* [CVE-2026-99275][IMPORTANT] smb: client: reject short SESSION_SETUP responses
@ 2026-11-08 2:11 21% AL-KERNEL
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From: AL-KERNEL @ 2026-11-08 2:11 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99275
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: smb: client: reject short SESSION_SETUP responses
Commit: df63e01933886153e40706483b9635b2456ad91f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df63e01933886153e40706483b9635b2456ad91f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99275
Analysis date: Sat, 07 Nov 2026 22:11:10 -0400
ActionableScore: 7
ActionableScore lower bound: unknown
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
A malicious SMB server can use a short SESSION_SETUP response and stale length metadata to make the client disclose up to approximately 64 KB of out-of-bounds kernel data in its NTLMv2 authentication reply.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99275 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99275
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
======================================================================
CVE-2026-99275 IMPORTANT https://www.kernelcve.org/list/?q=CVE-2026-99275 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=5 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H';CWE-125;*CWE-200;*CWE-20;CWE-130;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:L';BEST CVSS score: '9.1';DESCR 'The SMB client accepts a short STATUS_MORE_PROCESSING_REQUIRED SESSION_SETUP response before enforcing the minimum response size. The parser can read SecurityBufferLength from stale receive buffer bytes and use an attacker influenced value to read up to about 64 KB beyond the received object. Data obtained by this out of bounds read can be returned to a malicious or compromised SMB server in the NTLMv2 AUTHENTICATE target info and therefore provides a concrete kernel memory disclosure primitive. For the CVSS the PR:N value reflects that the server needs no authenticated privileges on the client and controls the protocol response. The base score uses UI:R because a user or administrator normally initiates access to the SMB share. The paranoid score uses UI:N when the client automatically reconnects to an already configured share or when a trusted server is compromised. Integrity impact is not assigned because no write primitive is present. Availability impact may occur if the out of bounds read reaches unmapped memory and faults the kernel. Automated impact decision path. The original SimpleNet classifier selected LOW. The primary LLM estimated paranoid CVSS 9.1 and the production ActionableScore returned 7 with recommendation Strong Important candidate. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage LOW to IMPORTANT because the CVSS score, vector or bug class threshold rule was applied. final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were INCREASED_TO_HIGH_BASED_ON_GUESSCVSS8.5PLUS and TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 4.3 and ActionableScore 5 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7) MAYBE DISK LEAK ERRORPATH IMPROVEONLY SERVERTOCLIENT LINUS INCREASED_TO_HIGH_BASED_ON_GUESSCVSS8.5PLUS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED - - checked
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ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=CONFIRMED
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 7.
* Conservative score: 7; no floor or cap applied.
* Raw paranoid score: 7.
* Paranoid score: 7; possible faulting was not scored without reliable-crash evidence.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
* +2 Remote network reachability: an attacker-controlled SMB server supplies the malformed SESSION_SETUP response once the client accesses it.
* +2 Concrete kernel disclosure primitive: stale `SecurityBufferLength` drives an out-of-bounds read of up to roughly 64 KB beyond the received object.
* +1 Practical exploit chain: short response acceptance → stale length → oversized decode → copied bytes returned in NTLMv2 AUTHENTICATE target information.
* +1 Confidentiality impact: the out-of-bounds data is transmitted to the server, not merely read internally.
* +1 Broad, exercised path: SESSION_SETUP is an ordinary stage of SMB client authentication for affected mounts.
* No write, UAF, controlled reclaim, integrity impact, authorization bypass, or LPE is shown.
* No crash/availability point: reaching unmapped memory may fault, but reliability is not established.
* NEW52/B5-R and NEW53/B3-PENDING do not apply: this is an OOB read/disclosure, not a retry-accumulated or shared-backing write.
3. Reachability analysis
The malicious server controls the protocol response without SMB authentication privileges. Victim interaction or an existing mount/reconnection is required, but the supplied evidence does not establish whether the complete client setup requires host administrator powers, host UID 0, a mount helper, or an ordinary user-accessible mount. Therefore both host-admin and UID requirements remain unknown. No namespace-scoped capability chain is shown. There is no race, special hardware, or retry requirement.
4. Primitive and severity reading
The primitive is a confirmed out-of-bounds kernel read followed by network exfiltration. The minimum-size ordering bug permits fixed response fields to come from stale receive-buffer bytes; the unchecked stale length then controls a later decoder read. This is CLASS_A because a concrete kernel-memory disclosure path is shown. The exact disclosed contents and repeatability are not characterized, so arbitrary kernel read, credential disclosure, and C:H are not independently proven. ImportantCandidate is YES and automatic downgrade is unsafe because disclosure past the receive object is operationally consumed and returned to the attacker.
5. One-sentence report phrase
A malicious SMB server can use a short SESSION_SETUP response and stale length metadata to make the client disclose up to approximately 64 KB of out-of-bounds kernel data in its NTLMv2 authentication reply.
6. Manual review advice
MANUAL CHECK REQUIRED
Confirm practical mount/reconnection reachability and characterize disclosed allocation contents and repeatability; the concrete network disclosure already warrants Important-class review.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: smb: client: reject short SESSION_SETUP responses
Commit: df63e01933886153e40706483b9635b2456ad91f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=df63e01933886153e40706483b9635b2456ad91f
Commit description:
smb2_check_message() accepts frames shorter than struct smb2_pdu when
the SMB2 status is non-zero, before the minimum-PDU check added by
b4694f2 . A 64-byte STATUS_MORE_PROCESSING_REQUIRED SESSION_SETUP
response is therefore accepted even though it cannot contain the
SecurityBufferOffset and SecurityBufferLength fields.
The parser then reads those fields from stale bytes left in the reused
receive buffer by an earlier frame. With a long stale
SecurityBufferLength, decode_ntlmssp_challenge() copies up to about
64 KB from beyond the receive object and sends the data back to the
server in the NTLMv2 AUTHENTICATE target info.
Reject short MPR SESSION_SETUP responses before the short-error path,
and validate SecurityBufferOffset + SecurityBufferLength against the
received response length before decoding the challenge.
The ordering bypass is not new: the short-error early return predates
b4694f2 , and the unvalidated SecurityBufferLength read predates
the 2016 raw NTLMSSP split in 166cea4 , so all supported stable
trees are affected. Reproduced on 6.6.87.2; the path is unchanged on
mainline.
Fixes: 5478f9b ("CIFS: Add session setup/logoff capability for SMB2")
Cc: [email protected]
Signed-off-by: Abdifatah Suruur <[email protected]>
Reviewed-by: Namjae Jeon <[email protected]>
Signed-off-by: Paulo Alcantara <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
fs/smb/client/smb2misc.c
fs/smb/client/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=df63e01933886153e40706483b9635b2456ad91f
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99275 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99275
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:H/I:N/A:L
The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:H
The Best / paranoid CVSS score: 9.1
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: unknown
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: NO
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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