[CVE-2026-63858][MODERATE 7.0] netfilter: nf_tables: add hook transactions for device deletions [ Upstream

[CVE-2026-63858][MODERATE 7.0] netfilter: nf_tables: add hook transactions for device deletions [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63858
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: netfilter: nf_tables: add hook transactions for device deletions [ Upstream
Commit: 4e69bfb32b2db323d9205fdb30e284481b37817c
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4e69bfb32b2db323d9205fdb30e284481b37817c
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63858
Analysis date: Sun, 19 Jul 2026 11:25:44 -0400
ActionableScore: 5
ActionableScore lower bound: 3
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
nf_tables hook deletion could corrupt RCU protected hook list traversal during concurrent netlink dumps, creating a local race that may crash the kernel and should be treated as actionable when nftables control plane access is available.

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

The original Linux kernel CVE announcement for CVE-2026-63858 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63858

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-63858	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H';CWE-362;*CWE-416;*CWE-664;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'nf_tables device hook deletion can break RCU protected hook list readers because the old transaction preparation moved hook objects away from the basechain or flowtable hook_list while netlink dump readers may still traverse that list. This is a local control plane race and RCU lifetime issue that can lead to reader confusion, stale list traversal, kernel crash, and a plausible use after free style failure if a reader observes a hook while deletion is in progress. For the CVSS the PR:L is used for the paranoid score because a local user or process with delegated CAP_NET_ADMIN in a network namespace may be able to update nf_tables objects, while full host root is not always required in practical deployments. The issue is not directly network reachable and is triggered by nftables device deletion or flowtable chain update operations rather than packet traffic. Impact is at least local denial of service via kernel crash and in worst case may allow confidentiality or integrity impact due to RCU list misuse and possible stale pointer traversal. YES REQUIRES MANUAL CHECK';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 4)	YES	REMOTE NETFILTER HARDWARE  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Weak or indirect corruption candidate: +1
  The old transaction preparation moved `nft_hook` objects between lists while netlink dump readers may still traverse the RCU protected `hook_list`. This is not a proven arbitrary write or controlled UAF reclaim primitive, but it is a plausible stale traversal or list consistency issue.

* Real lifetime or RCU corruption: +1
  The commit explicitly says the existing approach breaks readers of an RCU protected list. This is a lifetime and synchronization problem, not only ordinary bookkeeping.

* Reliable kernel crash or strong DoS concern: +1
  Broken RCU list traversal can plausibly cause reader confusion, invalid traversal, WARN/Oops, or kernel crash during concurrent dump and hook deletion.

* Broad or common subsystem exposure: +1
  `nf_tables` is a common kernel firewall subsystem, although this specific path is control plane rather than packet data path.

* Hard or timing dependent race: -1
  Triggering requires overlap between device hook deletion transaction preparation and a netlink dump reader.

* Requires CAP_NET_ADMIN or equivalent in typical deployments: -1
  Managing nftables chains, flowtables, and netdev hooks normally requires `CAP_NET_ADMIN`. I use `-1` rather than `-2` because this may be reachable from reduced privilege network namespaces or containerized admin contexts, not necessarily full host root.

Paranoid additional signals:

* Local unprivileged trigger in user namespace deployments: +1
  If unprivileged user namespaces are enabled and nftables is usable inside a created network namespace, a local user may obtain effective `CAP_NET_ADMIN` inside that namespace and trigger the control plane path.

* Cross namespace host kernel impact: +1
  A namespace local nftables operation can still exercise shared host kernel code and potentially crash the host kernel, so this is relevant for container or namespace isolation triage.

Paranoid cap reasoning:

* This is not scored as strong memory corruption or strong LPE. The patch supports RCU/list lifetime concern, but does not show attacker controlled reclaim, object replacement, type confusion, callback control, refcount takeover, or arbitrary write.
* Therefore the paranoid score is kept at 5 rather than Important class 7 plus.

## 3. Reachability analysis

The bug is triggered through nftables control plane operations involving device hook deletion for netdev chains or flowtable hooks. It is not directly network reachable and is not triggered by packet traffic alone.

In normal host deployments, the attacker needs `CAP_NET_ADMIN` or an equivalent privilege to modify the relevant nftables objects. In systems with unprivileged user namespaces and network namespaces enabled, the practical privilege requirement may be lower because a local user can sometimes create a namespace and gain namespace scoped `CAP_NET_ADMIN`.

The affected subsystem is commonly present, but the exact trigger requires a specific transaction path plus concurrent netlink dump traversal. Realistic exploitation is more likely to produce DoS than privilege escalation based on the provided patch context.

Call-site confidence: medium. The commit message clearly identifies RCU protected list readers and transaction hook movement, but the provided diff is partial and does not include all dump and transaction call paths.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Moderate. The issue is not just a validation failure or cosmetic list cleanup because it affects RCU protected list traversal and object lifetime during nftables transactions.

Theoretical memory corruption potential exists through stale list traversal or RCU lifetime misuse. Realistic exploitation evidence is weaker because there is no demonstrated reclaim primitive, arbitrary write, callback dispatch, or controlled object reuse in the provided patch.

Conservative handling can remain borderline due to `CAP_NET_ADMIN` and race requirements. Paranoid triage should not auto close it because nftables plus RCU lifetime bugs have a history of being underestimated, especially when namespaces reduce practical privilege requirements.

## 5. One-sentence report phrase

nf_tables hook deletion could corrupt RCU protected hook list traversal during concurrent netlink dumps, creating a local race that may crash the kernel and should be treated as actionable when nftables control plane access is available.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is an RCU protected list lifetime issue in `nf_tables`, with possible stale traversal during concurrent transaction preparation and dump readers. It is not proven LPE, but it is also not safe to auto close as a simple low severity control plane bug.

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

Patch: netfilter: nf_tables: add hook transactions for device deletions [ Upstream
Commit: 4e69bfb32b2db323d9205fdb30e284481b37817c
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=4e69bfb32b2db323d9205fdb30e284481b37817c

Changed files:
  include/net/netfilter/nf_tables.h

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=4e69bfb32b2db323d9205fdb30e284481b37817c

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

The original Linux kernel CVE announcement for CVE-2026-63858 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63858

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:H/PR:H/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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: 3
  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: NO
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).