[CVE-2026-74544][MODERATE REGULAR] net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds [ Upstream

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

CVE: CVE-2026-74544
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds [ Upstream
Commit: 313cb9ffc4101a885d791facf4b5ea3d5e06144d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=313cb9ffc4101a885d791facf4b5ea3d5e06144d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74544
Analysis date: Sat, 15 Aug 2026 10:54:19 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Ordinary Moderate / Low-like, paranoid borderline only
Manual review required: NO

Summary:
A local user who can create cls_u32 filters, including via user/network namespaces, can set an invalid offshift value that later triggers shift-out-of-bounds behavior during packet classification, causing an UBSAN report or possible kernel DoS on panic-configured systems.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74544 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74544

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: 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
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CVE-2026-74544	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-1335;*CWE-20;CWE-682;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.5';DESCR 'A shift-out-of-bounds bug in cls_u32 can occur because the user controlled tc_u32 offshift value was accepted without a bounds check and later used by u32_classify() when TC_U32_VAROFFSET is set. A local user who can create tc u32 filters can choose an invalid offshift and then send traffic through the classifier to trigger undefined shift behavior and a UBSAN report. For the CVSS the PR:L is used because reliable triggering requires local code or a process with CAP_NET_ADMIN in a network namespace, which may be reachable by unprivileged users when user/network namespaces are enabled. The issue is not directly network reachable because remote traffic alone cannot install the malicious classifier. Impact is primarily denial of service via warning or kernel crash on systems configured to panic on UBSAN or warnings. No concrete UAF, OOB write, information leak, or privilege escalation primitive is evident from the patch context.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like, paranoid borderline only (with actual score 3)	YES	REMOTE WRITE KASAN OOB SIMPLEFIX NETWORK PACKET  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Ordinary Moderate / Low-like, paranoid borderline only**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. The commit explicitly says this can be triggered by an unprivileged user via user/network namespaces.
* Availability impact realistic: +1. The demonstrated effect is UBSAN shift-out-of-bounds and possible warning/crash behavior, mainly DoS-like.

Paranoid additional signal:

* Reliable kernel crash / strong DoS: +1. Only in the paranoid interpretation, systems configured with UBSAN trap, panic_on_warn, or similar hardening/debug policies may convert the warning into a kernel panic.

Signals not applied:

* Remote reachable: +0. Remote packets alone cannot install the malicious cls_u32 filter.
* Generic memory corruption: +0. The patch shows shift-out-of-bounds undefined behavior, but no UAF, OOB write, double free, stale pointer, type confusion, or arbitrary write.
* Privilege escalation plausible: +0. No supported LPE primitive is visible.
* Confidentiality / Integrity: +0. No leak or write/corruption primitive is demonstrated.
* Requires admin/root penalty: not applied conservatively because user/network namespaces may allow an unprivileged local user to create the needed network context.

## 3. Reachability analysis

The bug is triggered through tc cls_u32 filter creation with an invalid offshift value, followed by packet classification through u32_classify() with TC_U32_VAROFFSET set. In normal host networking this usually requires CAP_NET_ADMIN, but with unprivileged user and network namespaces enabled, a local unprivileged user may gain CAP_NET_ADMIN inside a new netns and reach the vulnerable path.

The issue is not directly network-triggerable because the attacker must first configure the classifier locally. The packet-processing step is only the second stage that exercises the invalid shift. The path is not default-active unless such a tc filter is installed.

Call-site confidence: high, because the commit directly identifies u32_change() as the input path and u32_classify() as the later trigger path.

## 4. Severity interpretation

This behaves like an ordinary Moderate / Low-like local DoS issue, not an Important-class kernel vulnerability. The theoretical concern is C undefined behavior from an excessive shift exponent, but the concrete evidence is an UBSAN report and possible crash under specific kernel policies. There is no demonstrated memory corruption, no attacker-controlled overwrite, no information disclosure, and no privilege-escalation path.

The paranoid score is only borderline because local namespace reachability plus panic-on-warning style configurations can make the DoS more actionable.

## 5. One-sentence report phrase

A local user who can create cls_u32 filters, including via user/network namespaces, can set an invalid offshift value that later triggers shift-out-of-bounds behavior during packet classification, causing an UBSAN report or possible kernel DoS on panic-configured systems.

## 6. Manual review recommendation

NO MANUAL CHECK NEEDED

Reason: this is a validation-only fix for a local shift-out-of-bounds condition with no supported memory corruption or privilege-escalation primitive.

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

Patch: net/sched: cls_u32: validate offshift to prevent shift-out-of-bounds [ Upstream
Commit: 313cb9ffc4101a885d791facf4b5ea3d5e06144d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=313cb9ffc4101a885d791facf4b5ea3d5e06144d

Changed files:
  net/sched/cls_u32.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=313cb9ffc4101a885d791facf4b5ea3d5e06144d

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

The original Linux kernel CVE announcement for CVE-2026-74544 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74544

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:L/UI:N/S:U/C:N/I:N/A:L
  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: 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).