* [CVE-2026-90257][MODERATE REGULAR] Bluetooth: virtio_bt: avoid OOB read of build info string [ Upstream
@ 2026-09-18 3:41 21% AL-KERNEL
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From: AL-KERNEL @ 2026-09-18 3:41 UTC (permalink / raw)
To: kernel-cve
CVE: CVE-2026-90257
Priority: MODERATE REGULAR
AL-KERNEL base severity: MODERATE
KPANIC flag: NO
Patch: Bluetooth: virtio_bt: avoid OOB read of build info string [ Upstream
Commit: fb445b3466a8d1c7a0b0d0676fb475e3ce22d94e
Upstream patch: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb445b3466a8d1c7a0b0d0676fb475e3ce22d94e
Original CVE announcement: https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90257
Analysis date: Thu, 17 Sep 2026 23:41:28 -0400
ActionableScore: 3
ActionableScore lower bound: 2
Actionable bucket: Borderline / manual review recommended
Manual review required: YES
Summary:
No one-sentence report phrase was found.
======================================================================
ABOUT THIS REPORT
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90257 is available here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90257
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.
======================================================================
AL-KERNEL CLASSIFICATION RESULT
======================================================================
CVE-2026-90257 MODERATE https://www.kernelcve.org/list/?q=CVE-2026-90257 CHECK WITH IMPACT FROM ORIG NN MODERATE Maybe valid. Check manually. Hints by AL-KERNEL: The best (paranoid) CVSS is 'AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L';CWE-126;CWE-125;*CWE-200;Other CVSS 'AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:N';BEST CVSS score: '4.4';DESCR 'An OOB read in the Bluetooth virtio_bt setup path can occur because the Zephyr build information response is treated as an unbounded string starting after the status byte. A malicious or faulty virtio Bluetooth backend can return only a status byte or an unterminated short response, causing the kernel to read adjacent slab memory until a NUL byte is found. The leaked bytes can be written to the kernel log and firmware info debugfs output. For the CVSS the PR:L is used in the paranoid score because some virtualized or delegated environments may allow a less privileged local actor to influence the backend path or read the exposed log data, although typical backend control is more privileged. The issue is not network reachable and the main impact is limited confidentiality exposure, with possible local denial of service if the unbounded read faults.';YES REQUIRES MANUAL CHECK; ,and ActionableScore result is Borderline / manual review recommended (with actual score 3) YES OOB VIRT SIMPLEFIX SKB DEBUGFS DECREASED_TO_MODERATE_REGULAR_FROM_MODERATE7_BASED_ON_GUESSCVSS NO NO checked
======================================================================
ACTIONABLESCORE ANALYSIS
======================================================================
ActionableScore=3
ActionableScoreLower=2
PrimitiveClass=CLASS_C
RealUAF=NO
HostAdminRequired=UNKNOWN
NamespaceScopedPrivilege=UNKNOWN
ImportantCandidate=NO
ManualReview=RECOMMENDED
AutoDowngradeAllowed=YES
1. ActionableScore
* Raw conservative score: 2
* Conservative score: 2. No floor or cap applied.
* Raw paranoid score: 3
* Paranoid score: 3. No floor or cap applied.
* Final recommended bucket: **Borderline / manual review recommended**::
2. Signal breakdown
Conservative positive signals:
* +1 Confidentiality plausible. The OOB string walk can read adjacent slab bytes and those bytes are sent to kernel log and firmware info debugfs.
* +1 Device or backend mediated external influence. The vulnerable response is controlled by the virtio Bluetooth backend, but this is not direct network reachability.
Paranoid additional signal:
* +1 Weak availability concern. An unbounded kernel string read may plausibly fault or trigger hardening diagnostics in some builds, but a reliable host-wide crash is not directly demonstrated.
Non-awarded signals:
* +0 Real UAF not awarded. No stale lifetime or post-free object use is shown.
* +0 Strong corruption not awarded. The primitive is read-only OOB string consumption, not an OOB write, controlled reclaim, type confusion, or refcount corruption.
* +0 Strong LPE not awarded. No arbitrary write, callback control, credential manipulation, or privilege escalation chain is supported.
* +0 Remote reachability not awarded. Input comes from a virtio backend, not from ordinary network traffic.
* +0 Reliable kernel crash not awarded conservatively. The patch establishes disclosure through logging and debugfs, while crash is only a plausible secondary outcome.
* +0 Extra availability not awarded. No distinct persistent availability impact is established.
* +0 Broad/default exposure not awarded. virtio Bluetooth with a Zephyr style backend is deployment dependent.
* No negative privilege penalty applied. Host-admin-equivalent control of the backend is plausible in many deployments but not fully established by the supplied evidence.
3. Reachability analysis
The direct trigger is a malicious or faulty virtio Bluetooth backend returning a status-only or unterminated Build Information response. The vulnerable kernel then treats skb->data + 1 as an unbounded C string and consumes memory beyond the received payload.
This is not directly network reachable. It is also not an ordinary local syscall or netlink path. Practical triggering normally depends on who controls the virtio backend or virtual device model. That may be a host VMM, a device emulator, a delegated virtualization component, or another privileged management layer. The supplied evidence does not fully resolve whether the complete trigger requires host-admin-equivalent privilege, so HostAdminRequired=UNKNOWN.
Namespace scoped privilege is not established. No evidence shows that an attacker can create a namespace, obtain scoped capabilities, instantiate this backend path, and complete the trigger entirely below host admin.
Prior host or virtualization setup is likely required for the vulnerable virtio Bluetooth device to exist, but the exact administrative boundary is deployment dependent. No special physical hardware is required beyond the configured virtio Bluetooth path. No race is involved.
4. Primitive and severity interpretation
The exact primitive is a read-only out-of-bounds string read from adjacent kernel slab memory. The read data has a disclosure path because it reaches bt_dev_info and hci_set_fw_info, making it visible through kernel logs and firmware info debugfs depending on access policy.
This is not an active structured UAF and not fixed stale storage. It is an unbounded read from an skb payload boundary into adjacent allocation memory. There is no concrete repeated release or refcount operation. There is no first lifetime operation that may free an object and no second lifetime mutation through a dangling reference.
PrimitiveClass is CLASS_C because the established primitive is limited read-only OOB behavior with disclosure and possible invalid-state consequences, not a strong corruption or lifetime primitive. Demonstrated consequence is limited kernel memory disclosure. Speculative consequence is local DoS if the unbounded read faults, but this is not strong enough for Important classification.
ImportantCandidate=NO because the patch supports limited disclosure only and does not establish memory corruption, privilege escalation, authorization bypass, controlled reclaim, or a strong exploitation primitive. AutoDowngradeAllowed=YES because no unresolved Important-grade primitive remains. Operational severity is Low to Borderline Moderate, with manual review useful mainly to confirm backend attacker model and log/debugfs exposure.
5. One-sentence report phrase
A malicious or faulty virtio Bluetooth backend can return a short or unterminated Zephyr build information response that is printed as an unbounded string, causing a limited OOB read and possible kernel memory disclosure through logs or firmware info debugfs.
6. Manual review recommendation
MANUAL CHECK RECOMMENDED
Manual review is recommended because there is a real user-visible kernel memory disclosure path, but the attacker privilege boundary depends on who can control the virtio Bluetooth backend and who can read the exposed log or debugfs output.
7. Call-site confidence
Call-site confidence: high
======================================================================
UPSTREAM PATCH SUMMARY
======================================================================
Patch: Bluetooth: virtio_bt: avoid OOB read of build info string [ Upstream
Commit: fb445b3466a8d1c7a0b0d0676fb475e3ce22d94e
Upstream URL: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=fb445b3466a8d1c7a0b0d0676fb475e3ce22d94e
Changed files:
drivers/bluetooth/virtio_bt.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=fb445b3466a8d1c7a0b0d0676fb475e3ce22d94e
======================================================================
DETAILED REPORT METHODOLOGY
======================================================================
The original Linux kernel CVE announcement for CVE-2026-90257 can be found here:
https://lore.kernel.org/linux-cve-announce/?q=CVE-2026-90257
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:H/UI:N/S:U/C:L/I:N/A:N
The Best / paranoid CVSS vector: AV:L/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:L
The Best / paranoid CVSS score: 4.4
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: 2
Paranoid ActionableScore: 3
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 [email protected] (and both
send reply to CVE record itself too and see "reply" button below for howto reply).
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