[CVE-2026-64024][MODERATE 7.0] tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction [ Upstream

[CVE-2026-64024][MODERATE 7.0] tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64024
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction [ Upstream
Commit: e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64024
Analysis date: Sun, 19 Jul 2026 18:13:53 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-64024 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64024

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-2026-64024	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L';CWE-330;CWE-226;CWE-665;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '7.7';DESCR 'TCP used a per CPU tcp_tw_isn value to carry a TIME_WAIT derived ISN from tcp_v4_rcv or tcp_v6_rcv into tcp_conn_request. Several drop paths can leave this per CPU value stale, so the next SYN handled on the same CPU may consume an ISN that belongs to a different packet and may receive a predictable server ISN. For the CVSS the PR:N is used because a remote network peer can trigger the relevant TCP receive paths without local code execution. AC:H is used because reliable exploitation depends on steering packets to the same CPU and hitting a drop path before another SYN consumes the stale value. The issue is network reachable on systems exposing TCP listeners, but it is not a memory corruption bug. Impact is mainly confidentiality and integrity risk from weakened TCP ISN unpredictability, with possible low availability impact from bypassing normal request queue or syncookie handling in the affected path.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	REMOTE LEAK NETWORK SKB HARDWARE IPV6 KPANIC PACKET  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

1. ActionableScore
* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

2. Signal breakdown
* Remote reachable / network-triggerable: +2. The affected path is the TCP receive path and can be reached by remote packets sent to exposed TCP listeners.
* Confidentiality impact plausible: +1. Stale TIME_WAIT-derived ISN reuse may weaken TCP ISN unpredictability and can support TCP spoofing or session inference scenarios.
* Integrity impact plausible: +1. Predictable ISNs may enable packet injection or connection manipulation in favorable network conditions.
* Broad/default/common subsystem: +1. TCP is a default and broadly deployed subsystem.
* Common networking core path or packet-processing path: +1. The bug is in tcp_v4_rcv(), tcp_v6_rcv(), and tcp_conn_request().
* Hard or unreliable conditions: -1. Reliable exploitation depends on hitting a drop path and having the next SYN processed on the same CPU before the stale per-CPU value is overwritten or cleared.
* Practical exploit chain strongly implied: +1 for paranoid score only. The commit explicitly states ISN prediction as the security consequence, but practical off-path exploitation still depends on network positioning and timing.
* No memory corruption signal. The patch moves state from a per-CPU variable back to skb control block storage. It does not indicate UAF, OOB write, double-free, type confusion, or arbitrary write.

3. Reachability analysis
A remote peer can influence the TCP receive path by sending packets to a reachable TCP listener. No local account is required, so PR:N is appropriate for network-level triage. Namespaces do not materially reduce reachability because the vulnerable logic is in the common TCP stack for the namespace handling the listener. The path is default-enabled whenever TCP services are exposed. Practical exploitation is non-trivial because the attacker must create a stale tcp_tw_isn through one of the listed drop paths and then cause a relevant SYN to be processed on the same CPU.

4. Severity interpretation
This is not a generic kernel memory corruption issue and does not directly imply privilege escalation or kernel code execution. It behaves more like an actionable network protocol security flaw: the realistic impact is weakened TCP ISN unpredictability with plausible confidentiality and integrity consequences. Conservative handling is Actionable Moderate because it is remote, affects core TCP, and should not be auto-closed. The paranoid interpretation reaches the upper Actionable Moderate range because the commit explicitly frames the stale state leak as enabling ISN prediction.

5. One-sentence report phrase
A stale per-CPU tcp_tw_isn value in the TCP receive path can be consumed by a later SYN on the same CPU, potentially making TCP ISNs predictable and enabling network-level confidentiality or integrity attacks under timing-dependent conditions.

6. Manual review recommendation
MANUAL CHECK REQUIRED.
Reason: this is network reachable in a core TCP path and the stated impact is ISN prediction, so even without memory corruption it has plausible confidentiality and integrity impact that warrants manual triage.

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

Patch: tcp: fix stale per-CPU tcp_tw_isn leak enabling ISN prediction [ Upstream
Commit: e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd

Changed files:
  include/net/tcp.h
  net/ipv4/tcp.c
  net/ipv4/tcp_input.c
  net/ipv4/tcp_ipv4.c
  net/ipv6/tcp_ipv6.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=e47f7060eaf60894e3e4d0e3c4fe6e1f2eacfbdd

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

The original Linux kernel CVE announcement for CVE-2026-64024 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64024

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:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:L
  The Best / paranoid CVSS score: 7.7

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: 5
  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).