[CVE-2026-53070][MODERATE REGULAR] sctp: disable BH before calling udp_tunnel_xmit_skb() [ Upstream

[CVE-2026-53070][MODERATE REGULAR] sctp: disable BH before calling udp_tunnel_xmit_skb() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53070
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: sctp: disable BH before calling udp_tunnel_xmit_skb() [ Upstream
Commit: 790093245e35040c2adb15f48970020425aa3f47
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=790093245e35040c2adb15f48970020425aa3f47
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53070
Analysis date: Thu, 25 Jun 2026 15:17:45 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline, manual review recommended
Manual review required: YES

Summary:
SCTP over UDP transmit may run udp_tunnel_xmit_skb() without BH disabled, allowing CPU migration to break per CPU recursion accounting and causing packet drops or severe SCTP throughput degradation on exposed SCTP UDP encapsulation deployments.

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

The original Linux kernel CVE announcement for CVE-2026-53070 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53070

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 REGULAR
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-2026-53070	MODERATE	CHECK	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-362;CWE-667;**CWE-400;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L';BEST CVSS score: '5.9';DESCR 'SCTP over UDP transmit can call udp_tunnel_xmit_skb() or udp_tunnel6_xmit_skb() without bottom halves disabled. This can let execution move between CPUs while per CPU xmit recursion accounting is incremented and decremented, causing incorrect recursion level detection and packet drops in ip(6)_tunnel_xmit or __dev_queue_xmit. For the CVSS the PR:N is used for the paranoid score because a remote peer may trigger transmit on an exposed SCTP over UDP endpoint without local credentials. The issue is network reachable only when SCTP UDP encapsulation is enabled and usable by the attacker, which makes reliable triggering configuration dependent. Impact is denial of service through packet loss and degraded SCTP throughput, with no supported confidentiality or integrity impact from the patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended (with actual score 3) 	MAYBE	OOB NETWORK HARDWARE LINUS TEST IPV6 PACKET  DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=3
ActionableScoreLower=2

## 1. ActionableScore

* Conservative score: 2
* Paranoid score: 3
* Final recommended bucket: **Borderline, manual review recommended**::

## 2. Signal breakdown

* Remote reachable / network-triggerable: +2
  An exposed SCTP over UDP endpoint may be influenced by a remote peer to exercise this transmit path.

* Availability impact realistic: +1
  The demonstrated effect is incorrect recursion accounting and packet drops, with severe SCTP throughput degradation.

* Common networking packet-processing path: +1, paranoid only
  This is in the networking transmit data path, but not a broad default TCP/IP path, so I count it only for the paranoid score.

* Rare AND difficult-to-reach subsystem/configuration: -1
  SCTP over UDP encapsulation must be enabled and usable. This is not a common default Internet-facing configuration.

* No memory corruption: +0
  The patch fixes BH/preemption context around per-CPU recursion accounting. It does not indicate UAF, OOB write, double-free, or controlled corruption.

* No privilege escalation plausible: +0
  The observed impact is packet loss / throughput degradation, not object lifetime corruption or privilege boundary bypass.

## 3. Reachability analysis

A remote peer may trigger the affected path if SCTP over UDP encapsulation is enabled and the target participates in SCTP traffic over that UDP encapsulation. No local credentials are necessarily required for the paranoid interpretation, so remote PR:N is defensible for exposed deployments. However, the feature is configuration-dependent and not a common default public-facing service. Namespaces can make this easier to reproduce in test environments, but practical host impact depends on whether a real SCTP/UDP service is exposed.

## 4. Severity interpretation

This behaves like a network-reachable DoS / packet-loss bug, not a memory-safety vulnerability. The realistic impact is service degradation or loss of SCTP over UDP throughput due to incorrect per-CPU xmit recursion accounting. There is no supported confidentiality or integrity impact from the patch context. It is not an Important-class candidate, but because it is potentially network-triggerable in enabled deployments, it should not be blindly auto-closed without checking exposure.

## 5. One-sentence report phrase

SCTP over UDP transmit may run udp_tunnel_xmit_skb() without BH disabled, allowing CPU migration to break per CPU recursion accounting and causing packet drops or severe SCTP throughput degradation on exposed SCTP UDP encapsulation deployments.

## 6. Manual review recommendation

MANUAL CHECK RECOMMENDED

Reason: network reachability is plausible when SCTP over UDP encapsulation is enabled, but the bug is configuration-dependent and appears limited to availability impact without memory corruption or privilege escalation.

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

Patch: sctp: disable BH before calling udp_tunnel_xmit_skb() [ Upstream
Commit: 790093245e35040c2adb15f48970020425aa3f47
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=790093245e35040c2adb15f48970020425aa3f47

Changed files:
  net/sctp/ipv6.c
  net/sctp/protocol.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=790093245e35040c2adb15f48970020425aa3f47

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

The original Linux kernel CVE announcement for CVE-2026-53070 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53070

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:L
  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: 2
  Paranoid ActionableScore: 3

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).