[CVE-2026-72021][MODERATE 7.0] ipvs: use parsed transport offset in SCTP state lookup commit 2f75c0faa3361b28e36cc0512b3299e163e25789 upstream.

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

CVE: CVE-2026-72021
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipvs: use parsed transport offset in SCTP state lookup commit 2f75c0faa3361b28e36cc0512b3299e163e25789 upstream.
Commit: d2b8b1557ec07ea1bb5dddbceaf4dfe63d388e27
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2b8b1557ec07ea1bb5dddbceaf4dfe63d388e27
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72021
Analysis date: Sat, 15 Aug 2026 15:17:02 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A remote client can send IPv6 SCTP packets with extension headers to an exposed IPVS service and cause SCTP state lookup to read the chunk type from the wrong offset, leading to incorrect ESTABLISHED state, longer timeouts, counter skew, and possible load balancer resource pressure.

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

The original Linux kernel CVE announcement for CVE-2026-72021 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72021

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
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CVE-2026-72021	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H';CWE-682;*CWE-20;**CWE-400;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '8.2';DESCR 'IPVS SCTP state tracking for IPv6 can read the SCTP chunk type from the wrong offset when packets contain IPv6 extension headers. A remote client can send crafted IPv6 SCTP traffic to an exposed IPVS virtual service and make the state machine classify an incomplete handshake as ESTABLISHED, which can extend connection timeouts and update active or inactive destination counters incorrectly. For the CVSS the PR:N is used because no local account or authenticated control plane access is required when the IPVS SCTP service is reachable over the network. The issue is network reachable but depends on IPVS being configured for SCTP over IPv6 and on packets with extension headers reaching the load balancer. Impact is mainly integrity of load balancing state and availability through possible connection table pressure or backend counter skew, not kernel memory corruption or privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-72021 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	REMOTE READ DANGER NETFILTER SKB LINUS IPV6 KPANIC PACKET  KPANIC INCREASED_TO_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2. A remote client can send IPv6 SCTP packets with extension headers to an exposed IPVS virtual service.
* Integrity impact plausible: +1. The IPVS SCTP state machine can classify an incomplete SCTP handshake as ESTABLISHED and update destination counters incorrectly.
* Availability impact realistic: +1. Wrong ESTABLISHED state can extend connection lifetime and may create connection table pressure under sustained traffic.
* Common networking packet-processing path: +1. The bug is in IPVS packet/state processing for SCTP traffic.
* Rare AND difficult-to-reach subsystem/configuration: -1. Triggering requires IPVS configured for SCTP over IPv6 and packets with IPv6 extension headers.

Paranoid additional signal:

* Practical resource-pressure chain strongly implied: +1. The commit directly describes longer established timeouts and incorrect active/inactive counters before handshake completion, which can plausibly be abused for load balancer state pressure.

No memory corruption signals are awarded. The patch fixes a wrong transport offset and state-machine misclassification, not UAF, OOB write, double free, type confusion, or attacker-controlled kernel overwrite.

## 3. Reachability analysis

The bug can be triggered by a remote network client if an IPVS SCTP IPv6 virtual service is reachable and accepts packets containing IPv6 extension headers. No local user account or authenticated control-plane access is needed for the packet trigger itself, so PR:N is reasonable for CVSS-style exposure.

The path is not universally default-enabled because it depends on IPVS, SCTP service configuration, IPv6, and extension-header handling. Containers and namespaces mainly matter for who can configure IPVS, but exploitation of an already configured service is network-side. Realistic exploitation is plausible in environments using IPVS as a load balancer for SCTP over IPv6, but less likely on generic systems without this service.

Call-site confidence: high. The patch shows the caller passing `iph_len` from `sctp_state_transition()` into `set_sctp_state()`, and the commit explains that the surrounding IPVS path already parsed the real transport offset.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than an ordinary Moderate, because it is network reachable and affects packet-driven load balancer state. It is not a strong Important memory-corruption candidate because the primitive is a logic/state-machine error. The demonstrated impact is incorrect SCTP connection state, longer timeout selection, and incorrect destination counters, with possible availability degradation under sustained traffic.

Theoretical impact beyond DoS/state integrity is limited. There is no evidence of kernel memory corruption, privilege escalation, information disclosure, or arbitrary code execution.

## 5. One-sentence report phrase

A remote client can send IPv6 SCTP packets with extension headers to an exposed IPVS service and cause SCTP state lookup to read the chunk type from the wrong offset, leading to incorrect ESTABLISHED state, longer timeouts, counter skew, and possible load balancer resource pressure.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Network reachability plus a plausible state-exhaustion path makes this unsuitable for auto-close, even though the bug is not memory corruption and does not currently justify Important-class treatment by itself.

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

Patch: ipvs: use parsed transport offset in SCTP state lookup commit 2f75c0faa3361b28e36cc0512b3299e163e25789 upstream.
Commit: d2b8b1557ec07ea1bb5dddbceaf4dfe63d388e27
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=d2b8b1557ec07ea1bb5dddbceaf4dfe63d388e27

Commit description:

set_sctp_state() reads the SCTP chunk header again in order to drive the
IPVS SCTP state table. For IPv6 it computes the offset with
sizeof(struct ipv6hdr), while the surrounding IPVS code uses iph.len from
ip_vs_fill_iph_skb(), where ipv6_find_hdr() has already skipped
extension headers and found the real transport header.

This makes the state machine read from the wrong offset for IPv6 SCTP
packets that carry extension headers. For example, an INIT packet with an
8-byte destination options header can be scheduled correctly by
sctp_conn_schedule(), but set_sctp_state() reads the first byte of the
SCTP verification tag as a DATA chunk type. The connection then moves
from NONE to ESTABLISHED instead of INIT1, gets the longer established
timeout, and updates the active/inactive destination counters
incorrectly. This happens even though the SCTP handshake has not
completed.

Use the parsed transport offset passed down from ip_vs_set_state() for
the SCTP chunk-header lookup. For IPv4 and IPv6 packets without
extension headers this preserves the existing offset.

Fixes: 2906f66a5682 ("ipvs: SCTP Trasport Loadbalancing Support")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/netdev/20260705123040.35755-1-zhaoyz24@mails.tsinghua.edu.cn/
Reported-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Reported-by: Yuxiang Yang <yangyx22@mails.tsinghua.edu.cn>
Reported-by: Ao Wang <wangao@seu.edu.cn>
Reported-by: Xuewei Feng <fengxw06@126.com>
Reported-by: Qi Li <qli01@tsinghua.edu.cn>
Reported-by: Ke Xu <xuke@tsinghua.edu.cn>
Assisted-by: Claude Code:GLM-5.2
Signed-off-by: Yizhou Zhao <zhaoyz24@mails.tsinghua.edu.cn>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat -rw-r--r-- net/netfilter/ipvs/ip_vs_proto_sctp.c 15

Changed files:
  net/netfilter/ipvs/ip_vs_proto_sctp.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=d2b8b1557ec07ea1bb5dddbceaf4dfe63d388e27

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

The original Linux kernel CVE announcement for CVE-2026-72021 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72021

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:L/PR:N/UI:N/S:U/C:N/I:L/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:H
  The Best / paranoid CVSS score: 8.2

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: 4
  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).