[CVE-2026-63891][MODERATE 7.0] thunderbolt: property: Cap recursion depth in __tb_property_parse_dir() [ Upstream

[CVE-2026-63891][MODERATE 7.0] thunderbolt: property: Cap recursion depth in __tb_property_parse_dir() [ Upstream

[AL-KERNEL]

CVE: CVE-2026-63891
Priority: MODERATE 7.0
AL-KERNEL base severity: MODERATE
KPANIC flag: YES
Patch: thunderbolt: property: Cap recursion depth in __tb_property_parse_dir() [ Upstream
Commit: 2b5f47a710172c962ef42d1b732b04d2ad0dce21
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b5f47a710172c962ef42d1b732b04d2ad0dce21
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63891
Analysis date: Sun, 19 Jul 2026 18:30:17 -0400
ActionableScore: 5
ActionableScore lower bound: 4
Actionable bucket: Actionable Moderate at minimum
Manual review required: YES

Summary:
An unauthenticated Thunderbolt XDomain peer can craft recursive DIRECTORY property entries that exhaust the kernel stack during property parsing, causing an adjacent-triggered kernel crash without evidence of privilege escalation.

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

The original Linux kernel CVE announcement for CVE-2026-63891 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63891

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-2026-63891	MODERATE	CHECK WITH IMPACT FROM ORIG NN IMPORTANT	Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H';CWE-674;**CWE-400;*CWE-20;BEST CVSS score: '6.5';DESCR 'Thunderbolt XDomain property parsing can recurse without a depth limit because a DIRECTORY entry value is reused as the next directory offset. A crafted adjacent XDomain peer can chain DIRECTORY entries into a back reference loop and drive __tb_property_parse_dir until the kernel stack is exhausted and the guard page triggers a kernel crash. For the CVSS the PR:N is selected because the peer does not need authentication or local privileges on the victim once it can reach the Thunderbolt XDomain PROPERTIES_REQUEST control plane. The issue is adjacent network reachable through Thunderbolt links such as a cable, dock, in line device, or adjacent host, but it is not Internet reachable. Impact is denial of service only based on the patch context, with no supported UAF, OOB write, information leak, or privilege escalation primitive.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Actionable Moderate at minimum (with actual score 5)  SKIP CVE-2026-63891 UNKNOWN SKIP No affected files built, so skip this CVE NO  - - unknown 	MAYBE	OOB SIMPLEFIX IMPROVEONLY KPANIC 	-	-	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

* `+2 Remote reachable / network-triggerable`: applies as adjacent-network reachability. An unauthenticated Thunderbolt XDomain peer can trigger the parser through the PROPERTIES_REQUEST control-plane exchange. This is not Internet remote, but it is externally triggerable without a local account.
* `+1 Reliable kernel crash / strong DoS`: crafted DIRECTORY recursion can exhaust the kernel stack until the guard page fires.
* `+1 Availability impact realistic`: the failure mode is a high-confidence kernel crash or panic from attacker-controlled property data.
* `+1 Paranoid exposure adjustment`: Thunderbolt docks, cables, adjacent hosts, and inline devices are realistic attack surfaces on Thunderbolt-enabled systems.
* No memory corruption bonus: the patch shows unbounded recursion and stack exhaustion, not UAF, OOB write, double free, type confusion, arbitrary write, or page-cache corruption.
* No LPE bonus: there is no supported privilege-escalation primitive.
* No C/I impact: no information disclosure or integrity-modifying primitive is shown.

## 3. Reachability analysis

The trigger is an untrusted Thunderbolt XDomain peer, such as a cable, dock, inline inspector, or adjacent host, that can reach the XDomain PROPERTIES_REQUEST exchange. No authentication or local user privileges on the victim are required once that peer is connected and the path is enabled. This is best treated as adjacent-network or hardware-adjacent exposure, not normal IP network reachability. Operators can disable the affected XDomain path with `thunderbolt.xdomain=0`, so deployment exposure depends on Thunderbolt and XDomain being enabled.

Call-site confidence: high. The patch directly changes the recursive parser and the commit message describes the attacker-controlled recursion path and crash mechanism.

## 4. Severity interpretation

This behaves like an actionable Moderate DoS issue rather than an Important memory-corruption vulnerability. The practical impact is unauthenticated adjacent-triggered kernel stack exhaustion leading to a crash. Theoretical memory-corruption escalation is not supported by the patch context because the fix only caps recursion depth and there is no evidence of attacker-controlled overwrite, stale object reuse, UAF reclaim, callback control, or object confusion.

## 5. One-sentence report phrase

An unauthenticated Thunderbolt XDomain peer can craft recursive DIRECTORY property entries that exhaust the kernel stack during property parsing, causing an adjacent-triggered kernel crash without evidence of privilege escalation.

## 6. Manual review recommendation

MANUAL CHECK REQUIRED

Reason: externally influenced unauthenticated hardware-adjacent kernel parser DoS should not be auto-closed, even though the supported impact is DoS only and not LPE.

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

Patch: thunderbolt: property: Cap recursion depth in __tb_property_parse_dir() [ Upstream
Commit: 2b5f47a710172c962ef42d1b732b04d2ad0dce21
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=2b5f47a710172c962ef42d1b732b04d2ad0dce21

Changed files:
  drivers/thunderbolt/property.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=2b5f47a710172c962ef42d1b732b04d2ad0dce21

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

The original Linux kernel CVE announcement for CVE-2026-63891 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-63891

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:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
  The Best / paranoid CVSS score: 6.5

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: 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).