[CVE-2026-98255][MODERATE 7.0] tcp: exclude old ACKs from tcp fast path

[CVE-2026-98255][MODERATE 7.0] tcp: exclude old ACKs from tcp fast path

[AL-KERNEL]

CVE: CVE-2026-98255
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tcp: exclude old ACKs from tcp fast path
Commit: 07cacab90290f795bad2e566790f4ba8e84cdf5f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07cacab90290f795bad2e566790f4ba8e84cdf5f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98255
Analysis date: Sat, 10 Oct 2026 13:59:24 -0400
ActionableScore: 6
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

======================================================================
ABOUT THIS REPORT
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98255 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98255

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-2026-98255	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98255 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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:H/A:H';*CWE-20;CWE-345;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L';BEST CVSS score: '7.4';DESCR 'The TCP established fast path can accept a segment whose ACK is older than SND.UNA because the previous prediction condition rejects ACK values after SND.NXT but does not reject ACK values before SND.UNA. If the segment also carries the expected receive sequence number and payload, the payload can be processed and RCV.NXT can advance before the old ACK receives full validation in the slow path. This creates a network data injection and TCP state manipulation risk rather than a memory corruption issue. For the CVSS the PR:N is selected because no authenticated account or local execution context is required when an attacker can send forged TCP segments to the established connection. Attack complexity is high because an off path attacker normally needs the connection tuple and a valid receive sequence number, while an on path attacker or an attacker with suitable network visibility has a more practical trigger path. Impact includes modification of an established byte stream and possible connection desynchronization or disruption. No UAF, OOB access, information disclosure, or direct privilege escalation primitive is identified. The issue is remotely reachable through TCP traffic and should receive manual review.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	REMOTE SIMPLEFIX NETWORK SKB KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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ACTIONABLESCORE ANALYSIS
======================================================================

ActionableScore=6
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 6
* Conservative score: 6
* Raw paranoid score: 6
* Paranoid score: 6
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Remote reachable. Attacker-supplied TCP segments directly reach the vulnerable established-connection receive path without local execution or administrative configuration.
* +1 Common networking core path. The defect is in `tcp_rcv_established()` and affects the ordinary TCP receive fast path.
* +1 Integrity impact. Payload accompanying an ACK older than `SND.UNA` can be accepted and advance `RCV.NXT` before full ACK validation, providing a concrete stream-injection and connection-state modification primitive.
* +1 Practical exploit chain. The patch establishes the sequence of forged segment, fast-path acceptance, payload processing, and `RCV.NXT` advancement.
* +1 Availability impact. Incorrect `RCV.NXT` advancement can desynchronize or disrupt the affected connection independently of merely modifying its byte stream.
* No race penalty. This is a deterministic validation-order defect, not a narrow timing race.
* No privilege penalty. Sending the triggering packet does not require host UID 0 or host-administrator capabilities.
* No UAF, OOB access, heap corruption, arbitrary kernel write, controlled reclaim, information disclosure, or privilege escalation is established.
* B5-R does not apply because there is no retry-accumulated heap OOB write.
* B3-PENDING does not apply because no shared-backing or file-backed page write exists.
* B4-PENDING does not apply because this is TCP protocol validation bypass, not omission of a kernel credential, capability, or authorization-state guard.

3. Reachability analysis

A remote peer or packet-injection attacker can provide the malformed segment. The segment must target an existing TCP connection, carry the expected receive sequence number, and contain an ACK older than `SND.UNA`. An off-path attacker generally needs knowledge or prediction of the connection tuple and sequence state, while an on-path attacker has a substantially more practical trigger.

No local account, UID 0, capability, namespace privilege, prior privileged setup, special hardware, sysctl, or unusual subsystem is required. The complete trigger is packet based rather than namespace-capability based. Attack complexity limits practical exploitation but does not negate the directly established remote integrity primitive.

4. Primitive and severity interpretation

The exact primitive is acceptance and processing of attacker-supplied TCP payload under an invalid old-ACK state before the slow path performs RFC5961 validation. This is a protocol security-boundary and active connection-state integrity violation, not kernel memory corruption.

The demonstrated consequence is unauthorized advancement of `RCV.NXT`, potential insertion of bytes into an established stream, and possible connection desynchronization. Application-level command execution, credential compromise, kernel privilege escalation, and arbitrary host-state modification remain speculative because they depend on the protected application protocol and connection context.

`PrimitiveClass=CLASS_A` is used because a directly useful network integrity primitive is established rather than a crash-only invalid state. `ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` follow from the remotely reachable stream-injection primitive in a common TCP path. The absence of memory corruption prevents promotion to a high-end Important or Critical classification without additional application-level exploit evidence.

5. One-sentence report phrase

A forged TCP segment with an ACK older than SND.UNA can be processed by the established-connection fast path and advance RCV.NXT before full validation, enabling remote stream injection or connection disruption when the attacker knows valid connection sequence state.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The issue provides a concrete remote integrity primitive in the common TCP receive path, while its ultimate severity depends on whether realistic attackers can obtain sequence state and inject security-sensitive application data.

7. Call-site confidence

Call-site confidence: high

======================================================================
UPSTREAM PATCH SUMMARY
======================================================================

Patch: tcp: exclude old ACKs from tcp fast path
Commit: 07cacab90290f795bad2e566790f4ba8e84cdf5f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07cacab90290f795bad2e566790f4ba8e84cdf5f

Commit description:

Exclude old ACKs before SND.UNA from the tcp fast path
as well as ACKs after SND.NXT.

Such ACKs will fall through to the slow path, where tcp_ack()
performs the appropriate validation and challenge ACK handling
according to RFC5961 and Commit 3d501dd ("tcp: do not
accept ACK of bytes we never sent").

This prevents old ACKs from being accepted
or modifying connection state as part of the fast path before
appropriate ACK validation is applied.
In particular, this prevents payload carried by a segment with
an excessively old ACK from advancing RCV.NXT before the ACK
is rejected.

Fixes: 31770e3 ("tcp: Revert "tcp: remove header prediction"")
Reported-by: Amit Klein <amit.klein@mail.huji.ac.il>
Reported-by: Tamir Shahar <tamir.shahar1@mail.huji.ac.il>
Reported-by: Inbal Schussheim <inbal.lipshtat@mail.huji.ac.il>
Suggested-by: Eric Dumazet <edumazet@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Inbal Schussheim <inbal.lipshtat@mail.huji.ac.il>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260914090408.1435080-2-inbal.lipshtat@mail.huji.ac.il
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/tcp_input.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=07cacab90290f795bad2e566790f4ba8e84cdf5f

======================================================================
DETAILED REPORT METHODOLOGY
======================================================================

The original Linux kernel CVE announcement for CVE-2026-98255 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98255

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:H/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:H
  The Best / paranoid CVSS score: 7.4

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: 6

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 alexanjelausa@gmail.com (and both
send reply to CVE record itself too and see "reply" button below for howto reply).