[CVE-2026-63925][MODERATE 7.0] macsec: fix replay protection at XPN lower-PN wrap

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

CVE: CVE-2026-63925
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: macsec: fix replay protection at XPN lower-PN wrap
Commit: dd7306779c6ce1238f4cdc34f3c1f2246b854457
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd7306779c6ce1238f4cdc34f3c1f2246b854457
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63925
Analysis date: Sun, 19 Jul 2026 19:45:25 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not a Strong Important candidate on current evidence.
Manual review required: YES

Summary:
MACsec XPN replay protection can fail at lower PN wrap, allowing an adjacent attacker who captured the legitimate PN 0xffffffff frame to replay it repeatedly and bypass anti-replay integrity checks.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-63925 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63925

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-2026-63925	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:L';CWE-294;CWE-190;CWE-345;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '5.9';DESCR 'A replay protection bypass in MACsec XPN can occur when the lower packet number reaches 0xffffffff and pn plus 1 wraps to 0. In this state the receiver may fail to advance the upper PN half, so a legitimate captured frame with PN 0xffffffff can be accepted repeatedly instead of being rejected as a replay. An attacker needs adjacent Layer 2 access to the protected MACsec link and must be able to capture and reinject the specific frame around the lower PN wrap condition. For the CVSS the PR:N is used because no local account or privileges on the target system are required, but AV:A and AC:H are used because exploitation requires access to the local link and a rare protocol state. Impact is primarily integrity loss through replay of an accepted encrypted frame, with possible limited availability side effects depending on the carried traffic.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 5)	YES	SIMPLEFIX NETWORK LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS 	YES	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: **Strong Important candidate / Actionable Moderate at minimum**:: Actionable Moderate at minimum. Not a Strong Important candidate on current evidence.

## 2. Signal breakdown

Conservative signals:

* Network-triggerable but adjacent only: +2
  The issue is triggered by receiving replayed MACsec frames, but the attacker needs Layer 2 adjacency to the protected MACsec link, not Internet reachability.

* Cross-boundary security or policy bypass: +1
  The bug bypasses MACsec XPN replay protection, which is an intended cryptographic integrity boundary.

* Practical exploit path strongly implied: +1
  The commit explicitly describes capture of a legitimate PN 0xffffffff frame and indefinite replay afterward.

* Integrity impact plausible: +1
  Accepted replayed encrypted frames can violate traffic integrity and protocol assumptions.

* Packet-processing path: +1
  The vulnerable logic is in MACsec receive/decrypt post-processing.

Negative signals:

* Hard or rare condition: -1
  Exploitation requires the XPN association to reach the lower PN wrap point at 0xffffffff and the attacker must capture that specific legitimate frame.

* Configuration-dependent exposure: -1
  MACsec XPN must be configured and in use. This is not a default exposure on ordinary systems.

Paranoid adjustment:

* Paranoid score removes some deployment/configuration discount and treats the replay as operationally significant for protected L2 environments, giving 5.

No memory corruption signals apply. No UAF, OOB write, arbitrary write, refcount issue, or LPE primitive is shown.

## 3. Reachability analysis

An attacker does not need a local account on the target system. They need adjacent Layer 2 access to the MACsec-protected link and the ability to capture and reinject a legitimate frame with PN 0xffffffff on an XPN association.

Namespaces and containers are not the main factor here, because the trigger is packet-path behavior on a configured MACsec link. The affected path is not broadly default-enabled. It requires MACsec with XPN and anti-replay semantics in active use.

Realistic exploitation is constrained by the need to observe the rare lower-PN wrap condition, but the commit describes a concrete replay scenario once that frame is captured.

Call-site confidence: high. The affected function and state update logic are included in the patch context.

## 4. Severity interpretation

This behaves like an actionable Moderate cryptographic replay-protection bypass, not like ordinary low-risk cleanup and not like kernel memory corruption.

Theoretical impact is mainly integrity loss through replay of an otherwise valid encrypted frame. Availability impact is possible only as a side effect if replayed traffic disrupts higher-level protocols. There is no evidence of confidentiality loss, privilege escalation, arbitrary code execution, or kernel memory corruption.

Because this is network-adjacent and affects a security property of MACsec, it should not be auto-closed solely because the CVSS is below Important range.

## 5. One-sentence report phrase

MACsec XPN replay protection can fail at lower PN wrap, allowing an adjacent attacker who captured the legitimate PN 0xffffffff frame to replay it repeatedly and bypass anti-replay integrity checks.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not memory corruption, but it is a network-adjacent cryptographic replay-protection bypass with a concrete attacker model described in the commit.

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

Patch: macsec: fix replay protection at XPN lower-PN wrap
Commit: dd7306779c6ce1238f4cdc34f3c1f2246b854457
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=dd7306779c6ce1238f4cdc34f3c1f2246b854457

Commit description:

In macsec_post_decrypt(), when pn is U32_MAX, pn + 1 overflows u32 to 0
and the first branch never fires. If next_pn_halves.lower is also in the
upper half, pn_same_half(pn, lower) is true and the XPN else-if does not
fire either, leaving next_pn_halves unchanged. An attacker that captures
the legitimate frame carrying pn == 0xFFFFFFFF on an XPN association
can then replay it indefinitely, since lowest_pn never rises above
the captured pn and macsec_decrypt() reconstructs the same IV.

Extend the XPN else-if to also fire when pn + 1 wraps to 0, so receipt
of pn == U32_MAX advances next_pn_halves to (upper + 1, 0).

Fixes: a21ecf0 ("macsec: Support XPN frame handling - IEEE 802.1AEbw")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Link: https://patch.msgid.link/SYBPR01MB78813FD49E58F253B989F197AF012@SYBPR01MB7881.ausprd01.prod.outlook.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  drivers/net/macsec.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=dd7306779c6ce1238f4cdc34f3c1f2246b854457

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

The original Linux kernel CVE announcement for CVE-2026-63925 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63925

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