[CVE-2026-72243][MODERATE 7.0] selinux: check connect-related permissions on TCP Fast Open

[CVE-2026-72243][MODERATE 7.0] selinux: check connect-related permissions on TCP Fast Open

[AL-KERNEL]

CVE: CVE-2026-72243
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: selinux: check connect-related permissions on TCP Fast Open
Commit: 11406d0d7e11b4e525bb2ace2c70107031d058da
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=11406d0d7e11b4e525bb2ace2c70107031d058da
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72243
Analysis date: Sun, 16 Aug 2026 08:08:51 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
SELinux failed to apply connect-related permission checks for TCP Fast Open `sendmsg(MSG_FASTOPEN)`, allowing a local confined process to initiate TCP or MPTCP connections that normal SELinux policy would deny.

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

The original Linux kernel CVE announcement for CVE-2026-72243 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72243

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-2026-72243	MODERATE	CHECK WITH IMPACT FROM ORIG NN LOW	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N';CWE-862;CWE-863;CWE-284;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.2';DESCR 'SELinux did not check connect-related permissions when a TCP Fast Open sendmsg operation used MSG_FASTOPEN with a destination address. A local process in a confined SELinux domain could use this path to initiate a TCP or MPTCP connection even when normal connect permission to that peer or port should be denied by policy. For the CVSS the PR:L is selected because the attacker needs local code execution in a user or service context, but does not need administrative privileges. The issue is not directly network reachable from outside the system because the trigger is a local socket API call. Impact is an SELinux network policy bypass with possible confidentiality or integrity impact depending on what the blocked connection would have protected.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	LINUS  KPANIC INCREASED_FROM_LOW_BASED_ON_REQUIREMANUALCHECK KPANIC INCREASED_TO_HIGH_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN6 DECREASED_TO_MODERATE7_BASED_ON_FALSEPOSCHECKOFKP	NO	NO	checked

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

ActionableScore=7
ActionableScoreLower=6

## 1. ActionableScore

* Conservative score: 6
* Paranoid score: 7
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1
  A local process in a confined SELinux domain can call `sendmsg()` with `MSG_FASTOPEN`.

* Security-boundary bypass with practical sensitive-object access: +2
  This bypasses SELinux connect-related policy checks, including restrictions that may prevent a confined process from connecting to protected network services, ports, or peers.

* Cross-boundary isolation/policy bypass: +1
  The issue crosses an intended LSM enforcement boundary. It is not memory corruption, but it weakens a configured security policy.

* Confidentiality impact plausible: +1
  If SELinux policy was blocking access to sensitive internal services, the bypass may allow unauthorized reads or protocol-level data exposure.

* Integrity impact plausible: +1
  If the blocked endpoint accepts state-changing requests, the bypass may allow unauthorized writes or actions over the network connection.

* Broad/default/common subsystem exposure: +1 in paranoid score only
  SELinux is widely deployed on some enterprise and hardened systems. TCP Fast Open availability is configuration-dependent, so this is counted only for the higher triage score.

No memory corruption, UAF, OOB write, kernel crash, or direct LPE primitive is supported by the patch.

## 3. Reachability analysis

The trigger is local, through the socket API, not remotely reachable from outside the system. The attacker needs code execution in a local SELinux-confined user or service context, but does not need administrative privileges. Namespaces do not remove the issue if SELinux policy is enforcing on the affected process and the process can create TCP or MPTCP stream sockets. Practical impact depends on SELinux being enabled and enforcing, TCP Fast Open being usable, and the policy actually restricting connect permissions for the domain.

## 4. Severity interpretation

This behaves more like an actionable Moderate or low-end Important candidate than an ordinary Moderate. The bug is an access-control bypass, not a memory-safety issue. Realistic exploitation is policy-dependent, but the affected boundary is security-relevant because SELinux connect checks are explicitly intended to restrict network access by confined domains. Theoretical impact can include confidentiality or integrity loss through unauthorized network connections, but there is no evidence of kernel memory corruption, arbitrary code execution, or direct privilege escalation.

## 5. One-sentence report phrase

SELinux failed to apply connect-related permission checks for TCP Fast Open `sendmsg(MSG_FASTOPEN)`, allowing a local confined process to initiate TCP or MPTCP connections that normal SELinux policy would deny.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a real LSM policy bypass with deployment-specific impact. It should be checked against affected SELinux policies and confined services, even though it is not memory corruption and is not directly network-triggerable.

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

Patch: selinux: check connect-related permissions on TCP Fast Open
Commit: 11406d0d7e11b4e525bb2ace2c70107031d058da
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=11406d0d7e11b4e525bb2ace2c70107031d058da

Commit description:

Similar to Landlock, SELinux was not updated when TCP Fast Open
support was introduced to ensure connect-related permissions are
checked when using TCP Fast Open. Update its socket_sendmsg() hook to
call selinux_socket_connect() when MSG_FASTOPEN is passed.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/linux-security-module/20260616201615.275032-1-hexlabsecurity@proton.me/
Link: https://lore.kernel.org/linux-security-module/20260617180526.15627-2-matthieu@buffet.re/
Reported-by: Bryam Vargas <hexlabsecurity@proton.me>
Reported-by: Matthieu Buffet <matthieu@buffet.re>
Reported-by: Mikhail Ivanov <ivanov.mikhail1@huawei-partners.com>
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Tested-by: Bryam Vargas <hexlabsecurity@proton.me>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  security/selinux/hooks.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=11406d0d7e11b4e525bb2ace2c70107031d058da

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

The original Linux kernel CVE announcement for CVE-2026-72243 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-72243

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:L/PR:L/UI:N/S:U/C:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:C/C:L/I:L/A:N
  The Best / paranoid CVSS score: 5.2

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: 6
  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).