[CVE-2026-74689][MODERATE REGULAR] net/atm: fix slab-out-of-bounds read in vcc_setsockopt() [ Upstream

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

CVE: CVE-2026-74689
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/atm: fix slab-out-of-bounds read in vcc_setsockopt() [ Upstream
Commit: b3bcd5d65ac03c15787b2a5b36c718e26a689336
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3bcd5d65ac03c15787b2a5b36c718e26a689336
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74689
Analysis date: Sat, 22 Aug 2026 13:55:26 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
A local ATM socket setsockopt path can bypass or lose effective optlen validation and trigger a slab out of bounds read in `vcc_setsockopt()`, causing at least local DoS risk and limited confidentiality concern without evidence of privilege escalation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74689 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74689

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-2026-74689	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H';CWE-125;CWE-126;*CWE-20;CWE-130;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.3';DESCR 'vcc_setsockopt can copy more data than allowed by optlen because the old length check can be bypassed when the socket option level does not match the encoded option level. A cgroup BPF setsockopt filter can also shrink optlen after entry, and copy_from_sockptr on a kernel sockptr then uses memcpy and may read beyond the allocated option buffer. For the CVSS the PR:L is used because a local user or local process is needed to issue setsockopt on an ATM socket, while no full administrator privileges are normally required for the triggering process in the paranoid model. The issue is not directly network reachable and is triggered through a local socket control path rather than packet traffic. Impact is at least local denial of service via kernel crash or KASAN detected slab out of bounds read. Worst case confidentiality impact is limited because this is an out of bounds read primitive, but there is no demonstrated arbitrary write or direct privilege escalation path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 4)  SKIP CVE-2026-74689 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	READ KASAN OOB BPF SIMPLEFIX IMPROVEONLY LINUS SYZBOT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. A local process can reach `vcc_setsockopt()` through the ATM socket setsockopt path when ATM support is enabled and accessible.
* Weak memory safety primitive: +1. The concrete primitive is a slab out of bounds read caused by copying more bytes than `optlen` permits. This is not an OOB write, UAF, double free, or type confusion.
* Availability impact realistic: +1. The issue is syzbot/KASAN observable and can cause a kernel diagnostic crash in debug or panic on fault configurations, but production crash reliability is less certain.
* Broad/default exposure: +0. ATM is not a broadly deployed default subsystem.
* Rare subsystem penalty: +0. I do not subtract by default because if ATM is enabled, the path is reachable through an ordinary local socket control interface.

Paranoid additional signal:

* Confidentiality impact plausible: +1. The bug is an OOB read in kernel memory handling, so limited confidentiality concern is defensible, but there is no clear user visible leak path.

Not awarded:

* Remote reachable: +0. This is not packet-triggered.
* Strong LPE plausibility: +0. No reclaim, overwrite, callback control, type confusion, or refcount takeover is shown.
* Strong memory corruption: +0. The patch supports read-only OOB access, not write corruption.
* High-control API bonus: +0. ATM setsockopt is not comparable to io_uring, AF_ALG, or BPF data-plane shaping here.
* Reliable strong DoS: +0 separately. Avoiding double count because availability was already counted and the crash evidence is mainly KASAN.

## 3. Reachability analysis

The trigger is local through `setsockopt()` on an ATM socket. The most direct KASAN slab OOB read case involves `copy_from_sockptr()` operating on a kernel sockptr, which can happen when a cgroup BPF setsockopt filter shrinks `optlen` after entry. Attaching such BPF is privileged in typical deployments, but the process that later triggers the affected setsockopt path may be less privileged if it runs inside the configured cgroup.

The mismatched level case shows the original length check was logically bypassable, but without the kernel sockptr condition it is less clear that this becomes a reliable kernel memory disclosure rather than a failed or unsafe copy path. ATM itself is niche and often not enabled or used by default, which limits exposure. Namespace behavior does not obviously turn this into a container escape or capability bypass.

Call-site confidence: high. The vulnerable call site and fixed copy operations are visible in the supplied patch.

## 4. Severity interpretation

This behaves like a borderline actionable Moderate issue rather than an Important-class vulnerability. The theoretical concern is kernel OOB read, with limited confidentiality uncertainty and possible local DoS. The realistic evidence supports a KASAN-detected slab out of bounds read and crash risk, but not privilege escalation, arbitrary write, UAF, type confusion, or a demonstrated information leak.

The paranoid score reaches 4 because kernel OOB reads in socket option handling deserve manual review, especially with cgroup BPF changing `optlen`. It should not be auto-upgraded to Important without a concrete disclosure path or stronger corruption primitive.

## 5. One-sentence report phrase

A local ATM socket setsockopt path can bypass or lose effective optlen validation and trigger a slab out of bounds read in `vcc_setsockopt()`, causing at least local DoS risk and limited confidentiality concern without evidence of privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a kernel OOB read in socket option handling and the cgroup BPF optlen-shrink path may affect practical reachability. However, current evidence does not support treating it as strong memory corruption or LPE.

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UPSTREAM PATCH SUMMARY
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Patch: net/atm: fix slab-out-of-bounds read in vcc_setsockopt() [ Upstream
Commit: b3bcd5d65ac03c15787b2a5b36c718e26a689336
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3bcd5d65ac03c15787b2a5b36c718e26a689336

Changed files:
  net/atm/common.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=b3bcd5d65ac03c15787b2a5b36c718e26a689336

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

The original Linux kernel CVE announcement for CVE-2026-74689 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74689

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:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:H
  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: 3
  Paranoid ActionableScore: 4

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).