[CVE-2026-93783][MODERATE REGULAR] Bluetooth: RFCOMM: validate skb length in rfcomm_recv_frame [ Upstream

[CVE-2026-93783][MODERATE REGULAR] Bluetooth: RFCOMM: validate skb length in rfcomm_recv_frame [ Upstream

[AL-KERNEL]

CVE: CVE-2026-93783
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Bluetooth: RFCOMM: validate skb length in rfcomm_recv_frame [ Upstream
Commit: 67dc3b40fae71b6b11c71e7ff69bac0758818c05
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=67dc3b40fae71b6b11c71e7ff69bac0758818c05
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93783
Analysis date: Thu, 24 Sep 2026 21:10:07 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-93783 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93783

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-2026-93783	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93783 CHECK WITH IMPACT FROM ORIG NN LOW	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-191;*CWE-457;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'A Bluetooth RFCOMM frame parser can read beyond the skb buffer because rfcomm_recv_frame dereferences the RFCOMM header before validating the minimum skb length. A truncated frame can cause an out of bounds read of uninitialized kernel memory. A zero length frame can also underflow skb length and make skb_tail_pointer operate far beyond the original buffer. An attacker within Bluetooth range may trigger the issue by sending malformed RFCOMM traffic to a device that accepts the relevant connection. For the CVSS the PR:N metric is used because the trigger does not require local code execution or privileges on the target system. AV:A is used because Bluetooth proximity or equivalent access to the Bluetooth transport is required. The confirmed impact is at least denial of service through a possible kernel crash. For the paranoid score C:L is used because an out of bounds read of uninitialized memory is confirmed by KMSAN, but no direct information disclosure path or write primitive is demonstrated. Bluetooth pairing requirements and RFCOMM service policy may reduce practical exposure in many deployments.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	REMOTE READ KASAN OOB NETWORK SKB MEMORY  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::

No floor or cap was applied. The paranoid interpretation does not add a point because the patch establishes invalid reads and uninitialized-value use, but does not establish a disclosure path, write primitive, controlled corruption, or lifetime violation.

2. Signal breakdown

* +2 Directly network-triggerable adjacent input. RFCOMM frames are attacker-controlled Bluetooth traffic and can reach the vulnerable parser from a Bluetooth peer within radio range when the target accepts the RFCOMM connection.
* +1 Plausible kernel DoS. A zero-length skb causes length underflow to UINT_MAX and makes subsequent skb tail access operate far beyond the original buffer. This can realistically cause a kernel fault or unstable protocol processing.
* +0 Weak corruption concern. The patch establishes an out-of-bounds read and uninitialized memory consumption, but this is read-only behavior without a demonstrated attacker-controlled write or useful post-read primitive.

Important non-awarded signals:

* Controlled reclaim is not demonstrated.
* Arbitrary or directed kernel write is not demonstrated.
* A user-visible information disclosure path is not demonstrated. KMSAN confirms uninitialized-value use inside the kernel, not transmission of kernel bytes to the attacker.
* Integrity impact is not demonstrated.
* Privilege escalation is not demonstrated.
* No UAF, stale post-free object use, repeated release, refcount mutation, or object lifetime violation is established.
* No additional Availability point is awarded because the possible kernel crash is already counted as the DoS signal.
* Broad default exposure is not awarded. RFCOMM support may be present, but practical exposure depends on Bluetooth being enabled and an RFCOMM connection being accepted.

3. Reachability analysis

The attacker model is a Bluetooth-adjacent peer, not an Internet remote attacker. The malformed frame is processed after Bluetooth and RFCOMM transport handling reaches `rfcomm_recv_frame()`.

For the CVSS-style privilege model, no local account or host administrative privilege is required on the target. Therefore HostAdminRequired is NO. The attacker must nevertheless be within Bluetooth range or otherwise have equivalent access to the Bluetooth transport, and the target must accept the relevant RFCOMM connection.

Bluetooth pairing, link authentication, service policy, and RFCOMM channel configuration can materially reduce practical exposure. Those controls affect deployment reachability, but they do not change the fact that the parser accepts attacker-supplied protocol frames once a connection is established. No namespace-scoped privilege operation is involved.

4. Primitive and severity interpretation

The exact primitive is an unchecked short skb read. `rfcomm_recv_frame()` interprets `skb->data` as `struct rfcomm_hdr` and reads header fields before confirming that the skb contains the minimum RFCOMM header and FCS length. A truncated frame can therefore read outside the skb buffer.

A zero-length frame is stronger than an ordinary one-byte header overread because decrementing `skb->len` underflows the unsigned length value to UINT_MAX. Later skb tail pointer handling can then access memory far beyond the real packet buffer. KMSAN confirms that this path can consume uninitialized memory.

This is not an active structured-object UAF, not a double free, and not a repeated lifetime/refcount mutation. The supplied evidence establishes invalid read and crash-capable malformed-input handling, but not controlled kernel memory corruption, attacker-controlled replacement data, arbitrary read or write, disclosure, or a security-boundary bypass.

PrimitiveClass is CLASS_C because the demonstrated consequence is primarily invalid-state processing and potential kernel DoS. ImportantCandidate is NO because there is no meaningful exploitation primitive beyond the read-only out-of-bounds access and crash-capable behavior. AutoDowngradeAllowed is YES because no unresolved Class A or meaningful Class B lifetime/corruption leverage is established.

Operationally, this should be handled as a Bluetooth-adjacent DoS and kernel-memory-safety defect. It merits review where Bluetooth RFCOMM is enabled or exposed to untrusted nearby devices, but the supplied evidence does not support Important-class escalation.

5. One-sentence report phrase

A Bluetooth-adjacent attacker can send a truncated or zero-length RFCOMM frame that causes an out-of-bounds kernel read and possible denial of service, but no disclosure, write primitive, or privilege-escalation path is demonstrated.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review Bluetooth RFCOMM deployment exposure and verify whether target systems accept RFCOMM connections from untrusted or unpaired nearby devices. The bug is directly reachable by malformed adjacent network traffic and includes a length underflow, but the supplied patch does not establish a stronger exploitation primitive.

7. Call-site confidence

Call-site confidence: medium

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

Patch: Bluetooth: RFCOMM: validate skb length in rfcomm_recv_frame [ Upstream
Commit: 67dc3b40fae71b6b11c71e7ff69bac0758818c05
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=67dc3b40fae71b6b11c71e7ff69bac0758818c05

Changed files:
  net/bluetooth/rfcomm/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=67dc3b40fae71b6b11c71e7ff69bac0758818c05

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

The original Linux kernel CVE announcement for CVE-2026-93783 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93783

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:H
  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: 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).