[CVE-2026-63872][LOW] esp: fix page frag reference leak on skb_to_sgvec failure [ Upstream

[CVE-2026-63872][LOW] esp: fix page frag reference leak on skb_to_sgvec failure [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63872
Priority: LOW
AL-KERNEL base severity: LOW
KPANIC flag: NO
Patch: esp: fix page frag reference leak on skb_to_sgvec failure [ Upstream
Commit: e705b8ff4dd38fb8fe4e6fdc5378a86acea4feb5
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e705b8ff4dd38fb8fe4e6fdc5378a86acea4feb5
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63872
Analysis date: Sun, 19 Jul 2026 11:35:44 -0400
ActionableScore: 2
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate / Low-like
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-63872 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63872

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: LOW
Manual review required: NO

A detailed explanation of the methodology and priority rules is included
at the end of this message.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-63872	LOW	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.7';DESCR 'A page fragment reference leak exists in the IPsec/ESP output path when non-inplace encryption replaces old skb fragments with a new page and the second skb_to_sgvec() call fails before the AEAD request is fully initialized. The old source scatterlist still refers to the original skb page fragments, but the error path only frees the temporary scatterlist memory and kfree_skb() later releases only the new fragment. For the CVSS the PR:L is used because a local user or process may be able to generate traffic through an already configured ESP policy, while creating the IPsec/XFRM policy itself is normally administrative setup and is not counted as the exploit action. The issue is not directly network reachable because it is in the outbound ESP processing path, although a remote peer may indirectly cause replies over IPsec in some deployments. Impact is denial of service through memory pressure or resource exhaustion, with no supported UAF, OOB access, information leak, or privilege escalation primitive.';NO MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like (with actual score 2)	YES	REMOTE LEAK NETWORK SKB IPV6  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5 DECREASED_TO_LOW_FROM_MODERATE_BASED_ON_ACTIONABLESCORELESSTHAN3	NO	NO	checked

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

ActionableScore=2

1. ActionableScore
* Conservative score: 2
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like**

2. Signal breakdown
* Local unprivileged trigger: +1. A local process may generate traffic that traverses an already configured ESP/XFRM policy. Creating the policy is normally administrative, but sending matching traffic is not necessarily privileged.
* Real lifetime corruption: +1. The bug is a page-fragment reference lifetime leak: old skb page frags remain referenced after replacement and an error path fails to unref them.
* Availability impact realistic: +1. Repeated triggering could create memory pressure or resource exhaustion.
* Hard or unreliable special condition: -1. The vulnerable path requires non-inplace ESP output plus failure of the second skb_to_sgvec() call, which is not an easy or deterministic trigger.
* No generic memory corruption bonus. This is a leak of references, not UAF, double-free, OOB write, type confusion, or attacker-controlled overwrite.
* No privilege escalation bonus. The patch does not show reclaim, stale object reuse, writable freed object access, callback control, or refcount takeover.
* No remote-reachable bonus. The affected code is outbound ESP processing. A peer may indirectly influence traffic patterns, but the failure condition is local kernel processing and not a direct packet parser remote trigger.

3. Reachability analysis
A local user can potentially send traffic that matches an already configured IPsec ESP policy. Setting up XFRM/IPsec state usually requires CAP_NET_ADMIN or administrator-controlled configuration, so fully self-contained exploitation by an unprivileged user depends on preexisting policy. Containers or namespaces may matter only if the container has delegated network administration or can route traffic through host ESP policy. The path is not default-active on systems without IPsec ESP configuration. Realistic exploitation requires repeated access to the uncommon error path where destination skb_to_sgvec() fails after old frags were captured in the source scatterlist.

4. Severity interpretation
This behaves like an ordinary Moderate or Low-like resource leak rather than an Important kernel vulnerability. The theoretical impact is memory pressure or eventual DoS if the failure path can be repeatedly reached. There is no supported evidence of UAF, OOB access, information disclosure, integrity impact, or privilege escalation. The bug is worth fixing but does not appear to require urgent manual escalation based on the provided patch.

5. One-sentence report phrase
A page-fragment reference leak in the IPsec ESP output error path can retain old skb fragments after skb_to_sgvec() failure, potentially causing local resource exhaustion under specific configured ESP conditions.

6. Manual review recommendation
NO MANUAL CHECK NEEDED
This is a constrained reference leak in a non-default IPsec ESP path with no demonstrated memory corruption or privilege-escalation primitive.

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

Patch: esp: fix page frag reference leak on skb_to_sgvec failure [ Upstream
Commit: e705b8ff4dd38fb8fe4e6fdc5378a86acea4feb5
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=e705b8ff4dd38fb8fe4e6fdc5378a86acea4feb5

Changed files:
  net/ipv4/esp4.c
  net/ipv6/esp6.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=e705b8ff4dd38fb8fe4e6fdc5378a86acea4feb5

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

The original Linux kernel CVE announcement for CVE-2026-63872 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63872

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:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 4.7

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: unknown
  Paranoid ActionableScore: 2

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: LOW
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): LOW

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).