* [CVE-2026-98438][MODERATE REGULAR] mptcp: options: fix uninit-value in mptcp_write_data_fin
@ 2026-10-19 7:37 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-19 7:37 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98438
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: mptcp: options: fix uninit-value in mptcp_write_data_fin
Commit: 9a682067dd7b9dce6b94a0a05661dcc921446402
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9a682067dd7b9dce6b94a0a05661dcc921446402
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98438
Analysis date: Mon, 19 Oct 2026 03:37:55 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review required
Manual review required: YES
Summary:
An uninitialized 16-bit value from ext_copy may be exposed to a remote MPTCP peer through the DSS checksum of a zero-payload DATA_FIN, potentially leaking limited stale kernel data and disrupting termination of the affected connection.
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ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98438 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98438
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
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CVE-2026-98438 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98438 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, 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:L/I:N/A:L';*CWE-457;*CWE-200;CWE-682;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:L';BEST CVSS score: '6.5';DESCR 'An uninitialized value can be transmitted in the MPTCP DSS checksum when a DATA_FIN without payload requires creation of a new one byte mapping. Because ext_copy is no longer fully cleared, ext csum may contain stale kernel data when checksum support has been negotiated, allowing the remote MPTCP peer to observe a limited value from reused memory. An invalid checksum may also cause the peer to reject the DATA_FIN or delay connection termination, producing connection level availability impact. For the CVSS the PR:N is selected because a remote peer does not need an authenticated local account or victim side privileges to negotiate MPTCP checksum support and receive the outgoing option. The base score uses AC:H because reliable triggering depends on the checksum negotiation and the specific zero data DATA_FIN path. The paranoid score uses AC:L when an exposed MPTCP service allows the peer to arrange these conditions predictably. There is no supported UAF, OOB access, arbitrary write, host wide crash, or privilege escalation primitive. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 6.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review required. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were ActionableScore thresholds stage MODERATE_KPANIC to MODERATE because the ActionableScore threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 3.7 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review required (with actual score 3) MAYBE REMOTE READ SIMPLEFIX NETWORK LINUS PACKET DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* No floor or cap changed either score.
* Final recommended bucket: **Borderline / manual review required**::
2. Signal breakdown
Positive signals:
* +2 Remote reachability. An MPTCP peer can negotiate DSS checksum support and receive the affected DATA_FIN option over the network.
* +1 Confidentiality plausible. The uninitialized 16-bit `ext->csum` value is incorporated into network-visible output, providing a concrete but highly limited stale-data disclosure path.
Non-awarded signals:
* No memory-corruption point. The patch establishes an uninitialized read, not an OOB write, UAF, arbitrary write, or active-object corruption.
* No reliable kernel-crash point. The supported availability effect is rejection of DATA_FIN or delayed termination of one connection, not host-wide failure.
* No separate integrity point. The malformed checksum affects protocol processing but does not establish unauthorized modification of protected kernel or user state.
* No practical-exploit-chain point. Observing one stale checksum field does not independently establish disclosure of a sensitive pointer, key, credential, or other protected object.
* No common-networking-path point. MPTCP checksum negotiation is an optional protocol path and is narrower than ordinary TCP processing.
* No NEW52 B5-R signal. There is no retry-accumulated heap OOB write or later consumption of corrupted descriptors.
* No NEW53 B3-PENDING signal. There is no write to shared or file-backed storage.
* No V33 or V34 active-state signal. No attacker-influenced write modifies structured state consumed by a privileged downstream component.
* No B4-PENDING signal. No authorization guard changes from DENY to ALLOW.
3. Reachability analysis
The evaluated attacker is a remote MPTCP peer. The peer can negotiate checksum support and observe an outgoing DATA_FIN, without a local account, host UID 0, or administrative capability.
Reliable triggering additionally requires the zero-payload DATA_FIN path. Whether a peer can force this immediately depends on application behavior, but the vulnerable output remains network-visible when that path is reached. No user-namespace or network-namespace capability is required by the remote attacker.
MPTCP and checksum support must be enabled and used by the connection. No special hardware, race, retry loop, or narrow timing window is involved.
4. Primitive and severity interpretation
The exact primitive is transmission of an uninitialized 16-bit checksum seed or result from `ext_copy` through an MPTCP DSS option. This is a limited kernel-data disclosure and protocol-state defect, not a post-lifetime access or kernel-memory write.
The practical consequences supported by the patch are:
* exposure of a small stale value to the MPTCP peer;
* an invalid DSS checksum;
* possible rejection of DATA_FIN or delayed connection termination.
There is no evidence of UAF, OOB access, controlled reclaim, arbitrary kernel access, host panic, privilege escalation, or authorization bypass. NEW52, NEW53, V32, V33, V34, and B4-PENDING therefore do not apply.
`PrimitiveClass=CLASS_C` reflects constrained invalid-state and disclosure behavior without a stronger corruption primitive. `ImportantCandidate=NO` because no Important-grade boundary or memory-corruption mechanism is established. `AutoDowngradeAllowed=NO` is retained pending manual confirmation of what data can occupy `ext->csum`, because the uninitialized value is exposed across a network boundary even though the likely severity remains Moderate or Low-like.
5. One-sentence report phrase
An uninitialized 16-bit value from ext_copy may be exposed to a remote MPTCP peer through the DSS checksum of a zero-payload DATA_FIN, potentially leaking limited stale kernel data and disrupting termination of the affected connection.
6. Manual review recommendation
MANUAL CHECK REQUIRED
Confirm the storage origin and reuse history of `ext_copy`, the exact serialized checksum value, and whether repeated connections can expose correlated or security-sensitive kernel data.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: mptcp: options: fix uninit-value in mptcp_write_data_fin
Commit: 9a682067dd7b9dce6b94a0a05661dcc921446402
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9a682067dd7b9dce6b94a0a05661dcc921446402
Commit description:
When sending a DATA_FIN without data, and because the DATA_FIN occupies
1 octet of the connection-level sequence space [1], it is then required
to add a DSS mapping with specific values.
If the checksum has been negotiated, it also needs to be computed, and
included in the outgoing packet, and thus the initial csum data needs to
be reset to 0 as well. This is no longer the case since commit
cfcceb7 ("tcp: shrink per-packet memset in __tcp_transmit_skb()"),
because the whole ext_copy structure is no longer zeroed by default.
This seems to be the only case where use_map is changed and set
afterwards, so initialising the csum field only in this case, along with
other fields for this specific case.
Fixes: cfcceb7 ("tcp: shrink per-packet memset in __tcp_transmit_skb()")
Cc: [email protected]
Link: https://datatracker.ietf.org/doc/html/rfc8684#section-3.3.3 [1]
Link: https://sashiko.dev/#/patchset/20260812-net-next-mptcp-misc-feat-7-3-v1-0-1905a818f6cb%40kernel.org?part=2
Reviewed-by: Geliang Tang <[email protected]>
Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>
Link: https://patch.msgid.link/20260908-net-mptcp-misc-fixes-7-3-rc1-v2-13-df1de70348b6@kernel.org
Signed-off-by: Jakub Kicinski <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Changed files:
net/mptcp/options.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=9a682067dd7b9dce6b94a0a05661dcc921446402
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98438 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98438
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:L
The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:L
The Best / paranoid CVSS score: 6.5
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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