[CVE-2026-63984][MODERATE 7.0] ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress() [ Upstream

[CVE-2026-63984][MODERATE 7.0] ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63984
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress() [ Upstream
Commit: 75b3680047bf09af8e7e471a7a6ddf2ce5847f56
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75b3680047bf09af8e7e471a7a6ddf2ce5847f56
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63984
Analysis date: Sun, 19 Jul 2026 19:03:34 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An IPv6 RPL SRH integer truncation can wrap `hdrlen` to zero for crafted large segment counts, causing compressed-header placement to overlap decompressed routing-header data and corrupt forwarded packets in RPL-enabled IPv6 paths.

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

The original Linux kernel CVE announcement for CVE-2026-63984 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63984

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-2026-63984	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';CWE-190;CWE-681;*CWE-20;CWE-682;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:L';BEST CVSS score: '7';DESCR 'An integer truncation in the IPv6 RPL SRH decompression path can make hdrlen wrap from 256 to 0 when a crafted header encodes 128 addresses. The caller may then place the compressed header at the wrong offset, causing ipv6_rpl_srh_compress to write into an overlapping part of the decompressed routing header and corrupt the forwarded packet. For the CVSS the PR:N is used because a sender only needs network reachability to an RPL enabled IPv6 path and no local account on the target is required. The issue is network reachable in IPv6 deployments that process RPL source routing headers, but practical exposure is usually limited to specialized RPL or low power routing domains rather than the public Internet. Impact is at least packet integrity corruption and possible denial of service in the forwarding path. For the paranoid score, choose the highest still defensible interpretation supported by the bug class and patch context, with preference for manual review sensitivity over autoclosed false negatives.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE NETWORK LINUS IPV6 KPANIC PACKET  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN7	YES	NO	checked

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

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
  The bug is in IPv6 RPL SRH receive/decompress handling and can be reached by crafted network packets when RPL SRH processing is enabled.

* Constrained kernel buffer overwrite: +1
  The overflow causes the compressed header to be placed at the wrong offset and written into an overlapping region of the same packet/header buffer. This is not shown as arbitrary kernel memory corruption.

* Memory layout invariant restoration: +1
  The patch restores consistency between `n`, computed `hdrlen`, decompressed header size, and compressed-header placement.

* Integrity impact plausible: +1
  The described effect is corruption of the forwarded routing header.

* Rare AND difficult-to-reach subsystem/configuration: -1
  IPv6 RPL SRH is a specialized routing feature, typically used in constrained or specific RPL deployments, not broad Internet-facing default traffic.

Conservative total: 4

Paranoid additional signal:

* Availability impact realistic: +1
  Repeated malformed RPL SRH packets may plausibly disrupt forwarding behavior or cause packet-processing failures in affected RPL-enabled paths, even though a kernel crash is not demonstrated.

Paranoid total: 5

Not awarded:

* No strong generic memory corruption +2, because the overwrite appears bounded to an overlapping packet/header buffer region.
* No LPE signal, because there is no reclaim, UAF, arbitrary write, type confusion, callback control, or refcount takeover.
* No confidentiality signal in the conservative score, because no disclosure or read-back primitive is shown.

Call-site confidence: high, because the commit message describes both `ipv6_rpl_srh_decompress()` and the caller behavior in `ipv6_rpl_srh_rcv()`.

## 3. Reachability analysis

A remote sender can potentially trigger the condition by sending a crafted IPv6 RPL Source Routing Header with enough segments to make the computed `hdrlen` overflow the `__u8` field. No local account on the target is required if the attacker can reach an RPL-enabled IPv6 forwarding path.

The main limiting factor is deployment exposure. RPL is normally used in specialized low-power or constrained routing domains, not in typical public Internet server paths. Namespaces and containers are not the central factor here because the issue is primarily packet-processing reachability rather than a local userspace API.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary low-risk correctness fix. The bug is a network-reachable integer truncation in IPv6 header processing with a concrete packet-buffer overlap/write consequence.

It does not currently justify Important by itself because the patch and description do not show arbitrary kernel memory overwrite, UAF, privilege escalation, or information disclosure. The paranoid score is higher because network protocol parsing bugs with buffer-layout corruption symptoms deserve manual review to rule out deeper skb/header corruption effects.

## 5. One-sentence report phrase

An IPv6 RPL SRH integer truncation can wrap `hdrlen` to zero for crafted large segment counts, causing compressed-header placement to overlap decompressed routing-header data and corrupt forwarded packets in RPL-enabled IPv6 paths.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is network-reachable in affected RPL-enabled deployments and involves integer overflow plus packet-buffer corruption in IPv6 extension-header handling, even though the currently demonstrated primitive is constrained and does not prove arbitrary memory corruption or LPE.

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

Patch: ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress() [ Upstream
Commit: 75b3680047bf09af8e7e471a7a6ddf2ce5847f56
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=75b3680047bf09af8e7e471a7a6ddf2ce5847f56

Changed files:
  net/ipv6/exthdrs.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=75b3680047bf09af8e7e471a7a6ddf2ce5847f56

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

The original Linux kernel CVE announcement for CVE-2026-63984 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63984

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