* [CVE-2026-99273][MODERATE 7.0] ksmbd: reserve space for the UTF-16 terminator in stream info
@ 2026-11-08 2:27 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-08 2:27 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99273
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ksmbd: reserve space for the UTF-16 terminator in stream info
Commit: 06a751809ebd33e95b05bf733697318a17a4872b
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06a751809ebd33e95b05bf733697318a17a4872b
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99273
Analysis date: Sat, 07 Nov 2026 22:27:38 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
An authenticated SMB user with stream-write access can trigger a deterministic two-byte zero write past a ksmbd heap response buffer.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99273 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99273
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-2026-99273 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99273 CHECK WITH IMPACT FROM ORIG NN NONE | AEGIS IMPACT MODERATE AEGIS_AS=2 LEGACY=MODERATE_KPANIC RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE_KPANIC, Aegis: MODERATE) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-131;Other CVSS 'AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7.5';DESCR 'A size calculation error in ksmbd stream information handling leaves no room for the terminating UTF-16 zero written after a converted stream name. An SMB client able to control alternate data stream names can potentially make an entry exactly fill the 448 byte small response buffer and cause a two byte zero write beyond the allocated object. For the CVSS the PR:L value is selected because reliable triggering normally requires authenticated access to a writable SMB share and permission to create or control named streams. The confirmed primitive is a constrained kernel heap out of bounds write with likely denial of service impact. Corruption of an adjacent object and privilege escalation remain plausible worst case outcomes but are not proven by the patch. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 7.5 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to MODERATE_KPANIC because the ActionableScore threshold rule was applied. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4. The independent weaker Aegis shadow evaluated CVSS 3.1 and ActionableScore 2 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 4) MAYBE REMOTE DISK INIT SIMPLEFIX LEAK SKIP LINUS KPANIC INCREASED_TO_MODERATE7_FROM_LOW_BASED_ON_GUESSCVSS_AND_ACTIONABLESCOREHIGHER4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_B
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 4.
* Paranoid score: 4; stronger consequences would require unsupported adjacent-object evidence.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +1 constrained kernel-buffer overwrite: `smbConvertToUTF16()` writes a fixed two-byte zero beyond the 448-byte heap allocation when an entry exactly exhausts it.
* +2 remote/network reachability: an authenticated SMB client with writable-share and named-stream permissions can influence stream names and reach the response-building path.
* +1 plausible kernel crash/strong DoS: a slab OOB write can corrupt adjacent allocation data, although panic reliability is not demonstrated.
* No strong-corruption +2: payload is fixed zero, destination is exactly two bytes beyond a fixed response buffer, and no adjacent victim or target selection is shown.
* No separate integrity, disclosure, LPE, arbitrary-write, controlled-reclaim, or practical exploit-chain points.
* NEW52/B5-R does not apply: no retry-accumulated index or re-entry sequence exists.
* This is not an authorization flaw; B4-PENDING is inapplicable.
3. Reachability analysis
The grounded attacker is a remote authenticated SMB user permitted to create or control alternate data streams on a writable share. Host UID 0 or host-administrator capability is not required for that data-path operation. Share provisioning is prior administrative setup, but the later trigger itself is below host-admin. No namespace-capability model is established. Exact-length construction raises complexity but is deterministic, not a race.
4. Primitive and severity reading
The exact primitive is a confirmed, fixed-width two-byte heap OOB zero write caused by omitting terminator space from two size checks. It is constrained CLASS_B corruption, not a UAF and not merely an invalid read. The patch does not identify the adjacent slab object or prove later structured-state consumption, so arbitrary corruption, confidentiality impact, and LPE remain speculative. Nevertheless, a remotely influenced write outside the allocated object leaves material adjacent-object leverage unresolved; therefore ImportantCandidate is YES and automatic downgrade is unsafe.
5. One-sentence report phrase
An authenticated SMB user with stream-write access can trigger a deterministic two-byte zero write past a ksmbd heap response buffer.
6. Manual review advice
MANUAL CHECK REQUIRED
Determine the slab layout, adjacent victim types, overwrite repeatability, and whether corrupted adjacent state is subsequently consumed.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ksmbd: reserve space for the UTF-16 terminator in stream info
Commit: 06a751809ebd33e95b05bf733697318a17a4872b
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=06a751809ebd33e95b05bf733697318a17a4872b
Commit description:
smb_strtoUTF16() always writes a terminating zero UTF-16 unit after the
converted characters, but get_file_stream_info() budgets only
sizeof(struct smb2_file_stream_info) + streamlen * 2 for a stream entry
and sizeof(struct smb2_file_stream_info) + 7 * 2 for the trailing
"::$DATA" entry. When an entry exactly fills the 448-byte small response
buffer, the terminator is written two bytes past the allocation.
Reserve one extra UTF-16 unit in both budget checks. The terminator is
not part of the wire format, so the reported lengths are unchanged.
Cc: [email protected]
Signed-off-by: Abdifatah Suruur <[email protected]>
Signed-off-by: Namjae Jeon <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/smb/server/smb2pdu.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=06a751809ebd33e95b05bf733697318a17a4872b
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99273 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99273
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:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS vector: AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 7.5
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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