* [CVE-2026-98573][MODERATE 7.0] net: skbuff: fix pull-bound underflow in skb_checksum_setup_ipv6() [ Upstream
@ 2026-10-24 9:31 21% AL-KERNEL
0 siblings, 0 replies; 1+ results
From: AL-KERNEL @ 2026-10-24 9:31 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-98573
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net: skbuff: fix pull-bound underflow in skb_checksum_setup_ipv6() [ Upstream
Commit: 9c4e432132a97b82edf28fa172c39f743e6bfb26
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c4e432132a97b82edf28fa172c39f743e6bfb26
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98573
Analysis date: Sat, 24 Oct 2026 05:31:42 -0400
ActionableScore: 4
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES
Summary:
IPv6 checksum setup can underflow a pull bound and drive an oversized `skb_copy_bits()` heap write, with credible but incompletely proven guest or delegated-interface reachability.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98573 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98573
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-2026-98573 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-98573 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=2 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:L/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-191;CWE-681;*CWE-787;*CWE-20;Other CVSS 'AV:A/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8';DESCR 'An unsigned subtraction in skb_maybe_pull_tail can underflow when the skb linear head is larger than MAX_IPV6_HDR_LEN and IPv6 extension header parsing still needs data from nonlinear fragments. The wrapped value becomes a negative delta in __pskb_pull_tail and can reach skb_copy_bits as a very large copy length, creating a plausible out of bounds write into kernel memory rather than only a parser failure. For the CVSS the PR:L rating reflects the need to control a guest or local virtual network endpoint that can supply a specially arranged nonlinear skb and checksum metadata. AV:A is used because the clearest supported boundary is a virtual or adjacent networking path such as a guest backend, while ordinary remote packet reachability is not established. Impact is at least a host denial of service through memory corruption and may include confidentiality or integrity impact if the overwrite can be controlled. The paranoid HHH score is retained for manual review because a kernel memory overwrite can potentially support privilege escalation, although no reliable write primitive or code execution chain is proven. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 8 and the production ActionableScore returned 4 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were KP and FD specialist checks stage IMPORTANT to MODERATE_KPANIC because FD false positive review allowed a downgrade. 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 produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH. The independent weaker Aegis shadow evaluated CVSS 2.5 and ActionableScore 2 with primitive class CLASS_B and manual review REQUIRED. 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 / Strong Important candidate requiring manual review (with actual score 4) MAYBE REMOTE OOB SIMPLEFIX SKB LINUS IPV6 KPANIC DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH - - checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=4
PrimitiveClass=CLASS_A
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 base score: 4.
* Conservative score: 4; no floor or cap applied.
* Raw paranoid score: 4; no additional exploit effect is sufficiently established.
* Paranoid score: 4.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::
2. Signal breakdown
* +2 — Strong corruption primitive: unsigned pull-bound underflow becomes a negative `delta`; `skb_copy_bits()` can interpret it as a very large length and copy into the skb linear tail beyond its allocation.
* +1 — Reliable kernel crash/strong DoS is plausible from the oversized kernel copy.
* +1 — Common skb packet-processing path, specifically IPv6 checksum setup used by virtual-network backends.
* No privilege penalty: host-admin-only triggering is not positively shown.
* No remote/local-unprivileged bonus: the patch does not establish the complete interface and privilege chain for producing the required nonlinear skb and checksum metadata.
* No arbitrary-write, controlled target, controlled adjacent victim, disclosure, LPE, or additional integrity point is shown.
* No extra availability point: it would duplicate the same crash consequence.
* NEW52/B5-R does not apply: no retry/re-entry accumulation exists; this is a single unsigned-bound underflow.
* V32 checksum-header corruption does not apply: the defect is the pull/copy operation, not a two-byte checksum write into packet headers.
3. Reachability analysis
The vulnerable operation requires an skb with a linear head exceeding `MAX_IPV6_HDR_LEN`, nonlinear data needed during IPv6 extension-header parsing, and checksum-setup processing. Xen-netback provenance and the supplied hints make a guest or delegated virtual-network endpoint credible, but the visible patch lacks the caller chain proving who can construct all required skb metadata. Ordinary remote packet reachability, namespace-only capability sufficiency, prior setup requirements, and host-admin-only restriction therefore remain unresolved. No timing race or special hardware is required.
4. Primitive and severity reading
This is an attacker-influence-dependent heap OOB write from an oversized copy into skb tail storage, not a UAF or fixed in-buffer packet edit. The write primitive is CLASS_A, but payload, destination beyond the skb, and post-corruption object consumption are not controlled or identified. Crash is plausible; code execution, confidentiality loss, and guest escape remain speculative. ImportantCandidate is YES because a concrete kernel heap overwrite with a credible virtual-network trust boundary cannot safely be auto-downgraded despite incomplete reachability proof.
5. One-sentence report phrase
IPv6 checksum setup can underflow a pull bound and drive an oversized `skb_copy_bits()` heap write, with credible but incompletely proven guest or delegated-interface reachability.
6. Manual review advice
MANUAL CHECK REQUIRED
Confirm the Xen/vhost/virtio call chain, attacker control over nonlinear skb layout and checksum metadata, overwrite extent, and any adjacent structured-state consumers.
7. Call-site confidence
Call-site confidence: medium
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: net: skbuff: fix pull-bound underflow in skb_checksum_setup_ipv6() [ Upstream
Commit: 9c4e432132a97b82edf28fa172c39f743e6bfb26
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c4e432132a97b82edf28fa172c39f743e6bfb26
Changed files:
net/core/skbuff.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=9c4e432132a97b82edf28fa172c39f743e6bfb26
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-98573 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98573
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:L/PR:L/UI:N/S:U/C:L/I:L/A:H
The Best / paranoid CVSS vector: AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
The Best / paranoid CVSS score: 8
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: 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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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