[CVE-2026-74624][MODERATE 7.0] netfilter: nf_conntrack: defer invalid log until after unlock

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

CVE: CVE-2026-74624
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: netfilter: nf_conntrack: defer invalid log until after unlock
Commit: 63853eb20bba4e00b7cd0b8cfc19337bbaaf5037
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63853eb20bba4e00b7cd0b8cfc19337bbaaf5037
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74624
Analysis date: Sat, 22 Aug 2026 13:04:34 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A network-triggerable deadlock in TCP and SCTP conntrack invalid-packet logging can occur when nfnetlink_log and conntrack export are enabled, allowing malformed traffic to stall conntrack processing and cause denial of service.

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

The original Linux kernel CVE announcement for CVE-2026-74624 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74624

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.

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AL-KERNEL CLASSIFICATION RESULT
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CVE-2026-74624	MODERATE	CHECK WITH IMPACT FROM ORIG NN MODERATE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-667;*CWE-833;**CWE-400;BEST CVSS score: '5.9';DESCR 'TCP and SCTP conntrack paths can emit invalid packet logs while ct lock is still held. When invalid logging is routed to nfnetlink_log and conntrack export is enabled, the logging path can re enter conntrack netlink dumping and try to take the same ct lock again, causing a deadlock. For the CVSS the PR:N is used because a remote sender can trigger packet processing without a local account when the vulnerable logging configuration is present. The issue is network reachable through crafted or malformed TCP or SCTP traffic, but exploitation requires non default logging and export conditions, so AC:H is used. Impact is denial of service due to lockup or stalled packet processing, with no supported confidentiality or integrity impact.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE LOCK DEADLOCK DANGER NETFILTER SKB LOG TEST PACKET  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	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

Triggered signals:

* Remote reachable / network-triggerable: +2
  Invalid TCP or SCTP packets can reach the conntrack packet path from the network when the affected netfilter configuration is active.

* Reliable kernel crash / strong DoS: +1
  The issue is a lock inversion / deadlock on `ct->lock`, which can stall conntrack processing and potentially cause soft lockups or packet-processing hangs.

* Common networking core path or packet-processing path: +1
  The affected code is in `nf_conntrack` TCP/SCTP packet handling, a central netfilter packet-processing subsystem.

Paranoid-only signal:

* Availability impact realistic: +1
  In the vulnerable configuration, a deadlock in conntrack logging can affect system-wide packet filtering or networking responsiveness, not merely one failed packet.

Not awarded:

* No generic memory corruption. The patch shows deadlock avoidance, not UAF, OOB write, double free, refcount corruption, or type confusion.
* No privilege escalation plausible signal. There is no supported write primitive, stale object reuse, callback control, or credential/security-boundary manipulation.
* No confidentiality or integrity impact. The issue is DoS-only based on the patch.

## 3. Reachability analysis

A remote attacker may trigger the vulnerable path by sending invalid or malformed TCP or SCTP traffic that causes conntrack invalid-packet logging. The practical trigger depends on configuration: conntrack must be active, invalid logging must be enabled, logging must be routed through `nfnetlink_log`, and conntrack export must be enabled so that logging can re-enter conntrack netlink dumping.

No local account is required for the network packet trigger once the vulnerable configuration is present, so PR:N is reasonable for CVSS-style reachability. However, the required logging/export setup is not a universal default, which keeps the practical severity below Important.

Namespaces and containers mainly affect who can enable the logging configuration, but the packet trigger itself can be external once configured.

## 4. Severity interpretation

This behaves like an actionable Moderate network-triggerable DoS, not an Important memory-corruption issue. The realistic impact is deadlock or stalled packet processing due to logging while holding `ct->lock`.

Theoretical escalation is not supported by the patch. The fix only defers logging until after unlock and adds a lockdep assertion, which strongly indicates a locking-order DoS bug rather than a corruption primitive.

## 5. One-sentence report phrase

A network-triggerable deadlock in TCP and SCTP conntrack invalid-packet logging can occur when nfnetlink_log and conntrack export are enabled, allowing malformed traffic to stall conntrack processing and cause denial of service.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: the bug is network-triggerable in a common packet-processing subsystem, but only under specific logging and conntrack export configuration, so product exposure must be checked manually.

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

Patch: netfilter: nf_conntrack: defer invalid log until after unlock
Commit: 63853eb20bba4e00b7cd0b8cfc19337bbaaf5037
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=63853eb20bba4e00b7cd0b8cfc19337bbaaf5037

Commit description:

TCP and SCTP conntrack paths can emit invalid-packet logs while ct->lock
is still held.

When invalid logging is routed to nfnetlink_log and conntrack export is
enabled, the log path can re-enter conntrack netlink glue and dump the
same conntrack again. Protocol attribute dumping may take ct->lock, so
logging while holding that lock can deadlock.

Defer the TCP invalid logs by storing only the minimal log context while
ct->lock is held and emitting the log after unlocking. Also make the TCP
timeout-lowering invalid path return whether a log is needed, then emit
that log after unlocking.

Do the same for the SCTP invalid state-transition log that can be reached
while ct->lock is held.

Add a lockdep assertion to nf_ct_l4proto_log_invalid() so future callers
that log invalid conntracks while holding ct->lock are caught outside TCP
and SCTP as well.

Fixes: 628d694 ("netfilter: conntrack: reduce timeout when receiving out-of-window fin or rst")
Fixes: d9a6f0d ("netfilter: conntrack: prepare tcp_in_window for ternary return value")
Fixes: f71cb8f ("netfilter: conntrack: sctp: use nf log infrastructure for invalid packets")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Assisted-by: Codex:gpt-5.4
Signed-off-by: Zihan Xi <zihanx@nebusec.ai>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/netfilter/nf_conntrack_proto.c
  net/netfilter/nf_conntrack_proto_sctp.c
  net/netfilter/nf_conntrack_proto_tcp.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=63853eb20bba4e00b7cd0b8cfc19337bbaaf5037

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

The original Linux kernel CVE announcement for CVE-2026-74624 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74624

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 5.9

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).