[CVE-2026-93787][MODERATE 7.0] smb: client: bound dirent name against end of SMB response in cifs_filldir [ Upstream

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

CVE: CVE-2026-93787
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: bound dirent name against end of SMB response in cifs_filldir [ Upstream
Commit: 6e3b965267b7313903baba2739992b1e2e92d540
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e3b965267b7313903baba2739992b1e2e92d540
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93787
Analysis date: Thu, 24 Sep 2026 19:18:09 -0400
ActionableScore: 4
ActionableScore lower bound: 3
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-2026-93787 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93787

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-2026-93787	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-93787 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:N/UI:R/S:U/C:L/I:N/A:H';CWE-125;CWE-126;*CWE-20;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L';BEST CVSS score: '7.1';DESCR 'An out of bounds read in the SMB client directory enumeration path occurs because cifs_filldir trusts a server supplied FileNameLength without verifying that the file name remains within the SMB response buffer. A hostile SMB1 server can return an oversized directory entry name and trigger the invalid read when a client lists the mounted directory through getdents64. This may cause a KASAN failure or kernel crash and may expose adjacent kernel memory to local directory enumeration processing in the worst case. For the CVSS the PR:N is used because an attacker controlling the SMB server does not require an account or privileges on the client system. UI:R is used because a user or process must mount or access the attacker controlled SMB share and enumerate its directory. The issue is network reachable only for systems using the legacy SMB1 client path and connected to an attacker controlled or compromised SMB server. In practice this commonly requires access to an internal network or a trusted file sharing environment because SMB services should not be exposed to untrusted public networks.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4)	YES	READ KASAN OOB DISK SERVERTOCLIENT KPANIC 	NO	NO	checked

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::

2. Signal breakdown

* +2 Remote reachable input. An attacker controlled SMB1 server can supply the malformed TRANS2 directory response to a connected CIFS client.
* +1 Weak invalid-read primitive. The server controlled FileNameLength causes memchr to read beyond the kmalloc response buffer.
* +1 Paranoid only. The supplied reproducer reliably reaches a KASAN slab out of bounds report. Under KASAN or panic on warning configurations this can cause a kernel failure or host DoS.
* No controlled reclaim, UAF, write primitive, type confusion, or refcount mutation is demonstrated.
* No confidentiality point is awarded. The patch establishes an invalid read, but does not establish that bytes beyond the SMB response buffer are copied back to the remote SMB server or otherwise disclosed to the attacker.
* No integrity point is awarded. No out of bounds write or modification of protected kernel state is established.
* No extra availability point is awarded. The possible KASAN failure and kernel crash are the same potential consequence.
* No privilege penalty is applied. The attacker controlling the SMB server does not require host administrative privilege on the client. A user action or preexisting client mount is required.

3. Reachability analysis

The evaluated attacker is a hostile or compromised SMB1 server. The vulnerable operation is reached when a CIFS client enumerates a directory on an already mounted attacker controlled SMB share through getdents64.

HostAdminRequired is NO because the attacker does not need host administrative privilege on the affected client. The supplied context states that any user able to list the mounted directory can trigger the client-side processing. A victim user or automated process must access the SMB share, therefore user interaction or an existing active mount is required.

NamespaceScopedPrivilege is UNKNOWN. No namespace capability or namespace-specific privilege path is relevant to the demonstrated network-server trigger. SMB1 is a legacy protocol mode and the vulnerable path requires a client configured to use SMB1 and connected to an attacker controlled server. This limits practical exposure but is not scored as a negative point because the supplied evidence does not establish that the mode is rare or unreachable in the evaluated deployment.

4. Primitive and severity interpretation

The exact primitive is a server controlled out of bounds read from the SMB response slab buffer. cifs_filldir trusts de.namelen and passes the derived name range to later directory-entry validation code without confirming that the range remains below end_of_smb.

This is not a UAF, ownership violation, active-object lifetime violation, or out of bounds write. The demonstrated path is an invalid read in memchr while processing a directory entry. The KASAN report establishes that the read crosses the response allocation boundary, but does not establish attacker controlled replacement, arbitrary kernel read, kernel disclosure to the SMB server, corruption of active kernel objects, or privilege escalation.

PrimitiveClass is CLASS_C because the established effect is invalid-state behavior and potential crash or DoS rather than an independently useful kernel corruption primitive. ImportantCandidate is NO because the patch does not show meaningful exploitation leverage beyond the out of bounds read and possible kernel failure. AutoDowngradeAllowed is YES because no unresolved active UAF, write primitive, authorization bypass, shared ownership violation, or concrete lifetime corruption remains.

Operationally, this should receive manual review due to network reachability and a reliable KASAN reproducer, but the patch-supported evidence does not justify Important classification.

5. One-sentence report phrase

A hostile SMB1 server can send an oversized directory entry FileNameLength that causes the CIFS client to read beyond the SMB response buffer during directory enumeration, potentially resulting in a KASAN failure or kernel denial of service.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The issue is network reachable through an attacker controlled SMB1 server and has a reliable KASAN reproducer, but the supplied evidence establishes only an out of bounds read and does not demonstrate disclosure, write corruption, or privilege escalation.

7. Call-site confidence

Call-site confidence: high

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

Patch: smb: client: bound dirent name against end of SMB response in cifs_filldir [ Upstream
Commit: 6e3b965267b7313903baba2739992b1e2e92d540
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6e3b965267b7313903baba2739992b1e2e92d540

Changed files:
  fs/smb/client/readdir.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=6e3b965267b7313903baba2739992b1e2e92d540

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

The original Linux kernel CVE announcement for CVE-2026-93787 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-93787

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:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H
  The Best / paranoid CVSS score: 7.1

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: 3
  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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).