[CVE-2026-89666][MODERATE 7.0] nfsd: reject out-of-range nseconds in NFSv3 SETATTR and create ops

[CVE-2026-89666][MODERATE 7.0] nfsd: reject out-of-range nseconds in NFSv3 SETATTR and create ops

[AL-KERNEL]

CVE: CVE-2026-89666
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: reject out-of-range nseconds in NFSv3 SETATTR and create ops
Commit: 55341d8a5a0f5853e70d46f5a55cf06007808f1d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=55341d8a5a0f5853e70d46f5a55cf06007808f1d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89666
Analysis date: Sat, 12 Sep 2026 05:34:12 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89666 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89666

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-2026-89666	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89666 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:L';*CWE-20;CWE-1284;CWE-754;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '7.1';DESCR 'An NFSv3 server can accept a client supplied atime or mtime whose nseconds field is outside the valid timespec64 range. The value is valid on the wire but invalid for kernel timestamp storage, and it can be stored into inode metadata without being normalized. On filesystems such as ext4 or XFS bigtime this can corrupt the encoded on disk timestamp and make the stored seconds or year wrong when read back. For the CVSS the PR:L is used because a client normally needs access to the exported NFS namespace and permission to issue SETATTR or create operations. The issue is network reachable through NFSv3 procedures such as SETATTR, CREATE, MKDIR, SYMLINK, and MKNOD. Impact is mainly persistent integrity impact on file metadata, not kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate (with actual score 5)  SKIP CVE-2026-89666 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	OOB DISK NETWORK LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
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**::

No floor or cap was applied. The paranoid score is emitted as the final ActionableScore because it treats the persistent on-disk filesystem metadata corruption as operationally security-relevant enough to prevent automatic closure.

2. Signal breakdown

Positive signals:

* +2 Remote reachable / directly network-triggerable: an NFSv3 client can send SETATTR, CREATE, MKDIR, SYMLINK, or MKNOD requests with an out-of-range nseconds value to the NFS server.
* +1 Concrete persistent filesystem metadata corruption: invalid tv_nsec can be stored into inode atime or mtime and later mis-encoded on disk by filesystems such as ext4 or XFS bigtime.
* +1 Important filesystem/storage path: the corruption affects VFS inode timestamp metadata and filesystem on-disk timestamp encoding, not just transient protocol state.
* Paranoid +1 Integrity plausible: wrong persistent timestamps may affect audit, backup, replication, retention, or timestamp-based policy logic.

Negative or non-awarded signals:

* No RealUAF: no use-after-free, stale pointer, refcount misuse, or lifetime bug is established.
* No memory corruption bonus: the corruption is filesystem metadata corruption, not kernel heap or stack corruption.
* No controlled reclaim, arbitrary write, OOB write, or callback control is demonstrated.
* No privilege escalation signal is established.
* No confidentiality signal is awarded because no disclosure path is shown.
* No extra availability point is awarded because the patch does not establish kernel crash, filesystem shutdown, or host-wide DoS.
* No authorization bypass is awarded because the attacker still needs normal NFS access and permission for the relevant file or create operation.
* No race or timing penalty is applied because this is an input-validation bug, not a timing-dependent race.
* No host-admin penalty is applied because the vulnerable operation is reachable by an NFS client with export access, not by host administrator only.

3. Reachability analysis

The vulnerable path is reachable through NFSv3 server RPC procedures. A remote client that can access the exported namespace and has permission to perform SETATTR or object creation can supply an atime or mtime nseconds field outside the valid timespec64 range.

Host administrator privileges on the server are not required for the client-side trigger. The relevant attacker is a remote NFS client with sufficient export and file permissions, so HostAdminRequired=NO. This is not a namespace-scoped local capability issue, so NamespaceScopedPrivilege=NO.

There is likely prior administrative setup in the practical deployment sense because an NFS export must exist, but that is the normal exposure model of the service and not a per-attack host-admin prerequisite. No special hardware, rare execution mode, or race condition is required. Exposure depends on whether NFSv3 is enabled and reachable by untrusted or semi-trusted clients.

4. Primitive and severity interpretation

The exact primitive is improper validation of client-supplied timestamp nanoseconds in NFSv3 server operations. The value is valid as a wire uint32 but invalid as a timespec64 tv_nsec. If stored without normalization, it can corrupt persistent on-disk timestamp metadata because filesystem encoders expect tv_nsec to be less than NSEC_PER_SEC.

This is not an active structured UAF and not fixed stale storage. There is no concrete repeated release or refcount mutation. The demonstrated consequence is persistent incorrect metadata, especially wrong seconds or year values after ext4 or XFS bigtime encoding and read-back.

PrimitiveClass=CLASS_C because this is an invalid-state and persistent metadata-corruption issue without an independently useful kernel memory-corruption or authorization primitive. ImportantCandidate=NO because the evidence does not support LPE, arbitrary kernel R/W, protected cross-file overwrite, or a broad security-boundary bypass. AutoDowngradeAllowed=NO because the issue is network reachable and causes persistent filesystem metadata integrity impact, so automatic closure as ordinary low-risk noise would risk missing operational security impact.

Likely operational severity is Actionable Moderate. It is not a Strong Important candidate based on the supplied patch alone, but it should be manually reviewed where NFSv3 exports are reachable by untrusted clients or where timestamp integrity matters for audit, backup, replication, retention, or compliance workflows.

5. One-sentence report phrase

An NFSv3 client can provide out-of-range atime or mtime nanoseconds in SETATTR or create operations, causing persistent on-disk timestamp metadata corruption on filesystems such as ext4 or XFS bigtime when the server stores the invalid timespec64 value.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable through NFSv3 and affects persistent filesystem metadata integrity, even though it is not kernel memory corruption and does not currently show privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: reject out-of-range nseconds in NFSv3 SETATTR and create ops
Commit: 55341d8a5a0f5853e70d46f5a55cf06007808f1d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=55341d8a5a0f5853e70d46f5a55cf06007808f1d

Commit description:

A client can send an NFSv3 SETATTR, CREATE, MKDIR, SYMLINK or MKNOD
carrying an atime or mtime whose nseconds field is out of range. The
value is well-formed on the wire and decodes cleanly into a valid
uint32, but it is not a valid timespec64: tv_nsec must be less than
NSEC_PER_SEC.

Nothing in the setattr path clamps it. notify_change() runs the time
through timestamp_truncate(), which does not reduce tv_nsec below
NSEC_PER_SEC when the filesystem supports nanosecond granularity
(s_time_gran == 1), and the inode atime/mtime setters store it verbatim
(only ctime is normalized, via inode_set_ctime_to_ts()). The
un-normalized value then corrupts on-disk metadata: ext4's
ext4_encode_extra_time() shifts tv_nsec left by EXT4_EPOCH_BITS, which
overflows the 32-bit extra field and clobbers the seconds-epoch bits, so
the stored seconds (and thus the year) are wrong on read-back. XFS with
bigtime mis-stores the timestamp for the same reason.

Validate the client-supplied atime/mtime in the proc handlers and return
NFS3ERR_INVAL before anything is changed. RFC 1813 lists NFS3ERR_INVAL
for SETATTR and describes it as the error for a value the server 'can
not store ... in its own representation'; the client maps it to EINVAL.

Checking in the proc handlers, rather than in nfsd_setattr(), keeps the
rejection in front of object creation. The create operations create the
object before nfsd_create_setattr() runs, so a late failure would leave
the new object behind and turn a non-idempotent request into a namespace
change that reports failure. The check is therefore done up front, for
the create operations before the object is created.

tv_nsec is a long, so the comparison casts it to unsigned long (the same
width) rather than to u32, matching timespec64_valid(). A u32 cast would
truncate on 64-bit; the unsigned long cast also rejects a value that
became negative when an out-of-range u32 wire nseconds was assigned to a
32-bit long.

Only client-supplied times are checked: SET_TO_SERVER_TIME requests
carry no client value. The sattrguard3 ctime is deliberately left alone:
an out-of-range guard simply never matches the object's ctime and yields
NFS3ERR_NOT_SYNC via the existing guardtime comparison, which is the
protocol-correct outcome rather than rejecting the request.

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Signed-off-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260616054027.2360930-2-robbieko@synology.com
Signed-off-by: Chuck Lever <cel@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs3proc.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=55341d8a5a0f5853e70d46f5a55cf06007808f1d

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

The original Linux kernel CVE announcement for CVE-2026-89666 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89666

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