* [CVE-2026-89659][IMPORTANT] NFSD: Prevent client use-after-free during delegation revoke [ Upstream
@ 2026-09-12 7:50 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 7:50 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89659
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: NFSD: Prevent client use-after-free during delegation revoke [ Upstream
Commit: 3c0a53ee0b442348d8d2286d6960d3f07bb3a3d3
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3c0a53ee0b442348d8d2286d6960d3f07bb3a3d3
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89659
Analysis date: Sat, 12 Sep 2026 03:50:23 -0400
ActionableScore: 6
ActionableScore lower bound: 5
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-2026-89659 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89659
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89659 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-89659 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:L/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: '7.5';DESCR 'A use-after-free can occur in the NFSD NFSv4 delegation revoke path because a delegation stateid keeps only a raw pointer to its owning nfs4_client. nfs4_laundromat() can temporarily remove an expired delegation from cl_delegations before revoke_delegation() links it onto cl_revoked, leaving a window where client_has_state() may see no remaining state and allow free_client() to run. A remote NFSv4 client that can create and recall delegations may be able to race delegation expiry against client teardown, causing the server kernel to dereference a freed nfs4_client. For the CVSS the PR:L is used for the paranoid score because reliable triggering typically requires NFS client access to an exported filesystem and the ability to create delegation state, but it does not require local shell access on the server. The issue is network reachable through NFSD when NFSv4 delegations are enabled and accessible to the attacker. Impact is at least denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to use-after-free memory corruption, so manual review is required.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6) YES LOCK DANGER DISK UAF NETWORK LINUS KPANIC NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
Conservative floor applied: 4 -> 5. Reason: real active structured kernel UAF in NFSD is reachable by a remote NFSv4 client below host-admin privilege, and the stale object participates in nontrivial delegation revoke/client teardown processing rather than being fixed stale storage.
2. Signal breakdown
Positive conservative signals:
* +2 Real UAF / active lifetime violation: delegation stateid keeps a bare nfs4_client pointer, and free_client() can run while revoke_delegation() is about to dereference the client.
* +2 Remote reachable / network-triggerable: the vulnerable lifecycle is in NFSD/NFSv4 delegation handling and can be influenced by an NFSv4 client with access to the server.
* +1 Reliable kernel crash / strong DoS: dereferencing a freed nfs4_client, including lock/state access, can realistically crash the kernel.
Negative conservative signals:
* -1 Hard timing / lifecycle race: the trigger depends on delegation recall timeout, laundromat processing, and concurrent client teardown.
Paranoid additional signal:
* +1 Attacker-influenced active-object lifetime manipulation: a remote NFSv4 client can influence creation and expiry of delegation state and client teardown timing enough that unresolved post-lifetime behavior should not be treated as ordinary crash-only cleanup.
Important non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Callback or function pointer control not demonstrated.
* Confidentiality and integrity were not awarded as direct signals because no concrete disclosure, write primitive, or replacement-object control is shown.
* Concrete repeated release/refcount signal not awarded. The patch adds cl_rpc_users pinning, but the vulnerable path does not show first release then exact same stale reference receiving a second lifetime/refcount mutation.
* Structured stale-object leverage is not separately awarded beyond the paranoid lifetime-control point because the supplied patch does not show attacker-shaped reused fields being consumed.
* Extra Availability not awarded because the crash/DoS is already counted once.
* CLASS_A not used because no controlled corruption, demonstrated LPE, arbitrary R/W, or security-boundary bypass beyond the kernel UAF is established.
Repeated-release/refcount checklist:
* First release can end lifetime: client teardown can reach free_client().
* Exact same reference survives: delegation stateid retains a bare pointer to nfs4_client.
* Second operation is a real lifetime/refcount mutation: no, the stale operation described is dereference/use around revoke_delegation(), not a second concrete put/release/refcount mutation.
* +1 Concrete repeated release/refcount awarded: no.
* Paranoid repeated-refcount leverage awarded: no.
* FLOOR 5 applied: yes, but due to active structured remote UAF surface, not due to repeated refcount mutation.
3. Reachability analysis
The evaluated attacker is a remote NFSv4 client with access to an exported filesystem where delegations can be created and later recalled or expired. Host administrator privilege on the server is not required by the supplied evidence. The attacker does need network access and enough NFS authorization to participate as a client and obtain delegation state, so this is not an unauthenticated Internet-wide assumption.
The issue is not namespace-scoped in the relevant sense. The vulnerable operation is in the host NFSD server state machine, not an attacker-created namespace-local kernel object.
Prior host setup exists in the ordinary deployment sense: NFSD must be running, NFSv4 must be enabled, exports must exist, and delegations must be possible. This is not scored as host-admin-required for the attacker because it is the exposed service configuration, not a privilege needed to exploit an already deployed server.
The race is nontrivial because it involves laundromat expiration, delegation revocation, and client teardown. However, the lifecycle is plausibly repeatable by a client that can reconnect, create state, and trigger delegation recall/expiry conditions, so the paranoid score does not apply the timing penalty.
No special hardware requirement is present.
4. Primitive and severity interpretation
The exact primitive is a use-after-free of struct nfs4_client. A delegation stateid stores only a raw pointer to its owning client. nfs4_laundromat() can remove an expired delegation from cl_delegations before revoke_delegation() links it onto cl_revoked. During that window, client_has_state() can observe no remaining state, allowing teardown to reach free_client() while revoke_delegation() still plans to use the client pointer.
This is an active structured object UAF, not fixed stale storage. The stale object is an NFSD client structure involved in server state, locks, delegation lists, and teardown logic. The demonstrated consequence is kernel crash/DoS. The speculative consequence is stronger post-free corruption if reuse or stale structured processing can be influenced, but that is not proven by the patch.
This is CLASS_B. It is not CLASS_A because controlled reclaim, arbitrary write, callback control, credential corruption, or demonstrated privilege escalation is not established.
ImportantCandidate=YES because a remote reachable active-object UAF in NFSD leaves meaningful Important-vs-Moderate exploitation leverage unresolved and should not be automatically treated as ordinary crash-only cleanup.
AutoDowngradeAllowed=NO because automatic downgrade would discard a concrete network-reachable kernel lifetime primitive with unresolved post-lifetime behavior. This does not prove Important severity, but it requires manual review.
Likely operational severity: actionable Moderate at minimum, with Strong Important candidacy depending on whether further analysis shows useful reuse, controlled replacement, or exploitable stale structured processing beyond crash.
5. One-sentence report phrase
A remote NFSv4 client with delegation-capable access can race NFSD delegation revocation against client teardown, causing a use-after-free of nfs4_client and at least a server kernel crash, with unresolved memory-corruption leverage requiring manual review.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a real network-reachable active-object UAF in NFSD, and although the patch directly supports DoS, the post-lifetime exploitation leverage is not resolved enough for safe automatic downgrade.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: NFSD: Prevent client use-after-free during delegation revoke [ Upstream
Commit: 3c0a53ee0b442348d8d2286d6960d3f07bb3a3d3
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3c0a53ee0b442348d8d2286d6960d3f07bb3a3d3
Changed files:
fs/nfsd/netns.h
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=3c0a53ee0b442348d8d2286d6960d3f07bb3a3d3
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89659 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89659
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:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.5
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: 5
Paranoid ActionableScore: 6
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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