[CVE-2026-98306][MODERATE 7.0] seg6: set IPSKB_L3SLAVE from IP6SKB_L3SLAVE on IPIP decapsulation [ Upstream

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

CVE: CVE-2026-98306
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: seg6: set IPSKB_L3SLAVE from IP6SKB_L3SLAVE on IPIP decapsulation [ Upstream
Commit: 1d9f5c78903dd25a3556229eb716dd465c5f3573
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d9f5c78903dd25a3556229eb716dd465c5f3573
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98306
Analysis date: Sun, 11 Oct 2026 00:44:27 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98306 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98306

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
======================================================================

CVE-2026-98306	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98306 CHECK WITH IMPACT FROM ORIG NN MODERATE | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=1 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L';CWE-284;CWE-668;CWE-863;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '7.3';DESCR 'An SRv6 End.DX4 decapsulation path can lose the L3 slave marker when converting the outer IPv6 control block into the inner IPv4 control block. The decapsulated packet can therefore be processed as if it belonged to the default VRF, allowing an unbound socket outside the ingress VRF to receive traffic that should remain scoped to the enslaved interface. This creates a potential VRF isolation bypass with limited confidentiality and integrity impact, while the intended VRF bound socket may not receive the packet. For the CVSS the PR:N value is selected because a remote sender does not need an account or privileges on the target when the configured SRv6 endpoint is reachable. Exploitation requires a system using SRv6 End.DX4 with a VRF enslaved ingress interface and a packet addressed so that processing reaches local socket lookup. The issue is network reachable, although practical exposure is generally limited to routed SRv6 infrastructure or controlled service provider and data center networks. There is no memory corruption primitive or evidence of privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	REMOTE OOB NETWORK SKB IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

ActionableScore=5
PrimitiveClass=CLASS_A
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=NO
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=YES
ProtectedResourceImpact=CONFIRMED
B4PendingReview=YES
B4PendingConfirmed=YES

1. ActionableScore

* Raw conservative score: 5
* Conservative score: 5
* Raw paranoid score: 5
* Paranoid score: 5
* No floor or cap was applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

* +2 Concrete authorization decision bypass. Losing `IPSKB_L3SLAVE` changes socket lookup from rejecting an unbound default-VRF socket to matching it, despite the default VRF isolation policy.
* +1 Integrity impact. An attacker-controlled decapsulated packet can reach a socket outside its intended VRF scope.
* +1 Availability impact. The commit confirms that the intended socket bound to the enslaved interface does not receive affected packets.
* +1 Practical exploit chain. The established chain is SRv6 End.DX4 packet reception, IPIP decapsulation, loss of the L3-slave marker, local socket lookup under default-VRF semantics, and delivery to the wrong socket.
* No separate cross-boundary bonus was added because it would duplicate the authorization-bypass signal.
* No remote-reachability bonus was added because SRv6 End.DX4 and the VRF topology require prior privileged configuration.
* No confidentiality point was added. The patch proves wrong-domain delivery but does not establish disclosure of protected data to the attacker.
* No UAF, OOB access, arbitrary write, memory corruption, privilege escalation, or host crash is established.
* B5-R and B3-PENDING do not apply. There is neither an accumulated OOB write nor a shared-backing write.

3. Reachability analysis

A network sender can exercise the vulnerable data path after an administrator has configured an SRv6 End.DX4 endpoint on an interface enslaved to a VRF. The sender does not require host credentials, UID 0, or a capability on the target, so `HostAdminRequired=NO`.

The privileged SRv6 and VRF configuration is a deployment prerequisite rather than an attacker operation. It prevents awarding the unconditional remote-reachability bonus but does not make the actual packet trigger host-admin-only.

No namespace capability is required by the packet sender, so this is not a namespace-scoped privilege path. No special hardware or timing race is involved. The trigger is deterministic when the required network topology and local socket conditions exist.

4. Primitive and severity interpretation

The primitive is a security-domain metadata loss during IPv6-to-IPv4 decapsulation. A packet received in an L3-slave context is subsequently interpreted by IPv4 socket lookup as belonging to the default VRF.

The concrete DENY-to-ALLOW path is:

`IP6SKB_L3SLAVE` present on ingress
to marker lost while clearing `IPCB`
to `IPSKB_L3SLAVE` absent
to default-VRF unbound socket incorrectly matching the packet.

The protected resource is the socket namespace and routing-domain scope enforced by VRF policy. The fix cleanly preserves the mandatory state that causes socket lookup to reject the cross-VRF match unless `udp_l3mdev_accept` or `tcp_l3mdev_accept` explicitly permits it.

This is CLASS_A as a concrete authorization and isolation-policy bypass, not as memory corruption. B4-PENDING is confirmed because the network caller, incorrect decision, protected socket scope, below-host-admin trigger, and restored protection are all grounded in the commit. Stronger consequences such as sensitive-data theft, privilege escalation, or arbitrary kernel access remain unproven.

`ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` preserve the issue for human review. The likely operational severity is actionable Moderate, with Important classification possible where VRFs enforce a meaningful tenant or service boundary.

5. One-sentence report phrase

An SRv6 End.DX4 packet can lose its L3-slave marker during IPIP decapsulation, causing default-VRF socket lookup to accept traffic that should remain isolated to the ingress VRF.

6. Manual review recommendation

MANUAL CHECK REQUIRED

Confirm whether the affected deployment uses SRv6 End.DX4 with VRF-enslaved ingress interfaces and whether the crossed VRF boundary separates tenants, trust domains, or security-sensitive services.

7. Call-site confidence

Call-site confidence: high

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

Patch: seg6: set IPSKB_L3SLAVE from IP6SKB_L3SLAVE on IPIP decapsulation [ Upstream
Commit: 1d9f5c78903dd25a3556229eb716dd465c5f3573
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1d9f5c78903dd25a3556229eb716dd465c5f3573

Changed files:
  net/ipv6/seg6_local.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=1d9f5c78903dd25a3556229eb716dd465c5f3573

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

The original Linux kernel CVE announcement for CVE-2026-98306 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98306

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:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 7.3

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: 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).