* [CVE-2026-89699][MODERATE REGULAR] nfsd: validate symlink target length in NFSv4 CREATE
@ 2026-09-12 3:59 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 3:59 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89699
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: nfsd: validate symlink target length in NFSv4 CREATE
Commit: b24843ea3de1676b29902457b38507c95a80649b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b24843ea3de1676b29902457b38507c95a80649b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89699
Analysis date: Fri, 11 Sep 2026 23:59:53 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89699 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89699
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: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-89699 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89699 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:N/A:H';**CWE-400;CWE-770;CWE-789;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7.5';DESCR 'An NFSv4 server can allocate an excessively large buffer while decoding a CREATE operation for a symbolic link because the symlink target length is accepted from the wire without an early limit. A remote client can send multiple CREATE requests with oversized target lengths and cause several MiB of memory to be retained per COMPOUND operation until request teardown. For the CVSS the PR:N is used because allocation occurs while decoding network supplied operation data before the VFS rejects the oversized target. The issue is directly network reachable when NFSv4 server access is exposed to the attacker. Impact is denial of service through memory pressure or exhaustion. No confidentiality or integrity impact is indicated because the patch shows an oversized allocation without a memory corruption primitive. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) SKIP CVE-2026-89699 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES NO NO unknown MAYBE DISK SIMPLEFIX NETWORK INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=NONE
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::
The conservative and paranoid scores are identical. The patch establishes direct remote resource consumption, but does not establish memory corruption, a persistent leak, an isolation bypass, or a security-boundary violation.
2. Signal breakdown
* +2 Remote reachable / directly network-triggerable. An NFSv4 client can send an oversized NF4LNK CREATE operation to an exposed nfsd service. The allocation occurs while decoding network supplied operation data.
* +1 Availability impact realistic. Each malformed operation can retain an allocation up to the maximum RPC payload size, potentially several MiB, until COMPOUND teardown. Sustained concurrent requests can cause material memory pressure or memory exhaustion.
Important non-awarded signals:
* No UAF, stale pointer, refcount misuse, OOB access, or memory corruption is established.
* No arbitrary write, controlled reclaim, information disclosure, privilege escalation, or authorization bypass is demonstrated.
* No Concrete persistent kernel resource/reference leak signal is awarded. The allocated memory is retained only until normal COMPOUND request teardown and is not shown to be permanently leaked.
* No additional Reliable kernel crash point is awarded. The patch establishes memory pressure and possible exhaustion, not a deterministic kernel panic.
* No broad/default subsystem exposure point is awarded. NFSv4 server exposure depends on deployment and exported service configuration.
* No rare subsystem, race, hardware, or privilege penalty is applied because none is positively established.
3. Reachability analysis
The attacker is a remote NFSv4 client able to reach an exposed nfsd service. The vulnerable allocation occurs in `nfsd4_decode_create()` while parsing wire-format `CREATE` data for an NF4LNK symlink target, before the VFS performs the later pathname-length rejection.
Host-admin-equivalent privilege is not required to send the malformed network request to an exposed NFSv4 server. Therefore, `HostAdminRequired=NO` is supported by the commit description stating that a client can force the allocation.
The complete trigger is not shown to depend on attacker-controlled namespaces, network namespace capabilities, containers, special hardware, boot parameters, or sysctls. `NamespaceScopedPrivilege=UNKNOWN` because namespaces are not relevant to the supplied remote attack path.
The service must be running and reachable by the attacker. In many deployments NFS is restricted to internal, storage, or trusted networks, but such network restriction is deployment-specific and is not a valid negative scoring signal.
4. Primitive and severity interpretation
The established primitive is an oversized but bounded allocation driven by an NFSv4 `CREATE` request containing an excessive symlink target length. The requested length was accepted from the wire and could cause a `kmalloc` up to the maximum RPC payload size before the VFS later returned `ENAMETOOLONG`.
This is a resource-exhaustion issue, not an active-object lifetime violation. There is no freed object, dangling reference, stale structured object, second release, refcount mutation, object replacement, or post-free field consumption. `RealUAF=NO` and `PrimitiveClass=NONE` are therefore appropriate.
The demonstrated consequence is remote memory pressure and possible denial of service through sustained concurrent oversized requests. The speculative consequence is broader host memory exhaustion under sufficiently high request concurrency and available network bandwidth. No confidentiality or integrity consequence is supported because the patch does not establish an out-of-bounds access, invalid free, use-after-free, data disclosure, or attacker-controlled write.
`ImportantCandidate=NO` because this is pure transient resource consumption without a persistent leak, cross-container accounting bypass, memory corruption primitive, or security-boundary effect. `AutoDowngradeAllowed=YES` because the vulnerability can be handled as an ordinary resource-exhaustion issue after verifying local NFS exposure and capacity controls.
Likely operational severity is Moderate in exposed NFSv4 environments, but it is not an Important-class kernel-corruption candidate.
5. One-sentence report phrase
An exposed NFSv4 server can be forced to allocate several MiB per malformed symbolic link CREATE request before rejecting the target length, allowing remote denial of service through sustained memory pressure.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Review is recommended because the issue is directly network reachable and practical impact depends on nfsd exposure, RPC concurrency limits, request size limits, memory limits, and whether untrusted clients can access the NFSv4 service.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: validate symlink target length in NFSv4 CREATE
Commit: b24843ea3de1676b29902457b38507c95a80649b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b24843ea3de1676b29902457b38507c95a80649b
Commit description:
nfsd4_decode_create() accepts an unbounded cr_datalen from the wire for
NF4LNK symlink targets, allowing a client to force a kmalloc of up to
the maximum RPC payload size (several MiB) per COMPOUND op that persists
until compound teardown. The VFS rejects oversized targets with
ENAMETOOLONG, but the allocation has already occurred.
Reject cr_datalen == 0 early with nfserr_inval and cr_datalen greater
than NFS4_MAXPATHLEN (PATH_MAX) with nfserr_nametoolong to bound the
allocation.
Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: [email protected]
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <[email protected]>
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/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=b24843ea3de1676b29902457b38507c95a80649b
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89699 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89699
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:N/UI:N/S:U/C:N/I:N/A:L
The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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: unknown
Paranoid ActionableScore: 3
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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