* [CVE-2026-90104][MODERATE 7.0] NFSv4.1: zero referring call lists before decoding [ Upstream
@ 2026-09-17 17:30 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-17 17:30 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90104
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: NFSv4.1: zero referring call lists before decoding [ Upstream
Commit: f31f3c042e024aef437cda42f0424ae8d4594b6c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f31f3c042e024aef437cda42f0424ae8d4594b6c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90104
Analysis date: Thu, 17 Sep 2026 13:30:25 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
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-2026-90104 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90104
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-2026-90104 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90104 CHECK WITH IMPACT FROM ORIG NN MODERATE 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-824;*CWE-457;CWE-763;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'An uninitialized pointer bug in the NFSv4.1 callback sequence decoder can occur because decode_cb_sequence_args allocates csa_rclists without zeroing it and decode_rc_list leaves rcl_refcalls unset for a valid empty referring call list. A malicious or compromised NFSv4.1 server can cause the client to later pass stale slab contents to kfree during callback processing, which can crash the kernel and may be treated as a kernel memory corruption candidate. For the CVSS the PR:N is used because the attacker does not need privileges on the victim beyond network reachability as an NFS server endpoint already used by the client. The issue is network reachable through NFS callback processing, but practical exposure is usually limited to trusted storage or data center networks rather than the public Internet. Impact is at least denial of service via kernel crash. For the paranoid score, confidentiality and integrity are raised because invalid kfree of a stale kernel pointer can plausibly corrupt kernel memory and should not be auto closed without manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5) YES DISK SIMPLEFIX NETWORK LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=5
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
No floor or cap changed the score.
2. Signal breakdown
Positive signals:
* +2 Remote reachable / directly network-triggerable: a malicious or compromised NFSv4.1 server can send callback XDR data that reaches `decode_cb_sequence_args()` on the client.
* +1 Weak / indirect corruption candidate: `csa_rclists` was allocated without zeroing, leaving `rcl_refcalls` uninitialized for valid empty referring-call lists.
* +1 Reliable kernel crash / strong DoS: the uninitialized stale slab value can later be passed to `kfree()`, plausibly causing an invalid free, allocator fault, or kernel crash.
* +1 Weak LPE concern / unresolved allocator corruption concern: this is more than a NULL dereference because `kfree()` is a lifetime-mutating allocator operation applied to a stale uninitialized kernel pointer value. Controlled reclaim or arbitrary R/W is not shown, so this remains Class B rather than Class A.
Non-awarded signals:
* RealUAF not awarded: the patch establishes an uninitialized pointer / invalid free, not a directly established use-after-free of the same object.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Confidentiality and integrity bonuses not awarded because no disclosure path, controlled corruption target, or privilege-escalation chain is established.
* Concrete repeated release/refcount signal not awarded because there is no visible first valid release followed by a second release of the exact same dangling reference.
* Extra Availability not awarded because the same kernel crash is already counted.
* No host-admin penalty applied because the attacker model is a malicious or compromised NFS server endpoint, not a local host administrator on the victim.
3. Reachability analysis
The vulnerable path is in NFSv4.1 callback XDR decoding on the client side. A malicious or compromised NFSv4.1 server can provide a valid referring-call list with zero referring calls, causing `decode_rc_list()` to leave `rcl_refcalls` unset while later cleanup code still frees it.
The attacker does not need host-admin privileges on the victim once the victim is using the malicious or compromised NFS server. This is not ordinary public Internet exposure in most deployments, because NFS is commonly used inside storage, enterprise, or data-center networks. However, the trigger is network-originated from the server side and does not require local code execution on the victim.
Namespace-scoped privilege is not relevant. No race or timing dependency is established. The main prerequisite is that the NFS client has an active NFSv4.1 relationship where callback processing is reachable.
4. Primitive and severity interpretation
The exact primitive is an uninitialized kernel pointer field later passed to `kfree()`. The affected object is a decoded NFSv4.1 callback data structure. The stale value comes from slab contents because `kmalloc_objs()` did not zero the allocated array.
This is not a demonstrated Real UAF, but it is a concrete invalid-free / allocator-lifetime corruption candidate. The demonstrated consequence is kernel crash / DoS. The speculative consequence is broader allocator-state corruption if the stale slab value is a plausible kernel allocation pointer, but controlled target selection, controlled reclaim, arbitrary kernel read/write, or LPE is not established.
PrimitiveClass is CLASS_B because a real kernel lifetime/corruption primitive exists, but exploitation leverage beyond crash remains unresolved. ImportantCandidate is YES because a network-reachable invalid `kfree()` in kernel code should not be treated as ordinary crash-only Moderate without manual review. AutoDowngradeAllowed is NO because automatic downgrade would discard a concrete allocator-lifetime corruption primitive reachable through NFS callback processing.
Likely operational severity: actionable Moderate at minimum, with Important review justified due to remote server-to-client kernel invalid-free potential.
5. One-sentence report phrase
An uninitialized pointer in NFSv4.1 CB_SEQUENCE decoding can cause a malicious or compromised NFS server to make the client pass stale slab contents to kfree(), leading to a kernel crash and an unresolved allocator corruption concern.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a network-reachable invalid `kfree()` of stale uninitialized slab contents in kernel NFS client callback handling, with DoS demonstrated and memory-corruption leverage unresolved.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: NFSv4.1: zero referring call lists before decoding [ Upstream
Commit: f31f3c042e024aef437cda42f0424ae8d4594b6c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=f31f3c042e024aef437cda42f0424ae8d4594b6c
Changed files:
fs/nfs/callback_xdr.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=f31f3c042e024aef437cda42f0424ae8d4594b6c
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90104 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90104
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: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: unknown
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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