[CVE-2026-74724][MODERATE 7.0] ipvs: avoid out-of-bounds write in ip_vs_nat_icmp [ Upstream

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

CVE: CVE-2026-74724
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipvs: avoid out-of-bounds write in ip_vs_nat_icmp [ Upstream
Commit: 3b8f79af0e98f27b932b0b416e9c52b692d31ff9
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b8f79af0e98f27b932b0b416e9c52b692d31ff9
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74724
Analysis date: Sat, 22 Aug 2026 17:37:51 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An out-of-bounds write in the IPVS IPv4 ICMP NAT path can occur when a local attacker modifies the embedded IP header while it is being processed, allowing `ip_vs_nat_icmp()` to write beyond the validated packet area and potentially causing kernel memory corruption or DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74724 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74724

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-2026-74724	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';*CWE-787;CWE-367;CWE-362;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'An out-of-bounds write in the IPVS IPv4 ICMP NAT path can occur because ip_vs_nat_icmp() re reads the embedded IP ihl field after earlier validation. A local attacker may be able to modify the packet while it is being processed, causing the transport header offset to move beyond the validated area and allowing port rewrite code to write out of bounds. For the CVSS the PR:L is used because reliable triggering requires local code or a local process able to inject or manipulate packets while IPVS is processing them. The issue is not considered directly network reachable as a packet-only remote attack based on the patch context. Impact is at least a local denial of service via kernel memory corruption and in the paranoid case may allow confidentiality and integrity impact or privilege escalation, so it should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)  SKIP CVE-2026-74724 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: url YES  NO NO unknown 	MAYBE	REMOTE WRITE OOB SKB IPV6 KPANIC PACKET  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Local unprivileged trigger: +1. The commit explicitly says a local attacker can modify the packet while it is processed by IPVS.
* Generic memory corruption, strong corruption primitive: +2. The patch fixes an out-of-bounds write in `ip_vs_nat_icmp()`.
* Weak LPE concern: +1. The primitive is an OOB write in kernel networking code, but the write payload appears constrained to rewritten port values and the patch does not show arbitrary target or arbitrary data control.
* Common networking packet-processing path: +1. IPVS runs in the netfilter packet path for configured virtual services.
* Availability impact realistic: +1. An OOB write in skb packet processing can plausibly crash or destabilize the kernel.
* Hard or unreliable race / special timing required: -1. Triggering depends on modifying the packet while IPVS is processing it.

Paranoid additional signals:

* Confidentiality impact plausible: +1. Kernel memory corruption could theoretically expose data depending on what is corrupted.
* Integrity impact plausible: +1. The OOB write may theoretically corrupt kernel state beyond a pure crash, though no reliable overwrite primitive is demonstrated.

Not counted:

* Remote reachable: +0. The patch context describes a local attacker modifying the packet during processing, not a pure remote packet-only trigger.
* Strong LPE plausibility: +0. No reclaim, arbitrary write, type confusion, callback control, or attacker-controlled object replacement is shown.
* Call-site confidence: high. The relevant callers in `ip_vs_core.c` and `ip_vs_xmit.c` are present in the patch.

## 3. Reachability analysis

The realistic trigger is local. The attacker needs the affected IPVS path to be active and must be able to cause or manipulate packets that traverse IPv4 ICMP NAT handling. IPVS itself normally requires administrative setup, but exploitation may not require the attacker to configure IPVS if the service is already enabled.

Namespaces can matter. A container or delegated network namespace with packet injection or local network access to an already configured IPVS instance may reduce the practical privilege barrier. However, the patch does not support treating this as remotely network-triggerable from arbitrary external traffic alone.

## 4. Severity interpretation

This is more than an ordinary Moderate because the fixed bug is an explicit kernel out-of-bounds write in a networking packet path. Conservative handling is Actionable Moderate because exploitation requires a timing window and the write appears constrained.

The paranoid interpretation reaches Strong Important candidate because kernel OOB writes in packet processing can historically be underestimated, and limited corruption may still become useful after manual analysis. The realistic evidence still does not prove arbitrary code execution or a reliable privilege escalation chain.

## 5. One-sentence report phrase

An out-of-bounds write in the IPVS IPv4 ICMP NAT path can occur when a local attacker modifies the embedded IP header while it is being processed, allowing `ip_vs_nat_icmp()` to write beyond the validated packet area and potentially causing kernel memory corruption or DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED.

Reason: this is an explicit OOB write in kernel networking code with local attacker influence over packet state, even though the primitive appears timing-dependent and constrained.

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

Patch: ipvs: avoid out-of-bounds write in ip_vs_nat_icmp [ Upstream
Commit: 3b8f79af0e98f27b932b0b416e9c52b692d31ff9
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3b8f79af0e98f27b932b0b416e9c52b692d31ff9

Changed files:
  include/net/ip_vs.h
  net/netfilter/ipvs/ip_vs_core.c
  net/netfilter/ipvs/ip_vs_xmit.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=3b8f79af0e98f27b932b0b416e9c52b692d31ff9

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

The original Linux kernel CVE announcement for CVE-2026-74724 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74724

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:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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: 7

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).