[CVE-2026-98457][MODERATE 7.0] Bluetooth: L2CAP: validate frame length before control and FCS access

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

CVE: CVE-2026-98457
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Bluetooth: L2CAP: validate frame length before control and FCS access
Commit: 4373ba79a267947e771c438507f7b103e30bdb21
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4373ba79a267947e771c438507f7b103e30bdb21
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98457
Analysis date: Sat, 24 Oct 2026 04:28:58 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
A nearby Bluetooth peer can trigger bounded skb OOB reads through undersized L2CAP control or FCS fields, with possible fault but no demonstrated disclosure or write primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98457 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98457

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-2026-98457	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98457 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME V39_AEGIS_ONE_STEP_UP=MODERATE->MODERATE_KPANIC (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H';CWE-125;*CWE-20;CWE-191;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7.1';DESCR 'A short Bluetooth L2CAP ERTM or streaming frame can reach l2cap_data_rcv lacking enough bytes for the two byte or four byte control field. With CRC16 enabled another short frame can make the FCS length subtraction wrap and cause a two byte load past the logical end of the skb. The demonstrated primitive is an out of bounds read rather than an out of bounds write or UAF. For the CVSS the PR:N value is selected because a nearby Bluetooth peer can supply the malformed frame lacking a local account on the target. AV:A applies because exploitation requires Bluetooth radio proximity and a reachable L2CAP channel. The likely result is frame or channel failure, while a kernel crash is plausible if the read crosses accessible backing storage. Limited confidentiality impact is retained only in the paranoid vector because protocol behavior may depend on unintended bytes and no direct disclosure primitive is proven. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 7.1 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. final reconciliation stage MODERATE to MODERATE_KPANIC because the cross model evidence reconciliation rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V39_AEGIS_ONE_STEP_UP and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 4.6 and ActionableScore 4 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	MAYBE	REMOTE OOB SIMPLEFIX NETWORK SKB LINUS TEST PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 KPANIC V39_AEGIS_ONE_STEP_UP MANUAL_REVIEW_REQUIRED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; unsupported disclosure or panic effects are not scored.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Adjacent-network reachability: a nearby Bluetooth peer with a reachable L2CAP ERTM/streaming channel can supply the short frame without local credentials.
* +1 Weak structured-read concern: missing length validation permits confirmed bounded OOB reads while unpacking a two- or four-byte control field, and separately a two-byte FCS read after unsigned length underflow.
* +0 Crash/DoS: a fault is plausible if the read crosses inaccessible backing, but no reliable host crash is demonstrated.
* +0 Confidentiality: no unintended bytes are shown returning to the peer or otherwise forming a disclosure channel.
* +0 Integrity: there is no write, UAF, lifetime mutation, or protected-state modification.
* B5-R does not apply: requirement (1), a confirmed heap/slab OOB write, is absent. There is also no retry-accumulated offset.
* B3-PENDING does not apply because no shared-backing write occurs.
* No authorization guard changes from DENY to ALLOW; B4 pending profiles do not apply.

3. Reachability analysis

The supported attacker is a nearby Bluetooth peer able to send data on a reachable L2CAP channel. No host UID, capability, namespace privilege, or host-admin operation is needed for the malformed-frame trigger. Bluetooth hardware and an established/reachable channel are prerequisites, but the evidence does not establish rare hardware, privileged setup by the attacker, or a timing race.

4. Primitive and severity reading

The exact primitive is a limited OOB read from an skb: absent control bytes are unpacked, or CRC16 processing subtracts two from an insufficient length and subsequently loads the FCS beyond the logical frame end. This is neither active-object corruption nor stale storage. The demonstrated consequence is invalid parsing and possible fault; disclosure, kernel-memory modification, persistent corruption, and code execution are unproven. It is therefore CLASS_C, not an Important-preserving corruption primitive. Auto-downgrade remains allowed, although manual review is recommended for deployment-specific crash behavior.

5. One-sentence report phrase

A nearby Bluetooth peer can trigger bounded skb OOB reads through undersized L2CAP control or FCS fields, with possible fault but no demonstrated disclosure or write primitive.

6. Manual review advice

MANUAL CHECK RECOMMENDED

Confirm whether affected Bluetooth receive paths can turn the bounded read into a reproducible host fault or externally observable data-dependent response.

7. Call-site confidence

Call-site confidence: high

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

Patch: Bluetooth: L2CAP: validate frame length before control and FCS access
Commit: 4373ba79a267947e771c438507f7b103e30bdb21
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4373ba79a267947e771c438507f7b103e30bdb21

Commit description:

l2cap_data_rcv() unpacks either a two-byte or four-byte control field
without first ensuring that it is present. A short ERTM or streaming-mode
frame can therefore cause an out-of-bounds read.

There is a second short-frame case when CRC16 is enabled. After the
control field is pulled, l2cap_check_fcs() subtracts two from skb->len
without checking it. If fewer than two bytes remain, the subtraction
wraps; skb_trim() leaves the buffer unchanged and the subsequent FCS
load reads past the logical end of the frame.

Validate that the frame contains both its control field and, when
enabled, its FCS before either field is accessed.

Fixes: 1c2acff ("Bluetooth: Add initial support for ERTM packets transfers")
Fixes: fcc203c ("Bluetooth: Add support for FCS option to L2CAP")
Cc: stable@vger.kernel.org
Signed-off-by: Aldo Ariel Panzardo <qwe.aldo@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bluetooth/l2cap_core.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=4373ba79a267947e771c438507f7b103e30bdb21

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

The original Linux kernel CVE announcement for CVE-2026-98457 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98457

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: unknown
  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: 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).