[CVE-2026-89672][MODERATE 7.0] nfsd: gate nfs2 setacl by argp->mask

[CVE-2026-89672][MODERATE 7.0] nfsd: gate nfs2 setacl by argp->mask

[AL-KERNEL]

CVE: CVE-2026-89672
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: gate nfs2 setacl by argp->mask
Commit: e41d173d9dc735cecb15ab7aa63ecab09338f81b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e41d173d9dc735cecb15ab7aa63ecab09338f81b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89672
Analysis date: Fri, 11 Sep 2026 23:34:03 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
NFSACL v2 SETACL can silently remove access or default POSIX ACLs that were omitted from the request mask, allowing a remote authenticated NFS client with SETACL permission to weaken persistent filesystem access policy on affected exports.

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ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-89672 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89672

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-2026-89672	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-89672 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:L/UI:N/S:U/C:L/I:H/A:N';CWE-284;CWE-732;*CWE-20;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '7.1';DESCR 'NFSACL v2 SETACL handling can remove POSIX ACLs that were not present in the request because missing ACL pointers are passed as NULL to set_posix_acl, where NULL means delete this ACL type. A remote NFS client can trigger this through the NFSACL SETACL procedure by sending a mask that contains only one ACL arm or no ACL arm, causing the omitted access ACL or default ACL to be removed. For the CVSS the PR:L is used because the attacker normally needs valid NFS credentials and permission to perform SETACL on the target export or object. The issue is network reachable when the NFS ACL service is exposed to the client network. Impact is an access control policy bypass with possible confidentiality and integrity impact, especially on shared exports where default ACLs control permissions inherited by newly created files and directories.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	DISK NETWORK KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

## 2. Signal breakdown

Conservative signals:

* +2 Remote reachable / directly network-triggerable. The vulnerable operation is the NFSACL v2 SETACL procedure handled by nfsd and can be reached by an NFS client when the NFS ACL service and export are exposed.
* +2 Meaningful access-control policy bypass. The request mask says which ACL arm is present, but the old code called `set_posix_acl()` for both access and default ACLs unconditionally. A missing ACL pointer becomes `NULL`, and `set_posix_acl(..., NULL)` means delete that ACL type.
* +1 Persistent filesystem security-policy integrity impact. The bug can remove on-disk POSIX ACLs, including default directory ACLs that affect inherited permissions for future files and directories.

Paranoid additional signal:

* +1 Plausible confidentiality consequence. On shared NFS exports, silent removal of default or access ACLs can weaken access restrictions and may expose files or future objects to principals that should not have access. This is environment-dependent and therefore used only for the paranoid score.

Non-awarded or limited signals:

* RealUAF not awarded. No use-after-free, stale pointer lifetime violation, or double release is established.
* Memory corruption not awarded. The primitive is a protocol/ACL semantic bug, not kernel memory corruption.
* Reliable kernel crash / DoS not awarded. The patch does not indicate panic, NULL dereference, hang, or resource exhaustion.
* Strong LPE not awarded. No local privilege escalation, credential overwrite, arbitrary write, or root acquisition chain is established.
* Sensitive file / credential / FD theft not awarded. No specific protected file, credential, or FD theft path is supplied.
* Central authorization helper fan-out not awarded. The affected code is a specific NFSACL SETACL handler, not a broad kernel authorization predicate.
* No negative privilege penalty applied. The attacker needs valid NFS credentials and sufficient SETACL permission on the target object, but host-admin-equivalent privilege is not established.

## 3. Reachability analysis

The attacker is a remote NFS client that can send NFSACL v2 SETACL requests to an affected nfsd server. The practical trigger requires that the NFS server exports a filesystem with NFS ACL support and that the client has credentials accepted by the export and permission to perform SETACL on the target file or directory.

This is below host administrator privilege, so `HostAdminRequired=NO`. It is not an ordinary unauthenticated network attack because NFS authentication, export policy, and file ownership or ACL-change permission normally matter. Namespace-scoped privilege is not the relevant model here, so `NamespaceScopedPrivilege=NO`.

Prior host setup exists in the ordinary service sense: an administrator must have configured and exposed an NFS export and NFSACL service. That does not make the attacker host admin. No race, special timing, special hardware, or local kernel interface is required.

## 4. Primitive and severity interpretation

The exact primitive is a semantic access-control state corruption in NFSACL v2 SETACL handling. The decoder initializes only the ACL pointers whose mask bits are present. The vulnerable handler then treats omitted ACL arms as explicit `NULL` ACLs and passes them to `set_posix_acl()`, where `NULL` means remove this ACL type.

This is not an ordinary crash-only bug. It changes persistent filesystem authorization metadata contrary to protocol semantics. A request intended to update only the access ACL can remove the directory default ACL, and `mask=0` can remove both access and default ACLs.

Primitive class is `CLASS_A` because a meaningful security policy primitive is directly established: persistent ACLs can be deleted when the request did not include those ACLs. The demonstrated consequence is unauthorized or unintended weakening of ACL policy. The speculative consequence is broader confidentiality impact if removed ACLs expose existing or future objects on a shared export.

`ImportantCandidate=YES` because persistent ACL deletion on a network filesystem is a security-boundary and policy-bypass issue whose real impact depends heavily on export policy, authentication flavor, ownership, and default ACL usage. `AutoDowngradeAllowed=NO` because automatic closure would risk losing a meaningful authorization-policy primitive even though no memory corruption is involved.

Likely operational severity is Actionable Moderate, with Strong Important review justified for deployments where NFSACL v2 is exposed to many clients and default ACLs enforce meaningful multi-user separation.

## 5. One-sentence report phrase

NFSACL v2 SETACL can silently remove access or default POSIX ACLs that were omitted from the request mask, allowing a remote authenticated NFS client with SETACL permission to weaken persistent filesystem access policy on affected exports.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

The issue is not memory corruption, but it is a network-reachable persistent ACL policy corruption primitive. Manual review is required because severity depends on whether NFSACL v2 is enabled, which clients can issue SETACL, and whether default ACLs protect shared or sensitive export contents.

## 7. Call-site confidence

Call-site confidence: high

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

Patch: nfsd: gate nfs2 setacl by argp->mask
Commit: e41d173d9dc735cecb15ab7aa63ecab09338f81b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e41d173d9dc735cecb15ab7aa63ecab09338f81b

Commit description:

The NFSACL v2 SETACL path shares the decoder convention used by its
v3 sibling: nfsaclsvc_decode_setaclargs() fills in argp->acl_access
only when NFS_ACL is set in the request mask and argp->acl_default
only when NFS_DFACL is set, leaving the other pointer NULL because
the argument buffer is zeroed up to pc_argzero before decode.

nfsacld_proc_setacl() then hands both pointers to set_posix_acl()
unconditionally. set_posix_acl(idmap, dentry, type, NULL) is the VFS
"remove this ACL type" operation, so an omitted arm is
indistinguishable from an explicit request to delete that ACL. A
SETACL carrying only NFS_ACL silently strips the directory's default
ACL; mask=0 strips both.

This is the same defect just fixed in nfsd3_proc_setacl(); apply the
same remedy. Gate each set_posix_acl() call on its mask bit and
initialize error to 0 so that a request with neither bit set leaves
the on-disk ACLs untouched and returns success. The out_drop_lock
path and the unconditional posix_acl_release() in
nfsaclsvc_release_setacl() already tolerate the skipped arms.

Fixes: a257cdd ("[PATCH] NFSD: Add server support for NFSv3 ACLs.")
Cc: stable@vger.kernel.org
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/nfsd/nfs2acl.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=e41d173d9dc735cecb15ab7aa63ecab09338f81b

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

The original Linux kernel CVE announcement for CVE-2026-89672 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89672

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:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N
  The Best / paranoid CVSS score: 7.1

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: 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: 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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).