* [CVE-2026-89634][MODERATE 7.0] smb: client: fix ALIGN() overflow in symlink_data() error context loop
@ 2026-09-12 2:05 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-12 2:05 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-89634
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: smb: client: fix ALIGN() overflow in symlink_data() error context loop
Commit: 657372ba6ea50b4474d2a6a610097eae8cd1f3e9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=657372ba6ea50b4474d2a6a610097eae8cd1f3e9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89634
Analysis date: Fri, 11 Sep 2026 22:05:02 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89634 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89634
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-2026-89634 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-89634 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H';CWE-190;CWE-125;*CWE-835;Other CVSS 'AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H';BEST CVSS score: '7.1';DESCR 'SMB client symlink_data can mishandle a crafted ErrorDataLength because ALIGN() is applied before the raw length is checked. A malicious or compromised SMB server can send a very large ErrorDataLength such as 0xFFFFFFF9 so the aligned length wraps to zero and the parser may keep processing stale data or read beyond the intended error context. For the CVSS the PR:N because the attacker only needs to control or tamper with the SMB server response after the client reaches the server, while victim side mounting or access to the share is represented by UI:R. The issue is network reachable from the server side to the SMB client and the main impact is denial of service through a client kernel hang or crash. The paranoid score also allows C:L because stale or out of bounds reads are plausible, but it does not assume privilege escalation or arbitrary write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 4) YES OOB DISK SIMPLEFIX IMPROVEONLY SERVERTOCLIENT LINUS KPANIC DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH YES NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 4
* Conservative score: 4
* Raw paranoid score: 4
* Paranoid score: 4
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap was applied.
2. Signal breakdown
Positive signals:
* +2 Remote reachable / directly network-triggerable: a malicious or compromised SMB server can influence the SMB client parser by returning crafted symlink error-context data.
* +1 Reliable kernel crash / strong DoS: the supplied context supports an infinite loop, stale parser progress, OOB read, hang, or crash-class client-side kernel DoS.
* +1 Weak / indirect corruption candidate: the ALIGN() overflow can make the parser accept an invalid length and continue parsing stale or out-of-bounds context data, but no write primitive or useful disclosure path is established.
Non-awarded signals:
* RealUAF not awarded: no object lifetime violation or stale post-free object use is shown.
* CLASS_A not awarded: no arbitrary write, controlled reclaim, type confusion, callback control, ownership/COW violation, or authorization bypass is established.
* Confidentiality not awarded: an internal OOB read is plausible, but no path shows leaked data being returned to userspace, network, or another observable sink.
* Integrity not awarded: no unauthorized write or modification of protected kernel or filesystem state is shown.
* Extra Availability not awarded: the hang/crash consequence is already counted once.
* ImportantCandidate not awarded: the primitive remains parser invalid-state/OOB-read/DoS oriented without unresolved strong exploitation leverage.
* AutoDowngradeAllowed remains YES: the issue is network-exposed and worth review, but the established primitive is not strong enough to preserve Important-class handling by itself.
Negative signals:
* No privilege penalty: host-admin-equivalent privilege is not required from the attacker controlling the SMB server response.
* No race penalty: this is an ordered parser arithmetic bug, not a timing race.
* No niche-subsystem penalty: SMB client deployments are common enough that subsystem rarity should not reduce the score.
3. Reachability analysis
The evaluated attacker is a malicious or compromised SMB server, or an attacker able to tamper with SMB server responses. The victim client must reach or already use that SMB server, so user interaction or prior mount/access to the share is part of the scenario, but attacker-side privileges on the victim host are not required.
HostAdminRequired=NO because the attacker does not need host administrator rights on the victim to send the malformed response once the client connects. A local privileged mount setup may exist in some deployments, but that is a deployment prerequisite rather than an attacker privilege requirement in the supplied scenario.
NamespaceScopedPrivilege=NO because this is not a namespace-capability path. No relevant namespaced capability chain is involved.
No special hardware, boot option, sysctl, or race timing is required by the supplied patch. The practical exposure depends on whether the SMB client connects to untrusted or compromised SMB servers.
4. Primitive and severity interpretation
The exact primitive is an integer overflow in length alignment inside fs/smb/client/smb2file.c symlink_data(). ErrorDataLength is aligned before the raw value is checked against the remaining buffer. For values near UINT32_MAX, ALIGN(len, 8) can wrap to zero, causing the bounds check to pass and preventing the parser pointer from advancing as intended.
This is not an active structured-object UAF and not fixed stale storage. It is a parser arithmetic and bounds-validation flaw. The demonstrated consequence is client-side kernel DoS through stale parsing, possible infinite loop, OOB read, hang, or crash.
PrimitiveClass=CLASS_C because the behavior is primarily invalid parser state and DoS without an independently useful corruption primitive. Theoretical stronger outcomes are not scored because the patch does not establish controlled memory corruption, a write primitive, a disclosure sink, or privilege escalation.
ImportantCandidate=NO because no Important-grade primitive is established. ManualReview=RECOMMENDED because the issue is network-reachable from an SMB server to a kernel client parser and involves OOB/stale parsing behavior. AutoDowngradeAllowed=YES because no unresolved strong corruption, ownership, authorization, or lifetime primitive remains.
Likely operational severity: actionable for environments mounting SMB shares from untrusted, semi-trusted, or compromised servers, but not an Important-class candidate based on the supplied evidence alone.
5. One-sentence report phrase
A malicious SMB server can trigger an ALIGN() overflow in the SMB client symlink_data() error-context parser, causing stale or out-of-bounds parsing and likely client-side kernel DoS without a demonstrated write, disclosure, or privilege-escalation primitive.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: the bug is network-reachable from an SMB server and affects kernel parser bounds handling, but the established primitive is DoS/OOB-read oriented and does not show controlled corruption or privilege escalation.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: smb: client: fix ALIGN() overflow in symlink_data() error context loop
Commit: 657372ba6ea50b4474d2a6a610097eae8cd1f3e9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=657372ba6ea50b4474d2a6a610097eae8cd1f3e9
Commit description:
The check added by commit 7d9a7f1 ("smb/client: fix possible
infinite loop and oob read in symlink_data()") compared the post-ALIGN
length against the remaining buffer, but ALIGN() itself can overflow:
for ErrorDataLength near UINT32_MAX (e.g. 0xFFFFFFF9), ALIGN(x, 8)
wraps to 0, so the subsequent bounds check passes, and the loop
advances by zero bytes leaving 'p' pointing into stale data.
Fix by checking the raw ErrorDataLength against the remaining space
before applying ALIGN(), then checking again after. Since raw_len is
bounded by the buffer, raw_len + 7 cannot overflow, so the second check
is an exact post-alignment bounds guard.
Fixes: 76894f3 ("cifs: improve symlink handling for smb2+")
Cc: [email protected]
Signed-off-by: Frank Sorenson <[email protected]>
Signed-off-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/smb2file.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=657372ba6ea50b4474d2a6a610097eae8cd1f3e9
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-89634 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-89634
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:N/I:N/A:H
The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:N/A:H
The Best / paranoid CVSS score: 7.1
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: 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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