[CVE-2026-63952][IMPORTANT] memfd: deny writeable mappings when implying SEAL_WRITE [ Upstream

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

CVE: CVE-2026-63952
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: memfd: deny writeable mappings when implying SEAL_WRITE [ Upstream
Commit: b3f4f82d1315f1439059a83d1c22c51a5b43d99e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3f4f82d1315f1439059a83d1c22c51a5b43d99e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63952
Analysis date: Sun, 19 Jul 2026 22:01:40 -0400
ActionableScore: 7
ActionableScore lower bound: 6
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A local attacker can create a memfd that appears protected by F_SEAL_WRITE while retaining a writable mapping, allowing modification of data that another process may trust as immutable.

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

The original Linux kernel CVE announcement for CVE-2026-63952 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63952

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: IMPORTANT
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-2026-63952	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N';CWE-693;CWE-284;CWE-281;Other CVSS 'AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N';BEST CVSS score: '7.1';DESCR 'A memfd can appear to be protected by F_SEAL_WRITE while still being modified through a writable mapping that existed before F_SEAL_EXEC was applied. The root cause is that the implied write seal from F_SEAL_EXEC was added after the writable mapping denial check in memfd_add_seals(). A local attacker can create such a memfd and pass it to another process that trusts F_SEAL_WRITE as an immutability guarantee. For the CVSS the PR:L is used because an unprivileged local process can create and seal memfd objects, while practical privilege impact depends on a local consumer that accepts and trusts the file descriptor. The issue is not network reachable. Impact is primarily integrity bypass, and in a paranoid but defensible scenario it may contribute to local privilege escalation if a more privileged component validates the seal and later consumes attacker mutable code or data.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	SIMPLEFIX IMPROVEONLY KPANIC MEMORY 	YES	NO	checked

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ACTIONABLESCORE ANALYSIS
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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

Conservative scoring:

* Local unprivileged trigger: +1
  Unprivileged local processes can create memfd objects and add seals.

* Security-boundary / policy contract bypass: +2
  The bug breaks the F_SEAL_WRITE immutability contract. A memfd can appear write sealed while still being writable through an existing mapping.

* Practical exploit chain strongly implied: +1
  The commit explicitly describes passing such a memfd to another process that trusts the seal state.

* Integrity impact plausible: +1
  A trusted consumer may validate F_SEAL_WRITE and then consume attacker-modifiable data or code.

* Broad/default/common subsystem exposure: +1
  memfd is a common local Linux API and does not require rare hardware or a special driver.

Total conservative: 6

Paranoid additional interpretation:

* Privilege escalation plausible in consumer-dependent scenarios: +1
  If a privileged service, sandbox broker, loader, verifier, or IPC peer trusts F_SEAL_WRITE as an immutability guarantee, the attacker-controlled writable mapping can undermine that trust boundary.

Total paranoid: 7

Not counted:

* No generic kernel memory corruption.
  The patch does not show UAF, OOB write, double free, type confusion, arbitrary write, or page-cache corruption.

* No network reachability.
  The trigger is local memfd and sealing behavior.

* No availability impact.
  The issue is not primarily a crash or DoS bug.

## 3. Reachability analysis

The bug is reachable by a local user that can create an executable memfd, create or retain a writable mapping, and then apply F_SEAL_EXEC so that F_SEAL_WRITE is implied after the writable-mapping check has already been skipped. This does not require root or CAP_SYS_ADMIN in typical configurations.

Namespaces and containers can matter because memfd is commonly available inside containers and sandboxes. The most security-relevant scenario is a cross-process or sandbox boundary where one process passes a supposedly immutable sealed memfd to a more trusted process. The issue is not remotely reachable by packets or network protocol parsing, but it can affect local IPC designs that rely on memfd sealing for W^X or immutable-content guarantees.

## 4. Severity interpretation

This behaves more like an actionable Moderate or Important-candidate logic vulnerability than an ordinary Moderate bug. It is not kernel memory corruption and does not directly provide arbitrary kernel read or write. However, it violates a security-relevant kernel-enforced contract: F_SEAL_WRITE should mean the memfd cannot be modified.

The realistic impact is integrity bypass against software that trusts sealed memfd contents. The paranoid Important-candidate interpretation is defensible because privileged or sandbox-boundary consumers may rely on F_SEAL_WRITE before executing, mapping, verifying, or otherwise trusting the memfd contents.

## 5. One-sentence report phrase

A local attacker can create a memfd that appears protected by F_SEAL_WRITE while retaining a writable mapping, allowing modification of data that another process may trust as immutable.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Manual review is required because this is a deterministic local security-contract bypass, not a simple DoS or cleanup bug. The real severity depends on whether affected products or services use sealed memfd objects as a trust boundary for executable or security-sensitive content.

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

Patch: memfd: deny writeable mappings when implying SEAL_WRITE [ Upstream
Commit: b3f4f82d1315f1439059a83d1c22c51a5b43d99e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b3f4f82d1315f1439059a83d1c22c51a5b43d99e

Changed files:
  mm/memfd.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=b3f4f82d1315f1439059a83d1c22c51a5b43d99e

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

The original Linux kernel CVE announcement for CVE-2026-63952 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63952

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:N/I:H/A:N
  The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
  The Best / paranoid CVSS score: 7.1

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: IMPORTANT
KPANIC detected for this report: YES
Published priority for this report (same as in Subject): IMPORTANT

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).