[CVE-2026-98401][MODERATE 7.0] net: iptunnel: fix stale transport header during tunnel decapsulation [ Upstream

[CVE-2026-98401][MODERATE 7.0] net: iptunnel: fix stale transport header during tunnel decapsulation [ Upstream

[AL-KERNEL]

CVE: CVE-2026-98401
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: iptunnel: fix stale transport header during tunnel decapsulation [ Upstream
Commit: 8c3d4dfcecf7670c1d752f1ae74b21b2af5feac1
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c3d4dfcecf7670c1d752f1ae74b21b2af5feac1
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98401
Analysis date: Sun, 18 Oct 2026 13:36:10 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate / Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98401 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98401

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.

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

CVE-2026-98401	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98401 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME (final reconciliation agrees; DECISION: both 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:N/A:H';CWE-125;*CWE-20;CWE-682;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '5.9';DESCR 'A stale transport header can remain in an skb after tunnel decapsulation when skb data is advanced to the inner frame but transport_header still refers to the removed outer transport header. If the inner GSO frame is forwarded at layer 2 through a bridge or HSR, the normal IP receive path does not reset the offset. qdisc_pkt_len_segs_init can then interpret the negative transport offset as a very large unsigned header length and perform an out of bounds read, causing a KASAN fault, page fault, or kernel crash. Exploitation requires several conditions including tunnel encapsulation, GSO metadata, layer 2 forwarding, and qdisc processing, so AC:H is appropriate. For the CVSS the PR:N value is used because a network peer that can send suitable traffic to an exposed tunnel endpoint does not need an authenticated local user context on the target. AV:A is used because practical triggering depends on access to the relevant tunnel or virtual network path and on locally generated GRO or GSO metadata rather than arbitrary Internet traffic. Impact is primarily denial of service. The paranoid score allows limited confidentiality impact because an out of bounds kernel read occurs, but no disclosure channel, write primitive, UAF, or privilege escalation path is established. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.9 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. No severity changing production threshold or specialist override was recorded. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist has no review artifact for this run. The independent weaker Aegis shadow evaluated CVSS 5.3 and ActionableScore 1 with primitive class CLASS_C and manual review RECOMMENDED. Its independently reconciled impact was MODERATE_KPANIC. The shadow agreed with the final production impact and aggregation did not change that impact. The final reported impact is MODERATE_KPANIC.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Important candidate requiring manual review (with actual score 4)	MAYBE	KASAN NETWORK SKB KERNEL_PANIC_PLUS_UAF HARDWARE QDISC SYZBOT KPANIC PACKET 	-	-	guess

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

ActionableScore=4
ActionableScoreLower=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=UNKNOWN
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 3
* Conservative score: 3
* Raw paranoid score: 4
* Paranoid score: 4
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +1 Weak or indirect corruption candidate. Decapsulation leaves `skb->transport_header` inconsistent with the advanced `skb->data`, and the negative offset is later reinterpreted as a large unsigned header length.
* +1 Reliable kernel crash or strong DoS. The supplied trace demonstrates an out-of-bounds read followed by a KASAN or page fault in `qdisc_pkt_len_segs_init()`.
* +1 Common networking packet-processing path. The stale state reaches bridge or HSR forwarding, `__dev_queue_xmit()`, and qdisc processing.
* No negative timing point. Four conditions are required, but the evidence does not establish a narrow or uncontrollable race.
* No privilege penalty. Host-admin privilege is not positively established for the complete trigger.

Paranoid-only signal:

* +1 V33 active-state and trust-boundary review signal. Attacker-influenced GSO SKB state is consumed by privileged host networking after tunnel decapsulation, and the trace grounds a TUN-originated path. A delegated TUN, guest, or namespace boundary is credible but unresolved.

Not awarded:

* No UAF, OOB write, checksum write, controlled reclaim, arbitrary write, disclosure path, or privilege escalation is established.
* Confidentiality is not scored merely because an internal kernel OOB read occurs.
* No additional availability point is awarded for the same demonstrated crash.
* Direct remote reachability is not awarded. Ordinary network packets do not directly supply GSO metadata, and the complete remote construction path is unresolved.
* NEW52 B5-R does not apply because there is no retry-accumulated heap OOB write.
* NEW53 B3-PENDING does not apply because no shared-backing write occurs.
* NEW54 B4-PENDING does not apply because there is no authorization decision or DENY-to-ALLOW transition.

3. Reachability analysis

The demonstrated reproducer reaches the path through `tun_chr_write_iter()`, but its UID 0 status does not prove that host UID 0 is inherently required. A process with a previously delegated TUN file descriptor may be below host-admin privilege, and a guest or virtual-network backend may provide a similar boundary. The supplied evidence does not prove that an ordinary user can create the complete tunnel, GSO, bridge or HSR, and qdisc topology.

The relevant capability namespace and all setup requirements are therefore unresolved. `HostAdminRequired` and `NamespaceScopedPrivilege` remain `UNKNOWN`. Prior privileged setup may be required to create or delegate the TUN device and forwarding topology, but subsequent packet injection might be available to a less-privileged actor.

This is not a race. Triggering requires an encapsulated GSO packet, decapsulation through `gro_cells`, L2 forwarding that bypasses `ip_rcv_core()`, and later qdisc processing.

4. Primitive and severity interpretation

The exact primitive is a stale active SKB metadata offset followed by an unsigned length miscalculation and an out-of-bounds kernel read. The affected SKB is an active structured packet object, but there is no post-lifetime access and therefore no real UAF.

The demonstrated consequence is a host kernel Oops or crash. The unresolved non-panic concern is that attacker-influenced packet metadata crosses a plausible delegated TUN, guest, or namespace boundary and is trusted by privileged bridge and qdisc processing. However, no secondary write, header corruption, disclosure, policy bypass, or useful exploitation primitive is demonstrated.

This is `CLASS_C`, not Class A or Class B, because the established terminal operation is an invalid read and crash rather than a useful kernel corruption primitive. `ImportantCandidate=YES` is retained specifically under the V33 review rule, not as proof of Important severity. `AutoDowngradeAllowed=NO` because automatically closing the issue would discard the unresolved delegated-interface or guest-to-host trust-boundary question.

5. One-sentence report phrase

A stale transport header after tunnel decapsulation can make qdisc processing perform an out of bounds kernel read and crash the host, while the demonstrated TUN path leaves a delegated interface or guest boundary unresolved and requires manual review.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Confirm whether a delegated TUN file descriptor, guest networking backend, or attacker-created namespace can satisfy the complete GSO, tunnel, L2 forwarding, and qdisc sequence without host-admin-equivalent privileges.

7. Call-site confidence

Call-site confidence: high

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

Patch: net: iptunnel: fix stale transport header during tunnel decapsulation [ Upstream
Commit: 8c3d4dfcecf7670c1d752f1ae74b21b2af5feac1
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8c3d4dfcecf7670c1d752f1ae74b21b2af5feac1

Changed files:
  net/core/dev.c
  net/bridge/br_forward.c
  net/bridge/br_input.c
  net/core/gro_cells.c
  kernel/softirq.c
  drivers/net/tun.c
  drivers/net/gtp.c
  include/linux/skbuff.h

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=8c3d4dfcecf7670c1d752f1ae74b21b2af5feac1

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

The original Linux kernel CVE announcement for CVE-2026-98401 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98401

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:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H
  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: 3
  Paranoid ActionableScore: 4

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).