* [CVE-2026-90054][MODERATE REGULAR] tcp: fix corruption of urgent data on multi-segment retransmit [ Upstream
@ 2026-09-18 1:09 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 1:09 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90054
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: tcp: fix corruption of urgent data on multi-segment retransmit [ Upstream
Commit: 8eb51013b6308870479a64b4a2a018697b997849
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8eb51013b6308870479a64b4a2a018697b997849
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90054
Analysis date: Thu, 17 Sep 2026 21:09:56 -0400
ActionableScore: 3
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-2026-90054 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90054
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-90054 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90054 CHECK WITH IMPACT FROM ORIG NN IMPORTANT Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';CWE-682;CWE-691;CWE-440;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.8';DESCR 'TCP retransmission can corrupt urgent data semantics because __tcp_retransmit_skb may build a multi segment TSO skb while TCP urgent mode is active. GSO advances the TCP sequence number for each generated segment but copies urg_ptr unchanged, so the receiver reconstructs a moving urgent point instead of the single intended OOB byte. This can cause the real OOB byte to be delivered as normal in band data and can corrupt the TCP byte stream observed by the peer. For the CVSS the PR:N is used because a remote TCP peer does not need privileges on the vulnerable host, but reliable triggering requires a connection and an application path that uses TCP urgent data. The issue is network reachable in the sense that it affects TCP protocol behavior across the network, but it is not a kernel memory corruption or code execution primitive. Impact is mainly low integrity impact to the stream and possible limited availability impact if an application breaks on corrupted urgent data handling.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES REMOTE OOB DANGER NETWORK SKB LINUS PACKET LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 3
* Paranoid score: 3
* Final recommended bucket: **Borderline / manual review recommended**::
No floor or cap changed the score.
2. Signal breakdown
* +2 Remote/network protocol reachability: the bug is in TCP retransmission behavior and can be exercised over an established TCP connection when urgent data and retransmission conditions are present.
* +1 Integrity plausible: the urgent pointer is copied with the wrong per-segment semantics, so the peer may observe corrupted TCP urgent-data handling and the OOB byte may be delivered as normal in-band data.
Important non-awarded signals:
* No RealUAF: no freed object, stale pointer, refcount misuse, or lifetime violation is established.
* No kernel memory corruption: the patch changes retransmit length calculation only.
* No arbitrary write, controlled reclaim, OOB access, callback control, or privilege escalation primitive is demonstrated.
* No reliable kernel crash or host-wide DoS is shown.
* No extra Availability point is awarded because only application-level disruption is plausible, not a distinct kernel availability failure.
* No Strong semantic reinterpretation signal is awarded because the reinterpretation affects TCP urgent-data stream semantics, not kernel ownership, page state, pointers, descriptors, credentials, or another kernel security boundary.
* No Common networking core bonus is awarded separately because the affected path is specifically TCP urgent mode plus multi-segment retransmit, not ordinary TCP packet processing.
3. Reachability analysis
A remote TCP peer can participate in the affected protocol flow, but practical triggering requires an application that uses TCP urgent data and conditions that cause retransmission through a multi-segment TSO/GSO path. No local host-admin privilege is required by the remote peer, so HostAdminRequired=NO. Namespace privileges are not relevant to the evaluated trigger, so NamespaceScopedPrivilege=NO.
There is no evidence that a privileged local setup, special hardware, firmware compromise, or a kernel boot option is required. The relevant prerequisites are protocol/application state: TCP urgent mode, retransmission, and segmentation. This is not a race condition.
4. Primitive and severity interpretation
The exact primitive is protocol semantic corruption: `__tcp_retransmit_skb()` could retransmit multiple GSO segments while TCP urgent mode is active, and GSO copied `urg_ptr` unchanged while advancing sequence numbers. Since `urg_ptr` is relative to each segment sequence number, later segments reconstruct the urgent byte at the wrong absolute location.
This is not active structured kernel-object corruption and not fixed stale storage. It is not cleanup/destructor-only behavior. There is no concrete repeated release/refcount operation. The demonstrated consequence is corruption of TCP urgent-data semantics and possible application-visible stream corruption. The speculative consequence is application-specific integrity or availability impact if software relies on urgent data handling.
PrimitiveClass=CLASS_C because the established issue is protocol invalid-state/data semantics corruption without a kernel memory-corruption or authorization primitive. ImportantCandidate=NO because no Important-grade primitive is shown. AutoDowngradeAllowed=YES because no unresolved kernel corruption, lifetime, ownership, COW, authorization, or boundary-bypass primitive remains.
Likely operational severity is Low to borderline Moderate, with manual review recommended mainly because it is network protocol behavior and application impact depends on actual TCP urgent-data usage.
5. One-sentence report phrase
TCP urgent data can be corrupted on multi-segment retransmit because GSO copies urg_ptr unchanged across segments, allowing a remote TCP peer in a suitable urgent-data retransmission scenario to observe incorrect OOB versus in-band stream semantics.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Reason: the issue is network reachable at the TCP protocol level, but the established impact is limited to urgent-data stream integrity semantics and does not show kernel memory corruption, kernel crash, privilege escalation, or a security-boundary bypass.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: tcp: fix corruption of urgent data on multi-segment retransmit [ Upstream
Commit: 8eb51013b6308870479a64b4a2a018697b997849
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8eb51013b6308870479a64b4a2a018697b997849
Changed files:
net/ipv4/tcp_output.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=8eb51013b6308870479a64b4a2a018697b997849
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90054 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90054
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:N/I:L/A:N
The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L
The Best / paranoid CVSS score: 4.8
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: 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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