* [CVE-2026-90096][MODERATE REGULAR] fuse: invalidate the correct range after O_APPEND direct write [ Upstream
@ 2026-09-18 2:32 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 2:32 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90096
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: fuse: invalidate the correct range after O_APPEND direct write [ Upstream
Commit: 833963069adf86dcbdffd4e7d7b3171f95070b77
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=833963069adf86dcbdffd4e7d7b3171f95070b77
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90096
Analysis date: Thu, 17 Sep 2026 22:32:25 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90096 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90096
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-90096 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90096 CHECK WITH IMPACT FROM ORIG NN MODERATE 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:L/A:L';CWE-682;CWE-362;*CWE-664;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.5';DESCR 'FUSE O_APPEND direct writes can invalidate the wrong page cache range because fuse_direct_write_iter records the file position before generic_write_checks moves ki_pos to EOF. When the original file size is not page aligned, a concurrent buffered reader can fault the tail page back into the cache while the direct write is in progress. The incorrect post write invalidation can leave that page cached and cause later reads of the appended range to return stale data instead of the newly written content. For the CVSS the PR:L is selected because a local process needs access to the affected FUSE file and must be able to issue the relevant direct append write or participate in the concurrent access pattern. AC:H reflects the required timing window between the direct writer and buffered reader. The issue is not directly network reachable unless a separate remote service exposes file operations on the affected mount. Impact is primarily data inconsistency. A limited confidentiality or availability impact is retained in the paranoid score because stale bytes or application failures may be possible, but the source does not show kernel memory corruption or a credible privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-90096 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE WRITE OOB DISK HARDWARE DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Conservative positive signals:
* +1 Local unprivileged trigger. A process with ordinary access to the affected file can issue an O_APPEND direct write and participate in concurrent file access without host-admin privilege.
* +1 Important filesystem/storage path. The defect affects FUSE direct I/O and page-cache coherency, causing readers to receive content inconsistent with the completed write.
Paranoid-only signal:
* +1 Confidentiality plausible. The commit explicitly establishes read-back of stale bytes from the appended range. This supports limited disclosure review, although no cross-file or cross-principal disclosure is demonstrated.
Negative signals:
* No negative points. The required concurrency is repeatable through file operations, and the evidence does not establish a narrow or externally uncontrollable race.
* No host-admin penalty because the vulnerable operations are ordinary permitted file I/O after a FUSE mount exists.
* No rare-subsystem penalty. FUSE is established functionality, and no unusual hardware is required.
Material non-awarded signals:
* No UAF, OOB access, double release, arbitrary write, controlled reclaim, or kernel-memory corruption is established.
* Integrity was not awarded because backing storage is written at the correct offset and no unauthorized modification of another principal data is shown.
* Availability was not awarded because no kernel crash, filesystem shutdown, or persistent service failure is established.
* Dirty-Pipe-like ownership or COW violation was not awarded because no write authorized for one object is shown to modify independently owned storage.
* Remote reachability is not established.
* There is no concrete repeated release or refcount operation, so its dedicated signal and FLOOR 5 do not apply.
3. Reachability analysis
An ordinary local process can reach the vulnerable path if it has write permission on a file in an affected FUSE mount and can issue an O_APPEND direct write. A concurrent buffered read of the non-page-aligned tail is also required. The writer and reader may be separate processes, but meaningful adversarial impact requires the attacker to influence this concurrency while another process consumes the stale result.
No capability check or namespace-scoped privilege is part of the relevant file operation, so HostAdminRequired is NO and NamespaceScopedPrivilege is NO. An existing FUSE mount is required, but it may be provided by the system or created through ordinary user-accessible FUSE facilities. No special hardware, boot option, or device prerequisite is indicated.
The concurrency can be attempted repeatedly with normal reads and writes. The supplied evidence does not establish sufficiently narrow or uncontrollable timing to justify a race penalty. The issue is not directly network-triggerable, although a separate service could expose the affected file operations remotely.
4. Primitive and severity interpretation
The exact primitive is incorrect page-cache invalidation caused by retaining iocb->ki_pos before generic_write_checks moves it to EOF for O_APPEND. The direct write reaches the correct backing-store offset, but the post-write invalidation covers the wrong range. A tail page faulted back during the write can therefore remain cached and return stale bytes after the file size is extended.
This is active cache-coherency state, not stale post-lifetime storage. There is no object free, stale pointer dereference, refcount mutation, or cleanup/destructor lifetime defect. PrimitiveClass is CLASS_C because the demonstrated behavior is invalid data state and stale read-back without an independently useful kernel-corruption primitive.
The demonstrated consequence is stale data returned to a reader. Limited disclosure or application-level disruption is a defensible paranoid concern, but cross-principal disclosure, persistent corruption, privilege escalation, and kernel compromise remain speculative.
ImportantCandidate is NO because no Important-preserving ownership, authorization, lifetime, or memory-corruption primitive is established. AutoDowngradeAllowed is YES because the issue can be handled as a data-consistency defect after manual confirmation of whether a particular FUSE implementation can expose sensitive stale tail data. The likely operational severity is Low to Moderate.
5. One-sentence report phrase
A stale append position causes FUSE direct writes to invalidate the wrong page-cache range, allowing concurrent buffered readers to observe old data from the newly appended region.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm whether the affected FUSE server can populate bytes beyond the previous EOF with sensitive historical content and whether another security principal can read those stale bytes.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: fuse: invalidate the correct range after O_APPEND direct write [ Upstream
Commit: 833963069adf86dcbdffd4e7d7b3171f95070b77
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=833963069adf86dcbdffd4e7d7b3171f95070b77
Changed files:
fs/fuse/file.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=833963069adf86dcbdffd4e7d7b3171f95070b77
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90096 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90096
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:L/A:N
The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:L
The Best / paranoid CVSS score: 4.5
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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