[CVE-2026-72422][MODERATE 7.0] ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE [ Upstream

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

CVE: CVE-2026-72422
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE [ Upstream
Commit: c7bef84740d1d57848c74f6f5b996606e43ea4fe
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7bef84740d1d57848c74f6f5b996606e43ea4fe
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72422
Analysis date: Sat, 15 Aug 2026 03:18:43 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote unauthenticated SMB client may trigger a race in ksmbd SMB2 NEGOTIATE handling where `conn->preauth_info` is freed by one worker and read by another, causing a slab use-after-free read and kernel crash.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-72422 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72422

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-2026-72422	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	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-416;CWE-362;CWE-667;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free in the ksmbd SMB2 NEGOTIATE response path occurs because conn preauth_info can be freed by one worker while another worker reads conn preauth_info Preauth_HashValue without holding the connection lock. A remote client can trigger the race by driving concurrent SMB2 NEGOTIATE requests on the same connection, causing a slab use-after-free read and a kernel crash. For the CVSS the PR:N is used because SMB2 NEGOTIATE is processed before session authentication and the attacker only needs network reachability to the ksmbd service. The issue is network reachable over SMB and is most relevant when ksmbd is exposed to untrusted clients or reachable from a local or internal network. Impact is at least remote denial of service via kernel crash. In the paranoid score, the network reachable kernel heap UAF is treated as possible confidentiality and integrity impact and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-72422 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE DISK RACE LEAK UAF SKIP  KPANIC INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  SMB2 NEGOTIATE is reachable over the ksmbd SMB service before session authentication.

* Memory corruption, weak/indirect corruption candidate: +1
  This is a KASAN-confirmed slab use-after-free read, but the patch does not show attacker-controlled reclaim, overwrite, callback control, type confusion, or refcount takeover.

* Real lifetime corruption: +1
  `conn->preauth_info` can be freed by one worker while another worker dereferences it without holding `ksmbd_conn_lock`.

* Reliable kernel crash / strong DoS: +1
  The confirmed consequence is a kernel UAF read that can crash the system.

* Hard or unreliable race / special timing required: -1 in conservative score
  Triggering requires concurrent SMB2 NEGOTIATE handling on the same connection. The paranoid score does not subtract this because the race is directly described and remotely driveable by request concurrency.

* Race-UAF downgrade rule applied
  No LPE bonus, no confidentiality bonus, and no integrity bonus are counted for the conservative score because the patch only demonstrates stale read after free, not controlled reuse or write-after-free.

## 3. Reachability analysis

A remote unauthenticated client can potentially trigger the bug if the ksmbd service is enabled and reachable. The affected path is SMB2 NEGOTIATE, which occurs before authenticated session setup, so no SMB credentials are required. Namespaces and containers do not materially reduce the remote exposure once the host ksmbd listener is reachable. The subsystem is not universally enabled by default on all Linux systems, but it is realistic in NAS, appliance, file-server, and embedded SMB deployments.

Call-site confidence: high. The commit message and patch show the allocation/free side, the unlocked send-path read, and the locking fix at the dereference site.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a network-reachable pre-authentication kernel lifetime bug with confirmed UAF behavior. Realistically, the demonstrated impact is remote denial of service through a kernel crash. Theoretical escalation beyond DoS is not proven by the patch because there is no demonstrated attacker-controlled reclaim, write primitive, object replacement, or callback dispatch. For triage, the issue should remain actionable due to remote pre-auth reachability and kernel heap UAF semantics.

## 5. One-sentence report phrase

A remote unauthenticated SMB client may trigger a race in ksmbd SMB2 NEGOTIATE handling where `conn->preauth_info` is freed by one worker and read by another, causing a slab use-after-free read and kernel crash.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: network-reachable pre-auth kernel UAF with confirmed KASAN evidence should not be auto-closed, even though the currently supported practical impact is DoS rather than proven privilege escalation.

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

Patch: ksmbd: fix use-after-free of conn->preauth_info in concurrent SMB2 NEGOTIATE [ Upstream
Commit: c7bef84740d1d57848c74f6f5b996606e43ea4fe
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c7bef84740d1d57848c74f6f5b996606e43ea4fe

Changed files:
  fs/ksmbd/smb2pdu.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=c7bef84740d1d57848c74f6f5b996606e43ea4fe

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

The original Linux kernel CVE announcement for CVE-2026-72422 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72422

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:H/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: 4
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).