[CVE-2026-98442][IMPORTANT] net/sched: act_ct: fix helper UAF due to extensions realloc [ Upstream

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

CVE: CVE-2026-98442
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: NO
Patch: net/sched: act_ct: fix helper UAF due to extensions realloc [ Upstream
Commit: 1679bbf5fbeac52a310b1a58b2b301bbf388bdbf
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1679bbf5fbeac52a310b1a58b2b301bbf388bdbf
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98442
Analysis date: Mon, 19 Oct 2026 03:22:02 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
A conntrack helper expectation can retain a pointer into reallocated and freed extension storage, allowing later hlist removal to write through the stale pointer and potentially cause kernel memory corruption or a crash after attacker-controlled traffic reaches a preconfigured act_ct helper path.

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ABOUT THIS REPORT
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The original Linux kernel CVE announcement for CVE-2026-98442 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98442

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: IMPORTANT
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-2026-98442	IMPORTANT	https://www.kernelcve.org/list/?q=CVE-2026-98442 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=IMPORTANT RESULT=DIFF INDEPENDENT_LEGACY=IMPORTANT INDEPENDENT_RESULT=DIFF TEMP_KPANIC_MERGE=IMPORTANT_WINS_DISAGREEMENT PRE_MERGE_LEGACY=IMPORTANT (final Legacy and Aegis differ; FINAL: IMPORTANT, Aegis: MODERATE_KPANIC)	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'A use after free can occur in the net sched act_ct path because a conntrack helper may link an expectation into a list stored inside the connection extension area. A later extension addition can reallocate and free that area while the expectation still contains a pointer to the old list location. Removing the expectation can then access and potentially write through the stale hlist pointer, causing memory corruption or a kernel crash. For the CVSS the PR:N value is used because an attacker does not need privileges on the target after an administrator has configured the affected act_ct and conntrack helper path. A crafted network flow may reach the vulnerable path remotely, although exploitation depends on a suitable helper, expectation creation, and extension reallocation. The confirmed impact is denial of service. The stale list pointer and write during list removal also provide a plausible worst case path to confidentiality and integrity impact or privilege escalation, but reliable exploitation is not demonstrated. Automated impact decision path. The original SimpleNet classifier selected IMPORTANT. The primary LLM estimated paranoid CVSS 8.1 and the production ActionableScore returned 6 with recommendation Actionable Moderate / Strong Important candidate requiring manual review. The production rules then evaluated CVSS thresholds and ActionableScore thresholds. Recorded rule transitions were final reconciliation stage IMPORTANT to IMPORTANT because KP specialist did not establish a qualifying panic. The legacy KP specialist produced a review artifact and its decision was processed by the applicable KP rules. The legacy FD or DH downgrade specialist produced a review artifact and its decision was processed by the applicable downgrade and preservation rules. Exact triggered rule markers were TEMP_KPANIC_MERGE_IMPORTANT_WINS and KPANIC_UNSUPPORTED_REMOVED. The independent weaker Aegis shadow evaluated CVSS 2.2 and ActionableScore 4 with primitive class CLASS_B and manual review REQUIRED. Its independently reconciled impact was MODERATE_KPANIC. Cross model aggregation changed the production result under an enabled evidence gated merge rule. The final reported impact is IMPORTANT.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 6)	MAYBE	REMOTE UAF NETWORK SKB LINUS TEMP_KPANIC_MERGE_IMPORTANT_WINS KPANIC_UNSUPPORTED_REMOVED	-	-	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=6
ActionableScoreLower=5
PrimitiveClass=CLASS_B
RealUAF=YES
HostAdminRequired=NO
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=YES
ManualReview=REQUIRED
AutoDowngradeAllowed=NO
RequiresHostUID0=NO
AuthorizationDecisionBypass=NO
ProtectedResourceImpact=NONE
B4PendingReview=NO
B4PendingConfirmed=NO

1. ActionableScore

* Raw conservative score: 5.
* Conservative score: 5. No floor or cap applied.
* Raw paranoid score: 6.
* Paranoid score: 6. No floor or cap applied.
* Final recommended bucket: **Actionable Moderate / Strong Important candidate requiring manual review**::

2. Signal breakdown

Conservative signals:

* +2 Real UAF. An expectation retains `lnode.pprev` pointing to `master_help->expectations` inside an extension allocation that can be freed during extension reallocation.
* +1 Structured stale-object exploitation leverage. Later expectation removal uses the stale hlist linkage and can write through `pprev` into the freed extension address. This is more than a read-only stale dereference.
* +1 Reliable kernel crash or strong DoS. Accessing or modifying the freed extension storage can produce list corruption, an invalid memory access, or a kernel fault.
* +1 Common networking packet-processing path. The defect occurs while `act_ct` processes packets through conntrack helpers and expectations.

Additional paranoid signal:

* +1 Attacker-influenced repeatable lifetime activity. After an administrator installs the affected `act_ct` helper configuration, attacker-controlled flows can repeatedly create unconfirmed connections, invoke helpers, create expectations, and reach extension addition and expectation removal. The degree of allocator control remains unresolved.

Important non-awarded signals:

* Direct remote reachability +2 was withheld because the packet path first requires privileged `tc act_ct` and helper configuration.
* Strong exploitation leverage +2 was withheld because controlled reclaim, attacker-selected destination, arbitrary write, callback control, and refcount takeover are not demonstrated.
* Confidentiality and integrity points were not separately awarded. The post-free write is established, but modification of a named protected object or disclosure is not.
* No extra availability point was awarded for the same possible kernel crash.
* No repeated-release signal applies. The bug is a stale hlist pointer write, not two lifetime releases on the same reference.
* NEW52 B5-R does not apply because this is not a retry-accumulated indexed OOB write.
* NEW53 B3-PENDING does not apply because no shared or file-backed storage write is involved.
* V33 active-state review supports mandatory review, but no separate guest, namespace, or delegated-interface boundary is established strongly enough for an additional score.
* B4 authorization-state preservation does not apply. There is no DENY-to-ALLOW authorization decision.

3. Reachability analysis

A host administrator normally needs `CAP_NET_ADMIN` in the governing network namespace to install the affected traffic-control action and configure the conntrack helper. This is prior privileged setup rather than a privilege required from the subsequent packet sender.

Once that configuration exists, a network peer can potentially trigger the vulnerable sequence by sending traffic handled by a suitable helper that creates an expectation before `nf_conn_act_ct_ext_add()` reallocates the extension area. Therefore, the evaluated attacker does not require host UID 0 or host-administrator powers, but exposure is conditional on the configured data path.

The supplied evidence does not establish that the complete setup can be created using only attacker-obtainable user and network namespace capabilities. `NamespaceScopedPrivilege` therefore remains `UNKNOWN`. No special hardware, boot option, or timing race is required. Reliable triggering still depends on helper behavior, an unconfirmed connection, extension reallocation, and later expectation removal.

4. Primitive and severity interpretation

The primitive is a real UAF involving an active conntrack extension allocation. `nf_ct_expect_insert()` stores the expectation hlist linkage against a list head located inside `master_help`. A later extension addition can allocate replacement storage and free the old extension block without updating the expectation linkage.

When the expectation is removed, hlist deletion can write through the stale `lnode.pprev` pointer into the freed address and may also update adjacent linkage. This establishes an uncontrolled post-free write to the old extension location, not merely an invalid read. If that allocation has been reused, the operation can corrupt another object, but controlled reuse or a chosen victim is not demonstrated.

This is `CLASS_B`, not `CLASS_A`, because the target derives from the stale extension address and neither the write destination nor useful replacement contents are shown to be attacker-controlled. The demonstrated practical consequence is kernel memory corruption and possible host DoS. Privilege escalation remains a technically credible unresolved consequence rather than a demonstrated result.

`ImportantCandidate=YES` and `AutoDowngradeAllowed=NO` are appropriate because an active list pointer performs a concrete post-free write in a packet-processing path. Automatic crash-only classification would discard meaningful unresolved exploitation leverage.

5. One-sentence report phrase

A conntrack helper expectation can retain a pointer into reallocated and freed extension storage, allowing later hlist removal to write through the stale pointer and potentially cause kernel memory corruption or a crash after attacker-controlled traffic reaches a preconfigured act_ct helper path.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a real UAF with a concrete post-free hlist write, while reclaim control, practical remote exposure, and privilege-escalation leverage remain unresolved.

7. Call-site confidence

Call-site confidence: high

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

Patch: net/sched: act_ct: fix helper UAF due to extensions realloc [ Upstream
Commit: 1679bbf5fbeac52a310b1a58b2b301bbf388bdbf
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1679bbf5fbeac52a310b1a58b2b301bbf388bdbf

Changed files:
  net/sched/act_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=1679bbf5fbeac52a310b1a58b2b301bbf388bdbf

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

The original Linux kernel CVE announcement for CVE-2026-98442 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98442

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:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: IMPORTANT
KPANIC detected for this report: NO
Published priority for this report (same as in Subject): IMPORTANT

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).