[CVE-2026-74659][MODERATE REGULAR] net: bridge: mrp: fix uninitialised bytes on the wire

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

CVE: CVE-2026-74659
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: net: bridge: mrp: fix uninitialised bytes on the wire
Commit: 014c062d23c63ec77ef2cf17a0d9363c7441cc94
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=014c062d23c63ec77ef2cf17a0d9363c7441cc94
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74659
Analysis date: Sat, 22 Aug 2026 12:37:18 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline manual review recommended / Low practical impact
Manual review required: YES

Summary:
A bridge MRP MRA test frame construction bug can leak three uninitialized kernel bytes per emitted frame to an adjacent L2 observer, with limited confidentiality impact and no evidence of integrity, availability, or privilege escalation impact.

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

The original Linux kernel CVE announcement for CVE-2026-74659 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74659

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
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CVE-2026-74659	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:N/A:N';*CWE-200;CWE-908;*CWE-457;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.3';DESCR 'A small kernel memory disclosure exists in the bridge MRP MRA test frame path because br_mrp_alloc_test_skb can transmit 3 uninitialized bytes in every MRA MRP_Test frame. The leaked bytes come from an skb linear data area that is not cleared by dev_alloc_skb, specifically the missing sub TLV length byte and two alignment padding bytes. For the CVSS the PR:N is used for the paranoid score because an adjacent receiver only needs access to the same L2 segment when the victim bridge is already emitting MRA MRP_Test frames. The issue is network-adjacent rather than Internet routable because MRP frames are local Ethernet control traffic. Impact is limited confidentiality loss from small kernel memory disclosure. No integrity impact, availability impact, memory corruption, or privilege escalation path is indicated by the code context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline manual review recommended / Low practical impact (with actual score 3)  SKIP CVE-2026-74659 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	SIMPLEFIX SKB LINUS TEST  DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline manual review recommended / Low practical impact**::

## 2. Signal breakdown

Conservative signals:

* Network-adjacent observable disclosure: +2
  The leaked bytes are transmitted on the wire in MRA MRP_Test Ethernet control frames. This is not Internet-routable remote reachability, but an adjacent L2 receiver can observe the frames if the victim bridge emits them.

* Confidentiality impact plausible: +1
  Three uninitialized kernel bytes can be exposed per affected frame.

* Rare AND difficult-to-reach subsystem/configuration: -1
  This requires bridge MRP with the MRA ring-role path. It is not a common default network path.

Paranoid delta:

* Do not subtract rare configuration: +1 relative to conservative
  In deployments that actually use MRP/MRA, the affected frames may be emitted repeatedly and the disclosure is straightforward for an adjacent observer.

Not applied:

* No generic memory corruption. The bug is uninitialized memory disclosure only.
* No integrity impact. No write or ownership bypass is shown.
* No availability impact. No crash, stall, or resource exhaustion is indicated.
* No privilege escalation signal. The patch does not show UAF, OOB write, callback control, refcount abuse, or reclaim behavior.
* No common networking core path bonus. MRP is a specialized bridge redundancy protocol, not a central TCP/IP or netfilter packet path.

Call-site confidence: high. The changed code is the frame construction path and the commit directly identifies the leaked bytes.

## 3. Reachability analysis

The realistic attacker is on the same L2 segment and can observe MRP_Test frames emitted by the vulnerable bridge. The attacker does not need local code execution on the victim if the bridge is already configured to generate these frames. However, triggering the vulnerable path depends on bridge MRP/MRA configuration, which is normally administrative and not enabled by default on typical systems.

Namespaces or containers do not materially improve reachability unless they allow an untrusted actor to configure bridge MRP or observe the relevant L2 traffic. This is network-adjacent, not broadly remote over routed IP networks.

## 4. Severity interpretation

This behaves like a low-impact information disclosure rather than an Important-class kernel vulnerability. The theoretical issue is kernel memory disclosure, but the exposed amount is very small, limited to three uninitialized bytes per affected frame, and there is no supported memory corruption or privilege escalation primitive. It should not be treated as auto-closed without review because it is on-the-wire kernel memory disclosure, but the practical impact appears closer to ordinary Moderate or Low-like severity.

## 5. One-sentence report phrase

A bridge MRP MRA test frame construction bug can leak three uninitialized kernel bytes per emitted frame to an adjacent L2 observer, with limited confidentiality impact and no evidence of integrity, availability, or privilege escalation impact.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a network-adjacent kernel memory disclosure. The review can likely be short, focused on whether the affected product enables bridge MRP/MRA and whether untrusted adjacent hosts can observe the control frames.

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

Patch: net: bridge: mrp: fix uninitialised bytes on the wire
Commit: 014c062d23c63ec77ef2cf17a0d9363c7441cc94
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=014c062d23c63ec77ef2cf17a0d9363c7441cc94

Commit description:

br_mrp_alloc_test_skb() builds MRP test frames on an skb from
dev_alloc_skb(), which does not clear the linear data area.  On the MRA
ring-role branch the sub-option TLV header is appended with

	sub_tlv = skb_put(skb, sizeof(*sub_tlv));
	sub_tlv->type = BR_MRP_SUB_TLV_HEADER_TEST_AUTO_MGR;

so sub_tlv->length is never written, and the two trailing alignment bytes
are appended with a bare skb_put() that does not clear them either.  The
neighbouring oui and sub_opt regions are explicitly zeroed, so three
uninitialised bytes are left in every MRA MRP_Test frame that goes out.

Put the sub-option TLV header and the alignment padding in a single
skb_put_zero(), which clears both.  The AUTO_MGR sub-TLV carries no
payload, so the zeroed length field is already the value it should have.

Fixes: f745893 ("net: bridge: mrp: Update the Test frames for MRA")
Suggested-by: Nikolay Aleksandrov <razor@blackwall.org>
Cc: stable@vger.kernel.org
Signed-off-by: Baul Lee <baul.lee@xbow.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260729131941.10254-1-baul.lee@xbow.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/br_mrp.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=014c062d23c63ec77ef2cf17a0d9363c7441cc94

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

The original Linux kernel CVE announcement for CVE-2026-74659 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74659

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:H/PR:N/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS score: 4.3

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: 2
  Paranoid ActionableScore: 3

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).