Azure Linux VEX Attestations Explained: CVE-2025-39981 and Per Artifact Risk

Microsoft’s brief advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate for the product inventory Microsoft has completed so far, but it is not a categorical statement that no other Microsoft product could contain the same vulnerable code; the company explicitly committed to expand its machine‑readable VEX/CSAF attestations if additional Microsoft products are later found to ship the implicated library.

Background / Overview​

CVE-2025-39981 is a Linux kernel vulnerability described as “Bluetooth: MGMT: Fix possible UAFs.” The defect arises in the kernel’s Bluetooth management code (net/bluetooth/mgmt.c) where a struct mgmt_pending object can be freed while some worker or callback still processes it, producing a use‑after‑free (UAF) as captured by KASAN traces and reported stack traces in the public advisories. The vulnerability was published in October 2025 and has been cataloged in mainstream trackers and distribution advisories. Technically, the root cause is a lifetime/TOCTOU (time‑of‑check/time‑of‑use) problem: the code paths that add pending management commands to a list and the asynchronous completion or cancellation paths did not always synchronize list removal and free operations. Upstream fixes introduce a validation flag (for example, mgmt_pending_valid) and ensure list removal and freeing are done while holding the appropriate lock so that completion callbacks never reference freed objects. The practical result is an avoidance of slab UAFs and KASAN complaints; the fix is a correctness change that reduces crash/oom/oops risk in Bluetooth management paths. Multiple vendor trackers and distro advisories describe the same technical picture.

What Microsoft actually said — parsing the MSRC wording​

On CVE product‑mapping, Microsoft follows a machine‑readable CSAF/VEX attestation model and began publishing VEX attestations for Azure Linux in October 2025. The MSRC blog describing the rollout makes the approach explicit: Microsoft is starting with Azure Linux as the first product family for which it will publish VEX attestations and will expand that coverage to other Microsoft products over time. The MSRC page for CVE‑2025‑39981 (and the short advisory text you quoted) therefore reflects two linked facts: Microsoft found the implicated upstream component inside Azure Linux artifacts, and Microsoft will update the CVE/VEX mapping if additional internal products are shown to ship the component. Put simply: the MSRC attestation is an authoritative inventory statement for Azure Linux, not a universal exclusion for all Microsoft products. Several independent explainers and operational analyses emphasize this distinction and recommend treating the Azure Linux VEX attestation as a definitive “yes” for Azure Linux images while performing per‑artifact verification for other Microsoft‑supplied binaries and images.

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Is Azure Linux the only Microsoft product that includes the affected open‑source library?​

Short answer: No — Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the implicated library for this CVE, but absence of an attestation for other Microsoft products is not proof those products are clean. Microsoft’s published VEX/CSAF data and the MSRC advisory specifically state that the company will update the CVE/VEX record if additional Microsoft products are identified as carriers of the same upstream code. That phrasing is deliberate and standard for a phased inventory rollout.

Why the attestation doesn’t imply exclusivity​

• Kernel and distribution components are built artifact‑by‑artifact: whether a particular Microsoft product contains a given upstream source file depends on the kernel version, the exact upstream commit range used, and the kernel CONFIG_* flags chosen at build time. Different Microsoft artifacts (for example, the WSL2 kernel binary, linux‑azure kernel builds, Azure Marketplace images, or specialized VM node images) can therefore differ in whether they include the Bluetooth MGMT code that was fixed by the CVE.
• Vendors typically roll out VEX/CSAF attestations product‑by‑product. Microsoft chose to start with Azure Linux to provide immediate, machine‑readable signals to the large population of Azure VM customers; that practical sequencing shortens time to remediation for the largest affected surface while the vendor inventories the rest of its product catalog. It does not mean Microsoft scanned every internal artifact before publishing the first attestations.
• Third‑party and marketplace images hosted on Azure may ship their own kernel builds or static binaries. Those artifacts are the responsibility of their respective publishers; Microsoft’s Azure Linux attestation does not automatically extend to third‑party Marketplace images unless those vendors explicitly attest their images contain or lack the component.

Concrete examples of Microsoft artifacts that might, in principle, carry the same kernel code​

• WSL2 kernel images and the publicly maintained WSL2 kernel source tree (WSL uses a Microsoft‑maintained kernel binary for WSL2 instances). If that kernel build includes the bluetooth subsystem configured the same way, it could be a carrier.
• linux‑azure kernel packages used by certain Azure VM image pipelines or curated images. Those artifacts are distinct from the CBL‑Mariner/Azure Linux distribution but may share backports or stable upstream commits.
• Marketplace VM images, partner appliances, container base images or other Microsoft‑curated images that embed a kernel or kernel modules; each of these is an independently built artifact and must be evaluated separately.

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Cross‑checking the technical facts (why you should trust the CVE description)​

Multiple independent sources corroborate the technical description of CVE‑2025‑39981 as a Bluetooth MGMT use‑after‑free in the Linux kernel:

• The NIST/NVD entry for CVE‑2025‑39981 records the summary and links to vendor advisories; it is the canonical CVE registry entry used by many security tools.
• Distribution trackers (Debian security tracker, Ubuntu advisories, Oracle Linux CVE list) include the same technical KASAN trace excerpts and point to the same net/bluetooth/mgmt.c code paths and the upstream commits that introduced the fix. These distribution advisories are independently prepared by distro maintainers and package teams and confirm the fix landed in the kernel stable trees and was backported into distro packages.
• Commercial vulnerability databases and enterprise scanners (Tenable, Rapid7, Snyk, Red Hat advisories and others) have ingested the CVE and supplied mitigation advice and fixed‑package mappings for their supported distributions, reinforcing the public understanding of the vulnerability.

Together, these independent sources establish a consistent technical narrative: an asynchronous lifetime/locking bug in Bluetooth management code produced UAFs under certain workloads; upstream maintainer patches add checks and reorder removal/free operations to close the race.

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Operational impact and exploitability​

• Practical impact: the bug is primarily an availability/stability issue — kernel oopses, KASAN reports and potential host crashes are the likely symptoms. Public trackers describe local or privileged vectors rather than a trivial remote code‑execution route. The attack surface depends on whether untrusted workloads can trigger the mgmt command paths (for example, via socket calls to HCI/MGMT interfaces or by privileged local processes).
• Cloud risk: in multi‑tenant cloud hosts, any kernel crash on a VM host or node can have outsized fallout; therefore cloud operators should prioritize kernel updates for images Microsoft has attested as affected (Azure Linux) and verify other image classes separately.
• Exploit reports and EPSS: trackers show low-to-moderate EPSS/exploit activity at publication; however, UAFs in kernel space are historically risky and merit timely patching because, in some environments and with favorable heap grooming, a UAF can be escalated into a more powerful primitive. Treat the vulnerability as a medium‑to‑high operational priority where the affected kernel is part of a sensitive surface.

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Practical guidance for admins and operators​

Below is a prioritized checklist to determine exposure and remediate systems in your environment.

• Identify which images and kernel builds you run:
• For VMs and bare‑metal Linux hosts: run uname -r and check your distribution kernel package and changelog.
• For WSL2: run wsl --status and inspect the WSL2 kernel binary/version; check the WSL source or kernel binary used by your WSL distribution.
• For each artifact, verify the kernel source/config that produced it:
• If you maintain your own kernels, search the kernel source tree for net/bluetooth/mgmt.c and the commit IDs mentioned in the upstream fixes.
• If you rely on vendor packages, consult vendor advisories and the vendor’s VEX/CSAF file where available (Microsoft published these for Azure Linux beginning October 22, 2025).
• Prioritize patching:
• If you use official Azure Linux images, treat Microsoft’s VEX/CSAF attestation as authoritative and apply the vendor’s patched kernel images as they are published. Automate VEX ingestion where possible to speed triage.
• If you cannot patch immediately, apply compensations:
• Disable Bluetooth services or blacklist Bluetooth kernel modules on hosts that do not require Bluetooth.
• Restrict untrusted local workloads’ access to HCI/MGMT interfaces.
• Increase monitoring for kernel oopses, KASAN traces and abnormal HCI worker failures and preserve logs for forensic review.
• For Marketplace images or third‑party appliances:
• Contact the image vendor to request their SBOM, CVE/VEX attestation, or explicit patching guidance. Do not assume Microsoft’s Azure Linux attestation covers third‑party Marketplace images.
• Maintain an artifact inventory:
• Long tails come from custom appliances, old Marketplace images, and CI/CD pipelines that bake static kernel modules into appliances. Maintain a registry of images and their kernel provenance so you can answer “which images run kernel X with commit Y” rapidly.

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Strengths and limitations of Microsoft’s VEX/CSAF approach​

Strengths​

• Machine‑readable, automated signals: Publishing VEX/CSAF attestations for Azure Linux gives defenders a programmatic way to map CVEs to product artifacts and automate triage for affected SKUs. This reduces noise and speeds remediation for large fleets running the Microsoft‑maintained images.
• Transparency commitment: Microsoft’s public blog and MSRC updates show a clear commitment to expand attestations beyond Azure Linux. That policy commitment is useful for long‑term supply‑chain hygiene.

Limitations and residual risks​

• Phased coverage creates temporary “unknowns”: Until Microsoft completes inventory for every product family, customers running other Microsoft artifacts must do per‑artifact verification. Absence of an attestation is not evidence of absence.
• Per‑artifact variability: Kernel features are toggled at build time. The same vendor can ship multiple kernels with different CONFIG flags; a single product attestation therefore does not eliminate the need for host‑level or image‑level checks.
• Operational overhead: Customers must maintain inventories, check SBOMs, and reconcile vendor VEX data against their actual deployed artifacts. This is a one‑time effort to harden processes, but it is non‑trivial for organizations with many bespoke images.

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What we can and cannot verify right now​

• Verifiable (documented): Microsoft has attested that Azure Linux includes the implicated open‑source library and is therefore potentially affected, and Microsoft published the VEX pilot for Azure Linux beginning October 22, 2025. This is explicit in Microsoft’s public communications.
• Verifiable (documented): The technical nature of CVE‑2025‑39981 — an MGMT‑path use‑after‑free in the Bluetooth kernel code — is corroborated by NVD, Debian/Ubuntu/Oracle trackers, and enterprise vulnerability databases.
• Not verifiable (from public data): Whether every other Microsoft product (for example, every WSL2 kernel binary, linux‑azure build, Marketplace image or internal appliance) includes the vulnerable source file. That status is an artifact‑by‑artifact fact and requires either Microsoft to publish additional VEX attestations naming those product artifacts, or for customers/partners to inspect the individual images or their SBOMs. Microsoft has committed to update the CVE if additional products are identified, which means the only authoritative public confirmation for additional products will come from Microsoft’s expanded VEX/CSAF outputs or vendor advisories.

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Recommended short‑term action plan for WindowsForum readers and system operators​

• Step 1: Immediately inventory all Azure images and confirm which ones are Microsoft‑published Azure Linux images. If Azure Linux is in use, apply Microsoft’s published kernel updates or follow their distro advisory.
• Step 2: For every non‑Azure Linux Microsoft artifact you run (WSL2, linux‑azure images, Marketplace images, node images), perform a per‑artifact verification: check kernel version, inspect the kernel config (if available) for Bluetooth support, and search for net/bluetooth/mgmt.c or the mgmt_pending symbols in the image or kernel modules.
• Step 3: If you operate multi‑tenant hosts, prioritize hosts that expose local untrusted workloads to HCI/MGMT sockets or bridge Bluetooth device access to arbitrary tenants. Compensate where patching is delayed by restricting Bluetooth access or blacklisting modules.
• Step 4: Subscribe to Microsoft’s CSAF/VEX feed and your distro vendor advisories to receive automated updates. Microsoft will update CVE mappings if further products are affected; automation will catch those changes faster than manual monitoring.

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Conclusion​

Microsoft’s MSRC attestation that Azure Linux includes the implicated open‑source library is a useful, authoritative, machine‑readable signal for the many customers who run Microsoft‑published Azure Linux images. However, it should not be read as an exclusive claim that no other Microsoft product could contain the same vulnerable Bluetooth MGMT code. The technical presence of kernel code is a per‑artifact property — dependent on kernel version, upstream commit ranges, and build‑time configuration — and vendors commonly roll out VEX/CSAF attestations product‑by‑product. Until Microsoft publishes additional attestations or until image owners inspect their artifacts, operators must treat the Microsoft attestation as definitive for Azure Linux only and perform artifact‑level verification for WSL2, linux‑azure kernels, Marketplace images, and other Microsoft‑supplied artifacts. The pragmatic path forward for defenders is straightforward: apply patches for Azure Linux immediately where applicable, inventory and verify every other Microsoft artifact you run, and automate VEX/CSAF ingestion so you will be notified promptly if Microsoft expands the list of affected products. That combination of vendor‑driven automation and conservative per‑artifact verification is the only reliable way to close the “attestation gap” while preserving operational continuity.

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Source: MSRC Security Update Guide - Microsoft Security Response Center