[CVE-2026-53263][MODERATE REGULAR] 6lowpan: fix off-by-one in multicast context address compression [ Upstream

[CVE-2026-53263][MODERATE REGULAR] 6lowpan: fix off-by-one in multicast context address compression [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53263
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: 6lowpan: fix off-by-one in multicast context address compression [ Upstream
Commit: f24a58c72a45f4c109f3557a760cc4b60b7a6037
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f24a58c72a45f4c109f3557a760cc4b60b7a6037
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53263
Analysis date: Thu, 25 Jun 2026 10:43:00 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Borderline manual review recommended
Manual review required: YES

Summary:
An off-by-one in 6lowpan multicast context address compression leaves one byte of stack memory uninitialized and transmits it over adjacent 6lowpan traffic, causing a limited kernel information disclosure without a demonstrated crash or privilege escalation primitive.

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

The original Linux kernel CVE announcement for CVE-2026-53263 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53263

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-53263	MODERATE	CHECK	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:N/A:N';CWE-193;CWE-908;*CWE-200;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.3';DESCR 'An off by one in lowpan_iphc_mcast_ctx_addr_compress corrupts the RIID byte and leaves data 5 uninitialized before the compressed multicast address is sent with lowpan_push_hc_data. An adjacent 6lowpan peer or sniffer may observe one byte of kernel stack data in affected multicast compression traffic. For the CVSS the PR:N because the attacker does not need local account privileges on the victim, but does need reachability to the same 6lowpan link or the ability to observe that adjacent network. The issue is not Internet routable in the usual sense and practical exposure is limited to deployments using 6lowpan stateful multicast context compression. Impact is primarily confidentiality loss from kernel stack information disclosure, with no clear crash or privilege escalation primitive from the patch alone.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Borderline manual review recommended (with actual score 3)	YES	OOB LEAK LINUS  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Borderline manual review recommended**::

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  The leaked byte is transmitted over 6lowpan traffic via `lowpan_push_hc_data()`. This is not Internet-reachable in the usual sense, but it is observable by an adjacent 6lowpan peer or sniffer.

* Confidentiality impact plausible: +1
  `data[5]` is left uninitialized and then transmitted, creating a concrete kernel stack information disclosure primitive.

* Rare AND difficult-to-reach subsystem/configuration: -1
  6lowpan multicast context compression is a specialized low-power networking path and depends on affected runtime configuration and traffic.

Paranoid-only interpretation:

* Do not apply the rare-subsystem penalty: +1 relative to conservative
  If the affected device is deployed in an exposed 6lowpan mesh or sensor network, the adjacent-network disclosure may be operationally meaningful despite the niche subsystem.

Not triggered:

* Generic memory corruption: +0
  The off-by-one corrupts the protocol buffer layout, but the overwrite stays within the local 6-byte stack array and does not show an OOB write into adjacent kernel memory.

* Privilege escalation plausible: +0
  No reclaim, overwrite, type confusion, callback, refcount, or arbitrary write primitive is shown.

* Reliable kernel crash / strong DoS: +0
  The patch describes information disclosure and address compression corruption, not a reliable crash.

## 3. Reachability analysis

An adjacent 6lowpan participant or passive observer can potentially receive or observe the leaked byte when affected multicast context address compression is used. The attacker does not need a local account on the victim. Namespaces and containers are not the main factor because the relevant exposure is through the 6lowpan link. The path is not broadly default-exposed on ordinary Linux systems, but it is realistic for devices using 6lowpan in IoT, sensor, or low-power mesh deployments.

## 4. Severity interpretation

This behaves like an ordinary Moderate issue with manual-review value, not an Important-class vulnerability. The concrete primitive is a one-byte uninitialized kernel stack disclosure over adjacent network traffic. Theoretical follow-on value exists because stack leaks can assist exploitation of other bugs, but this patch does not show memory corruption outside the local buffer, privilege escalation, or reliable DoS.

## 5. One-sentence report phrase

An off-by-one in 6lowpan multicast context address compression leaves one byte of stack memory uninitialized and transmits it over adjacent 6lowpan traffic, causing a limited kernel information disclosure without a demonstrated crash or privilege escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is not high scoring by itself, but it is a network-adjacent kernel stack information leak, so it should not be blindly auto-closed without confirming exposure of 6lowpan multicast context compression in the affected product.

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

Patch: 6lowpan: fix off-by-one in multicast context address compression [ Upstream
Commit: f24a58c72a45f4c109f3557a760cc4b60b7a6037
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f24a58c72a45f4c109f3557a760cc4b60b7a6037

Changed files:
  net/6lowpan/iphc.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=f24a58c72a45f4c109f3557a760cc4b60b7a6037

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

The original Linux kernel CVE announcement for CVE-2026-53263 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53263

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:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.3

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: 2
  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).