[CVE-2026-98959][MODERATE 7.0] btrfs: fix xattr replace when multiple xattrs are packed in the same item [ Upstream

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-98959
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: btrfs: fix xattr replace when multiple xattrs are packed in the same item [ Upstream
Commit: 7a53bb4668b319fc6c8a3ce2c21635125c7d2712
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7a53bb4668b319fc6c8a3ce2c21635125c7d2712
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98959
Analysis date: Mon, 26 Oct 2026 21:26:10 -0400
ActionableScore: 6
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A permitted local xattr replacement can leave active Btrfs leaf metadata uninitialized, causing xattr loss and privileged tree/writeback processing to abort the transaction and force the filesystem read-only.

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

The original Linux kernel CVE announcement for CVE-2026-98959 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98959

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-98959	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98959 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H';CWE-665;CWE-908;*CWE-20;BEST CVSS score: '6.1';DESCR 'btrfs_setxattr can leave a newly placed btrfs_dir_item partly uninitialized when replacing one of several packed xattrs with a differently sized value. A local user who can set a user xattr on a file can trigger the known CRC32C name collision and cause xattr loss, tree checker corruption detection, transaction abort, and the affected Btrfs filesystem to be forced read only. For the CVSS the PR:L value reflects that exploitation requires a local user or process with permission to modify xattrs on a file, but does not normally require administrative privileges. There is no supported OOB, UAF, or arbitrary write primitive, so the grounded impact is filesystem integrity loss and high availability impact rather than kernel privilege escalation. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 6.1 and the production ActionableScore returned 6 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the manual review requirement was applied. ActionableScore thresholds stage MODERATE_KPANIC to IMPORTANT because the ActionableScore threshold rule was applied. KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK and INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 and DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP. The independent weaker Aegis shadow evaluated CVSS 6.1 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	OOB DANGER DISK SKIP KERNEL_PANIC_PLUS_UAF HARDWARE  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHERTHAN5 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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

ActionableScore=6
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw base score: 6.
* Conservative score: 6; no floor or cap applied.
* Raw paranoid score: 6.
* Paranoid score: 6; no additional speculative effects scored.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +1 Local unprivileged trigger: a user with ordinary permission to set `user.*` xattrs can invoke the faulty replacement; host administration is not required.
* +1 Weak/indirect structured corruption: the replacement leaves an active `btrfs_dir_item` partially uninitialized, including name length, key, flags, and transaction ID.
* +1 Filesystem metadata corruption reachable locally: malformed xattr metadata is installed in a Btrfs leaf.
* +1 Important filesystem path: corruption affects Btrfs tree metadata and transaction/writeback processing.
* +1 Integrity impact: the replaced xattr disappears and the leaf contains invalid structured metadata.
* +1 Strong DoS: tree-checker detection aborts the transaction and forces the affected filesystem read-only.
* No extra availability point: forced read-only and transaction abort are the same demonstrated failure chain.
* No OOB/UAF/arbitrary write, controlled payload target, cross-file ownership breach, disclosure, or LPE is shown.
* NEW52/B5-R does not apply: there is no confirmed retry-accumulated heap OOB write.
* No authorization decision changes from DENY to ALLOW; B4-PENDING is inapplicable.

3. Reachability analysis

The complete trigger is grounded through `setxattr(2)` on a file whose `user.*` xattrs the local actor may modify. It requires colliding xattr-name hashes, multiple packed xattrs, and replacement with a differently sized value, but no race, special hardware, namespace capability, or host-admin setup. The supplied reproducer used UID 0, but the syscall semantics and supplied evidence support a normal permitted file owner as the attacker.

4. Primitive and severity reading

The primitive is attacker-invoked corruption of active structured filesystem metadata, not a heap OOB write or UAF. The malformed item is subsequently consumed by `btrfs_listxattr`, the tree checker, and privileged writeback/transaction code. Proven effects are xattr loss, invalid leaf metadata, transaction abort, and forced read-only operation.

ImportantCandidate is YES under the active-state/trust-boundary rule: a below-host-admin file operation corrupts active filesystem state later consumed by privileged kernel paths, while broader cross-object or persistent integrity consequences remain unresolved. This does not prove Important severity, arbitrary disk corruption, or privilege escalation. Auto-downgrade is disallowed pending review of that concrete chain.

5. One-sentence report phrase

A permitted local xattr replacement can leave active Btrfs leaf metadata uninitialized, causing xattr loss and privileged tree/writeback processing to abort the transaction and force the filesystem read-only.

6. Manual review advice

MANUAL CHECK REQUIRED

Review the low-privilege-to-global-filesystem metadata boundary and whether malformed fields can affect additional Btrfs consumers beyond the demonstrated checker-detected shutdown.

7. Call-site confidence

Call-site confidence: high

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

Patch: btrfs: fix xattr replace when multiple xattrs are packed in the same item [ Upstream
Commit: 7a53bb4668b319fc6c8a3ce2c21635125c7d2712
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7a53bb4668b319fc6c8a3ce2c21635125c7d2712

Changed files:
  fs/btrfs/extent_io.c
  fs/btrfs/disk-io.c
  fs/btrfs/xattr.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=7a53bb4668b319fc6c8a3ce2c21635125c7d2712

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

The original Linux kernel CVE announcement for CVE-2026-98959 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98959

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: Not available
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 6.1

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: 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).