[CVE-2026-74621][MODERATE 7.0] net/sched: act_ct: fix sk_buff leak when the header checks reject a packet

[CVE-2026-74621][MODERATE 7.0] net/sched: act_ct: fix sk_buff leak when the header checks reject a packet

[AL-KERNEL]

CVE: CVE-2026-74621
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: net/sched: act_ct: fix sk_buff leak when the header checks reject a packet
Commit: 737873a59905a54ca0d2d127ef882f3f88bf4379
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=737873a59905a54ca0d2d127ef882f3f88bf4379
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74621
Analysis date: Sat, 22 Aug 2026 13:08:09 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: NO

Summary:
A malformed IPv4 or IPv6 packet reaching a configured net sched act_ct path can cause TC_ACT_CONSUMED to be returned while the skb is still owned by the action, leaking one sk_buff and data buffer per packet and enabling remote resource exhaustion DoS.

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

The original Linux kernel CVE announcement for CVE-2026-74621 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74621

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: NO

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-2026-74621	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';*CWE-401;*CWE-772;**CWE-400;Other CVSS 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '7.5';DESCR 'A resource leak in net sched act_ct can occur when malformed IPv4 or IPv6 fragment header checks reject a packet before the skb is handed to the conntrack defragmentation engine. The action then returned TC_ACT_CONSUMED even though it still owned the skb, so upper qdisc callers did not free it and one sk_buff plus its data buffer could be leaked per malformed packet. For the CVSS the PR:N value is used because an attacker only needs network reachability to an interface where tc action ct is configured and does not need a local account on the target. The issue is network reachable in deployments where clsact ingress or a similar tc path with action ct processes attacker controlled traffic. Impact is denial of service through unbounded kernel memory consumption. No confidentiality or integrity impact is assigned because the primitive is a resource leak rather than UAF, OOB access, information disclosure, or arbitrary write.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE LEAK UAF NETWORK SKB KERNEL_PANIC_PLUS_UAF LINUS IPV6 KPANIC PACKET  KPANIC DECREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCORELOWERTHAN6	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Malformed IPv4 or IPv6 packets can trigger the leak when they hit an interface with a tc clsact or similar path using `action ct`.

* Reliable kernel crash / strong DoS: +1
  The commit states the leak is unbounded and reproducible. One `sk_buff` plus its data buffer is leaked per malformed packet.

* Common networking core path or packet-processing path: +1
  The affected code is in the packet-processing path of `net/sched` with conntrack action integration.

Paranoid additional signal:

* Availability impact realistic: +1
  Sustained malformed traffic can realistically exhaust kernel memory on an exposed configured interface. This is stronger than a one-shot leak because the leak is per packet and unbounded.

Signals not applied:

* No generic memory corruption. This is an orphaned skb leak, not UAF, double free, OOB write, type confusion, or arbitrary write.
* No privilege escalation plausible. The patch does not show reclaim, overwrite, stale callback, object confusion, or refcount takeover.
* No confidentiality or integrity impact. The primitive is resource exhaustion only.
* No admin privilege penalty for the attacker. Admin is needed to configure `action ct`, but not to send the triggering packets once the interface is exposed.

## 3. Reachability analysis

The attacker can be remote if they can send malformed IPv4 or IPv6 traffic to an interface where `tc action ct` is configured, for example on a clsact ingress chain. No local account is required for the packet sender in that deployment, so practical trigger privileges are PR:N in CVSS terms.

The path is not necessarily enabled by default because it depends on tc configuration, but it is a realistic networking datapath in systems using traffic control plus conntrack actions. Containers or namespaces mainly matter for who can configure the tc rule, not for the remote packet trigger after configuration exists.

Call-site confidence: high. The commit explicitly explains that `sch_handle_ingress()`, `sch_handle_egress()`, and `tcf_qevent_handle()` skip freeing when `TC_ACT_CONSUMED` is returned, which directly supports the leak path.

## 4. Severity interpretation

This behaves like an actionable Moderate issue rather than ordinary Low or Important. The impact is only DoS, but it is remote packet-triggerable in affected configurations and leaks kernel memory per malformed packet without a natural bound.

It should not be treated as memory corruption or LPE. The theoretical risk does not extend beyond resource exhaustion based on the provided patch and call-path behavior. The paranoid score reaches 5 because remote unbounded memory exhaustion is operationally significant and should not be auto-closed.

## 5. One-sentence report phrase

A malformed IPv4 or IPv6 packet reaching a configured net sched act_ct path can cause TC_ACT_CONSUMED to be returned while the skb is still owned by the action, leaking one sk_buff and data buffer per packet and enabling remote resource exhaustion DoS.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Manual review is recommended because the issue is network-triggerable and exposure depends on whether `tc action ct` is deployed on reachable interfaces. It is not a memory-corruption or privilege-escalation candidate, but the unbounded per-packet kernel memory leak makes it actionable.

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

Patch: net/sched: act_ct: fix sk_buff leak when the header checks reject a packet
Commit: 737873a59905a54ca0d2d127ef882f3f88bf4379
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=737873a59905a54ca0d2d127ef882f3f88bf4379

Commit description:

tcf_ct_handle_fragments() runs its header sanity checks before handing
anything to the defragmentation engine:

	if (family == NFPROTO_IPV4)
		err = tcf_ct_ipv4_is_fragment(skb, &frag);
	else
		err = tcf_ct_ipv6_is_fragment(skb, &frag);
	if (err || !frag)
		return err;

tcf_ct_ipv4_is_fragment() returns -EINVAL or -ENOMEM;
tcf_ct_ipv6_is_fragment() adds -EPROTO when ipv6_find_hdr() fails. None of
them frees or queues the skb, so on that path the caller still owns it.

tcf_ct_act() however funnels every non-zero return into the
ownership-transfer exit:

	err = tcf_ct_handle_fragments(net, skb, family, p->zone, &defrag);
	if (err)
		goto out_frag;
	...
out_frag:
	if (err != -EINPROGRESS)
		tcf_action_inc_drop_qstats(&c->common);
	return TC_ACT_CONSUMED;

TC_ACT_CONSUMED means the action took ownership of the skb, so no caller
frees it - sch_handle_ingress(), sch_handle_egress() and
tcf_qevent_handle() all deliberately skip the free for that verdict. The
skb is therefore orphaned: one sk_buff plus its data buffer is leaked per
malformed packet, unbounded. Note the drop counter is already incremented
for these errors, so the statistics claim a drop that never happens.

Three different ownership states reach out_frag: today - the skb may be
queued by the defrag engine (-EINPROGRESS), already freed by
nf_ct_handle_fragments(), or still owned by us. Tell the caller which of
those it is, and free the packet ourselves in the last case, which
restores the TC_ACT_SHOT behaviour that predated the Fixes: commit.

Reproduced on v7.2-rc6 with a 54-byte frame carrying a 40-byte IPv6
header with nexthdr = 0 (hop-by-hop) and nothing after it, on a
clsact ingress chain with "action ct". kmemleak reports one leaked
232-byte skbuff_head_cache object plus its 704-byte data buffer per
packet; with this patch it reports none.

Fixes: 3f14b37 ("net/sched: act_ct: fix skb leak and crash on ooo frags")
Cc: stable@vger.kernel.org # v6.8+
Signed-off-by: Hyunjung Ko <hj351016@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Link: https://patch.msgid.link/20260806101235.809370-1-hj351016@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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=737873a59905a54ca0d2d127ef882f3f88bf4379

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

The original Linux kernel CVE announcement for CVE-2026-74621 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74621

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:L/PR:N/UI:N/S:U/C:N/I:N/A:L
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: 4
  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: 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).