[CVE-2026-97554][MODERATE REGULAR] smb: client: avoid using uninitialized SIDs in cifs_posix_to_fattr()

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

CVE: CVE-2026-97554
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: smb: client: avoid using uninitialized SIDs in cifs_posix_to_fattr()
Commit: c9a8b60ce140a68d172452f418137fc7ddbae7db
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9a8b60ce140a68d172452f418137fc7ddbae7db
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97554
Analysis date: Fri, 25 Sep 2026 06:59:26 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97554 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97554

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 REGULAR
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
======================================================================

CVE-2026-97554	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97554 CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N';*CWE-457;*CWE-200;CWE-908;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.3';DESCR 'A malformed SMB POSIX directory entry can cause cifs_posix_to_fattr to use uninitialized parsed owner and group SID data after posix_info_parse fails. The uninitialized stack data can be processed by sid_to_id and passed through request_key to a userspace idmap daemon, creating a limited kernel stack information disclosure risk. For the CVSS the PR:N rating is used because a remote SMB server can provide the malformed directory entry without requiring privileges on the victim system. The issue is network reachable only when the client mounts or accesses an attacker controlled or compromised SMB server. The base score uses AC:H because practical disclosure also depends on the local idmap request handling path being observable. Impact is limited confidentiality exposure. No integrity impact, denial of service, memory corruption, or privilege escalation primitive is demonstrated by the patch.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3)	YES	DISK SIMPLEFIX LEAK SERVERTOCLIENT  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	NO	NO	checked

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

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

1. ActionableScore

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

2. Signal breakdown

* +2 Remote reachable / directly network-triggerable. A malicious or compromised SMB server can provide a malformed POSIX directory entry to an already connected CIFS client during directory entry processing.
* +1 Confidentiality plausible. The patch establishes that uninitialized kernel stack bytes in parsed.owner and parsed.group can reach sid_to_id and request_key, which sends a SID representation to a userspace idmap daemon.
* No additional practical exploit-chain point. The supplied evidence does not establish that the remote SMB server can read the idmap request or that an unprivileged local attacker can observe the daemon input.
* No strong disclosure or Class A point. The available evidence supports a limited and indirect kernel-to-userspace disclosure path, but does not establish substantial attacker-readable disclosure.
* No integrity point. No write, ownership violation, refcount mutation, or authorization bypass is shown.
* No availability point. The patch does not demonstrate a crash, resource exhaustion, or persistent service failure.
* No memory-corruption or UAF point. The defect is use of uninitialized stack data and possible disclosure, not a stale lifetime object, OOB write, or controlled corruption.
* No negative privilege or niche-subsystem penalty. The evidence does not positively establish that host-admin privilege is required from the attacker, and SMB client usage is not inherently a rare subsystem.

3. Reachability analysis

A remote SMB server controls the malformed POSIX directory entry that causes posix_info_parse to fail. The vulnerable client path is reached when a CIFS client accesses directory entries from an SMB share using the relevant POSIX information path.

The attacker does not need local code execution on the victim merely to send the malformed server response. However, the supplied patch does not establish who configured the CIFS mount, whether the server was intentionally mounted by an administrator, whether a normal user can mount it, or whether the malicious server is a compromised trusted server. Therefore HostAdminRequired is UNKNOWN rather than YES or NO.

No namespace-specific capability path is involved in the supplied evidence. NamespaceScopedPrivilege is UNKNOWN.

No race, special timing, unusual hardware, boot option, or attacker-controlled allocator/reclaim condition is established.

4. Primitive and severity interpretation

The exact primitive is an uninitialized stack-data use after ignored error handling. posix_info_parse can fail without initializing the parsed SID structure, while cifs_posix_to_fattr continues into sid_to_id. The SID conversion path can pass garbage-derived data to request_key and then to a userspace idmap daemon.

This is not a UAF, double release, refcount mutation, active-object lifetime violation, or controlled kernel-memory corruption issue. It is also not an authorization bypass. The demonstrated consequence is a potential limited kernel stack information disclosure across the kernel-to-userspace idmap boundary.

The patch does not establish that the remote SMB peer can directly recover the disclosed bytes. It also does not establish arbitrary kernel disclosure, controlled out-of-bounds reads, integrity impact, privilege escalation, or a local attacker-controlled idmap daemon. Consequently, the issue is classified as CLASS_C invalid-state behavior with a plausible but constrained disclosure effect rather than CLASS_A.

ImportantCandidate is NO because the available evidence does not establish a strong disclosure primitive, a boundary bypass, or memory-corruption leverage requiring Important-versus-Moderate preservation. AutoDowngradeAllowed is YES because the practical attacker-readable disclosure path remains unresolved and no stronger kernel corruption primitive is present.

Likely operational severity is low to moderate. It should be reviewed to determine whether the deployed idmap daemon logs, exposes, or otherwise makes request_key descriptions available to an unprivileged local user or remote SMB peer.

5. One-sentence report phrase

A malicious SMB server can cause the CIFS client to pass uninitialized kernel stack SID data to a userspace idmap daemon after malformed POSIX directory entry parsing fails, creating a limited potential information disclosure path.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Confirm whether the idmap daemon, request_key handlers, audit logs, or error paths expose the generated SID string to an unprivileged local user or back to the remote SMB server.

7. Call-site confidence

Call-site confidence: medium

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

Patch: smb: client: avoid using uninitialized SIDs in cifs_posix_to_fattr()
Commit: c9a8b60ce140a68d172452f418137fc7ddbae7db
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=c9a8b60ce140a68d172452f418137fc7ddbae7db

Commit description:

cifs_posix_to_fattr() ignores the return value of posix_info_parse().
When a malformed POSIX directory entry is encountered (e.g. invalid
SID lengths from an untrusted server), posix_info_parse() returns -1
without populating the 'parsed' struct.  The uninitialized stack
memory in parsed.owner and parsed.group is then passed to
sid_to_id(), which processes the garbage bytes and passes them to
request_key() to construct a SID string, potentially leaking kernel
stack contents to the userspace idmap daemon.

Fix this by checking the return value and skipping the SID-to-id
mapping when parsing fails.  The remaining fattr fields (timestamps,
mode, etc.) are populated directly from the 'info' pointer so they
are unaffected.

Closes: https://sashiko.dev/#/patchset/20260906172005.627163-1-pc%40manguebit.org
Closes: https://sashiko.dev/#/patchset/20260906181540.647469-1-pc%40manguebit.org
Reviewed-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>
Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Shyam Prasad N <sprasad@microsoft.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Bharath SM <bharathsm@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=c9a8b60ce140a68d172452f418137fc7ddbae7db

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

The original Linux kernel CVE announcement for CVE-2026-97554 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97554

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:N/A:N
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS score: 5.3

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: unknown
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).