[CVE-2026-52935][MODERATE 7.0] xfrm: espintcp: do not reuse an in-progress partial send

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

CVE: CVE-2026-52935
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: espintcp: do not reuse an in-progress partial send
Commit: 6564e9c7af7e1dc7bfe7f3093b728abe484d7630
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6564e9c7af7e1dc7bfe7f3093b728abe484d7630
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52935
Analysis date: Wed, 24 Jun 2026 04:34:23 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An XFRM ESP-in-TCP partial-send state bug can reuse a live ctx partial object with a stale offset, causing an out-of-bounds read in the send path and potentially allowing remote DoS or limited data exposure in configured ESP-in-TCP deployments.

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

The original Linux kernel CVE announcement for CVE-2026-52935 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52935

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-2026-52935	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:H/I:N/A:H';CWE-125;CWE-362;*CWE-664;Other CVSS 'AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:H';BEST CVSS score: '7.4';DESCR 'xfrm espintcp can reuse ctx partial while a previous partial send is still in progress. This can attach a stale offset to a new sk_msg and lead to an out of bounds read in the send path. For the CVSS the PR:N is used because a network peer does not need local privileges on the target system, although the target must use ESP in TCP and the attacker must be able to influence a configured transport path. The issue is network reachable in affected IPsec ESP in TCP deployments, but exploitation is high complexity because it depends on an in progress partial send and timing or backpressure conditions. Impact is at least denial of service via kernel crash and may include confidentiality impact if out of bounds data is copied into the transmit path. Integrity impact is not assumed because the patch supports an out of bounds read primitive, not an out of bounds write or arbitrary write primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	REMOTE LOCK NETWORK LINUS KPANIC 	NO	NO	checked

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ACTIONABLESCORE ANALYSIS
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ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

* Conservative score: 4
* Paranoid score: 5
* Final recommended bucket: **Actionable Moderate at minimum**

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  Applies for systems using XFRM ESP-in-TCP where a peer can influence the transport path and partial-send conditions.

* Memory corruption, weak or indirect corruption candidate: +1
  The commit explicitly describes a memory-safety issue where stale partial-send state can lead to an out-of-bounds read. This is not an OOB write or UAF reclaim primitive.

* Reliable kernel crash / strong DoS: +1
  An OOB read in the kernel send path can plausibly cause an Oops or crash.

* Confidentiality impact plausible: +1
  Because the OOB read occurs in a send path, there is a plausible risk that unintended kernel-adjacent data could be copied into transmitted output, but this is not proven.

* Hard or timing-dependent condition: -1
  The trigger depends on an in-progress partial send, blocking behavior, and backpressure or timing conditions.

Conservative total: 4

Paranoid adjustment:

* Do not subtract the timing penalty in the paranoid score: +1 relative to conservative
  If TCP backpressure and repeated sends are naturally reachable in a real ESP-in-TCP deployment, the race-like partial-send condition may be reproducible enough to avoid the -1 reliability penalty.

Paranoid total: 5

Not counted:

* No privilege escalation bonus. The patch supports OOB read and DoS, not a write primitive, object replacement, callback control, refcount takeover, or type confusion.
* No integrity impact. The described primitive is read-only.
* No Dirty-Pipe-like bonus. This is not page-cache or COW ownership corruption.

## 3. Reachability analysis

A remote peer can plausibly influence this path only when ESP-in-TCP is configured and the attacker can interact with the affected IPsec transport. This is not a default Internet-wide exposure, but XFRM/IPsec should not be treated as an obscure local-only subsystem. The most realistic trigger requires a configured ESP-in-TCP connection plus conditions that leave a previous partial send in progress while a new send is attempted.

Local namespaces do not materially reduce the main remote interpretation. Local configuration of XFRM normally requires elevated privileges such as CAP_NET_ADMIN, but the relevant attacker model here is a peer influencing an already configured transport path.

## 4. Severity interpretation

This is stronger than an ordinary Moderate because the patch explicitly fixes a memory-safety bug in a network send path and names an out-of-bounds read. It is not a Strong Important candidate by itself because the primitive is read-only, timing-dependent, and does not show attacker-controlled overwrite, UAF reclaim, type confusion, or privilege escalation.

The realistic impact is DoS, with plausible confidentiality concern if OOB data can be transmitted. The theoretical worst case justifies manual review, but not an automatic Important classification.

## 5. One-sentence report phrase

An XFRM ESP-in-TCP partial-send state bug can reuse a live ctx partial object with a stale offset, causing an out-of-bounds read in the send path and potentially allowing remote DoS or limited data exposure in configured ESP-in-TCP deployments.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: this is a network-reachable memory-safety issue with an explicit out-of-bounds read in the send path. Even though exploitation appears timing-dependent and no write or LPE primitive is shown, the confidentiality and crash implications are sufficient to avoid auto-closure.

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

Patch: xfrm: espintcp: do not reuse an in-progress partial send
Commit: 6564e9c7af7e1dc7bfe7f3093b728abe484d7630
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=6564e9c7af7e1dc7bfe7f3093b728abe484d7630

Commit description:

espintcp keeps a single in-flight transmit in ctx->partial.
Before building a new sk_msg, espintcp_sendmsg() first tries to flush
that state through espintcp_push_msgs().

For blocking callers, espintcp_push_msgs() may return success even when
the previous partial send is still pending. espintcp_sendmsg() would
then reinitialize emsg->skmsg and reuse ctx->partial while the old
transfer still owns that state.

Do not rebuild the send message when ctx->partial is still in progress.
If espintcp_push_msgs() returns with emsg->len still set, fail the new
send instead of overwriting the live partial state.

This is a memory-safety fix: reusing the live partial-send state can
leave a stale offset attached to a new sk_msg and lead to an out-of-
bounds read in the send path.

tcp_sendmsg_locked() already handles waiting for send buffer memory, so
the fix here is just to preserve espintcp's one-message-at-a-time
transmit state.

Fixes: e27cca9 ("xfrm: add espintcp (RFC 8229)")
Cc: stable@kernel.org
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Zhengchuan Liang <zcliangcn@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Assisted-by: Codex:GPT-5.4
Signed-off-by: Wyatt Feng <bronzed_45_vested@icloud.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/xfrm/espintcp.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=6564e9c7af7e1dc7bfe7f3093b728abe484d7630

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

The original Linux kernel CVE announcement for CVE-2026-52935 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-52935

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:L/I:N/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:H
  The Best / paranoid CVSS score: 7.4

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: 4
  Paranoid ActionableScore: 5

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).