[CVE-2026-53211][MODERATE REGULAR] netfilter: nft_meta_bridge: fix stale stack leak via IIFHWADDR register

[CVE-2026-53211][MODERATE REGULAR] netfilter: nft_meta_bridge: fix stale stack leak via IIFHWADDR register

[AL-KERNEL]

CVE: CVE-2026-53211
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nft_meta_bridge: fix stale stack leak via IIFHWADDR register
Commit: 07acb9798477535933bd658ac9fa85b6cb10d995
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07acb9798477535933bd658ac9fa85b6cb10d995
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53211
Analysis date: Fri, 26 Jun 2026 04:22:17 -0400
ActionableScore: 4
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
`NFT_META_BRI_IIFHWADDR in nft_meta_bridge can leave 2 bytes of an nft_do_chain stack backed register uninitialized, allowing a local nftables rule controller to leak stale kernel stack data to userspace.`

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

The original Linux kernel CVE announcement for CVE-2026-53211 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53211

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-2026-53211	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:H/I:N/A:N';*CWE-457;*CWE-200;CWE-226;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '5.5';DESCR 'A stale stack information leak exists in nft_meta_bridge because NFT_META_BRI_IIFHWADDR declares a 6 byte Ethernet address but the nftables register span is rounded to 8 bytes. nft_meta_bridge_get_eval copies only ETH_ALEN bytes into the destination register, leaving 2 bytes of the second register uninitialized from the nft_do_chain stack. A downstream expression that consumes the full register span can expose those stale bytes to userspace. For the CVSS the PR:L is used because exploitation requires local ability to create or control nftables bridge rules, typically via CAP_NET_ADMIN or an equivalent delegated network administration context. The issue is not directly network reachable as packet traffic alone cannot create the malicious rule path. Impact is primarily confidentiality loss through a small kernel stack leak. Repeated leaks may disclose more sensitive kernel state, so the paranoid score treats confidentiality as high while keeping integrity and availability unchanged.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3)  SKIP CVE-2026-53211  SKIP The Fixes patch not applied yet, so unlikely that actual: cbd2257dc96e3e46217540fcb095a757ffa20d96 YES  NO NO unknown 	MAYBE	INIT SIMPLEFIX NETFILTER LEAK  INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended**

## 2. Signal breakdown

* Confidentiality impact plausible: +1
  The bug leaks 2 uninitialized bytes from the `nft_do_chain()` stack through an nftables register span.

* Common networking / netfilter path: +1
  The affected code is in netfilter bridge nftables evaluation, not a rare driver-only path.

* Local unprivileged trigger in some deployments: +1 paranoid
  Direct rule creation normally needs `CAP_NET_ADMIN`, but unprivileged user namespaces plus network namespaces may make nftables control reachable by local users on some systems.

* Local high-control netlink / nftables API: +1 paranoid
  nftables provides a rich local control-plane API where users can shape expressions, registers, and rule evaluation paths.

* Requires `CAP_NET_ADMIN` in typical deployments: -2 conservative, -1 paranoid
  In normal host configurations, creating the required nftables bridge rule requires administrative network privileges. The penalty is reduced in the paranoid score because namespace delegation may lower the practical barrier.

* No memory corruption signal: +0
  This is an uninitialized stack disclosure, not UAF, OOB write, double-free, or object lifetime corruption.

* No integrity or availability signal: +0
  The patch supports information disclosure only. There is no write primitive or crash condition.

## 3. Reachability analysis

The bug is triggered by creating an nftables bridge rule using `NFT_META_BRI_IIFHWADDR` and then consuming the full rounded register span. Packet traffic may be needed to evaluate the rule, but packet traffic alone cannot create the vulnerable rule path. In typical deployments, this requires `CAP_NET_ADMIN`. In systems with unprivileged user namespaces enabled, a local user may obtain `CAP_NET_ADMIN` inside a network namespace, which makes the paranoid interpretation more relevant. The issue is not remotely reachable in the usual network-only sense.

## 4. Severity interpretation

This behaves like a confidentiality-only kernel stack information leak. The leak is small, only 2 bytes per evaluation, so the conservative practical impact is closer to ordinary Moderate / Low-like. However, repeated kernel stack disclosures can assist exploitation of other bugs or reveal sensitive kernel state fragments, so it should not be automatically closed without review. There is no evidence of memory corruption, privilege escalation, or direct code execution from this patch alone.

## 5. One-sentence report phrase

`NFT_META_BRI_IIFHWADDR in nft_meta_bridge can leave 2 bytes of an nft_do_chain stack backed register uninitialized, allowing a local nftables rule controller to leak stale kernel stack data to userspace.`

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because this is a kernel stack information leak in netfilter. It is not an Important-class memory corruption issue by itself, but namespace exposure and repeated leakage may make it more actionable than an ordinary low-risk cleanup.

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

Patch: netfilter: nft_meta_bridge: fix stale stack leak via IIFHWADDR register
Commit: 07acb9798477535933bd658ac9fa85b6cb10d995
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=07acb9798477535933bd658ac9fa85b6cb10d995

Commit description:

NFT_META_BRI_IIFHWADDR declares its destination register with
len = ETH_ALEN (6 bytes), which the register-init tracking rounds up to
two 32-bit registers (8 bytes). nft_meta_bridge_get_eval() then does
memcpy(dest, br_dev->dev_addr, ETH_ALEN), writing only 6 bytes and
leaving the upper 2 bytes of the second register as uninitialised
nft_do_chain() stack. A downstream load of that register span leaks
those stale bytes to userspace.

Zero the second register before the memcpy so the full declared span is
written.

Fixes: cbd2257 ("netfilter: nft_meta_bridge: introduce NFT_META_BRI_IIFHWADDR support")
Cc: stable@vger.kernel.org
Signed-off-by: Davide Ornaghi <d.ornaghi97@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/bridge/netfilter/nft_meta_bridge.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=07acb9798477535933bd658ac9fa85b6cb10d995

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

The original Linux kernel CVE announcement for CVE-2026-53211 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53211

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:L/UI:N/S:U/C:L/I:N/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
  The Best / paranoid CVSS score: 5.5

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: 4

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).