* [CVE-2026-98287][MODERATE REGULAR] pppoatm: ensure a writable skb header and linear data [ Upstream
@ 2026-10-09 8:18 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-09 8:18 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98287
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: pppoatm: ensure a writable skb header and linear data [ Upstream
Commit: 208ccc4d08b1897451e9ba01c016bf93ff107966
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=208ccc4d08b1897451e9ba01c016bf93ff107966
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98287
Analysis date: Fri, 09 Oct 2026 04:18:09 -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-2026-98287 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98287
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-98287 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98287 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=0 LEGACY=MODERATE RESULT=DIFF (independent Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-125;*CWE-20;CWE-662;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '6.4';DESCR 'The PPPoATM transmit path can receive packets through PPP channel bridging without passing through ppp_start_xmit(), so the skb header may remain shared and the packet data may be empty or non linear. LLC encapsulation can then modify a shared skb header, while direct access to skb data can read outside the logical packet or inspect uninitialized headroom during the nospace recovery path. A malicious or malformed frame from a directly connected PPP peer may trigger packet corruption, an invalid read, or a kernel networking failure under the required bridging and PPPoATM configuration. For the CVSS the PR:N value is used because an adjacent PPP peer can supply frames without a local account or privileges on the target after an administrator has configured the bridge. The issue is adjacent network reachable rather than generally reachable over the Internet, and exploitation depends on a specialized PPP over ATM deployment and a suitable shared, empty, or non linear skb state. Impact is at least disruption of the affected network path and in the paranoid case may include limited confidentiality or integrity impact from uninitialized reads and shared buffer modification, with possible denial of service if the invalid state causes a kernel failure.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate (with actual score 1) MAYBE OOB SKB IMPROVEONLY LINUS PACKET LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN4 - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=1
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 1
* Conservative score: 1
* Raw paranoid score: 1
* Paranoid score: 1
* No floor or cap was applied.
* Final recommended bucket: **Ordinary Moderate**::
2. Signal breakdown
* +1 Constrained kernel buffer overwrite. LLC encapsulation performs a fixed four-byte write into allocated skb headroom without first ensuring that the shared skb backing storage is writable.
* +0 Limited invalid reads. `skb->data[0]` may be read for an empty or non-linear frame, and `skb->data[-1]` may inspect uninitialized headroom, but no disclosure path is established.
* +0 Crash impact. The patch identifies fragile skb state and possible unbalanced `skb_push()`, but does not establish a reliable kernel crash.
* +0 Remote reachability. PPP channel bridging provides a plausible adjacent input path, but the complete attacker-controlled trigger and its setup requirements are not established.
* No UAF, controlled reclaim, arbitrary write, attacker-selected target, disclosure, authorization bypass, or privilege escalation is demonstrated.
* No additional integrity point was awarded. The shared-buffer write is already represented by the constrained-overwrite signal, and modification of another security principal's protected data is not established.
* No availability point was awarded because a host-wide crash or persistent failure is not demonstrated.
* No rarity or hardware penalty was applied merely because PPPoATM is specialized.
B5-R evaluation:
* Gate 1 fails. There is no confirmed heap or slab out-of-bounds write. The LLC write remains within checked or reallocated headroom.
* Gate 2 fails. No operation index or offset is shown accumulating across retries. The old compression undo sequence is fragile, but retry-accumulated pointer growth is not established.
* Gate 3 is unresolved. Bridged packets reach the function, but attacker control of the complete malformed or shared-skb sequence and required setup privilege is not proven.
* Gate 4 fails. No OOB-corrupted descriptor, pointer, or structured state is shown being consumed later.
* Therefore B5-R Important preservation does not apply.
3. Reachability analysis
The vulnerable path is reachable through PPP channel bridging via `PPPIOCBRIDGECHAN`, bypassing the normal `ppp_start_xmit()` call that performs `skb_cow_head()`. An adjacent PPP peer may influence packet contents after a bridge has been configured, but the supplied evidence does not establish that the peer can reliably create the required shared, empty, or non-linear skb state.
The privilege required to configure the complete PPPoATM bridge is not visible, so `HostAdminRequired` remains `UNKNOWN`. The evidence also does not establish that all setup and trigger operations work in attacker-created namespaces, so `NamespaceScopedPrivilege` is `UNKNOWN`. No race is involved, and no timing penalty applies. ATM support and PPP bridging are prerequisites, but no rarity penalty is applied without evidence that the attacker cannot reach an existing deployment.
4. Primitive and severity interpretation
The concrete primitive is a bounded, fixed-content write into shared skb backing storage without copy-on-write preparation, combined with limited invalid reads and potentially incorrect skb pointer adjustment. This is not a heap OOB write because sufficient LLC headroom is checked or allocated before the four-byte `memcpy()`.
There is no stale post-lifetime object, UAF, repeated release, refcount mutation, or controlled replacement. The write target and payload are fixed, and no security-sensitive adjacent victim is identified. The behavior therefore fits `CLASS_C`: constrained packet-buffer corruption and invalid-state consumption without an independently useful exploitation primitive.
The demonstrated practical impact is malformed packet state or disruption of the affected PPPoATM path. Kernel failure is possible but not established as reliable. Privilege escalation, information disclosure, and broader integrity compromise remain speculative. `ImportantCandidate=NO` and `AutoDowngradeAllowed=YES` follow because the NEW52 B5-R gates are not satisfied and no unresolved Important-grade primitive remains.
5. One-sentence report phrase
PPPoATM channel bridging can expose shared or insufficiently linear skb state to bounded header modification and invalid reads, but the patch does not establish a retry-accumulated heap OOB write, UAF, disclosure, or privilege-escalation primitive.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Confirm the exact `PPPIOCBRIDGECHAN` privilege boundary and whether an adjacent peer can reliably generate shared, empty, or non-linear skb inputs, although the supplied patch does not qualify for B5-R preservation.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: pppoatm: ensure a writable skb header and linear data [ Upstream
Commit: 208ccc4d08b1897451e9ba01c016bf93ff107966
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=208ccc4d08b1897451e9ba01c016bf93ff107966
Changed files:
net/atm/pppoatm.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=208ccc4d08b1897451e9ba01c016bf93ff107966
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98287 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98287
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:L
The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS score: 6.4
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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