[CVE-2026-72020][MODERATE 7.0] ipvs: reset full ip_vs_seq structs in ip_vs_conn_new

[CVE-2026-72020][MODERATE 7.0] ipvs: reset full ip_vs_seq structs in ip_vs_conn_new

[AL-KERNEL]

CVE: CVE-2026-72020
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipvs: reset full ip_vs_seq structs in ip_vs_conn_new
Commit: 3bf9a260188b2a5449cbddc032a749ab433fe328
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3bf9a260188b2a5449cbddc032a749ab433fe328
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72020
Analysis date: Sun, 16 Aug 2026 03:03:32 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A malformed IPVS sync message can create a connection with sequence-adjustment flags set but uninitialized `ip_vs_seq` fields, allowing later helper processing to leak stale slab data into TCP seq/ack headers and corrupt forwarded TCP flows.

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

The original Linux kernel CVE announcement for CVE-2026-72020 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72020

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-2026-72020	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L';*CWE-457;CWE-908;*CWE-200;CWE-665;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L';BEST CVSS score: '6.3';DESCR 'IPVS can create a synced connection with sequence adjustment flags set while in_seq and out_seq still contain stale slab-data. A malformed sync message that omits IPVS_OPT_SEQ_DATA can leave init_seq and previous_delta uninitialized, and later IPVS application helper processing may use those values when rewriting TCP seq/ack numbers. This can expose stale kernel allocation bytes through forwarded TCP headers and can corrupt the forwarded TCP flow. For the CVSS the PR:N is used because a sender that can reach the IPVS sync channel does not need a local account on the target. The issue is best treated as adjacent or internal network reachable because IPVS sync traffic is normally limited to load balancer peers on a trusted network, not the public Internet. Impact is confidentiality and integrity low with availability low due to traffic corruption. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 7)	YES	REMOTE NETFILTER LINUS KPANIC PACKET  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

## 1. ActionableScore

ActionableScore=6
ActionableScoreLower=5

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  A malformed IPVS sync message can trigger the bad state if the attacker can reach the IPVS sync channel.
* Confidentiality impact plausible: +1
  Uninitialized `init_seq` and `previous_delta` can carry stale slab bytes into rewritten TCP seq/ack fields.
* Integrity impact plausible: +1
  Incorrect sequence adjustment can corrupt the forwarded TCP flow.
* Availability impact realistic: +1
  Flow corruption can break forwarded connections, although this is not shown as a reliable host-wide crash.
* Practical exploit chain strongly implied: +1
  The commit describes the concrete chain: malformed sync message without `IPVS_OPT_SEQ_DATA` -> SEQ flags preserved -> stale `ip_vs_seq` fields used by helpers.
* Rare AND difficult-to-reach subsystem/configuration: -1
  Requires IPVS sync backup behavior and later application-helper sequence adjustment, usually in load-balancer clusters rather than default hosts.

Conservative total: 5.

Paranoid difference:

* Do not apply the rare/configuration penalty when scoring an exposed IPVS cluster where sync traffic is reachable from an attacker-controlled adjacent/internal network.

Paranoid total: 6.

## 3. Reachability analysis

The likely trigger is a host that can send crafted IPVS sync messages to a backup/load-balancer node. No local account on the target is needed in that deployment model, so PR:N is reasonable for the network-side trigger.

This is not public-Internet reachable in typical deployments. IPVS sync traffic is normally restricted to trusted load-balancer peers or an internal/adjacent network. The affected path is also configuration-dependent: IPVS sync must be enabled, malformed sync state must preserve sequence-adjustment flags without sequence data, and a later packet must be processed by an IPVS application helper.

Namespaces mainly affect configuration and setup. Creating or configuring IPVS usually needs elevated privileges such as CAP_NET_ADMIN, but once the service is deployed, an external sender with access to the sync channel may trigger the issue without local privileges.

Call-site confidence: high. The commit message explicitly names `ip_vs_proc_conn()`, `IPVS_OPT_SEQ_DATA`, `vs_fix_seq()`, and `vs_fix_ack_seq()`.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate. It is not a strong Important-class memory corruption bug because the patch does not show UAF, OOB write, arbitrary write, controlled object replacement, or privilege-escalation primitive.

However, it is more than a harmless correctness issue. The bug uses uninitialized slab data in a network forwarding path and can expose stale kernel allocation bytes through TCP header rewriting while also corrupting forwarded traffic. Realistic exploitation depends on access to the IPVS sync channel and specific IPVS helper behavior, which keeps it below Strong Important.

## 5. One-sentence report phrase

A malformed IPVS sync message can create a connection with sequence-adjustment flags set but uninitialized `ip_vs_seq` fields, allowing later helper processing to leak stale slab data into TCP seq/ack headers and corrupt forwarded TCP flows.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is network-adjacent, involves uninitialized kernel memory exposure, and has a clearly described path from malformed sync input to user-visible TCP header corruption.

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

Patch: ipvs: reset full ip_vs_seq structs in ip_vs_conn_new
Commit: 3bf9a260188b2a5449cbddc032a749ab433fe328
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3bf9a260188b2a5449cbddc032a749ab433fe328

Commit description:

Commit 9a05475 ("ipvs: avoid kmem_cache_zalloc in
ip_vs_conn_new") changed ip_vs_conn_new() to allocate an ip_vs_conn
object with kmem_cache_alloc().  The function then initializes many
fields explicitly, but only resets in_seq.delta and out_seq.delta in the
two struct ip_vs_seq members.

That leaves init_seq and previous_delta uninitialized.  This is normally
harmless while the corresponding IP_VS_CONN_F_IN_SEQ or
IP_VS_CONN_F_OUT_SEQ flag is clear.  For connections learned from a sync
message, however, ip_vs_proc_conn() preserves those flags from
IP_VS_CONN_F_BACKUP_MASK and passes opt=NULL when the message omits
IPVS_OPT_SEQ_DATA.  In that case the new connection can be hashed with
SEQ flags set but with the rest of in_seq/out_seq still containing stale
slab data.

When a packet for such a connection is later handled by an IPVS
application helper, vs_fix_seq() and vs_fix_ack_seq() use
previous_delta and init_seq to rewrite TCP sequence numbers.  A malformed
sync message can therefore make forwarded packets carry stale slab bytes
in their TCP seq/ack numbers, and can also corrupt the forwarded TCP
flow.

Reset both struct ip_vs_seq members completely before publishing the
connection.  This matches the existing "reset struct ip_vs_seq" comment
and keeps the sequence-adjustment gates inactive unless valid sequence
data is installed later.

Fixes: 9a05475 ("ipvs: avoid kmem_cache_zalloc in ip_vs_conn_new")
Cc: stable@vger.kernel.org
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>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/ipvs/ip_vs_conn.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=3bf9a260188b2a5449cbddc032a749ab433fe328

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

The original Linux kernel CVE announcement for CVE-2026-72020 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72020

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:L/I:L/A:L
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
  The Best / paranoid CVSS score: 6.3

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: 5
  Paranoid ActionableScore: 6

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).