[CVE-2026-90237][MODERATE 7.0] netfilter: nft_ct: move custom expectation support to helper [ Upstream

[CVE-2026-90237][MODERATE 7.0] netfilter: nft_ct: move custom expectation support to helper [ Upstream

[AL-KERNEL]

CVE: CVE-2026-90237
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nft_ct: move custom expectation support to helper [ Upstream
Commit: af3fe52fd108fd38235e4813df62442ed0f1d8ff
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af3fe52fd108fd38235e4813df62442ed0f1d8ff
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90237
Analysis date: Fri, 18 Sep 2026 08:33:30 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
nft_ct custom expectation handling could leave stale expectation hlist linkage after conntrack extension reallocation, causing active conntrack list corruption and at least a local DoS, with unresolved kernel memory corruption leverage requiring manual review.

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

The original Linux kernel CVE announcement for CVE-2026-90237 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90237

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-2026-90237	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-90237 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-416;*CWE-664;*CWE-672;Other CVSS 'AV:L/AC:H/PR:H/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'nft_ct custom expectation support could insert expectations early for an unconfirmed conntrack and leave stale list linkage when the conntrack extension area is reallocated. The stale pprev pointer can reference the wrong hlist_head and may lead to expectation list corruption, use after free style behavior, or a kernel crash. For the CVSS the PR:L is used for the paranoid score because delegated CAP_NET_ADMIN, container root, or user namespace based network administration can be enough to configure nftables rules in some deployments. The issue is not directly network reachable as a standalone remote bug because creating the vulnerable nftables expectation object normally requires local control plane access. After such rules exist, packet processing can exercise the affected conntrack expectation path. Impact is at least denial of service, and in the paranoid interpretation may include confidentiality and integrity impact due to stale pointer and list corruption in kernel memory.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	YES	REMOTE OOB INIT NOMEMCHK NETFILTER SKB KPANIC PACKET  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	YES	NO	checked

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

ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO

## 1. ActionableScore

* Raw conservative score: 4
* Conservative score: 5
* Raw paranoid score: 6
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

Conservative floor applied: 4 -> 5.
Reason: a real active kernel lifetime/list-corruption primitive is established, involving stale expectation list linkage after conntrack extension reallocation. Host-admin-equivalent privilege is not positively established.

## 2. Signal breakdown

Conservative signals:

* +2 Real UAF / active lifetime violation: expectation entries can keep stale `.pprev` linkage to the wrong `hlist_head` after conntrack extension reallocation.
* +1 Secondary index / list staleness: the expectation hlist linkage is a secondary kernel structure that can become inconsistent with the moved conntrack extension state.
* +1 Reliable kernel crash / strong DoS: the commit describes splats and the stale list state can plausibly lead to list corruption or kernel crash.
* +1 Common packet-processing exposure after configuration: once the nftables expectation object exists, packet processing can exercise the affected conntrack expectation path.
* -1 Privilege-gated control plane: creating the nftables expectation object normally requires CAP_NET_ADMIN or equivalent network administration rights.

Raw conservative score: 2 + 1 + 1 + 1 - 1 = 4.
Applied Floor 5 because this is not a fixed stale buffer or simple NULL dereference. The stale state is part of active conntrack expectation list handling.

Paranoid additional signals:

* +1 Structured stale-object exploitation leverage: stale hlist linkage may be consumed by later list operations, which can affect pointer/list updates rather than only causing an immediate read fault.
* +1 Attacker-influenced repeatable active state: an attacker with the required nftables control can configure expectation rules and drive packet flows that create unconfirmed conntracks and exercise the vulnerable path repeatedly.

Raw paranoid score: 2 + 1 + 1 + 1 + 1 + 1 - 1 = 6.

Important non-awarded signals:

* Strong exploitation leverage +2 not awarded: controlled reclaim, arbitrary write, callback control, or demonstrated LPE is not established.
* Confidentiality and integrity bonuses not awarded: C/I impact is plausible only as a paranoid CVSS concern, not as an established ActionableScore primitive.
* Remote reachable +2 not awarded: this is not a standalone remote packet-only bug because nftables expectation configuration is required first.
* Concrete repeated release/refcount +1 not awarded: the patch does not show the same exact object reference being released twice after the first release may end lifetime.
* Extra availability point not awarded: crash/list corruption and DoS are the same availability consequence.
* Race timing penalty not awarded: the issue is primarily an ordered lifetime/list consistency problem around extension reallocation, not a proven narrow timing race.

Repeated-release/refcount check:

* First release can end lifetime: not established for the same exact reference.
* Exact same reference survives to second put/release: not established.
* Second operation is a concrete lifetime/refcount mutation: not established.
* Concrete repeated release/refcount +1: not awarded.
* Paranoid repeated-refcount +1: not awarded.
* Floor 5: applied for active stale list/lifetime corruption, not for repeated release.

## 3. Reachability analysis

The vulnerable configuration path is nftables control plane support for custom conntrack expectations. Creating such objects typically requires CAP_NET_ADMIN or equivalent rights. The patch evidence does not prove that host/init namespace CAP_NET_ADMIN is always required, and nf_tables/netfilter functionality can be namespace-scoped in some environments. However, the supplied evidence also does not prove that the complete trigger chain is available to an ordinary unprivileged user in attacker-created namespaces.

HostAdminRequired is therefore UNKNOWN.
NamespaceScopedPrivilege is also UNKNOWN.

The issue is not directly network reachable as a standalone remote vulnerability. Remote or local packet traffic can exercise the conntrack expectation path only after a suitable nftables expectation object has been installed. Prior configuration is therefore material.

No special hardware is required. No firmware mediation is involved. No narrow race timing is established from the supplied patch. The attacker-controlled parts are mainly nftables expectation parameters and packet flows once the required network-administration capability is available.

## 4. Primitive and severity interpretation

The exact primitive is stale active list linkage in conntrack expectation handling. The old code created expectations early for unconfirmed conntracks. If the conntrack extension area was later reallocated, expectation list entries could retain `.pprev` pointers to the wrong `hlist_head`. This is a real lifetime/list consistency violation and is best classified as CLASS_B.

This is an active structured-object case, not fixed stale storage. The stale state belongs to conntrack expectation/list infrastructure and may be consumed by later kernel list operations. The demonstrated consequence is splat, list corruption, and likely kernel crash or DoS. The speculative consequence is broader kernel memory corruption if stale hlist linkage is later used after extension storage movement, but controlled reclaim or arbitrary kernel read/write is not established.

This is not CLASS_A because no controlled object replacement, directed write primitive, callback control, credential manipulation, or demonstrated privilege escalation is shown.

ImportantCandidate is YES because the stale `.pprev` / hlist corruption is a concrete active kernel lifetime/list primitive with unresolved exploitation leverage beyond ordinary crash-only behavior.

AutoDowngradeAllowed is NO because automatically closing this as ordinary Moderate would discard a real active kernel corruption primitive involving stale list linkage after object reallocation.

Likely operational severity: actionable Moderate at minimum, with strong Important-candidate review justified if the relevant nftables/conntrack expectation path is reachable below full host administration in the evaluated product.

## 5. One-sentence report phrase

nft_ct custom expectation handling could leave stale expectation hlist linkage after conntrack extension reallocation, causing active conntrack list corruption and at least a local DoS, with unresolved kernel memory corruption leverage requiring manual review.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the patch establishes a concrete active kernel lifetime/list corruption primitive involving stale `.pprev` linkage after conntrack extension reallocation. Controlled exploitation is not proven, but the primitive is stronger than a simple crash-only NULL dereference.

## 7. Call-site confidence

Call-site confidence: medium

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

Patch: netfilter: nft_ct: move custom expectation support to helper [ Upstream
Commit: af3fe52fd108fd38235e4813df62442ed0f1d8ff
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=af3fe52fd108fd38235e4813df62442ed0f1d8ff

Changed files:
  include/net/netfilter/nf_conntrack_helper.h
  net/netfilter/nf_conntrack_helper.c
  net/netfilter/nft_ct.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=af3fe52fd108fd38235e4813df62442ed0f1d8ff

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

The original Linux kernel CVE announcement for CVE-2026-90237 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90237

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: 5
  Paranoid ActionableScore: 6

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).