[CVE-2026-68118][MODERATE 7.0] tcp: challenge ACK for non-exact RST in SYN-RECEIVED

[AL-KERNEL <alkernel@kernelcve.org>]

CVE: CVE-2026-68118
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tcp: challenge ACK for non-exact RST in SYN-RECEIVED
Commit: 234f9ffbd9b2c1b24ec67200ea3cff07401bec48
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=234f9ffbd9b2c1b24ec67200ea3cff07401bec48
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68118
Analysis date: Mon, 10 Aug 2026 10:06:35 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A TCP SYN-RECEIVED validation flaw allowed non-exact in-window RST packets to remove pending request sockets instead of triggering challenge ACKs, enabling remote connection setup DoS under tuple, timing, and sequence constraints.

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

The original Linux kernel CVE announcement for CVE-2026-68118 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68118

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-68118	MODERATE	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:N/A:H';CWE-345;*CWE-20;CWE-693;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'The TCP SYN-RECEIVED request socket path could accept a non-exact in-window RST and remove the pending request instead of replying with a challenge ACK. A remote attacker may use crafted or spoofed TCP RST packets to disrupt connection establishment for a victim flow if the attacker can match the 4-tuple, hit the SYN-RECEIVED timing window, and place SEG.SEQ inside the receive window. For the CVSS the PR:N is used because no authenticated user privileges are needed beyond network packet injection capability. The issue is network reachable, but reliable blind exploitation has high complexity due to tuple, timing, and sequence requirements. Impact is denial of service against TCP connection setup. For the paranoid score, availability is treated as high because repeated successful resets could degrade service availability, while confidentiality and integrity remain none because this is a protocol validation bug without a memory corruption or data disclosure primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE NETWORK SKB LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  TCP RST packets can reach the affected SYN-RECEIVED request-socket path over the network.

* Broad/default/common subsystem or broadly deployed exposure: +1
  TCP connection setup is a default and widely exposed kernel networking path.

* Common networking core path or packet-processing path: +1
  The bug is in TCP request-socket processing during handshake validation.

* Hard or unreliable race / special timing required: -1
  Reliable blind exploitation requires matching the flow tuple, hitting the SYN-RECEIVED window, and using an in-window sequence number.

* Availability impact realistic: +1
  The impact is connection setup disruption. This is real availability impact, but not kernel memory corruption or host compromise.

Paranoid additional interpretation:

* Paranoid availability weighting: +1 effective uplift
  If an attacker is on-path, near-path, or otherwise able to observe or infer connection parameters, repeated non-exact in-window RST injection could degrade service availability more reliably.

No memory corruption signals apply:

* No UAF, OOB write, double-free, stale callback, refcount misuse, page-cache corruption, or object lifetime corruption is shown.
* No confidentiality or integrity impact is supported by the patch.

## 3. Reachability analysis

An unauthenticated remote attacker can theoretically trigger the affected path by sending crafted or spoofed TCP RST packets toward a server while a connection is in SYN-RECEIVED. No local account or kernel-facing local API access is required, so PR:N is appropriate for CVSS-style reasoning.

The practical difficulty depends heavily on attacker position. A blind off-path attacker must guess or infer the 4-tuple, hit a narrow handshake timing window, and place SEG.SEQ inside the receive window. An on-path or traffic-observing attacker has a much more practical trigger condition.

Namespaces and containers do not materially reduce the network exposure of the TCP receive path, although local namespace packet injection may be relevant in test or multi-tenant environments. The vulnerable path is default TCP behavior, not a rare optional subsystem.

## 4. Severity interpretation

This behaves like an actionable network Moderate rather than an Important memory-safety issue. The bug is a TCP protocol validation flaw that can remove a pending request socket when RFC 5961 and RFC 9293 require a challenge ACK for a non-exact in-window RST.

The realistic impact is denial of service against connection establishment. There is no evidence of kernel memory corruption, privilege escalation, information disclosure, or persistent state corruption. However, because this is network reachable and affects a core TCP handshake path, it should not be auto-closed as a routine low-risk correctness fix.

## 5. One-sentence report phrase

A TCP SYN-RECEIVED validation flaw allowed non-exact in-window RST packets to remove pending request sockets instead of triggering challenge ACKs, enabling remote connection setup DoS under tuple, timing, and sequence constraints.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable in a core TCP path and may affect exposed services, even though the impact appears limited to DoS and no memory corruption primitive is present.

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

Patch: tcp: challenge ACK for non-exact RST in SYN-RECEIVED
Commit: 234f9ffbd9b2c1b24ec67200ea3cff07401bec48
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=234f9ffbd9b2c1b24ec67200ea3cff07401bec48

Commit description:

The SYN-RECEIVED request-socket path in tcp_check_req() accepts an
in-window RST without requiring SEG.SEQ to exactly match RCV.NXT.  A
non-exact RST therefore removes the request instead of eliciting a
challenge ACK.

RFC 9293 section 3.10.7.4 applies the RFC 5961 reset check in
SYN-RECEIVED: an exact RST resets the connection, while a non-exact
in-window RST must trigger a challenge ACK and be dropped.

Apply that check before the ACK-field validation, following the RFC
sequence-number, RST, then ACK processing order.  Factor the per-netns
challenge ACK quota out of tcp_send_challenge_ack() so request sockets
can share it.  Use the request socket's send_ack() callback and its own
out-of-window ACK timestamp to send and rate-limit the response.

Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reported-by: Ao Wang <wangao@seu.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Fixes: 282f23c ("tcp: implement RFC 5961 3.2")
Cc: stable@vger.kernel.org
Signed-off-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20260717081443.809393-2-yangyx22@mails.tsinghua.edu.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  include/net/tcp.h
  net/ipv4/tcp_input.c
  net/ipv4/tcp_minisocks.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=234f9ffbd9b2c1b24ec67200ea3cff07401bec48

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

The original Linux kernel CVE announcement for CVE-2026-68118 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-68118

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:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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: 4
  Paranoid ActionableScore: 5

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).