* [CVE-2026-89671][MODERATE 7.0] nfsd: gate nfs3 setacl by argp->mask
@ 2026-09-11 22:15 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-11 22:15 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89671
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: nfsd: gate nfs3 setacl by argp->mask
Commit: 68a80b26efdff1d09f8ae1773c6a915abb9e191d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68a80b26efdff1d09f8ae1773c6a915abb9e191d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89671
Analysis date: Fri, 11 Sep 2026 18:15:44 -0400
ActionableScore: 7
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-2026-89671 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89671
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-2026-89671 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89671 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:H/I:H/A:N';CWE-281;CWE-284;CWE-269;Other CVSS 'AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '8.1';DESCR 'NFSv3 SETACL handling in nfsd can silently remove POSIX ACLs because missing ACL arms are represented as NULL pointers, and set_posix_acl interprets NULL as a request to delete that ACL type. A remote authenticated NFS client can trigger this by sending a SETACL request where the mask omits NFS_ACL or NFS_DFACL, causing the server to remove an access ACL or a default ACL that the client did not explicitly provide. For the CVSS the PR:L is used because the attacker needs network access to the NFS export and enough user level authorization to issue SETACL against the target object, but administrator privileges are not normally required for owned files or directories. The issue is network reachable through the NFS control path and is not triggered by local kernel code execution. Impact is an access control policy bypass that can affect confidentiality and integrity of shared files, especially on multi user NFS exports where default ACL inheritance is used to enforce security policy.';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
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
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
* Raw paranoid score: 7
* Paranoid score: 7
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
No floor or cap was applied.
2. Signal breakdown
Conservative positive signals:
* +2 Remote reachable / directly network-triggerable. The vulnerable operation is in the NFS server NFSv3 ACL RPC path and can be reached by a remote NFS client that can issue SETACL requests.
* +2 Security-boundary / access-control policy bypass. Missing mask gating causes an omitted ACL arm to be reinterpreted as a request to remove that ACL type, changing externally meaningful POSIX ACL access-control state.
* +1 Integrity plausible. The bug can silently modify on-disk ACL policy by removing access or default ACLs that the client did not explicitly request to remove.
Paranoid additional signals:
* +1 Confidentiality plausible. Removing an access ACL or default ACL can weaken restrictions on shared NFS content and may expose data to principals that should have remained restricted.
* +1 Important filesystem/storage path. POSIX ACL metadata on an exported filesystem directly controls access decisions for shared files and directories.
Non-awarded signals:
* RealUAF not awarded. No use-after-free, stale pointer use, double free, refcount misuse, or memory lifetime corruption is established.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated.
* Kernel crash or DoS not demonstrated.
* Privilege escalation not demonstrated.
* Sensitive file or credential theft not awarded because no specific protected file, credential, or FD theft chain is supplied.
* Practical root exploit chain not awarded.
* Extra availability not awarded because the issue is access-control policy corruption, not host crash or resource exhaustion.
3. Reachability analysis
A remote authenticated NFS client can reach the vulnerable path by sending an NFSv3 SETACL request to an exported filesystem where ACL support is enabled. The attacker needs sufficient NFS-level authorization to issue SETACL against the target object, but host administrator privilege is not established and is not normally required for ACL operations on objects the user owns or can administer.
HostAdminRequired is NO because the complete relevant trigger is a remote NFS client operation below host-admin-equivalent privilege. NamespaceScopedPrivilege is NO because this is not primarily a namespace-scoped local capability issue.
Prior server-side setup is required in the normal deployment sense: the server must export NFSv3 with ACL handling available and the attacker must have network reachability to that export. This is not treated as a negative score because it is the ordinary exposure model for the affected nfsd path. No special hardware, boot mode, race timing, or local code execution is required.
4. Primitive and severity interpretation
The exact primitive is semantic reinterpretation of an omitted protocol field as an ACL deletion request. In the decoder, acl_access and acl_default are populated only when the corresponding mask bits are set. In nfsd3_proc_setacl, the old code called set_posix_acl unconditionally, and set_posix_acl with NULL means remove this ACL type. Therefore, a request that omits NFS_ACL or NFS_DFACL can remove access or default ACL state instead of leaving it unchanged.
This is not memory corruption and not a UAF. It is a filesystem access-control policy integrity bug with potential confidentiality consequences. The demonstrated consequence is silent removal of POSIX ACLs. The speculative stronger consequence is exposure or modification of shared files due to weakened ACL policy, especially where default ACL inheritance is relied on for multi-user access control.
PrimitiveClass is CLASS_A because a concrete security-boundary/access-control policy primitive is established. ImportantCandidate is YES because Class A exists and the issue is network reachable through NFS. AutoDowngradeAllowed is NO because automatically treating this as ordinary Moderate could discard a real ACL policy bypass affecting confidentiality and integrity.
Likely operational severity is at least Actionable Moderate. It can be a Strong Important candidate in environments where NFSv3 ACLs enforce isolation for shared data.
5. One-sentence report phrase
NFSv3 SETACL handling in nfsd can silently remove POSIX access or default ACLs when a remote authorized client omits the corresponding mask bit, causing an access-control policy bypass with integrity and possible confidentiality impact.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Manual review is required because the issue is network reachable and directly affects filesystem access-control policy, even though it is not a memory corruption bug.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: nfsd: gate nfs3 setacl by argp->mask
Commit: 68a80b26efdff1d09f8ae1773c6a915abb9e191d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=68a80b26efdff1d09f8ae1773c6a915abb9e191d
Commit description:
nfsd3_proc_setacl() calls set_posix_acl() unconditionally for both
ACL_TYPE_ACCESS and ACL_TYPE_DEFAULT, passing argp->acl_access and
argp->acl_default verbatim. The NFSv3 ACL decoder only populates
those pointers when the corresponding mask bit is set:
nfs3svc_decode_setaclargs()
if (args->mask & NFS_ACL) decode into acl_access
if (args->mask & NFS_DFACL) decode into acl_default
/* otherwise the pointer stays NULL (pc_argzero) */
nfsd3_proc_setacl()
set_posix_acl(.., ACL_TYPE_ACCESS, argp->acl_access)
set_posix_acl(.., ACL_TYPE_DEFAULT, argp->acl_default)
set_posix_acl(idmap, dentry, type, NULL) is the VFS "remove this
ACL type" operation. A NULL pointer that means "the client did not
send this arm" is therefore indistinguishable from "the client
asked to remove this ACL". A SETACL with mask=NFS_ACL silently
drops the directory's default ACL; mask=0 drops both.
The sibling nfsd3_proc_getacl() already consults argp->mask before
touching each arm; mirror that in setacl.
Fix by wrapping each set_posix_acl() call in the matching mask bit
check and initializing error to 0 before inode_lock so that a
request with neither bit set leaves the on-disk ACLs untouched and
returns nfs_ok. The out_drop_lock path and the unconditional
posix_acl_release() at out: are preserved; both NULL-tolerate the
skipped arms.
Fixes: a257cdd ("[PATCH] NFSD: Add server support for NFSv3 ACLs.")
Cc: [email protected]
Assisted-by: kres:claude-opus-4-7
Reported-by: Chris Mason <[email protected]>
Signed-off-by: Chris Mason <[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/nfs3acl.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=68a80b26efdff1d09f8ae1773c6a915abb9e191d
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89671 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89671
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:H/I:H/A:N
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: 5
Paranoid ActionableScore: 7
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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