[CVE-2026-63913][MODERATE 7.0] netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check

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

CVE: CVE-2026-63913
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check
Commit: 2006979a15af5404bf932a325357683c0bac1656
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2006979a15af5404bf932a325357683c0bac1656
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63913
Analysis date: Sun, 19 Jul 2026 19:49:28 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A logic flaw in netfilter TCP conntrack allows crafted invalid-sequence RST traffic to force a tracked connection into TCP_CONNTRACK_CLOSE without validating packet direction, potentially causing premature NAT or conntrack entry teardown and network DoS.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63913 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63913

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: NO

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-2026-63913	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:H';*CWE-20;CWE-345;CWE-754;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.5';DESCR 'A logic flaw in the netfilter TCP conntrack state machine can allow an invalid sequence RST packet to force a tracked connection into TCP_CONNTRACK_CLOSE without confirming that the packet is in the expected reply direction. This can prematurely terminate a conntrack or NAT entry after a crafted SYN and invalid RST sequence, making connection teardown easier than intended. For the CVSS the PR:N is used because the attacker does not need local privileges when crafted TCP packets can reach the conntrack or NAT path. AC:H is used because reliable impact against a victim flow usually depends on matching the relevant flow state and packet context. The issue is network reachable on systems using TCP conntrack or NAT, but it is not a memory corruption bug. Impact is denial of service and limited integrity impact against connection tracking state rather than privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE DANGER SIMPLEFIX NETFILTER NETWORK KPANIC PACKET 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Remote reachable / network-triggerable: +2
  Crafted TCP packets can reach the conntrack/NAT packet path when the system uses TCP conntrack or NAT.

* Common networking core path or packet-processing path: +1
  The bug is in netfilter conntrack TCP state handling, which is a central packet-processing path on firewall/NAT systems.

* Integrity impact plausible: +1
  The attacker can alter conntrack state by forcing TCP_CONNTRACK_CLOSE earlier than intended, affecting NAT/session state correctness.

* Availability impact realistic: +1 in paranoid score only
  Premature NAT entry teardown can disrupt active or establishing connections. This is not a kernel crash, so I count it only for the paranoid/actionable score.

Subtracted signals:

* Hard or unreliable sequencing / context requirement: -1
  Reliable impact depends on packet ordering and matching a relevant conntrack state after SYN observation, so this is not a trivial one-packet universal DoS.

Not triggered:

* No generic memory corruption.
* No UAF, OOB write, double free, refcount takeover, or page-cache corruption.
* No privilege escalation primitive.
* No kernel crash or host-wide panic shown by the patch.

## 3. Reachability analysis

The issue can be triggered by a remote or adjacent attacker able to send crafted TCP packets through a firewall/NAT path using nf_conntrack. No local account is required in the network-facing interpretation, so PR:N is reasonable for CVSS-style reasoning.

Namespaces and containers do not matter for the basic remote packet path, but local containerized workloads may also trigger it if their traffic traverses host conntrack/NAT. The affected functionality is common on Linux firewalls, routers, container hosts, and NAT gateways, though impact depends on whether the attacker can influence packet flow state sufficiently.

## 4. Severity interpretation

This behaves like an actionable Moderate networking logic flaw rather than an Important memory-safety vulnerability. The practical impact is premature conntrack/NAT teardown, causing connection disruption and limited state-integrity impact. There is no evidence of memory corruption, kernel code execution, information disclosure, or local privilege escalation.

The paranoid score reaches 5 because the bug is network reachable in a common packet-processing subsystem and can affect NAT/firewall state. It should not be auto-closed as a trivial Moderate, but it also does not justify Important-class handling without evidence of broader connection hijacking, persistent firewall bypass, or memory corruption.

## 5. One-sentence report phrase

A logic flaw in netfilter TCP conntrack allows crafted invalid-sequence RST traffic to force a tracked connection into TCP_CONNTRACK_CLOSE without validating packet direction, potentially causing premature NAT or conntrack entry teardown and network DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network-reachable netfilter/NAT behavior can have deployment-specific impact, especially on firewalls, NAT gateways, and container hosts. However, the patch supports a logic/state DoS only, not memory corruption or privilege escalation.

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

Patch: netfilter: conntrack: tcp: do not force CLOSE on invalid-seq RST without direction check
Commit: 2006979a15af5404bf932a325357683c0bac1656
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2006979a15af5404bf932a325357683c0bac1656

Commit description:

An unintended behavior in the TCP conntrack state machine allows a
connection to be forced into the CLOSE state using an RST packet with an
invalid sequence number.

Specifically, after a SYN packet is observed, an RST with an invalid SEQ
can transition the conntrack entry to TCP_CONNTRACK_CLOSE, regardless of
whether the RST corresponds to the expected reply direction. The relevant
code path assumes the RST is a response to an outgoing SYN, but does not
validate packet direction or ensure that a matching SYN was actually sent
in the opposite direction.

As a result, a crafted packet sequence consisting of a SYN followed by an
invalid-sequence RST can prematurely terminate an active NAT entry. This
makes connection teardown easier than intended.

So, tighten the state transition logic to ensure that RST-triggered
CLOSE transitions only occur when the RST is a valid response to a
previously observed SYN in the correct direction.

Cc: stable@vger.kernel.org
Fixes: 9fb9cbb ("[NETFILTER]: Add nf_conntrack subsystem.")
Signed-off-by: Hamza Mahfooz <hamzamahfooz@linux.microsoft.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nf_conntrack_proto_tcp.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=2006979a15af5404bf932a325357683c0bac1656

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

The original Linux kernel CVE announcement for CVE-2026-63913 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63913

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