[CVE-2026-46243][IMPORTANT] smb: client: reject userspace cifs.spnego descriptions

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

CVE: CVE-2026-46243
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: smb: client: reject userspace cifs.spnego descriptions
Commit: 7713bd320ed4fc3d08a227cd8e41242219a16981
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7713bd320ed4fc3d08a227cd8e41242219a16981
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46243
Analysis date: Mon, 01 Jun 2026 12:52:51 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local userspace process could craft `cifs.spnego` key descriptions containing authority-bearing fields that `cifs.upcall` treats as kernel-originating input, potentially causing SMB/CIFS Kerberos credential confusion and unauthorized confidentiality or integrity impact.

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

The original Linux kernel CVE announcement for CVE-2026-46243 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46243

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-2026-46243	IMPORTANT	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-863;CWE-287;CWE-807;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N';BEST CVSS score: '7.1';DESCR 'The cifs.spnego key description contains authority-bearing fields such as pid, uid, creduid, and upcall_target that cifs.upcall treats as kernel-originating inputs. Before this change, a local userspace process could create or request cifs.spnego keys with crafted descriptions through request_key(2) or add_key(2), which may cause credential confusion in the SMB/CIFS Kerberos upcall path. For the CVSS the PR:L is used because an unprivileged local process can reach the key creation interface once the key type is available, while reliable impact depends on CIFS Kerberos upcall configuration. The issue is not network reachable by itself and requires local code execution on the client. Impact is potential unauthorized confidentiality and integrity impact against SMB/CIFS authentication or mounted resource access, not a kernel memory corruption primitive or a direct denial of service.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DISK SERVERTOCLIENT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	NO	NO	guess

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

ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. Userspace can reach `request_key(2)` or `add_key(2)` and create or request `cifs.spnego` keys once the key type is available.
* Security-boundary bypass with practical sensitive-object access: +2. The key description carries authority-bearing fields such as `pid`, `uid`, `creduid`, and `upcall_target`, and `cifs.upcall` treats them as kernel-originating inputs.
* Confidentiality impact plausible: +1. Crafted descriptions may affect which credential context is used for SMB/CIFS Kerberos authentication.
* Integrity impact plausible: +1. Credential confusion may allow unauthorized SMB/CIFS authenticated access or operations under the wrong identity.

Paranoid additional signals:

* Practical exploit chain strongly implied: +1. The commit directly states that userspace can supply fields that should only originate from CIFS and that the helper trusts them.
* Broad/common enterprise exposure: +1. CIFS/SMB with Kerberos upcalls is common in enterprise Linux environments, even if not universally enabled by default.

Not counted:

* No generic memory corruption. The patch fixes an authorization/input-origin validation issue, not UAF/OOB/write corruption.
* No remote network trigger. The attacker needs local userspace execution on the SMB client.
* No reliable kernel crash or DoS signal.

## 3. Reachability analysis

The bug is locally reachable by a userspace process through Linux keyring operations when the `cifs.spnego` key type is registered and CIFS Kerberos upcall support is present. It does not require packet-level network reachability to the client, and it is not directly triggered by an SMB server alone.

In typical deployments, practical impact depends on CIFS mounts, `cifs.upcall`, Kerberos/SPNEGO configuration, and keyring permissions. Containers and restricted service accounts may matter because the attacker does not need full host root if they can access the relevant keyring interface and the key type is available. This should not be treated as PR:H merely because CIFS mount setup is often administrative.

## 4. Severity interpretation

This behaves more like an actionable access-control or confused-deputy vulnerability than an ordinary Moderate cleanup. There is no demonstrated memory corruption or direct LPE primitive, but the affected data is authority-bearing and trusted by a privileged authentication helper path.

Realistic exploitation is configuration-dependent, but the security boundary is meaningful: userspace can inject fields that should be kernel-originated. That supports at least Actionable Moderate handling, and paranoid triage can treat it as a Strong Important candidate because confidentiality and integrity impact against SMB/CIFS authentication is plausible.

## 5. One-sentence report phrase

A local userspace process could craft `cifs.spnego` key descriptions containing authority-bearing fields that `cifs.upcall` treats as kernel-originating input, potentially causing SMB/CIFS Kerberos credential confusion and unauthorized confidentiality or integrity impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

This is not memory corruption, but it is a credential and authorization-boundary issue in an authentication upcall path. The final severity depends heavily on CIFS/Kerberos deployment details and whether crafted key descriptions can produce unauthorized access to sensitive SMB resources.

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

Patch: smb: client: reject userspace cifs.spnego descriptions
Commit: 7713bd320ed4fc3d08a227cd8e41242219a16981
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7713bd320ed4fc3d08a227cd8e41242219a16981

Commit description:

cifs.spnego key descriptions contain authority-bearing fields such as
pid, uid, creduid, and upcall_target that cifs.upcall treats as
kernel-originating inputs. However, userspace can also create keys of
this type through request_key(2) or add_key(2), allowing those fields to
be supplied without CIFS origin.

Only accept cifs.spnego descriptions while CIFS is using its private
spnego_cred to request the key.

Fixes: f1d662a ("[CIFS] Add upcall files for cifs to use spnego/kerberos")
Assisted-by: avom-custom-harness:gpt-5.5-qwen3.6-mod-mix
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
Signed-off-by: Steve French <stfrench@microsoft.com>
[Salvatore Bonaccorso: Apply changes to fs/cifs/cifs_spnego.c instead of
fs/smb/client/cifs_spnego.c before 38c8a9a ("smb: move client and server
files to common directory fs/smb") in v6.4-rc1 and backported to v6.1.36]
Signed-off-by: Salvatore Bonaccorso <carnil@debian.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/cifs/cifs_spnego.c
  fs/smb/client/cifs_spnego.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=7713bd320ed4fc3d08a227cd8e41242219a16981

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

The original Linux kernel CVE announcement for CVE-2026-46243 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-46243

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:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
  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: 5
  Paranoid ActionableScore: 7

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).