[CVE-2026-77773][IMPORTANT] <title>smb: client: reject userspace cifs.spnego descriptions</title>

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

CVE: CVE-2026-77773
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: <title>smb: client: reject userspace cifs.spnego descriptions</title>
Commit: 3da1fdf4efbc490041eb4f836bf596201203f8f2
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3da1fdf4efbc490041eb4f836bf596201203f8f2
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-77773
Analysis date: Fri, 29 May 2026 08:56:49 -0400
ActionableScore: 8
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local user could forge `cifs.spnego` key descriptions containing authority-bearing fields consumed by `cifs.upcall`, potentially causing CIFS Kerberos authentication to use attacker-controlled identity context and exposing SMB resources under the wrong credentials.

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

The original Linux kernel CVE announcement for CVE-2026-77773 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-77773

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-77773	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:L';CWE-287;CWE-863;*CWE-20;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '7.3';DESCR 'CIFS cifs.spnego key descriptions include authority-bearing fields such as pid, uid, creduid, and upcall_target that cifs.upcall treats as kernel-originating input. In the vulnerable state, a local user could create or request a cifs.spnego key through add_key(2) or request_key(2) and supply those fields without CIFS origin. This is a local authentication and authorization confusion issue that can make the upcall helper act on attacker-controlled identity context, with plausible confidentiality and integrity impact through misuse of SMB or Kerberos credential selection. For the CVSS the PR:L is used because an unprivileged local process can reach the key creation or request interface, while no remote network packet alone triggers the vulnerable path. The issue is not directly network reachable, but it can affect network filesystem authentication when CIFS Kerberos or SPNEGO upcalls are configured.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 8)	MAYBE	DISK SERVERTOCLIENT KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN7	-	-	checked

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

ActionableScore=8
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative score signals:

* Local unprivileged trigger: +1. Userspace can reach `add_key(2)` or `request_key(2)` without needing full host root.
* Security-boundary bypass with practical sensitive-object access: +2. The bug allows userspace to forge authority-bearing fields that `cifs.upcall` treats as kernel-originating.
* Sensitive file / credential / file-descriptor theft primitive: +2. The affected path can plausibly influence SMB/Kerberos credential selection and access to network filesystem resources.
* Practical exploit chain strongly implied: +1. The patch directly blocks userspace-created `cifs.spnego` descriptions because fields like `uid`, `creduid`, `pid`, and `upcall_target` are trusted by the helper.
* Confidentiality impact plausible: +1. Forged credential context can plausibly expose SMB data under the wrong identity.
* Integrity impact plausible: +1. Wrong credential selection can plausibly allow unauthorized writes to SMB resources.
* Configuration dependency: -1. Practical impact requires CIFS Kerberos/SPNEGO upcalls to be configured and used.

Paranoid difference:

* Paranoid score removes the -1 deployment penalty because CIFS/SMB with Kerberos is common in enterprise environments, and the primitive is an authentication/authorization confusion rather than a mere crash.

No memory corruption, UAF, OOB write, or kernel code execution primitive is counted.

## 3. Reachability analysis

A local unprivileged user can potentially trigger the vulnerable path through key creation or key request interfaces. The bug is not directly network reachable by a remote packet, but it affects authentication for a network filesystem when CIFS Kerberos/SPNEGO upcalls are active.

Namespaces and containers may matter because reduced-capability users, service accounts, or container-like environments may still have access to keyring operations while not being fully trusted host administrators. This should not be treated as PR:H. The realistic exploitation condition is the presence of CIFS mounts or CIFS authentication flows using `cifs.upcall`.

## 4. Severity interpretation

This behaves more like an Important-class authentication and authorization confusion issue than an ordinary Moderate DoS. There is no demonstrated memory corruption, but the security impact is not limited to crashes. The key risk is that userspace can supply fields that the upcall helper interprets as kernel-authenticated authority context.

Theoretical impact is high because credential selection and SMB access decisions may be affected. Realistic impact depends on CIFS/SPNEGO deployment, keyring behavior, and how `cifs.upcall` maps the forged fields to credentials.

## 5. One-sentence report phrase

A local user could forge `cifs.spnego` key descriptions containing authority-bearing fields consumed by `cifs.upcall`, potentially causing CIFS Kerberos authentication to use attacker-controlled identity context and exposing SMB resources under the wrong credentials.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is not a memory corruption bug, but it is a security-boundary and credential-selection issue in an authentication path. Manual review is needed to determine how broadly the forged `pid`, `uid`, `creduid`, and `upcall_target` fields can affect real CIFS/Kerberos deployments.

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

Patch: <title>smb: client: reject userspace cifs.spnego descriptions</title>
Commit: 3da1fdf4efbc490041eb4f836bf596201203f8f2
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3da1fdf4efbc490041eb4f836bf596201203f8f2

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: f1d662a7d5e5 ("[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>

Changed files:
  fs/smb/client/cifs_spnego.c

Diff excerpt:

diff --git a/fs/smb/client/cifs_spnego.c b/fs/smb/client/cifs_spnego.c
index 3a41bbada04c7..44c4072756804 100644
--- a/fs/smb/client/cifs_spnego.c
+++ b/fs/smb/client/cifs_spnego.c
@@ -8,6 +8,7 @@
  */

 #include <linux/list.h>
+#include <linux/cred.h>
 #include <linux/slab.h>
 #include <linux/string.h>
 #include <keys/user-type.h>
@@ -40,12 +41,27 @@ cifs_spnego_key_destroy(struct key *key)
 	kfree(key->payload.data[0]);
 }

+static int
+cifs_spnego_key_vet_description(const char *description)
+{
+	/*
+	 * cifs.spnego descriptions are authority-bearing inputs to cifs.upcall.
+	 * They are only valid when produced by CIFS while using the private
+	 * spnego_cred installed below.  Do not let userspace create this type
+	 * of key through request_key(2)/add_key(2), since the helper treats
+	 * pid/uid/creduid/upcall_target as kernel-originating fields.
+	 */
+	if (current_cred() != spnego_cred)
+		return -EPERM;
+	return 0;
+}

 /*
  * keytype for CIFS spnego keys
  */
 struct key_type cifs_spnego_key_type = {
 	.name		= "cifs.spnego",
+	.vet_description = cifs_spnego_key_vet_description,
 	.instantiate	= cifs_spnego_key_instantiate,
 	.destroy	= cifs_spnego_key_destroy,
 	.describe	= user_describe,
generated by cgit 1.3-korg (git 2.53.0) at 2026-05-29 12:40:08 +0000

Full patch:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3da1fdf4efbc490041eb4f836bf596201203f8f2

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

The original Linux kernel CVE announcement for CVE-2026-77773 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-77773

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:L/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:L
  The Best / paranoid CVSS score: 7.3

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).