[CVE-2026-97964][MODERATE REGULAR] ppp_synctty: ensure a writeable skb header [ Upstream

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

CVE: CVE-2026-97964
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ppp_synctty: ensure a writeable skb header [ Upstream
Commit: 3af7797fbabc84992e0ca5ad2bc0ca8537d053ef
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3af7797fbabc84992e0ca5ad2bc0ca8537d053ef
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97964
Analysis date: Fri, 25 Sep 2026 10:50:36 -0400
ActionableScore: 1
ActionableScore lower bound: unknown
Actionable bucket: Ordinary Moderate
Manual review required: NO

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-97964 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97964

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 REGULAR
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
======================================================================

CVE-2026-97964	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-97964 CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N';*CWE-664;CWE-707;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N';BEST CVSS score: '4.3';DESCR 'A shared skb header can be modified in the synchronous PPP transmit path because ppp_sync_txmunge checks headroom but does not perform copy on write before prepending PPP address and control bytes. A PPPoE frame can reach this path through PPP channel bridging while a clone of the same skb remains queued to an AF_PACKET socket. The write can alter packet data still visible to the packet capture consumer. For the CVSS the PR:N is used because a remote PPPoE peer does not need privileges on the affected host once the vulnerable PPP bridge configuration exists. The attack vector is adjacent network AV:A because PPPoE normally operates on a local Layer 2 access network. The practical impact is limited integrity impact on packet data observed by AF_PACKET. The patch context does not demonstrate an out of bounds write use after free kernel crash or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1)	YES	OOB SIMPLEFIX NETWORK SKB LINUS  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4	YES	NO	checked

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

ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=NO

1. ActionableScore

* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* Final recommended bucket: **Ordinary Moderate**::

2. Signal breakdown

* +1 Constrained shared SKB buffer modification. The patch establishes that ppp_sync_txmunge can prepend two fixed PPP address and control bytes to a cloned SKB header without first ensuring copy on write ownership. The visible effect is modification of packet bytes still observable through an AF_PACKET clone.
* No remote reachable +2. A remote or adjacent PPPoE peer can provide the triggering frame only after a PPP channel bridge to a synchronous tty endpoint has already been configured locally. The supplied evidence does not establish that an attacker can perform the required bridge setup.
* No integrity impact +1. The altered data belongs to an attacker supplied network frame, and the patch does not establish modification of protected kernel state, another principals file data, credentials, policy, or independently permissioned data.
* No confidentiality impact +1. No readback or disclosure primitive is established.
* No strong exploitation leverage +2. The write is limited to fixed PPP header bytes in a valid SKB buffer. No arbitrary target selection, controlled reclaim, object replacement, callback influence, or directed kernel write is shown.
* No reliable kernel crash or availability point. The patch does not describe an out of bounds access, UAF, crash, panic, resource exhaustion, or device failure.
* No privilege escalation point. The patch does not establish a capability bypass, privilege boundary bypass, or a path from packet modification to privileged behavior.

3. Reachability analysis

A network adjacent PPPoE peer can supply a packet that reaches the vulnerable operation if the affected host already has PPP channel bridging configured to a synchronous tty channel and an AF_PACKET consumer receives a cloned copy of the packet.

The attacker does not require host administrator privileges merely to send a PPPoE frame in such a configured deployment. Therefore HostAdminRequired is NO. However, the bridge topology is a material precondition and likely requires prior local administrative configuration. The supplied patch does not show whether an unprivileged local attacker can create the PPP bridge, control the synchronous tty channel, or arrange the required AF_PACKET clone.

NamespaceScopedPrivilege is UNKNOWN. No supplied capability checks or full namespace scoped setup path establishes that all required PPP bridging and tty operations can be performed in an attacker controlled namespace.

No special hardware, race condition, firmware interaction, or boot time option is established by the patch.

4. Primitive and severity interpretation

The exact primitive is a shared SKB ownership and copy on write violation. A cloned SKB can retain visibility of a buffer header while ppp_sync_txmunge prepends fixed PPP address and control bytes to that same buffer without calling skb_cow_head.

This is not a UAF, stale lifetime reference, refcount mutation, OOB write, arbitrary kernel write, or active object replacement. The affected storage is packet buffer data, and the demonstrated consequence is incorrect packet contents visible to an AF_PACKET consumer.

The bug is classified as CLASS_C because the demonstrated effect is constrained packet representation corruption without an independently useful kernel memory corruption primitive. ImportantCandidate is NO because no meaningful exploitation leverage beyond alteration of the cloned packet view is established.

AutoDowngradeAllowed is NO because the patch directly demonstrates a shared buffer ownership and COW violation. Even though the currently supported operational impact is low, deployments using AF_PACKET for monitoring, filtering, accounting, or packet analysis may need manual confirmation that corrupted packet visibility cannot affect a higher level security decision.

5. One-sentence report phrase

A PPP channel bridging path can modify a cloned SKB header without copy on write, causing fixed PPP header bytes to become visible as altered packet data to an AF_PACKET consumer, but no kernel memory corruption, crash, or privilege escalation primitive is established.

6. Manual review recommendation

MANUAL CHECK RECOMMENDED

The practical impact depends on whether the deployment combines PPPoE, PPP channel bridging to synchronous tty, and AF_PACKET consumers whose decisions depend on the original packet representation.

7. Call-site confidence

Call-site confidence: medium

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

Patch: ppp_synctty: ensure a writeable skb header [ Upstream
Commit: 3af7797fbabc84992e0ca5ad2bc0ca8537d053ef
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3af7797fbabc84992e0ca5ad2bc0ca8537d053ef

Changed files:
  drivers/net/ppp/ppp_synctty.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=3af7797fbabc84992e0ca5ad2bc0ca8537d053ef

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

The original Linux kernel CVE announcement for CVE-2026-97964 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-97964

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:L/PR:N/UI:N/S:U/C:N/I:L/A:N
  The Best / paranoid CVSS score: 4.3

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: 1

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).