[CVE-2026-74425][MODERATE REGULAR] afs: handle CB.InitCallBackState3 requests without a server record

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

CVE: CVE-2026-74425
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: afs: handle CB.InitCallBackState3 requests without a server record
Commit: 42e3917cdbdc3d35e191c525687a6d5427f237fd
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=42e3917cdbdc3d35e191c525687a6d5427f237fd
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74425
Analysis date: Sun, 16 Aug 2026 06:12:15 -0400
ActionableScore: 4
ActionableScore lower bound: 3
Actionable bucket: Borderline, manual review recommended / Actionable Moderate consideration
Manual review required: YES

Summary:
AFS CB.InitCallBackState3 can dereference a missing server record for an unmatched RxRPC callback request, allowing a reachable remote peer to crash the kernel through a NULL pointer dereference.

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

The original Linux kernel CVE announcement for CVE-2026-74425 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74425

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 REGULAR
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-2026-74425	MODERATE	CHECK WITH IMPACT FROM ORIG NN NONE	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';**CWE-476;CWE-703;CWE-754;BEST CVSS score: '7.5';DESCR 'AFS CB.InitCallBackState3 handling can dereference a missing server record when an incoming RxRPC callback request is not mapped to a known AFS server. A remote peer that can reach the AFS callback service may trigger this path with an unmatched callback request, causing a NULL pointer dereference and kernel crash. For the CVSS the PR:N is selected because the attacker does not need local privileges on the victim and only needs network reachability to the AFS callback service. The issue is network reachable in AFS deployments where callback traffic from untrusted or spoofable peers can reach the client. Impact is denial of service only via kernel crash. No evidence of UAF, OOB access, information disclosure, or privilege escalation is supported by this patch context.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline, manual review recommended / Actionable Moderate consideration (with actual score 4)  SKIP CVE-2026-74425 UNKNOWN SKIP The Fixes patch not applied yet, so unlikely that actual: 40e8b52fe8c8ab6920ea5f59c5469b6918cce624 YES  NO NO unknown 	MAYBE	DANGER DISK SIMPLEFIX SKIP LINUS  INCREASED_TO_MODERATE_FROM_LOW_BASED_ON_GUESSCVSS DECREASED_TO_MODERATEREG_BASED_ON_ACTIONABLESCORELESSTHAN5	-	-	checked

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

ActionableScore=4
ActionableScoreLower=3

## 1. ActionableScore

* Conservative score: 3
* Paranoid score: 4
* Final recommended bucket: **Borderline, manual review recommended / Actionable Moderate consideration**::

## 2. Signal breakdown

Conservative signals:

* Remote reachable / network-triggerable: +2
  The affected path is an AFS cache manager callback handler over RxRPC. A peer that can reach the callback service may send an unmatched CB.InitCallBackState3 request.

* Reliable kernel crash / strong DoS: +1
  The old code dereferences `call->server->_uuid` without checking `call->server`. If no server record is attached, this is a NULL pointer dereference in kernel context.

* Rare AND difficult-to-reach subsystem/configuration: -1
  AFS is not commonly enabled or exposed on most general-purpose systems. Practical exposure depends on AFS client usage and callback reachability.

* Availability impact realistic: +1
  Counted in the paranoid score only. If the callback service is reachable, the NULL dereference is a direct kernel crash path.

Not counted:

* Generic memory corruption: +0
  This is a NULL pointer dereference, not UAF, OOB write, double free, type confusion, or arbitrary write.

* Privilege escalation plausible: +0
  No reclaim, stale object reuse, writable stale object, callback hijack, or refcount takeover is shown.

* Confidentiality / Integrity: +0
  The supported impact is DoS only.

## 3. Reachability analysis

The bug can be triggered by a remote peer able to send AFS/RxRPC callback traffic to the affected client. No local account on the victim is required if the callback service is reachable, so the practical CVSS-style PR interpretation is PR:N. Containers and namespaces are not the main factor here because the trigger is network-side callback handling. The main limiting condition is deployment exposure: AFS is relatively uncommon, and many systems will not run an AFS client or expose callback traffic to untrusted networks.

Call-site confidence: medium. The patch shows the direct dereference site and the missing NULL check, but broader callback service exposure depends on runtime AFS configuration.

## 4. Severity interpretation

This behaves like a network-reachable DoS issue, not an Important-class memory corruption vulnerability. The theoretical impact is limited by the primitive: the patch supports a NULL dereference, but not UAF, OOB access, attacker-controlled overwrite, information leak, or privilege escalation. Because the path is network reachable when AFS callbacks are exposed, it should not be auto-closed without checking affected product exposure.

## 5. One-sentence report phrase

AFS CB.InitCallBackState3 can dereference a missing server record for an unmatched RxRPC callback request, allowing a reachable remote peer to crash the kernel through a NULL pointer dereference.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Network reachability makes this worth manual triage even though the primitive is DoS-only. The key product-specific question is whether AFS client callback handling is enabled and reachable from untrusted peers.

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

Patch: afs: handle CB.InitCallBackState3 requests without a server record
Commit: 42e3917cdbdc3d35e191c525687a6d5427f237fd
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=42e3917cdbdc3d35e191c525687a6d5427f237fd

Commit description:

The cache manager callback path now attaches the server record to an
incoming call through the rxrpc peer's app data.  That association is
not guaranteed to exist for every callback request, and most callback
handlers already tolerate that case.

Make CB.InitCallBackState3 follow the same pattern by checking whether a
server record was attached before using it.  If the peer is not mapped
to a server record, trace the request and ignore it, matching the
existing behaviour for other unmatched callback requests.

This keeps the callback handler consistent with the rest of the cache
manager service and avoids depending on peer state that may not be
available for a given request.

Fixes: 40e8b52 ("afs: Use the per-peer app data provided by rxrpc")
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: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Nan Li <tonanli66@gmail.com>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260622090856.2746629-2-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Changed files:
  fs/afs/cmservice.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=42e3917cdbdc3d35e191c525687a6d5427f237fd

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

The original Linux kernel CVE announcement for CVE-2026-74425 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-74425

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: Not available
  The Best / paranoid CVSS vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 7.5

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: 3
  Paranoid ActionableScore: 4

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: NO
Published priority for this report (same as in Subject): MODERATE REGULAR

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).