[CVE-2026-74662][IMPORTANT] inet: frags: publish queues before arming timer

[CVE-2026-74662][IMPORTANT] inet: frags: publish queues before arming timer

[AL-KERNEL]

CVE: CVE-2026-74662
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: inet: frags: publish queues before arming timer
Commit: 9f904dd3e455750e5d4ec9b2f134835811b85a2f
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f904dd3e455750e5d4ec9b2f134835811b85a2f
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74662
Analysis date: Sat, 22 Aug 2026 15:56:55 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in inet fragment queue creation can arm the timer before rhashtable publication, allowing immediate timer expiry with zero or negative fragment timeout to leave a stale hash node and create a network-triggerable DoS or UAF candidate.

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

The original Linux kernel CVE announcement for CVE-2026-74662 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74662

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-2026-74662	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	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-362;*CWE-416;*CWE-672;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '8.1';DESCR 'inet fragment queue creation arms a timer before the queue is visible in the rhashtable. If the namespace fragment timeout is zero or negative, the timer can expire first and mark the queue complete, drop the anticipated hash reference, and leave a stale hash node after creation continues. This is a race and stale reference condition in the IP fragment reassembly path and can become a use after free candidate when later lookups or cleanup traverse the stale node. For the CVSS the PR:N is used for the paranoid network scenario because once the target namespace has this timeout setting, a remote sender can trigger queue creation with fragmented IP traffic without credentials. The issue is network reachable under that vulnerable configuration, but exploitation is configuration dependent and likely requires non default fragment timeout settings. Impact is at least denial of service via kernel crash. In the worst still defensible case, stale hash references may allow broader memory corruption impact, so manual review is recommended.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE LOCK SIMPLEFIX NETWORK TIMER LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 8
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

### Conservative score signals

* Remote reachable / network-triggerable: +2
  IP fragment reassembly is reachable through fragmented network traffic if the affected namespace has a zero or negative fragment timeout.

* Memory corruption, weak/indirect corruption candidate: +1
  The stale rhashtable node and unbalanced hash reference create a stale lifetime condition. The patch supports a UAF candidate, but does not show attacker-controlled reclaim, overwrite, type confusion, or callback control.

* Real lifetime corruption: +1
  The bug is explicitly about publishing a completed fragment queue after the timer has dropped the anticipated hash reference, leaving stale lifetime state.

* Reliable kernel crash / strong DoS: +1
  Later lookup or cleanup of the stale hash node can plausibly crash the kernel.

* Common networking core path or packet-processing path: +1
  inet fragment queues are part of common IPv4/IPv6 packet reassembly infrastructure.

* Configuration / timing constraint: -1
  Practical remote triggering depends on a zero or negative fragment timeout, which is a special runtime condition and likely non-default.

Conservative total: 5

### Paranoid score signals

* Remote reachable / network-triggerable: +2
  Once the vulnerable timeout configuration exists, a remote sender can create fragment queues with fragmented traffic without credentials.

* Memory corruption, strong corruption primitive: +2
  For paranoid triage, the stale rhashtable node after reference imbalance is treated as a stronger UAF style lifetime corruption candidate in a network packet path.

* Real lifetime corruption: +1
  The root cause is reference lifetime imbalance between timer expiry, rhashtable publication, and final queue release.

* Reliable kernel crash / strong DoS: +1
  A stale published node can lead to kernel crash during later traversal or cleanup.

* Common networking core path or packet-processing path: +1
  The affected logic is in inet fragment reassembly, not a rare driver-only path.

* Confidentiality impact plausible: +1
  Only for paranoid scoring, stale object traversal could theoretically expose broader memory corruption consequences.

* Integrity impact plausible: +1
  Only for paranoid scoring, a UAF-class stale node in a network path warrants manual review for possible corruption beyond DoS.

* Configuration / timing constraint: -1
  The zero or negative timeout condition remains an important practical limiter.

Paranoid total: 8

## 3. Reachability analysis

A remote attacker can plausibly trigger the affected path by sending fragmented IP traffic, but only if the target namespace fragment timeout is zero or negative. Setting that timeout likely requires local administrative control such as CAP_NET_ADMIN, so the strongest network scenario depends on a preexisting vulnerable configuration rather than attacker-controlled configuration.

Namespaces matter because fragment timeout is namespace-scoped. A container or delegated network namespace with CAP_NET_ADMIN could potentially create the vulnerable condition locally, while an external attacker benefits only if such a namespace or host configuration already exists.

The affected code is in common inet fragment reassembly infrastructure. The trigger condition is not default-like, but the subsystem itself is central networking code. Call-site confidence: medium, because the patch and commit clearly describe the timer, rhashtable publication, reference imbalance, and stale node consequence, but not a demonstrated reclaim or privilege escalation primitive.

## 4. Severity interpretation

This is not an ordinary low-risk DoS. Conservatively, it is an Actionable Moderate issue because it combines a network packet path with a real lifetime/reference bug and plausible kernel crash.

The realistic demonstrated impact is DoS under a special timeout configuration. The theoretical worst case is broader UAF-style memory corruption from a stale rhashtable node in a common packet-processing path. The patch does not demonstrate attacker-controlled reclaim, arbitrary write, type confusion, or a practical LPE chain, so the conservative score should not be treated as confirmed Important. However, the paranoid score is high enough for Important-candidate manual review.

## 5. One-sentence report phrase

A race in inet fragment queue creation can arm the timer before rhashtable publication, allowing immediate timer expiry with zero or negative fragment timeout to leave a stale hash node and create a network-triggerable DoS or UAF candidate.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a race and real lifetime corruption issue in a common network fragment reassembly path, with a stale rhashtable node that is plausibly UAF-relevant. The main practical limiter is the non-default timeout condition, but the memory lifetime class and network reachability make autoclose unsafe.

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

Patch: inet: frags: publish queues before arming timer
Commit: 9f904dd3e455750e5d4ec9b2f134835811b85a2f
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9f904dd3e455750e5d4ec9b2f134835811b85a2f

Commit description:

inet_frag_create() arms the fragment queue timer before inserting the
queue into the fqdir rhashtable. If the namespace fragment timeout is
zero or negative, the timer can run before the queue is published.

The timer callback then marks the queue complete, tries to remove a node
that is not in the hash table yet, and drops the anticipated hash
reference. Creation can subsequently publish the completed queue without
restoring that reference, leaving a stale hash node after the caller drops
the remaining reference.

Publish the queue first and arm the timer while holding the queue lock.
This makes timer expiry wait until the queue is visible in the hash table,
so inet_frag_kill() can remove the node and balance the hash reference.

Fixes: 648700f ("inet: frags: use rhashtables for reassembly units")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Signed-off-by: Ren Wei <enjou1224z@gmail.com>
Link: https://patch.msgid.link/bf66785e7c0c139d7a1900e2f01faeeab344b960.1784948849.git.zhilinz@nebusec.ai
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/ipv4/inet_fragment.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=9f904dd3e455750e5d4ec9b2f134835811b85a2f

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

The original Linux kernel CVE announcement for CVE-2026-74662 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74662

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: 8

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: YES
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).