[CVE-2026-74705][MODERATE 7.0] udp: fix potential use-after-free in tunnel segmentation [ Upstream

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

CVE: CVE-2026-74705
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: udp: fix potential use-after-free in tunnel segmentation [ Upstream
Commit: 6a733a38b983d8c2e222f13968209010cf44de87
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6a733a38b983d8c2e222f13968209010cf44de87
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74705
Analysis date: Sat, 22 Aug 2026 18:32:18 -0400
ActionableScore: 7
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A potential use-after-free in UDP tunnel segmentation occurs because `__skb_udp_tunnel_segment()` saved the UDP header pointer before `pskb_may_pull()` could reallocate `skb->head`, allowing attacker-influenced tunnel traffic to trigger a stale pointer dereference and likely kernel crash under specific GSO/tunnel conditions.

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

The original Linux kernel CVE announcement for CVE-2026-74705 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74705

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-2026-74705	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';*CWE-416;CWE-825;*CWE-664;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H';BEST CVSS score: '6.4';DESCR 'A potential use-after-free exists in UDP tunnel segmentation because __skb_udp_tunnel_segment() saved the UDP header pointer before pskb_may_pull() ensured that the tunnel header was present in the skb head. If the pull reallocates skb head, the saved UDP header pointer can become stale and later be dereferenced during checksum or length handling. For the CVSS the PR:N is used because a remote or adjacent sender may not need local privileges on the target if traffic can enter a configured UDP tunnel or forwarding path. The attack complexity is high because triggering depends on UDP tunnel segmentation, GSO behavior, and skb layout that causes head reallocation. Impact is at least denial of service via kernel crash and in the paranoid case may include limited confidentiality or integrity impact due to the UAF class, but the patch does not show a clear arbitrary write primitive. Practical exposure is usually limited to systems using UDP tunnels such as VXLAN or similar encapsulation paths rather than every Internet facing host.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	REMOTE UAF NETWORK SKB LINUS KPANIC  KPANIC LOWERED_FROM_HIGH_BASED_ON_GUESSCVSS  KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=7
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **7**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* **Remote reachable / network-triggerable: +2**
  The affected path is UDP tunnel segmentation. It can be reached by packets entering a configured UDP tunnel or forwarding path, although this is closer to adjacent or deployment-dependent network reachability than generic Internet reachability.

* **Memory corruption, weak/indirect corruption candidate: +1**
  The patch explicitly describes a potential UAF caused by saving `udp_hdr(skb)` before `pskb_may_pull()`. However, the visible primitive is a stale header pointer dereference, not a demonstrated controlled reclaim, arbitrary write, callback overwrite, or type confusion.

* **Real lifetime corruption: +1**
  If `pskb_may_pull()` reallocates `skb->head`, the saved UDP header pointer can reference old freed storage.

* **Reliable kernel crash / strong DoS: +1**
  A stale pointer dereference in packet segmentation can plausibly crash the kernel or trigger memory-safety diagnostics.

* **Common networking core path or packet-processing path: +1**
  This is in UDP offload / tunnel segmentation code, a networking data path relevant to tunnel deployments.

Negative signals:

* **Special triggering conditions: -1**
  Triggering requires UDP tunnel segmentation, suitable GSO/offload behavior, and an skb layout where `pskb_may_pull()` reallocates or invalidates `skb->head`.

* **Constrained primitive / no demonstrated attacker-controlled reuse: -1**
  The patch does not show attacker-controlled object replacement, write-after-free, or a controllable overwrite target.

Paranoid additions:

* **Stronger UAF concern for manual-review sensitivity: +1**
  Because the bug is explicitly a stale pointer after possible skb head reallocation in a network path, it deserves escalation above ordinary DoS-only handling.

* **Limited confidentiality/integrity concern: +1**
  A UAF class issue can sometimes exceed pure DoS, but here this is kept limited because no arbitrary read/write or reclaim primitive is shown.

## 3. Reachability analysis

A remote or adjacent host may trigger the bug if it can send traffic that enters a configured UDP tunnel or forwarding path using UDP tunnel segmentation. This is not universally exposed on every Internet-facing Linux system, but it is realistic for VXLAN-like or similar tunnel deployments.

No local user account on the target is necessarily required if the traffic path is reachable, so the practical PR interpretation is close to network or adjacent unauthenticated reachability. Namespaces and containers matter mostly for local generation of tunnel traffic or configuration, but the more important exposure is whether the host processes attacker-influenced tunnel packets through this segmentation path.

The path is not a generic default receive path for all UDP packets. It depends on tunnel/offload configuration and skb layout conditions, which keeps the conservative score below high-end Important.

## 4. Severity interpretation

This is more than an ordinary Moderate because it is a network-path UAF candidate in skb/UDP tunnel segmentation code. Conservative handling is **Actionable Moderate** because the demonstrated primitive is constrained and no controlled reuse or arbitrary write is shown.

The paranoid interpretation reaches **Strong Important candidate** because kernel networking UAF issues have historically been under-triaged when only the immediate symptom is described as a stale dereference or crash. The realistic evidence supports DoS strongly and only limited C/I concern.

## 5. One-sentence report phrase

A potential use-after-free in UDP tunnel segmentation occurs because `__skb_udp_tunnel_segment()` saved the UDP header pointer before `pskb_may_pull()` could reallocate `skb->head`, allowing attacker-influenced tunnel traffic to trigger a stale pointer dereference and likely kernel crash under specific GSO/tunnel conditions.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

This is a network-reachable UAF candidate in skb tunnel segmentation code. Even though the visible primitive is constrained and likely DoS-first, the combination of UAF plus packet-processing reachability should not be auto-closed.

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

Patch: udp: fix potential use-after-free in tunnel segmentation [ Upstream
Commit: 6a733a38b983d8c2e222f13968209010cf44de87
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6a733a38b983d8c2e222f13968209010cf44de87

Changed files:
  net/ipv4/udp_offload.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=6a733a38b983d8c2e222f13968209010cf44de87

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

The original Linux kernel CVE announcement for CVE-2026-74705 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74705

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:A/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS vector: AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS score: 6.4

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: 7

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).