[CVE-2026-64439][IMPORTANT] crypto: krb5 - filter out async aead implementations at alloc

[CVE-2026-64439][IMPORTANT] crypto: krb5 - filter out async aead implementations at alloc

[AL-KERNEL]

CVE: CVE-2026-64439
Priority: IMPORTANT
AL-KERNEL base severity: IMPORTANT
KPANIC flag: YES
Patch: crypto: krb5 - filter out async aead implementations at alloc
Commit: ef6feb77e2d91761427c5b773edc9c97e1b706ad
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ef6feb77e2d91761427c5b773edc9c97e1b706ad
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64439
Analysis date: Sat, 25 Jul 2026 06:45:22 -0400
ActionableScore: 8
ActionableScore lower bound: 5
Actionable bucket: Strong Important candidate / Actionable Moderate at minimum
Manual review required: YES

Summary:
A use-after-free in the Kerberos5 AEAD crypto path can occur when an async AEAD backend returns EINPROGRESS and the krb5 helper frees the buffer before async completion, allowing rxrpc, AFS, or Ceph crypto paths to trigger kernel memory corruption and likely DoS, with possible confidentiality or integrity impact in worst case.

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

The original Linux kernel CVE announcement for CVE-2026-64439 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64439

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.

======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================

CVE-2026-64439	IMPORTANT	CHECK WITH IMPACT FROM ORIG NN MODERATE	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:H/A:H';*CWE-416;CWE-362;CWE-754;Other CVSS 'AV:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H';BEST CVSS score: '8.1';DESCR 'A use-after-free can occur in the Kerberos5 crypto path because krb5 AEAD helpers treat crypto_aead_encrypt and crypto_aead_decrypt as synchronous operations while an async AEAD backend may return EINPROGRESS and complete later. The krb5 code can then free the sensitive buffer while the async worker still holds and later dereferences the freed slab object. This is reachable through rxrpc/AFS/Ceph crypto users when an async AEAD provider is selected for the krb5 enctype name. For the CVSS the PR:N because a remote peer may trigger the affected network protocol path without a local user account when the service is exposed. AC:H is used because exploitation depends on a non-default async provider selection and timing of async completion. Impact is at least denial of service via kernel crash and in the paranoid case may include confidentiality and integrity impact due to kernel use-after-free memory corruption. These services are usually deployed on internal storage or authentication networks rather than the public Internet.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Strong Important candidate / Actionable Moderate at minimum (with actual score 7)	YES	DANGER UAF LINUS KPANIC  KPANIC INCREASED_TO_HIGH_BASED_ON_GUESSCVSS	YES	NO	checked

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

ActionableScore=8
ActionableScoreLower=5

## 1. ActionableScore

* Conservative score: **5**
* Paranoid score: **8**
* Final recommended bucket: **Strong Important candidate / Actionable Moderate at minimum**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: **+2**. The affected krb5 crypto helpers are reachable through rxrpc, AFS, and Ceph crypto paths when those services are exposed.
* Generic memory corruption, strong primitive: **+2**. The commit explicitly describes a slab use-after-free after an async AEAD backend returns `EINPROGRESS`.
* Real lifetime corruption: **+1**. The freed buffer is still referenced by an async backend worker.
* Reliable kernel crash / strong DoS: **+1**. KASAN reports slab-use-after-free, and the likely practical result is kernel crash.
* Rare AND difficult-to-reach subsystem/configuration: **-1**. Triggering requires an async AEAD provider to be selected for the krb5 enctype name, which is not a default assumption.

Conservative total: **5**

Paranoid additions:

* Weak LPE concern: **+1**. The UAF involves async crypto completion using a freed slab buffer, which may create a limited memory-corruption concern even without a demonstrated reclaim primitive.
* Confidentiality impact plausible: **+1**. The freed buffer may contain sensitive crypto material or decrypted/encrypted data.
* Integrity impact plausible: **+1**. Async AEAD processing may write into memory after the buffer has been freed and potentially reused.

Paranoid total: **8**

## 3. Reachability analysis

A remote peer may trigger the affected path if rxrpc, AFS, or Ceph krb5 crypto functionality is reachable and the system selects an async AEAD implementation for the krb5 encryption name. No local account is necessarily required in the network-exposed service scenario, so PR:N is defensible for paranoid triage. The main limiting factor is configuration: the async AEAD provider selection appears non-default or environment-dependent, and the KASAN reproducer used a synthetic async backend. Namespaces and containers do not materially reduce the risk if the vulnerable network service is exposed from the host or a privileged service context.

Call-site confidence: **medium**. The commit names reachable users, but the provided diff only shows the allocation-mask change.

## 4. Severity interpretation

This is not an ordinary Moderate resource leak or validation-only bug. It is a real UAF caused by a synchronous krb5 helper freeing a buffer while an async crypto worker still references it. Realistic exploitation evidence currently supports crash/DoS most strongly, while privilege escalation remains plausible but not demonstrated. Because the bug combines network-adjacent reachability, kernel UAF, sensitive crypto buffers, and async lifetime mismatch, it should be treated as **Actionable Moderate at minimum** and a **Strong Important candidate** under paranoid triage.

## 5. One-sentence report phrase

A use-after-free in the Kerberos5 AEAD crypto path can occur when an async AEAD backend returns EINPROGRESS and the krb5 helper frees the buffer before async completion, allowing rxrpc, AFS, or Ceph crypto paths to trigger kernel memory corruption and likely DoS, with possible confidentiality or integrity impact in worst case.

## 6. Manual review recommendation

**MANUAL CHECK REQUIRED**

Reason: this is an explicitly described slab use-after-free in a crypto path reachable from networked subsystems, with non-trivial configuration constraints but enough memory-lifetime risk to avoid auto-closure.

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

Patch: crypto: krb5 - filter out async aead implementations at alloc
Commit: ef6feb77e2d91761427c5b773edc9c97e1b706ad
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ef6feb77e2d91761427c5b773edc9c97e1b706ad

Commit description:

krb5_aead_encrypt(), krb5_aead_decrypt() in rfc3961_simplified.c and
rfc8009_encrypt(), rfc8009_decrypt() in rfc8009_aes2.c set a NULL
completion callback and treat any negative return from
crypto_aead_{encrypt,decrypt}() as terminal, falling through to
kfree_sensitive(buffer).  When the encrypt_name resolves to an
async AEAD instance the request returns -EINPROGRESS, the buffer
is freed while the backend's worker still holds a pointer, and the
worker dereferences the freed slab on completion.

KASAN report under UML+SLUB with a synthetic async aead backend
bound to krb5->encrypt_name:

  BUG: KASAN: slab-use-after-free in t5_stub_complete+0x7d/0xc7

The helpers were written synchronously, so filter the async
instances out at allocation time instead of plumbing
crypto_wait_req() through every call site.

Reachable via net/rxrpc/rxgk.c, fs/afs/cm_security.c and
net/ceph/crypto.c on systems with an async AEAD provider bound to
the krb5 enctype name.

Fixes: 00244da ("crypto/krb5: Implement the Kerberos5 rfc3961 encrypt and decrypt functions")
Fixes: 6c3c0e8 ("crypto/krb5: Implement the AES enctypes from rfc8009")
Cc: stable@vger.kernel.org
Suggested-by: Herbert Xu <herbert@gondor.apana.org.au>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  net/rxrpc/rxgk.c
  fs/afs/cm_security.c
  net/ceph/crypto.c
  crypto/krb5/krb5_api.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=ef6feb77e2d91761427c5b773edc9c97e1b706ad

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

The original Linux kernel CVE announcement for CVE-2026-64439 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-64439

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:A/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:H
  The Best / paranoid CVSS vector: AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H
  The Best / paranoid CVSS score: 8.1

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: 5
  Paranoid ActionableScore: 8

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).