* [CVE-2026-89701][MODERATE REGULAR] nfsd: validate nseconds in TIME_DELEG decode paths
@ 2026-09-12 7:30 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 7:30 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89701
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: validate nseconds in TIME_DELEG decode paths
Commit: 7e7b93da7fa2e77f977096251899d1410986adf6
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7e7b93da7fa2e77f977096251899d1410986adf6
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89701
Analysis date: Sat, 12 Sep 2026 03:30:46 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89701 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89701
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-2026-89701 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89701 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N';*CWE-20;CWE-1284;CWE-840;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.3';DESCR 'An NFS server can accept malformed TIME_DELEG_ACCESS or TIME_DELEG_MODIFY timestamp attributes because the decoded nseconds value is not checked against NSEC_PER_SEC. A remote NFSv4 client that can issue SETATTR requests may cause an invalid timespec to reach notify_change and filesystem metadata handling. This is a network reachable input validation issue and may result in incorrect timestamp metadata. No memory corruption primitive, kernel crash path, information disclosure, or privilege escalation path is demonstrated by the affected code. For the CVSS the paranoid score uses PR:N only for deployments with anonymous or overly permissive writable NFS exports. Typical deployments require authenticated access and permission to modify file attributes. ';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 2) SKIP CVE-2026-89701 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK SIMPLEFIX NETWORK LINUS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=2
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2
* Raw paranoid score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +2 Remote reachable input: A remote NFSv4 client can supply the malformed TIME_DELEG timestamp attribute to an exposed NFS server through the SETATTR path. The callback decoder is also reachable from a peer participating in the NFSv4 callback protocol.
* +0 Memory corruption: No UAF, OOB access, invalid pointer dereference, stale object, refcount misuse, or write primitive is established.
* +0 Integrity impact: The patch shows that an invalid nanoseconds value can reach notify_change and filesystem metadata handling, but it does not establish unauthorized modification of another principal's protected data or a security-boundary bypass.
* +0 Availability impact: No kernel crash, filesystem shutdown, resource exhaustion, or persistent DoS mechanism is demonstrated.
* +0 Privilege escalation: No credential, capability, authorization, ownership, or policy-bypass primitive is present.
* No negative privilege penalty applied: Host-admin-equivalent privilege is not required. A remote NFS client with access to an exported object can normally issue SETATTR-related operations without controlling the host.
* No niche-subsystem penalty applied: Delegated timestamps may not be universally deployed, but the supplied evidence does not establish that the feature is unusually difficult for an attacker to reach once the affected NFS service is enabled.
3. Reachability analysis
The relevant attacker is a remote NFSv4 client communicating with an enabled NFS server. The SETATTR path requires access to an exported filesystem object and permission to modify its attributes, but this is below host-admin-equivalent privilege and can be available to an ordinary remote user of the NFS service.
The malformed field is protocol input. No local syscall, host capability, namespace capability, special hardware, race, sysctl, boot option, or prior host-admin action by the attacker is needed beyond the normal prerequisite that the server exports an affected NFSv4 service and supports delegated timestamp attributes.
Namespace-scoped privilege is not relevant to the complete trigger. The vulnerable operation is handled by the host NFS server and is reached through network protocol processing.
4. Primitive and severity interpretation
The exact primitive is missing range validation for a protocol-supplied uint32_t nanoseconds field before assignment to tv_nsec. Values greater than or equal to NSEC_PER_SEC violate the Linux timespec invariant and may be passed into notify_change and downstream filesystem metadata handling.
This is an invalid-state and input-validation defect, not a memory-lifetime or memory-corruption bug. The patch does not establish an active structured stale object, fixed stale storage, UAF, double release, second refcount mutation, arbitrary write, controlled reclaim, disclosure, authorization bypass, or cross-boundary ownership violation.
The demonstrated consequence is acceptance of malformed timestamp metadata. A filesystem may reject, normalize, or persist such a value depending on downstream behavior, but no concrete crash or protected-object modification is established by the supplied code. Therefore PrimitiveClass is NONE, ImportantCandidate is NO, and automatic downgrade is allowed. Manual review remains recommended because the input is network-reachable and downstream filesystem handling may vary.
5. One-sentence report phrase
An NFSv4 server may accept malformed delegated timestamp nanoseconds values and pass an invalid timespec to filesystem metadata handling, but the supplied patch establishes no memory corruption, privilege escalation, or kernel crash primitive.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The issue is remotely reachable in affected NFSv4 deployments, but the patch demonstrates only malformed timestamp validation failure and not a concrete confidentiality, integrity, availability, or privilege-escalation consequence.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: validate nseconds in TIME_DELEG decode paths
Commit: 7e7b93da7fa2e77f977096251899d1410986adf6
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7e7b93da7fa2e77f977096251899d1410986adf6
Commit description:
The xdrgen-based TIME_DELEG_ACCESS and TIME_DELEG_MODIFY decode arms
store a raw uint32_t nseconds directly into tv_nsec without enforcing
nseconds < NSEC_PER_SEC. The legacy nfsd4_decode_nfstime4 has this
check but the TIME_DELEG paths do not. A malformed timespec can
propagate through notify_change() to disk.
Add range checks in both nfs4xdr.c (SETATTR path) and
nfs4callback.c (CB_GETATTR path).
Fixes: 6ae30d6 ("nfsd: add support for delegated timestamps")
Cc: [email protected]
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Jeff Layton <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Chuck Lever <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/nfsd/nfs4callback.c
fs/nfsd/nfs4xdr.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=7e7b93da7fa2e77f977096251899d1410986adf6
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89701 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89701
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:N/UI:N/S:U/C:N/I:L/A:N
The Best / paranoid CVSS score: 5.3
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: 2
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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