[CVE-2026-90234][MODERATE 7.0] NFS: Return a delegation the client fails to record [ Upstream

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

CVE: CVE-2026-90234
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFS: Return a delegation the client fails to record [ Upstream
Commit: 92533e49fe3c455088657b862cf3562260c0ce25
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=92533e49fe3c455088657b862cf3562260c0ce25
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90234
Analysis date: Thu, 17 Sep 2026 15:24:01 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-90234 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90234

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.

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

CVE-2026-90234	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90234 CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-703;CWE-404;CWE-362;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'An NFSv4 client can drop a server granted delegation without recording it locally and without returning it with DELEGRETURN. If the server later recalls that delegation, the client has no matching stateid and may answer CB_RECALL with NFS4ERR_BADHANDLE, which can leave revoked delegation state visible in later SEQUENCE replies. The client state manager can then loop over TEST_STATEID recovery and fail to clear the condition, causing denial of service or persistent degradation for affected NFS mounts. For the CVSS the PR:N is used because the attacker does not need privileges on the victim when acting as a malicious or compromised NFS server already used by the client. The issue is network reachable in NFSv4 deployments but normally requires access to the storage network or control of a trusted NFS server. Impact is availability only. No direct memory corruption or privilege escalation primitive is indicated by the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	DANGER DISK RACE NETWORK LINUS KPANIC 	NO	NO	checked

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

ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 4
* Conservative score: 4. No floor or cap applied.
* Raw paranoid score: 4
* Paranoid score: 4. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

Positive signals:

* +2 Remote reachable / network protocol mediated trigger. A malicious or compromised NFSv4 server already used by the client can influence OPEN delegation replies and later recall behavior over the network.
* +1 Concrete persistent protocol state / reference leak. The client can drop a granted delegation locally without sending DELEGRETURN, leaving the server with stale delegation state that is later moved to revoked state.
* +1 Availability impact. Subsequent SEQUENCE replies can keep reporting SEQ4_STATUS_RECALLABLE_STATE_REVOKED, causing the client state manager to loop issuing TEST_STATEID and degrading or blocking affected NFS client recovery.

Negative or non-awarded signals:

* 0 RealUAF not awarded. The patch does not establish use after free, double free, stale heap object use, or refcount mutation on freed storage.
* 0 Controlled reclaim not demonstrated. No attacker controlled kernel object replacement or post free reuse primitive is shown.
* 0 Arbitrary read or write not demonstrated. The issue is protocol state inconsistency, not direct memory corruption.
* 0 Confidentiality and integrity not awarded. The established consequence is availability degradation, not protected data disclosure or unauthorized modification.
* 0 Race penalty not applied. The commit says the window is easy to reach after write delegation behavior changed, so there is no evidence of a hard one shot race.
* 0 Important candidate not awarded. The issue is network mediated and operationally relevant, but the primitive remains Class C DoS / invalid protocol state without memory corruption or security boundary bypass.

3. Reachability analysis

The evaluated attacker is a malicious or compromised NFSv4 server, or an attacker able to tamper with trusted NFS server traffic. The attacker does not need host administrator privileges on the victim client, so HostAdminRequired is NO.

The complete trigger depends on the victim already using an NFSv4 mount where delegations are enabled and where the attacker can influence server replies and recall behavior. This is a prior deployment condition, not a local victim privilege requirement. The issue is not namespace scoped and does not depend on attacker created user or network namespaces.

The trigger is network mediated through NFSv4 OPEN delegation handling and CB_RECALL / SEQUENCE state recovery. It is normally relevant to storage or data center networks rather than public Internet exposure. No special hardware, boot option, or kernel memory corruption prerequisite is indicated.

4. Primitive and severity interpretation

The exact primitive is an NFSv4 protocol state lifetime mismatch. The client receives a delegation from the server but, on specific error paths in nfs_inode_set_delegation(), fails to record it locally and also fails to return it with DELEGRETURN.

This leaves the server believing the client still owns a delegation while the client has no matching local stateid. If the server later recalls that delegation, the client can answer CB_RECALL with NFS4ERR_BADHANDLE. The server may then revoke the delegation and repeatedly report SEQ4_STATUS_RECALLABLE_STATE_REVOKED, while the client state manager loops on TEST_STATEID recovery.

This is not active structured kernel memory corruption. It is not fixed stale storage. It is not a cleanup UAF and there is no concrete repeated release/refcount operation. The demonstrated consequence is denial of service or persistent degradation of affected NFS mounts. A broader host compromise, privilege escalation, or confidentiality/integrity primitive is speculative and not supported by the patch context.

PrimitiveClass is CLASS_C because the established behavior is invalid protocol state and DoS without an independently useful corruption primitive. ImportantCandidate is NO because no Class A or unresolved Class B corruption primitive exists. AutoDowngradeAllowed is YES because the issue can be treated as non-Important after review, although network-mediated NFS exposure makes manual triage recommended.

5. One-sentence report phrase

An NFSv4 client can fail to return a server granted delegation that it could not record locally, leaving stale revoked delegation state that may cause repeated recovery loops and denial of service for affected NFS mounts.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is network mediated through NFSv4 server interaction and can cause persistent client-side recovery degradation, but there is no evidence of memory corruption, privilege escalation, or confidentiality/integrity impact.

7. Call-site confidence

Call-site confidence: medium

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

Patch: NFS: Return a delegation the client fails to record [ Upstream
Commit: 92533e49fe3c455088657b862cf3562260c0ce25
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=92533e49fe3c455088657b862cf3562260c0ce25

Changed files:
  fs/nfs/delegation.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=92533e49fe3c455088657b862cf3562260c0ce25

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

The original Linux kernel CVE announcement for CVE-2026-90234 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90234

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 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: 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).