[CVE-2026-89633][IMPORTANT] smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2() [ Upstream

[CVE-2026-89633][IMPORTANT] smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-89633
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2() [ Upstream
Commit: 672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89633
Analysis date: Fri, 11 Sep 2026 23:51:12 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-89633 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89633

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: IMPORTANT
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-2026-89633	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-89633 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-125;*CWE-20;CWE-119;*CWE-787;CWE-125;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H';BEST CVSS score: '9.8';DESCR 'An OOB read and OOB write in the SMB client path occurs because coalesce_t2() computes source and target data pointers directly from server supplied DataOffset values without validating them against buffer bounds. A malicious or compromised SMB server can return crafted SMB1 Trans2 response fragments so that memcpy reads from outside the secondary response buffer or writes outside the target response buffer in kernel memory. For the CVSS the PR:N is selected because the attacker does not need privileges on the victim system, only the ability to act as or tamper with the SMB server endpoint reached by the client. The base score uses UI:R because a client normally has to mount or access the SMB share, while the paranoid score can use UI:N for preconfigured or automatic mounts. Impact is kernel memory corruption with likely denial of service and a plausible path to confidentiality and integrity impact or privilege escalation. SMB/CIFS shares are often deployed on internal enterprise networks rather than the public Internet, but any reachable malicious server or MITM on the client path can be relevant.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 8)	YES	WRITE OOB DISK IMPROVEONLY SERVERTOCLIENT LINUS KPANIC 	NO	NO	checked

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

ActionableScore=8
ActionableScoreLower=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 7
* Applied conservative floor/cap: CLASS_A Important-preservation floor for remotely influenced kernel heap OOB write: 6 -> 7
* Raw paranoid score: 8
* Paranoid score: 8
* Applied paranoid floor/cap: none
* Final recommended bucket: **Strong Important candidate**::

2. Signal breakdown

Conservative signals:

* +2 Strong corruption primitive. Server-supplied DataOffset is used to compute source and target pointers and can cause an attacker-influenced kernel heap OOB write in memcpy.
* +2 Remote reachable / network influenced. A malicious or compromised SMB server, or a MITM on the SMB path, can supply the malformed Trans2 response fragments.
* +1 Reliable kernel crash / strong DoS. OOB write or OOB read in kernel heap parsing can realistically crash the client kernel.
* +1 Integrity plausible. The target-side OOB write can modify kernel heap memory using data influenced by the SMB response path.
* +1 Important filesystem/storage path. This is in the kernel SMB/CIFS client handling remote filesystem responses.
* -1 Legacy/non-default execution mode. The affected path is SMB1 Trans2, which is legacy and often disabled or avoided in modern deployments.

Paranoid additional or changed signals:

* +1 Memory layout invariant restoration. The fix restores bounds consistency between DataOffset, the actual SMB buffer, and the coalesced target buffer before memcpy and header mutation.
* Legacy penalty withheld in paranoid score because environments that still use SMB1 mounts remain directly exposed once a client reaches a malicious server.

Important non-awarded signals:

* RealUAF not awarded. This is OOB read/write, not a lifetime or stale-reference bug.
* Controlled reclaim not demonstrated.
* Arbitrary write not demonstrated. The primitive is an attacker-influenced OOB heap write, not proven arbitrary kernel write.
* Practical LPE not demonstrated.
* Confidentiality as a conservative signal not awarded. The OOB read is real, but a direct attacker-visible disclosure path is not fully established from the supplied patch.
* Extra availability not awarded. The same crash is already counted once.
* No namespace-scoped privilege conclusion. Namespaces are not material to the supplied trigger.

3. Reachability analysis

The attacker model is a malicious or compromised SMB server, or a network attacker able to tamper with SMB1 server responses on the client path. The vulnerable operation is reached when the Linux SMB/CIFS client processes fragmented SMB1 Trans2 responses and coalesces secondary response data.

The attacker does not need privileges on the victim host. For the CVSS-like attacker model this maps to PR:N, because the malicious endpoint controls protocol fields consumed by the victim kernel client. A user action or prior configuration may be needed for the client to connect to or mount the SMB share, but once the client reaches the attacker-controlled server, the malformed response is network supplied.

HostAdminRequired=NO because host-admin-equivalent privilege by the attacker is not required to supply the malicious server response. NamespaceScopedPrivilege=UNKNOWN because namespace capability scope is not relevant or established here. Prior privileged setup may exist in some deployments if an administrator configured an SMB mount, while other environments may allow user-driven or automatic access to SMB resources.

No special hardware is required. No race is required. The main deployment limiter is SMB1 Trans2 usage, which is legacy, but the primitive is strong when the path is enabled and reachable.

4. Primitive and severity interpretation

The exact primitive is a server-controlled bounds failure in coalesce_t2(). DataOffset is used to derive data_area_of_src and data_area_of_tgt without validating that either pointer remains inside the corresponding SMB buffer. The subsequent memcpy can read past the secondary response buffer or write before or beyond the target response buffer in kernel heap memory.

This is active kernel heap memory corruption, not a fixed crash-only invalid dereference. It is not a UAF and there is no concrete repeated release/refcount operation. The demonstrated consequence is at least kernel crash or memory corruption. The speculative but technically defensible consequence is stronger exploitation through kernel heap corruption, depending on allocator state, exact overwrite placement, and whether useful target objects can be corrupted.

PrimitiveClass=CLASS_A because an attacker-influenced OOB write into kernel heap memory is directly established. ImportantCandidate=YES because this is a network-influenced kernel memory corruption primitive in a filesystem client. AutoDowngradeAllowed=NO because automatically treating this as ordinary Moderate would lose a concrete strong corruption primitive. Operational severity is Strong Important candidate, with possible higher concern in environments with automatic SMB1 mounts or exposure to untrusted SMB servers.

5. One-sentence report phrase

A malicious SMB1 server can provide crafted Trans2 DataOffset values that make the Linux SMB client perform an out-of-bounds heap read or write during response coalescing, causing kernel memory corruption and likely denial of service with plausible integrity impact.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a concrete network-influenced kernel heap OOB write, and the complete exploitation leverage beyond crash is unresolved but security-relevant.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb: client: fix OOB read/write from unvalidated DataOffset in coalesce_t2() [ Upstream
Commit: 672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e

Changed files:
  fs/smb/client/smb1transport.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=672cf86aa6aa0fb4012ce4c3b3498df42ad67a4e

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

The original Linux kernel CVE announcement for CVE-2026-89633 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89633

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:N/UI:R/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 9.8

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: 7
  Paranoid ActionableScore: 8

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).