[CVE-2026-53236][MODERATE 7.0] tcp: restrict SO_ATTACH_FILTER to priv users [ Upstream

[CVE-2026-53236][MODERATE 7.0] tcp: restrict SO_ATTACH_FILTER to priv users [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53236
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: tcp: restrict SO_ATTACH_FILTER to priv users [ Upstream
Commit: 3747de241a66ef2c7032d2cc2b826a47c5fa0f6a
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3747de241a66ef2c7032d2cc2b826a47c5fa0f6a
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53236
Analysis date: Thu, 25 Jun 2026 07:08:37 -0400
ActionableScore: 6
ActionableScore lower bound: 5
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
Unprivileged SO_ATTACH_FILTER on TCP sockets could expose TCP sequence and acknowledgment numbers through a cBPF side channel, allowing a local process to assist TCP session integrity attacks without CAP_NET_ADMIN.

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

The original Linux kernel CVE announcement for CVE-2026-53236 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53236

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-2026-53236	MODERATE	CHECK	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:N';*CWE-200;CWE-862;CWE-203;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N';BEST CVSS score: '5.3';DESCR 'Unprivileged SO_ATTACH_FILTER on TCP sockets can expose TCP sequence and acknowledgment numbers through a cBPF side channel. A local process can attach a classic BPF filter to its own TCP socket and use the observed TCP metadata to assist attacks against TCP connection integrity. For the CVSS the PR:L is used because local unprivileged code execution is required to call setsockopt on a TCP socket, but no administrative capability was required before the fix. The issue is not directly network reachable by itself, although a remote peer or network attacker may be part of a practical attack setup. Impact is mainly confidentiality loss of TCP metadata with possible integrity impact against TCP sessions. No kernel memory corruption or direct privilege escalation primitive is indicated.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 6)	YES	REMOTE SIMPLEFIX LEAK  KPANIC INCREASED_TO_MODERATE70_BASED_ON_ACTIONABLESCOREHIGHEREQTHAN5	NO	NO	checked

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

ActionableScore=6
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: 5
* Paranoid score: 6
* Final recommended bucket: **Actionable Moderate at minimum**::

## 2. Signal breakdown

* Local unprivileged trigger: +1. Before the fix, an unprivileged local process could call setsockopt with SO_ATTACH_FILTER on a TCP socket.
* Confidentiality impact plausible: +1. The commit explicitly says TCP sequence and acknowledgment numbers may be leaked through a cBPF side channel.
* Integrity impact plausible: +1. Leaked TCP sequence or acknowledgment numbers can assist TCP session injection, spoofing, or manipulation attacks, although this is not a direct kernel integrity primitive.
* Broad/default/common subsystem: +1. TCP sockets and SO_ATTACH_FILTER are part of common networking functionality.
* Common networking core path: +1. The change is in net/core/sock.c and affects TCP socket option handling.
* Security-boundary bypass concern, paranoid only: +1. The patch converts this operation into a CAP_NET_ADMIN-gated action, implying the old behavior exposed network metadata that should be treated as privileged for TCP sockets.

No memory corruption, UAF, OOB write, arbitrary write, kernel crash, or direct privilege escalation primitive is shown.

## 3. Reachability analysis

The bug is triggered locally by a process that can create or access a TCP socket and call setsockopt(SO_ATTACH_FILTER). It is not directly remote reachable because a remote host cannot attach the cBPF filter by itself. A remote peer or network attacker may still be part of a practical attack chain if leaked seq/ack values are used against TCP session integrity.

Namespaces matter because the fix checks CAP_NET_ADMIN in the socket network namespace user_ns. In containerized or delegated networking environments, a process with CAP_NET_ADMIN in its namespace may still be allowed after the fix, while ordinary unprivileged users are blocked.

The affected functionality is broadly available in normal kernels and does not require rare hardware or non-default execution modes.

## 4. Severity interpretation

This behaves more like an actionable Moderate than an ordinary Low/Moderate cleanup. The issue is not memory corruption and does not directly provide LPE or kernel code execution. However, it is a real unprivileged side-channel against TCP connection metadata in a core networking path, with plausible downstream integrity impact against TCP sessions.

The theoretical worst case is stronger TCP session manipulation when the leaked seq/ack data is combined with a suitable network attack position. The realistic evidence supports confidentiality loss of TCP metadata and possible connection integrity assistance, not host compromise.

## 5. One-sentence report phrase

Unprivileged SO_ATTACH_FILTER on TCP sockets could expose TCP sequence and acknowledgment numbers through a cBPF side channel, allowing a local process to assist TCP session integrity attacks without CAP_NET_ADMIN.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is not a simple DoS or validation-only cleanup. It is a core TCP side-channel with explicit seq/ack leakage and possible integrity impact, so it should not be auto-closed despite the absence of memory corruption.

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

Patch: tcp: restrict SO_ATTACH_FILTER to priv users [ Upstream
Commit: 3747de241a66ef2c7032d2cc2b826a47c5fa0f6a
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=3747de241a66ef2c7032d2cc2b826a47c5fa0f6a

Changed files:
  net/core/sock.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=3747de241a66ef2c7032d2cc2b826a47c5fa0f6a

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

The original Linux kernel CVE announcement for CVE-2026-53236 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53236

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:L/I:L/A:N
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:H/A:N
  The Best / paranoid CVSS score: 5.3

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: 6

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).