[CVE-2026-64007][IMPORTANT] netfilter: synproxy: refresh tcphdr after skb_ensure_writable [ Upstream

[CVE-2026-64007][IMPORTANT] netfilter: synproxy: refresh tcphdr after skb_ensure_writable [ Upstream

[AL-KERNEL]

CVE: CVE-2026-64007
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: netfilter: synproxy: refresh tcphdr after skb_ensure_writable [ Upstream
Commit: 9902a1058992de5d95656b64a3bd95c077f7ba2c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9902a1058992de5d95656b64a3bd95c077f7ba2c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64007
Analysis date: Sun, 19 Jul 2026 20:05:48 -0400
ActionableScore: 9
ActionableScore lower bound: 7
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A stale TCP header pointer in netfilter SYNPROXY can survive `skb_ensure_writable()` and be used for a checksum write after the old skb head is freed, allowing a remote packet to cause kernel memory corruption or DoS when SYNPROXY is exposed.

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

The original Linux kernel CVE announcement for CVE-2026-64007 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64007

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: IMPORTANT
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-2026-64007	IMPORTANT	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:H/I:H/A:H';*CWE-416;CWE-825;*CWE-787;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A stale TCP header pointer in the netfilter SYNPROXY timestamp adjustment path can be used after skb_ensure_writable makes the skb linear and writable. On cloned or non linear skbs this may free the old skb head, while the cached th pointer is still later used for inet_proto_csum_replace4 on th check. A remote packet can reach this path when SYNPROXY is configured and exposed on a firewall or gateway. For the CVSS the PR:N is used because the sender does not need local credentials on the target and only needs network reachability to the protected service path. Impact is at least remote denial of service via kernel crash. The stale pointer write into freed skb head is a memory corruption candidate, so the paranoid score treats confidentiality and integrity as high for manual review sensitivity.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 9)	YES	REMOTE NETFILTER UAF SKB LINUS IPV6 KPANIC PACKET 	NO	NO	checked

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

ActionableScore=9
ActionableScoreLower=7

## 1. ActionableScore

* Conservative score: 7
* Paranoid score: 9
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A remote packet can reach this path when netfilter SYNPROXY is configured on a firewall or gateway.

* Memory corruption, strong corruption primitive: +2
  The commit explicitly describes a stale `tcphdr` pointer after `skb_ensure_writable()` may free the old `skb->head`, followed by a write through `th->check`.

* Real lifetime corruption: +1
  This is a stale pointer after free / write-after-free condition involving `skb->head`.

* Reliable kernel crash / strong DoS: +1
  A write into freed slab memory in a packet-processing path is a realistic kernel crash condition.

* Common networking / packet-processing path: +1
  The affected code is in netfilter SYNPROXY packet handling.

* Corruption primitive constrained: -1
  The write is limited to the TCP checksum field update and does not show arbitrary target or arbitrary payload control.

Paranoid additional signals:

* Weak LPE concern: +1
  The primitive is a network-reachable write-after-free candidate, but the patch does not demonstrate attacker-controlled reclaim or arbitrary write.

* Confidentiality impact plausible: +1
  Plausible only under memory-corruption escalation assumptions.

* Integrity impact plausible: +1
  Plausible because the bug can write into freed kernel memory, although control appears constrained.

## 3. Reachability analysis

The trigger is remote in deployments where SYNPROXY rules are configured and exposed to traffic. The attacker does not need local credentials on the target, so the practical PR is none for the packet sender. The path is not enabled on all systems by default because SYNPROXY requires firewall configuration, but it is a real packet-processing path on systems using SYNPROXY for TCP protection. Namespaces do not materially reduce the attacker requirement for the remote packet path, although local configuration of SYNPROXY itself requires administrative privileges.

Call-site confidence: high. The commit message names the callers, explains `skb_header_pointer()`, `skb_ensure_writable()`, `pskb_expand_head()`, and the stale `th` use in `inet_proto_csum_replace4()`.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch context explicitly identifies a write into freed slab memory, not only a NULL dereference or malformed checksum. Realistic exploitation is still constrained by SYNPROXY deployment, skb clone or non-linear conditions, and limited checksum-write semantics. Conservative handling should treat it as Actionable Moderate or Important candidate. Paranoid handling should keep it in Strong Important territory because it is remotely reachable kernel memory corruption in a packet-processing path.

## 5. One-sentence report phrase

A stale TCP header pointer in netfilter SYNPROXY can survive `skb_ensure_writable()` and be used for a checksum write after the old skb head is freed, allowing a remote packet to cause kernel memory corruption or DoS when SYNPROXY is exposed.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the issue is network reachable in configured SYNPROXY deployments and the patch explicitly describes stale-pointer write-after-free into freed `skb->head` memory.

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

Patch: netfilter: synproxy: refresh tcphdr after skb_ensure_writable [ Upstream
Commit: 9902a1058992de5d95656b64a3bd95c077f7ba2c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9902a1058992de5d95656b64a3bd95c077f7ba2c

Changed files:
  net/netfilter/nf_synproxy_core.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=9902a1058992de5d95656b64a3bd95c077f7ba2c

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

The original Linux kernel CVE announcement for CVE-2026-64007 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64007

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:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 7
  Paranoid ActionableScore: 9

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).