[CVE-2026-74598][MODERATE 7.0] ipv6: fix Route Information option length validation

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

CVE: CVE-2026-74598
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: ipv6: fix Route Information option length validation
Commit: 2f6f94eda12430fb41b24b44a71e2ea4e93561d7
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f6f94eda12430fb41b24b44a71e2ea4e93561d7
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74598
Analysis date: Sat, 22 Aug 2026 16:01:08 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
An IPv6 Router Advertisement Route Information option length validation bug allows an adjacent network attacker to trigger a bounded OOB read whose bytes become visible in the installed route prefix, with possible route state manipulation but no demonstrated write or privilege escalation primitive.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-74598 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74598

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-2026-74598	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	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-125;CWE-126;*CWE-20;*CWE-200;Other CVSS 'AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '6.3';DESCR 'An off-by-one validation bug in IPv6 Router Advertisement Route Information option handling can make rt6_route_rcv accept an option that is shorter than RFC 4191 allows. ipv6_addr_prefix then copies prefix bytes beyond the end of the option, so up to 8 adjacent bytes can be read and embedded into the installed IPv6 route prefix. For the CVSS the PR:N is used because the attacker does not need privileges on the victim host and only needs the ability to send Router Advertisements on the same L2 segment. The issue is adjacent-network reachable rather than Internet-routable because IPv6 Router Advertisements are link-local control traffic. Impact is limited information disclosure and possible routing state manipulation. Availability impact is low in the paranoid score because malformed RA-installed routes may degrade connectivity, but the patch supports an OOB read primitive rather than UAF, OOB write, or arbitrary write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE WRITE OOB DANGER NETWORK SKB IPV6 KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  The trigger is an IPv6 Router Advertisement with a malformed Route Information option. It is not Internet-routable, but it is network-adjacent reachable on the same L2 segment.

* Confidentiality impact plausible: +1
  The bug causes an OOB read of up to 8 bytes past the Route Information option, and the copied bytes become visible in the installed IPv6 route prefix.

* Integrity impact plausible: +1
  The attacker can influence installation of an incorrect IPv6 route prefix using adjacent bytes from the packet or skb tail room.

* Availability impact realistic, paranoid only: +1
  Malformed RA-installed routes can plausibly degrade connectivity, but this is weaker than a direct crash or persistent host-wide DoS.

* Broad/default/common subsystem exposure: +1
  IPv6 Router Advertisement processing is a common network path on hosts where RA acceptance is enabled.

* Common networking core path or packet-processing path: +1
  The affected code is in IPv6 route handling for Neighbor Discovery Router Advertisement options.

No strong memory corruption, UAF, OOB write, arbitrary write, or LPE primitive is supported by the patch. This is a bounded OOB read with visible disclosure through route state.

## 3. Reachability analysis

An attacker needs the ability to send IPv6 Router Advertisements on the same local L2 segment as the victim. No privileges are required on the victim host. This is AV:A style exposure rather than full AV:N Internet reachability because Router Advertisements are link-local control traffic and are normally not routed. Containers or namespaces do not materially lower victim-side privileges, since the attacker is external to the host but adjacent on the network. The path is realistic on systems accepting IPv6 RA and Route Information options.

## 4. Severity interpretation

This is above an ordinary Moderate because it is network-adjacent reachable and has a concrete OOB read with user-visible disclosure through installed route output. It does not look like Important-class memory corruption because there is no write primitive, UAF, reclaim, type confusion, callback control, or arbitrary kernel memory exposure. Conservative handling is Actionable Moderate. Paranoid handling reaches Strong Important candidate mainly to avoid auto-closing a network-adjacent OOB read affecting route state.

## 5. One-sentence report phrase

An IPv6 Router Advertisement Route Information option length validation bug allows an adjacent network attacker to trigger a bounded OOB read whose bytes become visible in the installed route prefix, with possible route state manipulation but no demonstrated write or privilege escalation primitive.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: adjacent-network reachability plus a concrete OOB read and visible route-state impact make this unsuitable for automatic closure, even though the primitive is limited and does not currently support LPE or arbitrary memory corruption.

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

Patch: ipv6: fix Route Information option length validation
Commit: 2f6f94eda12430fb41b24b44a71e2ea4e93561d7
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2f6f94eda12430fb41b24b44a71e2ea4e93561d7

Commit description:

rt6_route_rcv() validates the Route Information option (RFC 4191) length
against the prefix length, but both checks are off by one.

rinfo->length is the ND option length in units of 8 octets and it
*includes* the 8-byte option header, so an option carrying N bytes of
prefix has length == 1 + N/8.  RFC 4191 section 2.3 requires length 3
when Prefix Length is greater than 64, and 2 or 3 when it is greater
than 0.  The code accepts length >= 2 and length >= 1 respectively.

ipv6_addr_prefix() then copies prefix_len/8 bytes out of rinfo->prefix,
so a Router Advertisement with (prefix_len=128, length=2) or
(prefix_len=64, length=1) makes the kernel read up to 8 bytes past the
end of the option.  Those bytes end up in the prefix of the route that
gets installed, so they are visible to userspace:

  # RA with a Route Information option (prefix_len=128, length=2)
  # followed by a source link-layer address option, 01 01 de ad be ef ca fe
  $ ip -6 route show
  2001:db8:dead:beef:101:dead:beef:cafe via fe80::1234 dev veth0 proto ra
                     ^^^^^^^^^^^^^^^^^^ the next option, read out of bounds

When the Route Information option is the last one in the packet, those
eight bytes come from the skb tail room instead.

Reject the option lengths RFC 4191 does not allow.

Fixes: 70ceb4f ("[IPV6]: ROUTE: Add experimental support for Route Information Option in RA (RFC4191).")
Cc: stable@vger.kernel.org
Signed-off-by: Yuejie Shi <syjcnss@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260730035310.74584-1-syjcnss@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv6/route.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=2f6f94eda12430fb41b24b44a71e2ea4e93561d7

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

The original Linux kernel CVE announcement for CVE-2026-74598 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74598

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:L/PR:N/UI:N/S:U/C:L/I:L/A:N
  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: 6
  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).