[CVE-2026-98374][MODERATE 7.0] tcp: fix use-after-free of retransmit_skb_hint in tcp_send_synack() [ Upstream

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

CVE: CVE-2026-98374
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tcp: fix use-after-free of retransmit_skb_hint in tcp_send_synack() [ Upstream
Commit: fad6d429651e748365e93ca54645accc212f0018
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fad6d429651e748365e93ca54645accc212f0018
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98374
Analysis date: Wed, 07 Oct 2026 21:35:24 -0400
ActionableScore: 5
ActionableScore lower bound: unknown
Actionable bucket: Actionable Moderate / Strong Important candidate requiring manual review
Manual review required: YES

Summary:
No one-sentence report phrase was found.

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

The original Linux kernel CVE announcement for CVE-2026-98374 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98374

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-2026-98374	MODERATE	https://www.kernelcve.org/list/?q=CVE-2026-98374 CHECK WITH IMPACT FROM ORIG NN IMPORTANT | AEGIS IMPACT MODERATE_KPANIC AEGIS_AS=4 LEGACY=MODERATE_KPANIC RESULT=SAME (legacy and Aegis reconciliation agree; DECISION: both MODERATE_KPANIC)	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-362;CWE-825;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '7';DESCR 'A use after free in the TCP retransmit path occurs when tcp_send_synack replaces a cloned SYN skb but leaves retransmit_skb_hint pointing to the freed skb. The stale hint is later used as the root for retransmit queue traversal and can cause access to freed kernel memory. For the CVSS the PR:L is selected for the paranoid score because a local unprivileged process must create a TCP Fast Open connection with sendmsg MSG_FASTOPEN. Triggering also requires a crafted ICMP fragmentation needed condition and a simultaneous open timing window, so AC:H applies. The issue is not shown to be directly exploitable by a remote attacker without a cooperating local TFO client. Impact is at least a local denial of service via kernel crash and in the worst case may allow confidentiality integrity and availability impact due to the confirmed kernel use after free.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate / Strong Important candidate requiring manual review (with actual score 5)	MAYBE	REMOTE READ DANGER UAF NETWORK SKB LINUS KPANIC  DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFDH	-	-	checked

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

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

1. ActionableScore

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

2. Signal breakdown

* +2 Real UAF / active lifetime violation. `tcp_send_synack()` frees the original cloned SYN skb through `tcp_rtx_queue_unlink_and_free()` while `tp->retransmit_skb_hint` can retain the exact freed skb pointer. Later TCP paths dereference this stale pointer. The supplied KASAN report confirms a slab use after free.
* +1 Local unprivileged trigger. The commit explicitly identifies an unprivileged TCP Fast Open client using `sendmsg(MSG_FASTOPEN)` as able to create the relevant SYN and arm the retransmit hint.
* +1 Attacker-influenced active-object lifetime manipulation. The attacker can create the TFO SYN skb, influence the ICMP fragmentation-needed processing that arms the hint, and repeatedly attempt the connection sequence that replaces and frees the armed SYN skb. This is distinct from the UAF itself because the attacker influences the relevant object lifetime transition.
* +1 Reliable kernel crash / strong DoS. The supplied KASAN trace shows an actual stale-object access in `tcp_mark_skb_lost()` through `tcp_simple_retransmit()` and `tcp_v4_err()`. A kernel fault or panic is a realistic consequence.

Non-awarded material signals:

* +0 Controlled reclaim or replacement is not demonstrated. The patch does not show attacker-controlled allocation reuse at the freed `sk_buff` address.
* +0 Arbitrary or directed kernel write is not demonstrated.
* +0 Structured stale-object exploitation leverage is not separately awarded. The stale skb is used as the root of an rbtree traversal, but supplied evidence does not establish attacker-controlled replacement contents or meaningful post-free field control after reuse.
* +0 Weak LPE concern is not separately awarded. A UAF alone does not demonstrate privilege escalation.
* +0 Confidentiality and integrity impact are not awarded. The supplied evidence establishes a stale read and crash path, not disclosure, unauthorized modification, or a controlled memory-corruption primitive.
* +0 Remote reachability is not awarded. Network traffic participates through ICMP, but the described complete trigger requires a cooperating local unprivileged TFO client. The patch does not establish a remote-only trigger.
* +0 No hard-race penalty is applied. The sequence includes TFO, ICMP processing, and simultaneous open, but the supplied evidence does not establish a narrow uncontrollable one-shot race. The local client can repeatedly attempt the relevant connection sequence.
* +0 No extra Availability point is awarded. The same demonstrated stale access and kernel crash are already counted by the reliable DoS signal.
* +0 No repeated release or refcount mutation signal applies. The stale reference is dereferenced during retransmit processing, not passed through a second `put`, `free`, or refcount-decrement operation.

3. Reachability analysis

The evaluated attacker is a local unprivileged process using a normal TCP socket and `sendmsg(MSG_FASTOPEN)`. The commit directly states that an unprivileged TFO client can arm the vulnerable retransmit hint using an attacker-supplied ICMP fragmentation-needed message.

The complete sequence requires local TCP Fast Open activity, an ICMP message accepted as relevant to the connection, and a simultaneous-open condition that causes the SYN skb to be replaced and freed. The local process does not require host-admin-equivalent privilege to create and operate a TCP socket. Therefore `HostAdminRequired=NO`.

The supplied evidence does not establish that the full sequence can be executed solely with namespace-scoped privileges. No network namespace, user namespace, capability check, or namespace-local ICMP injection path is provided. Therefore `NamespaceScopedPrivilege=UNKNOWN`.

No special hardware is required. TCP and retransmission queues are broadly deployed kernel functionality, although the vulnerable sequence depends on TCP Fast Open, ICMP fragmentation-needed handling, and simultaneous open behavior. This is not established as a remote-only attack because a local TFO client is part of the trigger chain.

4. Primitive and severity interpretation

The exact primitive is a real slab use after free of an active `struct sk_buff`. The original cloned SYN skb is freed, while `tp->retransmit_skb_hint` retains its address. Later code reads the dangling hint, uses it as the starting node for retransmit rbtree processing, and reaches `tcp_mark_skb_lost()`.

This is an active structured-object UAF, not fixed stale DMA storage and not an ordinary cleanup-only dereference. The stale object participates in TCP retransmit queue traversal and packet-loss processing. However, the supplied evidence does not show controlled replacement of the freed skb, attacker-shaped reclaimed object contents, callback dispatch, arbitrary write, information disclosure, or a demonstrated privilege-escalation chain.

There is no concrete repeated release or refcount mutation. The first lifetime operation can free the original SYN skb. The exact stale pointer survives in `retransmit_skb_hint`. The later operation is a stale structured-object read and traversal, not a second `put`, `free`, or refcount mutation.

The issue is therefore `CLASS_B`. It is stronger than a simple NULL dereference because a confirmed freed heap object is accessed by active TCP retransmission logic. It is not `CLASS_A` because controlled reclaim, directed corruption, arbitrary kernel read or write, callback influence, or actual privilege escalation is not established.

The demonstrated practical consequence is local kernel DoS. The speculative consequence is memory-corruption-based confidentiality or integrity impact if a future analysis demonstrates useful skb reuse or attacker-controlled post-free object interpretation. That stronger outcome is not scored as established.

`ImportantCandidate=YES` because the issue is a confirmed attacker-influenced active TCP UAF reachable by an unprivileged local client, and its exploitation leverage beyond the observed crash remains unresolved. `AutoDowngradeAllowed=NO` because automatically closing it as an ordinary crash-only issue would discard a concrete active-object lifetime-corruption primitive in common TCP retransmission handling.

Likely operational severity is Actionable Moderate. It should receive manual exploitation review before being downgraded. Current evidence does not justify claiming confirmed LPE or remote compromise.

5. One-sentence report phrase

A local unprivileged TCP Fast Open client can trigger a confirmed use after free in TCP retransmit queue handling through a stale retransmit_skb_hint pointer, causing kernel denial of service and requiring manual review for unresolved post-free exploitation leverage.

6. Manual review recommendation

MANUAL CHECK REQUIRED

The patch establishes a real attacker-influenced active TCP skb use after free that reaches retransmit queue traversal after the original skb lifetime ends. Controlled reclaim and privilege escalation are not demonstrated, but the active stale-object processing is sufficiently meaningful that automatic downgrade is unsafe.

7. Call-site confidence

Call-site confidence: medium

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

Patch: tcp: fix use-after-free of retransmit_skb_hint in tcp_send_synack() [ Upstream
Commit: fad6d429651e748365e93ca54645accc212f0018
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fad6d429651e748365e93ca54645accc212f0018

Changed files:
  net/ipv4/tcp_input.c
  net/ipv4/tcp_ipv4.c
  net/ipv4/tcp_output.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=fad6d429651e748365e93ca54645accc212f0018

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

The original Linux kernel CVE announcement for CVE-2026-98374 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-98374

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:L/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: unknown
  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).