly allocating skbs until OOM.
* Requires admin/root/CAP_NET_ADMIN in typical deployments: -2. Bridge CFM configuration is performed through local netlink control-plane operations that normally require CAP_NET_ADMIN.
* Availability impact realistic: +1. The described failure mode is system memory exhaustion, not just a warning or local command failure.
* No memory corruption bonus. The patch shows input validation and resource exhaustion only, with no UAF, OOB write, double free, refcount misuse, type confusion, or arbitrary write.
* No remote/network-triggerable bonus. The bug is configured through netlink and is not triggered by ordinary received packets.

Paranoid adjustment:

* Paranoid score raises the result to 2 because CAP_NET_ADMIN may be delegated to containers, network namespaces, or service accounts that are not equivalent to full host root in practical risk terms.
* No higher score is justified because the primitive remains resource exhaustion only.

## 3. Reachability analysis

The realistic trigger is a local process able to configure bridge CFM via netlink. In normal host deployments this requires CAP_NET_ADMIN, so a fully unprivileged local user cannot trigger it directly. In containerized or delegated network-administration setups, a reduced-privilege root or service account with CAP_NET_ADMIN may be able to reach the path, which is why the paranoid interpretation is slightly higher. The affected path is not a packet receive path and is not remotely reachable by sending network traffic. Exploitation conditions are straightforward once configuration access exists, because the invalid interval causes a tight delayed-work loop and skb allocation pressure.

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like local resource-exhaustion issue, not an Important-class vulnerability. The practical impact is denial of service through OOM. There is no evidence of memory corruption, confidentiality impact, integrity impact, privilege escalation, or cross-object overwrite. Theoretical concern is limited to host-wide availability if a delegated network administrator can configure the feature.

## 5. One-sentence report phrase

An invalid bridge CFM CCM interval can make ccm_tx_work_expired requeue immediately and allocate skbs in a tight loop until OOM, causing local denial of service for users with bridge CFM configuration privileges.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

This can be auto-closed or handled as ordinary Moderate in most triage flows because it is a validation-only resource exhaustion bug requiring CAP_NET_ADMIN in typical deployments, with no supported memory corruption or privilege escalation primitive.

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

Patch: bridge: cfm: reject invalid CCM interval at configuration time [ Upstream
Commit: 2870056a78961e0fecd652362ee9d3fcfd24a8a6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2870056a78961e0fecd652362ee9d3fcfd24a8a6

Changed files:
  net/bridge/br_cfm.c
  net/bridge/br_cfm_netlink.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=2870056a78961e0fecd652362ee9d3fcfd24a8a6

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

The original Linux kernel CVE announcement for CVE-2026-64537 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64537

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:H
  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: 1
  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).