* [CVE-2026-89637][MODERATE 7.0] smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2 [ Upstream
@ 2026-09-11 22:07 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-09-11 22:07 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89637
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2 [ Upstream
Commit: 9eed72e9534b10a6d9f8f5146feff3db53aebdba
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9eed72e9534b10a6d9f8f5146feff3db53aebdba
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89637
Analysis date: Fri, 11 Sep 2026 18:07:42 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89637 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89637
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-2026-89637 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89637 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:R/S:U/C:H/I:H/A:H';*CWE-416;*CWE-401;CWE-703;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:H';BEST CVSS score: '8.8';DESCR 'A use-after-free in the SMB1/CIFS client path can occur when a valid primary TRANSACT2 response is followed by a malformed secondary T2 response. The error path can leave server smallbuf or bigbuf pointing to memory that is later freed by the user thread, after which the demux thread may reuse the dangling buffer pointer for the next packet. A malicious or compromised SMB server, or a network attacker able to tamper with the SMB session, can trigger the condition through crafted response ordering. For the CVSS the PR:N is used because the attacker does not need an account on the client system and only needs control of the SMB server side or the ability to modify traffic for the client connection. UI:R is used because the client typically must connect to or use the malicious SMB1 resource. Impact is at least remote denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact because the bug is a network reachable UAF in kernel buffer handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate (with actual score 7) YES DISK LEAK UAF SERVERTOCLIENT LINUS PACKET KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS8.5PLUS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=7
ActionableScoreLower=6
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
1. ActionableScore
* Raw conservative score: 6
* Conservative score: 6. No floor or cap changed the score.
* Raw paranoid score: 7
* Paranoid score: 7. No floor or cap changed the score.
* Final recommended bucket: **Strong Important candidate**::
2. Signal breakdown
Conservative signals:
* +2 Real UAF / active lifetime violation: the commit directly states that server->smallbuf or server->bigbuf can remain dangling after the user thread frees mid->resp_buf, and the demux thread later reuses the dangling pointer.
* +2 Remote reachable / network-triggerable: a malicious or compromised SMB server, or a MITM able to modify the SMB session, can send the valid primary TRANSACT2 response followed by malformed secondary T2 data.
* +1 Reliable kernel crash / strong DoS: the established consequence is kernel use-after-free in the SMB client receive path, with realistic kernel crash potential.
* +1 Constrained post-free buffer reuse/write concern: the stale pointer is reused by the demux receive path for a later packet, which means attacker-controlled network data may be processed through freed kernel buffer storage. This is not arbitrary write, but it is more than a shallow stale read.
Paranoid additional signal:
* +1 Attacker-influenced repeatable lifetime/use sequence: the attacker can influence the response ordering and later packet contents over the same SMB session, making the post-free reuse scenario meaningfully reviewable beyond crash-only behavior.
Non-awarded signals:
* Controlled reclaim not demonstrated.
* Arbitrary or directed kernel write not demonstrated.
* Callback or function pointer control not demonstrated.
* Concrete repeated release/refcount mutation not established.
* Strong CLASS_A primitive not established.
* Extra availability point not awarded because the same crash is already counted.
* Confidentiality and integrity are not scored as demonstrated consequences, although post-free network-buffer reuse leaves unresolved corruption leverage.
* SMB1 legacy exposure is noted, but no negative point is applied because the supplied evidence does not prove a rare or attacker-unreachable deployment once the victim client uses SMB1.
Repeated release/refcount check:
* First release can end lifetime: mid->resp_buf can be freed by the user thread.
* Exact same reference reaches second lifetime mutation: not established.
* Second operation is a real release/refcount mutation: no.
* Concrete repeated release/refcount +1 awarded: no.
* Paranoid repeated-refcount leverage +1 awarded: no.
* FLOOR 5 applied: no explicit floor needed because arithmetic already exceeds 5.
3. Reachability analysis
The evaluated attacker is a malicious SMB server or a network attacker able to tamper with an SMB1/CIFS client session. The attacker does not need an account or privileges on the victim host, so HostAdminRequired=NO. User interaction or prior victim behavior is practically relevant because the client must connect to or use the malicious or compromised SMB1 resource, but this does not make the attacker host-admin privileged.
This is not a namespace-scoped capability issue. No CAP_NET_ADMIN, user namespace, or container-local privilege is required for the remote server-side trigger described in the commit. NamespaceScopedPrivilege=NO.
Prior setup exists only in the operational sense that SMB1/CIFS must be enabled and the victim must reach the attacker-controlled SMB endpoint. SMB1 is a legacy protocol and many modern environments disable it, which reduces exposure, but systems that still mount or access SMB1 shares remain reachable through network responses. No special hardware is required. The trigger is an ordered protocol-state sequence rather than a hard timing race.
4. Primitive and severity interpretation
The exact primitive is a real kernel use-after-free in the SMB1/CIFS client multi-response TRANSACT2 handling path. A valid primary response sets mid->resp_buf and mid->multiRsp. A malformed secondary response can cause cifs_check_trans2() to return false, after which handle_mid() may overwrite mid->resp_buf and fail to detach server->smallbuf or server->bigbuf. The user thread later frees the response buffer, leaving the server buffer pointer dangling, and the demux thread can reuse it for the next packet.
This is closer to fixed stale buffer storage than an active structured object with callback fields. However, the stale storage is reused in an active network receive path and can carry attacker-controlled packet contents, so it is not merely a cleanup-only stale dereference. No concrete second put/free/refcount mutation is established.
PrimitiveClass=CLASS_B because a real kernel lifetime violation and post-free buffer reuse are established, but controlled reclaim, arbitrary write, callback control, and demonstrated privilege escalation are not established. The demonstrated consequence is remote DoS through kernel crash. The speculative consequence is stronger memory corruption if the freed buffer is reclaimed or reused in an exploitable way, but that remains unresolved and requires manual analysis.
ImportantCandidate=YES because this is a network-reachable kernel UAF in client-side filesystem/network code with attacker-controlled protocol data and unresolved post-free reuse leverage. AutoDowngradeAllowed=NO because automatically closing it as ordinary Moderate would discard a concrete remote UAF primitive with meaningful unresolved exploitation questions.
5. One-sentence report phrase
A malicious SMB1 server can trigger a CIFS client use-after-free by sending a malformed secondary TRANSACT2 response after a valid primary response, leaving server buffer pointers dangling and allowing the demux thread to reuse freed kernel memory, causing DoS and requiring review for possible stronger memory corruption impact.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Reason: this is a network-reachable real kernel UAF with attacker-controlled SMB response ordering and later packet influence over the stale buffer reuse path. Controlled reclaim or LPE is not demonstrated, but the primitive is too concrete to auto-downgrade.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: smb: client: fix UAF and buffer leak in cifs_check_trans2() for malformed secondary T2 [ Upstream
Commit: 9eed72e9534b10a6d9f8f5146feff3db53aebdba
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9eed72e9534b10a6d9f8f5146feff3db53aebdba
Changed files:
fs/smb/client/smb1transport.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=9eed72e9534b10a6d9f8f5146feff3db53aebdba
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89637 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89637
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:L/I:L/A:H
The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 8.8
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: 6
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: 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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