[CVE-2026-90203][MODERATE 7.0] Squashfs: check block offset is not negative [ Upstream

[CVE-2026-90203][MODERATE 7.0] Squashfs: check block offset is not negative [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90203
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: Squashfs: check block offset is not negative [ Upstream
Commit: b169185d5c672b989985c6c2e38cafab2548ba88
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b169185d5c672b989985c6c2e38cafab2548ba88
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90203
Analysis date: Fri, 18 Sep 2026 03:54:59 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90203 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90203

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 7.0
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-2026-90203	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90203 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:H';CWE-125;*CWE-20;CWE-129;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:N/A:L';BEST CVSS score: '7.1';DESCR 'A Squashfs image can encode a negative block offset that reaches squashfs_copy_data during file reads. The missing negative offset check can make the kernel read before the decompressed fragment buffer and copy out-of-bounds data into the file read path, which may expose adjacent kernel memory and may also crash the system. For the CVSS the PR:N is used in the paranoid user-assisted scenario where an attacker supplied filesystem image is mounted by an administrator or automount flow, and the attacker only needs access to read the crafted file after mount. Direct mounting of the crafted image normally requires CAP_SYS_ADMIN, so the base score uses PR:H. The issue is local and is not network reachable by itself. Impact is primarily confidentiality loss from an out-of-bounds read, with possible denial of service if the invalid access crashes the kernel.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	YES	OOB DISK SIMPLEFIX LINUS KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

No floor or cap changed the score.

2. Signal breakdown

Conservative signals:

* +1 Read-only OOB primitive. A negative Squashfs on-disk offset can reach `squashfs_copy_data()` and make it access memory before the decompressed fragment buffer.
* +1 Confidentiality plausible. The affected path is a file read path, so out-of-bounds bytes may be copied back as file data to the reader.
* +1 Reliable kernel crash / strong DoS plausible. The invalid access can fault or otherwise crash the kernel depending on the surrounding allocation and address.
* +1 Important filesystem parser/storage path. Squashfs parses attacker-controlled filesystem metadata after mount, and filesystem parser bugs with kernel memory exposure require manual triage.

Paranoid additional signal:

* +1 User-assisted or pre-mounted local trigger. The commit explicitly states that once the crafted filesystem is mounted, an unprivileged user can trigger the OOB access by reading the crafted file.

Non-awarded signals:

* RealUAF not awarded. No stale object or post-lifetime reference use is established.
* Concrete repeated release/refcount not awarded. There is no second put, release, free, or refcount mutation.
* Arbitrary write not demonstrated.
* Controlled reclaim not demonstrated.
* Integrity not awarded. The primitive is an out-of-bounds read, not a write.
* Remote reachability not established. The issue is local filesystem access, not network packet processing.
* Extra Availability not awarded beyond the same possible crash.
* NamespaceScopedPrivilege not confirmed. The input does not establish that the full mount and trigger chain works in attacker-created namespaces.

3. Reachability analysis

The direct setup requires a crafted Squashfs filesystem and a mount operation. The commit states that mounting such a filesystem requires `CAP_SYS_ADMIN`, so an ordinary unprivileged user cannot normally create the vulnerable mounted state alone.

However, the commit also states that once the crafted filesystem is mounted, an unprivileged user can trigger the out-of-bounds access by reading the crafted file. This supports a user-assisted or pre-mounted scenario where an attacker supplies the image and a privileged mount path, administrator action, image-processing service, or automount-like workflow creates the mounted state.

HostAdminRequired is therefore set to UNKNOWN rather than YES or NO for the overall triage model. Host-admin privilege is established for direct mounting, but below-host-admin triggering after mount is also explicitly established.

No direct network reachability is shown. No special hardware requirement is shown. No race or timing requirement is involved.

4. Primitive and severity interpretation

The exact primitive is an attacker-influenced negative offset read from Squashfs metadata that can make `squashfs_copy_data()` read before the intended decompressed fragment buffer.

This is not a UAF and not a write primitive. It is a filesystem parser out-of-bounds read with a plausible user-visible disclosure path through file reads. The demonstrated consequence is kernel out-of-bounds access, with likely confidentiality impact and possible DoS if the invalid access faults.

PrimitiveClass is CLASS_A because a user-visible kernel memory disclosure primitive is technically supported by the patch context. It is not classified as CLASS_B lifetime corruption because no lifetime bug exists, and not CLASS_C because the issue is not limited to crash-only behavior.

ImportantCandidate is YES because kernel memory disclosure from a crafted filesystem image should not be automatically downgraded without manual review, especially when unprivileged triggering is possible after mount.

AutoDowngradeAllowed is NO because automatic closure would risk missing a meaningful confidentiality primitive in a filesystem parser. The likely operational severity is Actionable Moderate, with Important review justified depending on deployment exposure to untrusted Squashfs images.

5. One-sentence report phrase

A crafted Squashfs image can encode a negative block offset that makes `squashfs_copy_data()` read before the decompressed fragment buffer, potentially exposing kernel memory through file reads or causing a local kernel crash after the image is mounted.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a filesystem parser out-of-bounds read with a plausible kernel memory disclosure path, and the commit explicitly states that unprivileged users can trigger it after the crafted filesystem is mounted.

7. Call-site confidence

Call-site confidence: high

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

Patch: Squashfs: check block offset is not negative [ Upstream
Commit: b169185d5c672b989985c6c2e38cafab2548ba88
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b169185d5c672b989985c6c2e38cafab2548ba88

Changed files:
  fs/squashfs/cache.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=b169185d5c672b989985c6c2e38cafab2548ba88

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

The original Linux kernel CVE announcement for CVE-2026-90203 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90203

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:H/UI:N/S:U/C:H/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:N/UI:R/S:U/C:H/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: 4
  Paranoid ActionableScore: 5

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: YES
Published priority for this report (same as in Subject): MODERATE 7.0

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).