[CVE-2026-53239][MODERATE 7.0] xfrm: policy: fix use-after-free on inexact bin in xfrm_policy_bysel_ctx() [ Upstream

[CVE-2026-53239][MODERATE 7.0] xfrm: policy: fix use-after-free on inexact bin in xfrm_policy_bysel_ctx() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-53239
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: xfrm: policy: fix use-after-free on inexact bin in xfrm_policy_bysel_ctx() [ Upstream
Commit: 8fc536e9f6856230f19c7d13e71af064b6a77b22
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8fc536e9f6856230f19c7d13e71af064b6a77b22
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53239
Analysis date: Thu, 25 Jun 2026 10:47:52 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
A race in `xfrm_policy_bysel_ctx()` can leave an inexact XFRM policy bin pointer usable after concurrent SPD hash rebuild frees it through RCU, allowing a local user with XFRM policy control or delegated `CAP_NET_ADMIN` to trigger a kernel UAF and likely DoS.

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

The original Linux kernel CVE announcement for CVE-2026-53239 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53239

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-2026-53239	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:H/I:H/A:H';*CWE-416;CWE-362;CWE-667;Other CVSS 'AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '7';DESCR 'A race in xfrm_policy_bysel_ctx() can leave an inexact policy bin pointer usable after xfrm_hash_rebuild() has flushed and freed the same bin through kfree_rcu(). A local attacker who can issue the relevant XFRM netlink policy operations may race XFRM_MSG_DELPOLICY against XFRM_MSG_NEWSPDINFO and trigger a use-after-free in the policy pruning path. For the CVSS the PR:L is used because CAP_NET_ADMIN may be available to a container root, netns root, or delegated network administration service account, which is weaker than full host administrator control in practical risk terms. The issue is not directly network reachable because the trigger is local XFRM control plane activity rather than inbound IPsec packet traffic. Impact is at least local denial of service via kernel crash. In the paranoid interpretation, the UAF may allow confidentiality and integrity impact or privilege escalation and should be reviewed manually.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)	YES	LOCK RACE UAF HARDWARE LINUS KPANIC 	NO	NO	checked

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

ActionableScore=5
ActionableScoreLower=4

## 1. ActionableScore

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

## 2. Signal breakdown

Triggered signals:

* Memory corruption, weak primitive: +1
  The patch fixes an explicit UAF where `bin` can be freed by `kfree_rcu()` in `__xfrm_policy_inexact_flush()` and then later used by `xfrm_policy_inexact_prune_bin()`.

* Real lifetime corruption: +1
  This is a real lifetime race involving a stale pointer after concurrent policy deletion and SPD hash rebuild.

* Reliable kernel crash / strong DoS: +1
  A stale freed `xfrm_pol_inexact_bin` used in pruning can realistically lead to kernel crash or memory safety failure.

* Local unprivileged or reduced-privilege trigger in some deployments: +1
  Direct triggering requires XFRM netlink policy operations, normally gated by `CAP_NET_ADMIN`, but this may be available to container root, netns root, or delegated network-management services.

* Broad/core networking subsystem exposure: +1
  XFRM is a core Linux networking/IPsec subsystem and should not be treated as rare-only, even though policy management is control-plane rather than packet receive path.

Negative or limiting signals:

* Race/timing dependency: -1
  The issue requires racing `XFRM_MSG_DELPOLICY` with `XFRM_MSG_NEWSPDINFO`, although the commit describes a relatively wide unlocked window.

Race-UAF downgrade applied:

* No attacker-controlled reclaim, stale object replacement, write-after-free, callback dispatch, or type confusion is shown by the patch.
* Therefore this should not be scored as a strong LPE primitive by default.
* Conservative score is capped at 4 and paranoid score at 5 under the provided race-UAF rule.

## 3. Reachability analysis

The bug is triggered through local XFRM netlink control-plane operations, not by inbound IPsec packets directly. In typical host deployments, creating or deleting XFRM policies and requesting SPD info requires `CAP_NET_ADMIN`, so a fully unprivileged host user may not reach it directly.

Namespaces matter. If unprivileged user namespaces and network namespaces are enabled, or if a container or service has delegated `CAP_NET_ADMIN`, the attacker may be able to exercise this path without full host-root privileges. Because the UAF occurs in host kernel code, a namespace-scoped trigger can still affect the whole host.

The path is not Internet-remote by itself. Realistic exploitation requires local code execution or delegated network-administration capability and a successful race between policy deletion and SPD hash rebuild.

## 4. Severity interpretation

This is stronger than an ordinary Moderate cleanup because the patch fixes an explicit UAF in kernel networking lifetime handling. However, the demonstrated primitive is still a race-dependent stale pointer use without evidence of controlled reclaim, attacker-controlled replacement object, arbitrary write, callback corruption, or type confusion.

Realistically, this behaves like an actionable local kernel DoS with memory-corruption concern. The paranoid interpretation keeps it at Actionable Moderate because kernel UAFs in networking control-plane code can be underestimated, especially with namespace delegation. It is not a strong Important candidate on the provided evidence alone.

## 5. One-sentence report phrase

A race in `xfrm_policy_bysel_ctx()` can leave an inexact XFRM policy bin pointer usable after concurrent SPD hash rebuild frees it through RCU, allowing a local user with XFRM policy control or delegated `CAP_NET_ADMIN` to trigger a kernel UAF and likely DoS.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

This is an explicit kernel UAF race in a core networking subsystem, and namespace or delegated `CAP_NET_ADMIN` exposure may make it reachable by reduced-privilege users even though no strong LPE primitive is demonstrated in the patch.

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

Patch: xfrm: policy: fix use-after-free on inexact bin in xfrm_policy_bysel_ctx() [ Upstream
Commit: 8fc536e9f6856230f19c7d13e71af064b6a77b22
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=8fc536e9f6856230f19c7d13e71af064b6a77b22

Changed files:
  net/xfrm/xfrm_policy.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=8fc536e9f6856230f19c7d13e71af064b6a77b22

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

The original Linux kernel CVE announcement for CVE-2026-53239 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-53239

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:H
  The Best / paranoid CVSS vector: AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 7

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).