* [CVE-2026-89660][MODERATE 7.0] NFSD: Prevent client use-after-free during admin state revocation [ Upstream
@ 2026-09-12 1:50 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 1:50 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89660
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSD: Prevent client use-after-free during admin state revocation [ Upstream
Commit: 549bd9868e9d77b07ea94870940d64342829c6ad
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=549bd9868e9d77b07ea94870940d64342829c6ad
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89660
Analysis date: Fri, 11 Sep 2026 21:50:02 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89660 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89660
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-2026-89660 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89660 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A race in NFSD state revocation can leave a stateid pointing at an nfs4_client that has already been freed. The issue occurs because nfsd4_revoke_states drops client_lock while revoke_one_stid still uses the client pointer, and a concurrent client teardown can free the same client during that window. A remote NFSv4 client may be able to participate in the race by creating client state and then driving client teardown while administrative state revocation is running on the server. For the CVSS the PR:N is used for the paranoid score because some NFS deployments expose the service to clients without a separate local account on the target, while the base score uses PR:L for environments where creating useful NFSv4 state requires authorized client access. The issue is network reachable when NFSD is exposed to the attacker, but practical exploitation is likely timing dependent and may require a local or internal storage network scenario. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the primitive is a kernel use-after-free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES DANGER DISK UAF NETWORK LINUS KPANIC DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
ActionableScoreLower=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4. Applied REAL UAF FLOOR 4 because a real stale post-lifetime nfs4_client use is established and host-admin-only reachability is not positively proven.
* Raw paranoid score: 4
* Paranoid score: 5. Applied REAL UAF FLOOR 5 because the stale object is an active structured NFSD client object, the remote NFSv4 client can influence client state and teardown, and stale use continues into state revocation rather than being only a fixed-buffer fault.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
Positive signals:
* +2 Real UAF / lifetime violation. A stateid keeps only a bare pointer to nfs4_client, and nfsd4_revoke_states can drop client_lock while revoke_one_stid still dereferences clp after concurrent teardown may have freed it.
* +1 Reliable kernel crash / strong DoS. A freed nfs4_client dereference in kernel NFSD state handling can realistically cause Oops or panic.
* +1 paranoid attacker-influenced active-object lifetime. A remote NFSv4 client can create state and participate in client teardown through NFSv4 client lifecycle operations, while revocation consumes the stale client pointer.
Negative signals:
* No host-admin penalty applied. Admin state revocation suggests an administrative component, but the supplied evidence does not positively establish that the complete attacker-controlled trigger requires host-admin-equivalent privilege.
* No race penalty applied. The bug is a race, but the evidence does not prove a narrow one-shot timing window that is materially uncontrollable or non-repeatable.
* No rare subsystem penalty applied. NFSD is optional, but NFS server deployment is a normal production configuration when enabled.
Important non-awarded signals:
* Controlled reclaim is not demonstrated.
* Arbitrary write is not demonstrated.
* Confidentiality and integrity bonuses are not awarded solely because UAF could theoretically corrupt reused memory.
* Concrete repeated release/refcount signal is not awarded. The patch adds cl_rpc_users pinning, but the vulnerable operation shown is stale dereference/use of clp, not a concrete second put/release/refcount mutation on the exact same dangling reference.
* Structured stale-object leverage is not separately awarded beyond the paranoid lifetime-control point because controlled replacement or attacker-shaped reclaimed contents are not established.
* Extra availability is not awarded because the same crash is already counted once.
* Direct remote reachable +2 is not awarded conservatively because nfsd4_revoke_states is an admin state revocation path, not a simple packet-processing operation directly reached by one network packet.
3. Reachability analysis
The affected code is NFSD NFSv4 server state management. A remote NFSv4 client can create client state and can participate in teardown through NFSv4 client lifecycle behavior. The vulnerable revocation walk is described as admin state revocation, so the complete trigger may be compound: remote client state plus server-side administrative revocation or expiry activity.
HostAdminRequired is UNKNOWN. The patch text strongly suggests an administrative revocation path, but it does not prove that an attacker must possess host root or host-admin-equivalent privilege to make the vulnerable race relevant. NamespaceScopedPrivilege is NO because this is not presented as a namespace-capability-triggered path.
Prior setup exists in the sense that NFSD and NFSv4 service must be enabled and reachable. No special hardware is required. Race repeatability is plausible because clients and state can be recreated, but full attacker control over both revocation and teardown is not established from the supplied patch.
4. Primitive and severity interpretation
The exact primitive is a kernel use-after-free of struct nfs4_client during NFSD state revocation. The stale object is an active structured object, not fixed stale storage. The stale path dereferences the client pointer and uses fields such as cl_minorversion while revoking a stateid.
This is not merely an ordinary stale cleanup dereference, because the object is part of active NFSv4 client and stateid management. However, it is also not CLASS_A because the patch does not establish controlled reclaim, arbitrary read/write, callback control, credential manipulation, or demonstrated privilege escalation.
PrimitiveClass is CLASS_B. Demonstrated consequence is at least host kernel DoS. Speculative consequence is stronger memory corruption or privilege impact if reclaimed memory can be shaped, but that is not established. ImportantCandidate is YES because a real active-object UAF exists in network server state handling and exploitation leverage remains unresolved. AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would discard a concrete kernel lifetime primitive requiring manual review.
5. One-sentence report phrase
A race in NFSD admin state revocation can leave a stateid with a stale nfs4_client pointer, allowing revoke_one_stid to dereference a freed client during concurrent teardown and causing at least a kernel crash with unresolved UAF exploitation risk.
6. Manual review recommendation
MANUAL CHECK REQUIRED
A real active-object UAF is explicitly established in NFSD server state handling, and although controlled reclaim or LPE is not shown, the network-facing state lifecycle and unresolved post-lifetime behavior should not be automatically downgraded.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: NFSD: Prevent client use-after-free during admin state revocation [ Upstream
Commit: 549bd9868e9d77b07ea94870940d64342829c6ad
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=549bd9868e9d77b07ea94870940d64342829c6ad
Changed files:
fs/nfsd/nfs4state.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=549bd9868e9d77b07ea94870940d64342829c6ad
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89660 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89660
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:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 8.1
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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