[CVE-2026-52915][MODERATE 7.0] netfilter: ip6t_hbh: reject oversized option lists

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

CVE: CVE-2026-52915
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: ip6t_hbh: reject oversized option lists
Commit: 2d523ba48d4ecc46acfb6aba548292cfcce1ac02
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d523ba48d4ecc46acfb6aba548292cfcce1ac02
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52915
Analysis date: Wed, 24 Jun 2026 05:00:55 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A missing bounds check in `ip6t_hbh` allows a local actor able to install IPv6 xtables rules to create an oversized Hop-by-Hop option list, causing an out of bounds read in the packet matching path and potential kernel DoS, with limited side effect concerns under a paranoid interpretation.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-52915 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52915

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-2026-52915	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H';CWE-129;CWE-125;*CWE-20;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.6';DESCR 'A bounds check bug in ip6t_hbh allows userspace to install an IPv6 Hop-by-Hop xtables match with optsnr larger than the fixed opts array. The packet matching path can then read past opts(16), producing an array-index-out-of-bounds condition and a potential kernel warning or crash depending on runtime hardening. For the CVSS the PR:L is used for the paranoid score because delegated CAP_NET_ADMIN in a network namespace or container network admin context may be enough to install the malformed rule without full host root. The issue is not directly network reachable because the malformed rule must first be installed locally, although later IPv6 traffic can exercise the matching path. Impact is at least local denial of service. In the paranoid interpretation, the out-of-bounds read may have limited confidentiality or integrity relevance through adjacent kernel match data or rule behavior, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE WRITE KASAN OOB SIMPLEFIX NETFILTER NETWORK IPV6 PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1. User supplied `optsnr` can exceed the fixed `opts[16]` array and cause an out of bounds read in the packet matching path.
* Common packet processing path: +1. The bug is in netfilter IPv6 Hop-by-Hop xtables matching, which is part of packet filtering.
* Broad deployed subsystem: +1. Netfilter and xtables are common even if this exact match is not always used.
* Requires CAP_NET_ADMIN in typical deployments: -2. Installing malformed ip6tables or xtables rules normally requires administrative network privileges.
* Availability concern: +2 total practical weighting in the conservative bucket is not used as a separate strong crash bonus because the evidence is UBSAN OOB read and warning, not a universally reliable panic.

Paranoid added signals:

* Local unprivileged or reduced privilege trigger possibility: +1. In systems with user namespaces, containers, or delegated network administration, CAP_NET_ADMIN may be available without full host root.
* Reliable DoS concern: +1. Once the malformed rule is installed, matching IPv6 traffic can repeatedly exercise the OOB read and may crash kernels with UBSAN trap or panic on warning policies.
* Limited confidentiality or integrity relevance: +1 combined. The primitive is read only OOB, but adjacent match data could theoretically affect rule behavior or expose limited kernel state through side effects. This is not treated as strong memory corruption.

Not counted:

* No remote reachable bonus. Remote traffic may exercise an already installed malformed rule, but remote packets alone cannot install it.
* No strong LPE bonus. The patch shows an OOB read, not UAF reclaim, write after free, arbitrary write, callback corruption, or type confusion.
* No strong generic memory corruption bonus. This is an array over index read, not an OOB write or heap overwrite.

## 3. Reachability analysis

The bug is triggered by installing an IPv6 Hop-by-Hop xtables match with `optsnr` greater than `IP6T_OPTS_OPTSNR`. In normal host deployments this requires CAP_NET_ADMIN, so conservative privilege is high. In containerized or user namespace environments, network administration privileges may be delegated to a less trusted local actor, making the paranoid interpretation closer to local low privilege.

The packet matching path is not changed by the patch. After a malformed rule is installed, IPv6 traffic can exercise the invalid option list. This does not make the vulnerability directly remote because rule installation is still the controlling step.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than a clear Important vulnerability. The concrete primitive is an out of bounds read from a fixed kernel array in a packet matching path. Realistic impact is DoS through UBSAN warning, panic on warning, or unstable rule matching behavior. Theoretical limited confidentiality or integrity impact is possible but not strongly demonstrated by the patch.

It should not be auto closed because the bug is in netfilter packet processing and the paranoid score remains defensible under delegated CAP_NET_ADMIN or namespace scenarios.

## 5. One-sentence report phrase

A missing bounds check in `ip6t_hbh` allows a local actor able to install IPv6 xtables rules to create an oversized Hop-by-Hop option list, causing an out of bounds read in the packet matching path and potential kernel DoS, with limited side effect concerns under a paranoid interpretation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED. YES REQUIRES MANUAL CHECK.

Reason: this is not a pure validation warning. The malformed rule reaches a packet processing path and produces a real out of bounds read, while namespace delegated CAP_NET_ADMIN can reduce the practical privilege barrier.

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

Patch: netfilter: ip6t_hbh: reject oversized option lists
Commit: 2d523ba48d4ecc46acfb6aba548292cfcce1ac02
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2d523ba48d4ecc46acfb6aba548292cfcce1ac02

Commit description:

struct ip6t_opts stores at most IP6T_OPTS_OPTSNR option descriptors,
but hbh_mt6_check() does not reject larger optsnr values supplied from
userspace.

Validate optsnr in the rule setup path so only match data that fits the
fixed-size opts array can be installed. This follows the existing xtables
pattern of rejecting invalid user-provided counts in checkentry() and
keeps the packet matching path unchanged.

`struct ip6t_opts` has a fixed `opts[IP6T_OPTS_OPTSNR]` array,
where `IP6T_OPTS_OPTSNR` is 16, then off-by-one array access is possible:

[  137.924693][ T8692] UBSAN: array-index-out-of-bounds in ../net/ipv6/netfilter/ip6t_hbh.c:110:29
[  137.926167][ T8692] index 16 is out of range for type '__u16 [16]'

Fixes: 1da177e ("Linux-2.6.12-rc2")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Zhengchuan Liang <zcliangcn@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/netfilter/ip6t_hbh.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=2d523ba48d4ecc46acfb6aba548292cfcce1ac02

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

The original Linux kernel CVE announcement for CVE-2026-52915 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52915

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:L/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.6

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: 3
  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: NO
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).