[CVE-2026-89726][MODERATE REGULAR] lib/ucs2_string.c: fix out-of-bounds read in ucs2_strnlen()

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

CVE: CVE-2026-89726
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: lib/ucs2_string.c: fix out-of-bounds read in ucs2_strnlen()
Commit: 709eb41adaf78d59d4579a13a898125919b69bcc
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=709eb41adaf78d59d4579a13a898125919b69bcc
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89726
Analysis date: Sat, 12 Sep 2026 01:57:37 -0400
ActionableScore: 0
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89726 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89726

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-89726	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89726 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';CWE-125;CWE-193;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'An off by one out of bounds read exists in ucs2_strnlen because the old loop dereferences the next UCS2 character before checking whether maxlength has already been reached. A caller that provides a non NUL terminated UCS2 buffer can cause one additional 16 bit read beyond the specified bound. The read value is not returned or used as a control primitive. No direct information disclosure, memory write, or privilege escalation primitive is demonstrated by the patch. For the CVSS the PR:L assessment assumes a local user can influence an externally reachable kernel caller of this helper, but the exact reachable call path requires call site review. Reliable triggering also requires the extra read to reach an inaccessible memory boundary, therefore AC:H is appropriate. The practical impact is likely a local kernel fault or denial of service. The paranoid score uses A:H only for environments where the reachable path can reliably produce a kernel Oops or panic. The issue is not inherently network reachable.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 0)  SKIP CVE-2026-89726 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB SIMPLEFIX IMPROVEONLY  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	-	-	checked

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

ActionableScore=0
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 0
* Conservative score: 0
* Raw paranoid score: 0
* Paranoid score: 0
* Final recommended bucket: **Ordinary Moderate**::

The patch establishes a one element out of bounds read, but does not establish an attacker reachable caller, an information disclosure path, a memory write, a lifetime violation, or a reproducible kernel crash. No floor or cap was applied.

2. Signal breakdown

* +0 Limited out of bounds read: `ucs2_strnlen()` may read one extra `ucs2_char_t` when the supplied buffer is not NUL terminated within `maxlength`. This is a real bounds violation but is read only.
* +0 Weak corruption concern: the additional value is not returned, copied to userspace, used for control flow, or otherwise exposed by this helper.
* +0 Kernel crash or strong DoS: not awarded. A fault is possible only if the extra two byte read crosses into an inaccessible memory page, but no supplied caller establishes attacker control over such placement.
* +0 Local unprivileged trigger: not awarded. The supplied patch does not identify an externally reachable call path or privilege boundary.
* +0 Confidentiality impact: not awarded. There is no demonstrated read back or disclosure primitive.
* +0 Integrity impact: not awarded. The patch contains no write, ownership, refcount, or state corruption primitive.
* +0 Strong exploitation leverage: not awarded. Controlled reclaim, arbitrary write, active object replacement, callback influence, and privilege escalation are not established.
* +0 Remote reachability: not awarded. `ucs2_strnlen()` is a generic helper and no network reachable caller is supplied.
* +0 Availability impact: not awarded. The possible fault condition is not demonstrated through a reachable call site.
* No negative privilege penalty was applied. Host administrative privilege is not proven, but below host admin reachability is also not proven.

3. Reachability analysis

The vulnerable operation is a generic kernel library helper operating on a UCS2 buffer supplied by its caller. The patch does not show its active call sites, whether any caller receives attacker controlled data, whether the supplied buffer length is attacker influenced, or whether a local user can arrange for the final valid UCS2 element to end at a page boundary.

A malformed non NUL terminated buffer alone causes the extra read, but this does not automatically imply an externally reachable security impact. To turn the read into a fault, the byte range immediately after the bounded UCS2 buffer must be inaccessible. No network protocol, device interface, filesystem parser, EFI variable path, or other attacker controlled interface is established in the supplied material.

HostAdminRequired is therefore UNKNOWN. NamespaceScopedPrivilege is also UNKNOWN because no namespace relevant caller or capability check is present. No prior host setup, special hardware, boot option, sysctl, race, or repeatable attacker controlled lifetime transition is demonstrated.

4. Primitive and severity interpretation

The exact primitive is a one `ucs2_char_t` read beyond the caller supplied maximum length due to evaluation order in the old loop condition:

`*s++ != 0 && length < maxlength`

When `length == maxlength`, the dereference happens before the length comparison rejects the iteration. The fixed condition checks the length first.

This is not a UAF, double free, refcount issue, active structured object misuse, ownership violation, or security boundary bypass. It is a bounded read only off by one condition. The additional read does not itself disclose its value and does not modify memory.

The practical patch supported consequence is limited to a possible invalid kernel read. A kernel Oops or DoS is plausible only if a reachable caller can provide a non NUL terminated buffer whose boundary is adjacent to inaccessible memory. That caller level condition is not established here.

PrimitiveClass is CLASS_C because the established behavior is an invalid read with at most a potential crash consequence. ImportantCandidate is NO because there is no meaningful unresolved corruption, lifetime, authorization, ownership, or post free exploitation primitive. AutoDowngradeAllowed is YES because the patch establishes neither an active corruption primitive nor a security boundary bypass.

5. One-sentence report phrase

An off by one read in ucs2_strnlen can access one UCS2 character beyond maxlength for a non NUL terminated buffer, but the supplied patch does not establish a reachable disclosure, write, privilege escalation, or reliable kernel crash path.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Review active `ucs2_strnlen()` call sites to determine whether externally controllable UCS2 data can reach this helper with an exact page boundary allocation and cause a kernel fault. The current patch alone does not justify Important treatment or block automatic downgrade after that call site review.

7. Call-site confidence

Call-site confidence: low

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

Patch: lib/ucs2_string.c: fix out-of-bounds read in ucs2_strnlen()
Commit: 709eb41adaf78d59d4579a13a898125919b69bcc
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=709eb41adaf78d59d4579a13a898125919b69bcc

Commit description:

Patch series "lib/ucs2_string.c: fix out-of-bounds read in
ucs2_strnlen()", v2.

This series fixes an off-by-one out-of-bounds read in ucs2_strnlen().

The first patch is the real fix, the second patch comes as a bonus and
fixes the code indentation.

This patch (of 2):

ucs2_strnlen() checks the current character before checking whether the
caller-provided maximum length has been reached.  If the input is not
NUL-terminated within that bound, the loop can read one ucs2_char_t past
the limit.

Test the length before dereferencing to prevent an off-by-one
out-of-bounds read.

Link: https://lore.kernel.org/20260723-fix-ucs2_strnlen-v2-0-9ea94e32a358@kernel.org
Link: https://lore.kernel.org/20260723-fix-ucs2_strnlen-v2-1-9ea94e32a358@kernel.org
Fixes: 1da177e ("Linux-2.6.12-rc2")
Signed-off-by: Vincent Mailhol <mailhol@kernel.org>
Cc: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  lib/ucs2_string.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=709eb41adaf78d59d4579a13a898125919b69bcc

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

The original Linux kernel CVE announcement for CVE-2026-89726 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89726

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:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: 0

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).