* [CVE-2026-98261][MODERATE 7.0] cifs: Fix server use-after-free in cifs_chan_skip_or_disable()
@ 2026-10-10 19:18 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-10 19:18 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98261
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: cifs: Fix server use-after-free in cifs_chan_skip_or_disable()
Commit: 0338489960ddad6368bce55e8adbc176c523093d
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0338489960ddad6368bce55e8adbc176c523093d
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98261
Analysis date: Sat, 10 Oct 2026 15:18:02 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98261 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98261
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-2026-98261 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98261 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC) 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-672;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free occurs in cifs_chan_skip_or_disable() because cifs_put_tcp_session() can release the final reference to a secondary channel and also drop its reference to the primary server. The function then passes the potentially freed server object to cifs_signal_cifsd_for_reconnect() and reads server primary_server, creating a concrete stale object access with possible read and write effects. A malicious or compromised SMB server may be able to reach this path by changing multichannel support after secondary channels have been established. For the CVSS the PR:N value is used because a remote SMB server does not need privileges on the affected client once the CIFS session exists. AC:H reflects the required multichannel state, channel teardown conditions, and final reference lifetime. Impact is at least remote denial of service through a kernel crash. In the worst still defensible case, reuse of the freed object may affect confidentiality and integrity or permit privilege escalation, but reliable exploitation beyond a crash requires manual confirmation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) MAYBE DISK UAF SERVERTOCLIENT LINUS KPANIC KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 4
* Raw paranoid score: 3
* Paranoid score: 4
* Applied conservative REAL UAF FLOOR 4: 3 -> 4.
* Applied paranoid REAL UAF FLOOR 4: 3 -> 4.
* No cap was applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Positive signals:
* +2 Real UAF. `cifs_put_tcp_session(server, ...)` can drop the final channel reference and free both `server` and its referenced primary server. The code then uses `server`.
* +1 Realistic kernel crash. Passing the stale object to `cifs_signal_cifsd_for_reconnect()` and reading `server->primary_server` can produce an Oops or other fatal kernel fault.
Not awarded:
* Remote reachability +2 was withheld. An SMB peer influences channel support, but the path requires an existing CIFS multichannel session and the supplied evidence does not establish a direct unauthenticated trigger without prior client configuration.
* Strong or structured stale-object leverage was not awarded. No controlled reclaim, attacker-shaped replacement object, callback control, arbitrary write, or directed read is demonstrated.
* Attacker-influenced repeatable lifetime manipulation was not awarded. The server can influence protocol state, but reliable repeated control over the final-reference teardown and post-free replacement is not established.
* Confidentiality and integrity points were not awarded. A possible stale helper write or theoretical reuse is insufficient to establish disclosure or unauthorized modification.
* No extra Availability point was awarded because it would duplicate the same possible kernel crash.
* No race penalty was applied. This is an ordered release followed by stale use, not a demonstrated narrow timing race.
* No uncommon-subsystem penalty was applied merely because the vulnerable path requires CIFS multichannel.
Repeated-release assessment:
* The first `cifs_put_tcp_session()` can end the object lifetime: yes.
* The exact `server` reference survives and is subsequently used: yes.
* A second release or refcount mutation on the same stale reference occurs: no.
* Concrete repeated release/refcount +1: not awarded.
* Paranoid repeated-refcount leverage +1: not applicable.
* FLOOR 5: not applied because the later operations are reconnect signaling and field access, not a second concrete lifetime mutation, and meaningful post-reuse leverage is not established.
3. Reachability analysis
The relevant attacker model is a malicious or compromised SMB server interacting with a client that already has a multichannel CIFS session. Such a server can influence whether a secondary channel remains supported, so the remote peer itself does not require host UID 0 or host-administrator powers.
The client-side CIFS mount and multichannel setup are prior deployment prerequisites and may have been configured by an administrator. This prevents treating the issue as directly network-triggerable for the scoring bonus, but it does not make the remote server a host administrator.
No namespace capability is part of the demonstrated trigger. No special hardware, boot option, firmware compromise, or attacker-created namespace is required. The bug is not a timing race, although reaching the dangerous case requires the dropped reference to be the final reference.
4. Primitive and severity interpretation
The exact primitive is a post-final-put use-after-free of an active structured `TCP_Server_Info` object. After `cifs_put_tcp_session()` potentially destroys the secondary channel and releases its primary-server reference, the stale channel pointer is passed to reconnect signaling code and dereferenced to obtain `primary_server`.
This is stronger than an invalid pointer with no lifetime explanation, so it is CLASS_B and `RealUAF=YES`. It is not a concrete repeated-release case because the stale path does not perform another visible put, free, or refcount decrement on the same reference.
The demonstrated practical consequence is a kernel fault or denial of service. Memory reuse could theoretically produce more complex corruption, but controlled reclaim, useful replacement contents, arbitrary read or write, disclosure, and privilege escalation are not established. The channel-disable context and lack of a demonstrated exploitation primitive beyond stale access make this a constrained CLASS_B case.
`ImportantCandidate=NO` because the evidence does not establish meaningful reuse, callback, write, disclosure, boundary-bypass, or repeated lifetime-mutation leverage. `AutoDowngradeAllowed=YES` because the unresolved evidence currently supports a crash-oriented channel-teardown UAF rather than an Important-preserving corruption primitive. Human review remains recommended due to the real network-influenced kernel lifetime violation.
B5-R, B3-PENDING, and B4-PENDING do not apply. There is no retry-accumulated OOB write, shared-backing write, or authorization decision bypass.
5. One-sentence report phrase
A malicious or compromised SMB server may trigger a client-side use-after-free during CIFS multichannel disable handling, potentially crashing the kernel, although controlled reclaim or a useful post-free corruption primitive is not established.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
The UAF and final-reference teardown are concrete, but review is needed to determine whether reconnect signaling performs exploitable writes or whether the freed server object can be predictably reclaimed rather than merely causing a crash.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: cifs: Fix server use-after-free in cifs_chan_skip_or_disable()
Commit: 0338489960ddad6368bce55e8adbc176c523093d
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=0338489960ddad6368bce55e8adbc176c523093d
Commit description:
When a secondary channel is no longer supported by the server,
cifs_chan_skip_or_disable() drops the channel reference with
cifs_put_tcp_session() and then continues to use the server pointer by
calling cifs_signal_cifsd_for_reconnect() on it and reading its
primary_server pointer. cifs_put_tcp_session() can drop the last
reference of the channel and tear it down, so both the channel and the
primary server (whose reference is also dropped by
cifs_put_tcp_session()) can be freed before they are signaled for
reconnect.
Signal the channel and the primary server and capture the primary
server pointer before dropping the channel reference with
cifs_put_tcp_session().
Fixes: f591062 ("cifs: handle servers that still advertise multichannel after disabling")
Cc: [email protected]
Signed-off-by: Wentao Liang <[email protected]>
Signed-off-by: Paulo Alcantara <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/smb/client/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=0338489960ddad6368bce55e8adbc176c523093d
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98261 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98261
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:L/I:L/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: 4
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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