* [CVE-2026-99041][MODERATE REGULAR] ip6_gre: remove obsolete ERSPAN PMTU update [ Upstream
@ 2026-10-27 7:26 21% AL-KERNEL
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From: AL-KERNEL @ 2026-10-27 7:26 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-99041
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: ip6_gre: remove obsolete ERSPAN PMTU update [ Upstream
Commit: b14a682ad7d45345701e08f9279d483a1da8b36c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b14a682ad7d45345701e08f9279d483a1da8b36c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99041
Analysis date: Tue, 27 Oct 2026 03:26:53 -0400
ActionableScore: 3
ActionableScore lower bound: unknown
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
An unprivileged namespace user can route a metadata-dst skb through native ip6erspan and trigger a NULL-device dereference causing a host kernel panic.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99041 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99041
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: YES
A detailed explanation of the methodology and priority rules is included
at the end of this message.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-99041 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-99041 CHECK WITH IMPACT FROM ORIG NN LOW | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 LEGACY=MODERATE RESULT=DIFF (final Legacy and Aegis differ; FINAL: MODERATE, Aegis: MODERATE_KPANIC) Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-843;*CWE-20;BEST CVSS score: '5.5';DESCR 'A local unprivileged process can create user and network namespaces and configure tc tunnel_key plus mirred to redirect an skb with a METADATA_IP_TUNNEL dst to a native ip6erspan device. The obsolete ERSPAN PMTU path treats this metadata dst as a normal route and reads dst_dev(dst) even though the metadata object has no output device, causing a NULL pointer dereference and a host kernel panic. For the CVSS the PR:L value is selected because local code execution and namespace setup are required, even though the demonstrated process starts with UID and GID 65534 and CapEff 0. The trigger is not directly reachable through ordinary remote IPv6 or GRE traffic. The demonstrated primitive is a NULL dereference with availability impact and there is no supported UAF, OOB access, information leak, or write primitive that would justify confidentiality or integrity impact. Automated impact decision path. The original SimpleNet classifier selected MODERATE_KPANIC. The primary LLM estimated paranoid CVSS 5.5 and the production ActionableScore returned 3 with recommendation Borderline / manual review recommended. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were CVSS and initial triage stage MODERATE_KPANIC to MODERATE because the CVSS score, vector or bug class threshold rule was applied. The legacy KP specialist has no review artifact for this run. The legacy FD or DH downgrade specialist has no review artifact for this run. Exact triggered rule markers were DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH and V41_INHERITED_KPANIC_PROMOTION_BLOCKED and MANUAL_REVIEW_REQUIRED. The independent weaker Aegis shadow evaluated CVSS 5.5 and ActionableScore 2 with primitive class CLASS_C and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. The models disagreed but the aggregation rules retained the production result. The final reported impact is MODERATE.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) MAYBE REMOTE KASAN NULLPTR NETWORK SKB HARDWARE IPV6 PACKET DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS_AND_CIANNH V41_INHERITED_KPANIC_PROMOTION_BLOCKED MANUAL_REVIEW_REQUIRED - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=NO
NamespaceScopedPrivilege=YES
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO
1. ActionableScore
* Raw base score: 3.
* Conservative score: 3; no floor or cap applied.
* Raw paranoid score: 3.
* Paranoid score: 3; no distinct stronger primitive supports an increase.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
* +1 — Local unprivileged trigger: demonstrated from UID/GID 65534, CapEff 0, using self-created user and network namespaces.
* +1 — Local unprivileged high-control data-plane API: the actor configures `tunnel_key` and `mirred` so an skb carrying `METADATA_IP_TUNNEL` reaches native ip6erspan.
* +1 — Reliable kernel crash/strong DoS: `dst_dev(dst)->mtu` dereferences a NULL device, with a demonstrated KASAN fault and host panic.
* +0 — No separate availability point; it would duplicate the same panic.
* +0 — No UAF, OOB access, write primitive, disclosure, authorization bypass, or corrupted-state consumer is shown.
* +0 — NEW52/B5-R does not apply: there is no retry-accumulated heap OOB write.
* +0 — Remote reachability is not shown; ordinary IPv6/GRE packets do not establish the required metadata-dst setup.
3. Reachability analysis
The complete trigger was demonstrated by an initially unprivileged local process that created user and network namespaces, obtained namespace-scoped network administration rights, configured tc actions and an ip6erspan device, and injected the triggering skb. Thus host-admin power and host UID 0 are not required for the evidenced configuration. No separate host-privileged setup, special hardware, or timing race is shown. Systems disabling unprivileged user namespaces may restrict exposure, but that deployment restriction is not established as part of the vulnerable interface.
4. Primitive and severity reading
The skb’s metadata destination is incorrectly treated as a normal route destination. Its absent output device produces a direct NULL-pointer dereference during ERSPAN transmission. This is CLASS_C crash behavior, not active-state corruption, UAF, type-confusion leverage surviving the fault, or a write primitive. The proven effect is host availability loss across a user/net namespace boundary; confidentiality, integrity, and privilege escalation are unsupported. ImportantCandidate is therefore NO, and auto-downgrade remains allowed.
5. One-sentence report phrase
An unprivileged namespace user can route a metadata-dst skb through native ip6erspan and trigger a NULL-device dereference causing a host kernel panic.
6. Manual review advice
MANUAL CHECK RECOMMENDED
Confirm product exposure to unprivileged user/network namespaces and whether the panic is fatal under the deployed oops policy.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: ip6_gre: remove obsolete ERSPAN PMTU update [ Upstream
Commit: b14a682ad7d45345701e08f9279d483a1da8b36c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b14a682ad7d45345701e08f9279d483a1da8b36c
Changed files:
net/ipv6/ip6_gre.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=b14a682ad7d45345701e08f9279d483a1da8b36c
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-99041 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-99041
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: Not available
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
The Best / paranoid CVSS score: 5.5
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: 3
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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