[CVE-2026-72400][MODERATE REGULAR] seg6: validate SRH length before reading fixed fields [ Upstream

[CVE-2026-72400][MODERATE REGULAR] seg6: validate SRH length before reading fixed fields [ Upstream

[AL-KERNEL]

CVE: CVE-2026-72400
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: seg6: validate SRH length before reading fixed fields [ Upstream
Commit: 715eb12e453df752f1b4baaf972c3acff0ab9402
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=715eb12e453df752f1b4baaf972c3acff0ab9402
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72400
Analysis date: Sun, 16 Aug 2026 06:54:04 -0400
ActionableScore: 2
ActionableScore lower bound: 0
Actionable bucket: Ordinary Moderate / Low-like but manual review recommended for BPF reachable OOB read
Manual review required: YES

Summary:
A BPF reachable SEG6 validation bug allowed seg6_validate_srh() to read fixed SRH fields before confirming that the supplied length covered struct ipv6_sr_hdr, causing a limited local out-of-bounds read with possible DoS in privileged or delegated BPF configurations.

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

The original Linux kernel CVE announcement for CVE-2026-72400 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72400

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 REGULAR
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-2026-72400	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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:N/A:L';CWE-125;*CWE-20;CWE-130;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '4.4';DESCR 'seg6_validate_srh reads fixed IPv6 SRH fields such as srh type before checking that the supplied length is large enough for struct ipv6_sr_hdr. A local actor able to load and attach the relevant BPF LWT or SEG6 local program can pass a short BPF supplied buffer and length through bpf_lwt_push_encap or END_B6 actions, causing an out-of-bounds read in the kernel validator. For the CVSS the PR:L is used for the paranoid score because delegated BPF or network administration capability may be available to a non-root service account or containerized workload in some deployments. The issue is not directly network reachable and is triggered through local BPF control plane execution rather than inbound packet traffic. Impact is mainly limited to a local denial of service or a limited read side effect. There is no strong evidence of a write primitive or direct privilege escalation.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Ordinary Moderate / Low-like but manual review recommended for BPF reachable OOB read (with actual score 1)	YES	REMOTE BPF SIMPLEFIX NETWORK LINUS IPV6  LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=2
ActionableScoreLower=0

## 1. ActionableScore

* Conservative score: 0
* Paranoid score: 2
* Final recommended bucket: **Ordinary Moderate / Low-like but manual review recommended for BPF reachable OOB read**

## 2. Signal breakdown

Conservative signals:

* Requires admin/root/CAP_* in typical deployments: -2. Loading and attaching the relevant BPF LWT or SEG6 local program typically requires CAP_BPF, CAP_NET_ADMIN, CAP_SYS_ADMIN, or root-controlled setup.
* Weak corruption (+0). This is a limited read-only out-of-bounds access before length validation, with no write primitive, no UAF, no reclaim, no callback control, and no object replacement.
* Availability impact realistic: +1 not awarded conservatively. The patch shows an OOB read beyond caller-supplied length, but does not show a reliable panic path outside debug or hardened detection.
* Rare/difficult-to-reach configuration: -1 would be plausible because this requires SEG6 plus BPF encap/helper usage, but the score is already floored at 0.

Paranoid signals:

* Confidentiality impact plausible: +1. The validator reads bytes beyond the declared SRH buffer and the result may influence validation behavior, giving at most a very small read-side oracle.
* Availability impact realistic: +1. On hardened/debug builds or unlucky memory layout, the invalid read could plausibly lead to a kernel warning, fault, or local DoS.
* Requires admin/root/CAP_* penalty not fully applied in the paranoid score because delegated BPF or network administration capability may exist in some containerized or service-account deployments. This is not treated as fully unprivileged reachability.

Not awarded:

* Remote reachable/network-triggerable: +0. Inbound packets alone do not trigger the bug without local BPF/SEG6 configuration.
* Local unprivileged trigger: +0. Unprivileged BPF is disabled in the evaluated product assumption.
* Generic memory corruption strong primitive: +0. No OOB write, UAF, double free, type confusion, or arbitrary write is shown.
* Privilege escalation plausible: +0. No concrete LPE primitive is supported by the patch.

## 3. Reachability analysis

The bug is reachable through the BPF SEG6 encap path, specifically via bpf_lwt_push_encap() or SEG6 local BPF END_B6 / END_B6_ENCAP actions that pass a BPF supplied pointer and length into seg6_validate_srh(). A short length such as 2 bytes can cause srh->type to be read before the validator checks that len covers struct ipv6_sr_hdr.

In typical deployments this requires privileged BPF and network configuration capabilities, so conservative reachability is local privileged. Namespace or container delegation can reduce the practical privilege barrier if CAP_NET_ADMIN or BPF-related privileges are granted to a workload, but this is not equivalent to a default unprivileged local trigger. The path is not directly network reachable and depends on SEG6/BPF configuration being installed.

Call-site confidence: high, because the commit message identifies the BPF helper call path and the patch directly fixes the first read in seg6_validate_srh().

## 4. Severity interpretation

This behaves like an ordinary Moderate or Low-like issue in conservative product triage because it is a privileged BPF-triggered, limited OOB read with no demonstrated write primitive, UAF, object lifetime corruption, or privilege escalation path.

The paranoid score remains nonzero because kernel OOB reads in BPF-reachable helper paths deserve manual attention, especially where delegated capabilities or hardened-kernel crash behavior may affect real deployments. However, the evidence does not support Important-class treatment.

## 5. One-sentence report phrase

A BPF reachable SEG6 validation bug allowed seg6_validate_srh() to read fixed SRH fields before confirming that the supplied length covered struct ipv6_sr_hdr, causing a limited local out-of-bounds read with possible DoS in privileged or delegated BPF configurations.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a kernel OOB read reachable from a BPF helper path, but it should not be treated as an Important candidate unless additional evidence shows unprivileged reachability, a real information leak, a reliable crash, or a stronger memory corruption primitive.

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

Patch: seg6: validate SRH length before reading fixed fields [ Upstream
Commit: 715eb12e453df752f1b4baaf972c3acff0ab9402
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=715eb12e453df752f1b4baaf972c3acff0ab9402

Changed files:
  net/ipv6/seg6.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=715eb12e453df752f1b4baaf972c3acff0ab9402

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

The original Linux kernel CVE announcement for CVE-2026-72400 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72400

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:L
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L
  The Best / paranoid CVSS score: 4.4

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: 0
  Paranoid ActionableScore: 2

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 REGULAR

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).