[CVE-2026-31402][IMPORTANT] nfsd: fix heap overflow in NFSv4.0 LOCK replay cache [ Upstream

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

CVE: CVE-2026-31402
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: nfsd: fix heap overflow in NFSv4.0 LOCK replay cache [ Upstream
Commit: c9452c0797c95cf2378170df96cf4f4b3bca7eff
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9452c0797c95cf2378170df96cf4f4b3bca7eff
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31402
Analysis date: Thu, 21 May 2026 16:57:42 -0400
ActionableScore: 12
ActionableScore lower bound: 11
Actionable bucket: High-end Important / possible Critical candidate
Manual review required: YES

Summary:
A remotely triggerable NFSv4.0 LOCK replay-cache bug can copy an oversized denied-lock response into a fixed 112-byte heap buffer, causing a slab out-of-bounds write and possible kernel memory corruption.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-31402 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31402

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: IMPORTANT
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
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CVE-2026-31402	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-122;CWE-120;BEST CVSS score: '9.8';DESCR 'A remote heap out-of-bounds write exists in the NFSv4.0 server replay cache because LOCK denied responses can exceed the fixed 112 byte replay buffer. An attacker can use two cooperating NFSv4.0 clients so one creates a lock with a large owner string and the other triggers a conflicting LOCK denial, causing nfsd4_encode_operation to copy an oversized encoded response into rp_ibuf. For the CVSS the PR:N is used because the described trigger does not require a local account or administrative privileges on the target beyond network reachability to the NFS server. The issue is network reachable when NFSv4.0 is exposed to the attacker. Impact is at least remote denial of service via kernel memory corruption and in worst case may allow confidentiality or integrity impact because the primitive is a remote heap out-of-bounds write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is High-end Important / possible Critical candidate (with actual score 12)	YES	WRITE OOB DANGER DISK NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=12
ActionableScoreLower=11

## 1. ActionableScore

* Conservative score: 11
* Paranoid score: 12
* Final recommended bucket: **High-end Important / possible Critical candidate**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. The issue is in `nfsd` NFSv4.0 request handling and is described as remotely triggerable by network NFS clients.
* Memory corruption, strong corruption primitive: +2. The bug is a slab out-of-bounds write into adjacent heap memory.
* Reliable kernel crash / strong DoS: +1. A heap overflow in kernel server code is a reliable crash or corruption condition.
* Weak to strong privilege escalation concern: +1 conservative. The overflow is attacker-influenced but no full exploit primitive is demonstrated.
* Confidentiality impact plausible: +1. Remote heap corruption can plausibly corrupt objects that later expose kernel or filesystem state.
* Integrity impact plausible: +1. Adjacent heap object corruption can alter kernel state or NFS server state.
* Availability impact realistic: +1. Remote kernel crash or service disruption is realistic.
* Broad/default/common subsystem: +1. NFS server is a common enterprise storage service, though not enabled on all hosts.
* Important filesystem/storage path: +1. The bug is in server-side filesystem access and lock handling.
* Practical exploit chain strongly implied: +1. The commit describes a concrete two-client trigger pattern using a large lock owner and a conflicting LOCK denial.

Paranoid additional signal:

* Strong LPE/RCE plausibility: +1 additional over conservative. A remote heap OOB write of up to about 944 bytes with attacker-influenced encoded response data is strong enough to treat as possible code execution or serious kernel object corruption during manual triage, even though exploitation is not proven.

## 3. Reachability analysis

The bug can be triggered by remote NFSv4.0 clients if the NFS server is exposed and accepts the relevant LOCK operations. No local account on the target is required in the described scenario, so PR:N is appropriate for CVSS-style reasoning. Containers and namespaces are not the key factor because the vulnerable component is the host NFS server. The path is not Internet-universal by default, but NFS is commonly deployed in internal and datacenter networks, and exposure to untrusted clients makes this highly actionable.

## 4. Severity interpretation

This behaves like a high-end Important or possible Critical kernel vulnerability, not an ordinary Moderate. The realistic demonstrated impact is remote kernel heap corruption and likely DoS. The theoretical worst case includes confidentiality and integrity compromise because the primitive is a remote slab out-of-bounds write, but practical exploitability beyond crash is not proven by the patch alone.

## 5. One-sentence report phrase

A remotely triggerable NFSv4.0 LOCK replay-cache bug can copy an oversized denied-lock response into a fixed 112-byte heap buffer, causing a slab out-of-bounds write and possible kernel memory corruption.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: remote unauthenticated heap OOB write in a common storage server path with plausible C/I/A impact and a concrete trigger pattern.

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

Patch: nfsd: fix heap overflow in NFSv4.0 LOCK replay cache [ Upstream
Commit: c9452c0797c95cf2378170df96cf4f4b3bca7eff
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9452c0797c95cf2378170df96cf4f4b3bca7eff

Changed files:
  fs/nfsd/nfs4xdr.c
  fs/nfsd/state.h

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=c9452c0797c95cf2378170df96cf4f4b3bca7eff

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

The original Linux kernel CVE announcement for CVE-2026-31402 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-31402

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 11
  Paranoid ActionableScore: 12

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).