CVE-2025-39925: Linux CAN J1939 race fix and Azure Linux attestation

The Linux kernel fix tracked as CVE‑2025‑39925 addresses a race/reference-counting bug in the CAN J1939 protocol stack: the subsystem lacked a NETDEV_UNREGISTER notification handler to undo an extra reference held by j1939_sk_bind, which could leave a virtual CAN device (for example vcan0) stuck with an elevated usage count and block unregister_netdevice from completing. Microsoft’s Security Response Center (MSRC) has published a product-level attestation that Azure Linux includes the implicated open‑source component and is therefore potentially affected, and the company says it will update the CVE/VEX entry if additional Microsoft products are identified as carriers.

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Background / Overview​

The bug fixed by the upstream kernel commit for CVE‑2025‑39925 appears in the CAN (Controller Area Network) J1939 protocol implementation inside the Linux networking stack. The technical root cause is straightforward: code paths that bind sockets to J1939 interfaces acquired an extra reference to the private j1939_priv structure (via j1939_sk_bind, but the subsystem lacked a NETDEV_UNREGISTER notification handler to release that reference when a net_device is unregistered. As a result, unregister_netdevice could wait indefinitely because the device’s usage count failed to drop to the value required to allow teardown. Operationally, this is a local robustness/availability issue rather than a remote code‑execution hole. When it triggers, systems may see stuck device-unregister operations, kernel warnings, or inability to unload virtual CAN devices; cloud or container platforms that create and destroy network devices frequently (for example in ephemeral testbeds, container networking, or virtualized setups) are the most likely contexts where this defect would surface. Several distributor trackers list the commit and the fixed kernel versions; those mappings are the canonical way operators judge whether a given kernel build is fixed.

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Microsoft’s public statement and what it actually means​

Microsoft’s publicly visible update for this CVE contains two linked assertions:

• Microsoft has inspected the Azure Linux distribution artifacts and found that the implicated upstream kernel code appears in those builds; therefore Azure Linux is listed as “potentially affected.”
• Microsoft is publishing machine‑readable CSAF/VEX attestations (a phased rollout that began with Azure Linux) and has committed to update CVE product mappings in its VEX/CSAF feed should internal inventories later identify additional Microsoft products that ship the same upstream component.

Read together, those statements are an authoritative, product‑by‑product inventory disclosure for Azure Linux — and nothing more. They confirm Microsoft’s inventory result for the Azure Linux family; they do not — and should not be read to — assert that every other Microsoft product is free of the vulnerable upstream code. The absence of another Microsoft product from that initial attestation is absence of attestation, not evidence of absence.
Two practical consequences follow:

• If you run Azure Linux images or Microsoft‑published images explicitly covered by Microsoft’s VEX/CSAF data, treat those artifacts as in‑scope and remediate accordingly.
• If you run other Microsoft artifacts that ship or embed Linux kernels (for example WSL2 kernels, linux‑azure kernels, Marketplace VM images, AKS node images, or various container base images derived from CBL‑Mariner lineage), you must perform artifact‑level verification: presence of a kernel file, the kernel configuration option that built the subsystem, and/or module presence is what determines actual exposure — not the vendor’s initial product list.

This interpretive point has been stated repeatedly by independent trackers and by analysts summarizing Microsoft’s new CSAF/VEX approach: Microsoft started by attesting Azure Linux and will expand product attestations over time; that phased approach is deliberate.

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

Short answer: No — not necessarily. Long answer: Azure Linux is the only Microsoft‑distributed product Microsoft has publicly attested so far to include the vulnerable j1939 kernel code; that is the definitive, machine‑readable signal for Azure Linux customers. But whether any other Microsoft product includes the same upstream sources depends on build provenance, kernel version, and kernel configuration for each Microsoft artifact — things that must be checked on an artifact‑by‑artifact basis. Why that nuance matters:

• Microsoft ships several distinct Linux artifacts and kernel builds: the Azure Linux distribution itself (CBL‑Mariner lineage/azure‑tuned kernels), the Windows Subsystem for Linux (WSL2) kernel binaries, linux‑azure kernels used by specific VM SKUs, curated Marketplace images and partner appliances, and container base images. Each artifact is built at different times and with possibly different CONFIG_* flags; the vulnerable j1939 code may be present in some artifacts and absent from others.
• Microsoft’s VEX/CSAF attestation is a recording of what Microsoft has validated for a given product family. Starting attestation coverage with Azure Linux is practical — it’s Microsoft’s own Linux distribution and therefore an obvious first target for inventory — but it does not substitute for host-level verification across a diverse estate.

Put another way: the correct operational posture is to treat Azure Linux as a confirmed hit and to verify other Microsoft images you run, especially if those images include kernels or kernel modules compiled from upstream trees that could predate the fix. The vendor’s pledge to update the CVE attestation if more products are found affected is welcome, but it will always lag real‑time deployments and third‑party images that may not be Microsoft‑built.

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Cross‑checking the technical facts (independent sources)​

To avoid relying on a single feed, the core technical claims and the fixed commit range for CVE‑2025‑39925 are confirmed by multiple independent sources:

• The NVD entry documents the vulnerability description and maps it to the upstream kernel fix. The NVD description matches the upstream commit message detailing the missing NETDEV_UNREGISTER handler and explains the unregister_netdevice usage‑count hang symptom.
• Debian’s security tracker maps affected and fixed package versions across Debian branches and references the kernel stable commits that carry the upstream fix. This gives defenders a way to map their distribution package versions to the upstream remedy.
• Ubuntu’s security notice for the CVE provides the same technical summary and lists which Ubuntu kernel packages require evaluation or patching in each release.
• Public CVE aggregators and distribution advisories (for example CVE Details, CVE mirror sites and distributor trackers) record the commit references and fixed kernel versions as well. Those entries align with the kernel stable trees and the upstream patch commits.

Cross‑referencing these sources confirms three important, load‑bearing facts you can depend on operationally:

• The bug is a kernel‑level reference/unregister logic issue in the J1939 CAN subsystem (not a userland library).
• The symptom described by syzbot/unregister_netdevice traces — a device that fails to drop usage count and blocks unregister — is the observed failure mode and the reason the fix adds a NETDEV_UNREGISTER handler.
• Distribution advisories and kernel stable trees list fixed versions and backports; remediation is a kernel update or distribution package that includes the backport.

If anything in your environment is inconsistent with those three points, treat that as a red flag and investigate further.

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Practical verification steps (a short runbook)​

To decide whether a particular system or image you run is affected, verify kernel presence, module configuration, and kernel version. These steps apply to VMs, container hosts, and WSL2 instances.

• Identify the running kernel on the host:
• uname -sr && uname -r
• Check for built‑in or modular presence of the CAN/J1939 subsystem:
• grep -i j1939 /boot/config-$(uname -r) || zgrep -i j1939 /proc/config.gz
• ls /lib/modules/$(uname -r)/kernel/drivers/net/can || find /lib/modules/$(uname -r) -type f -name 'j1939'
• If j1939 is a module, see if it is loaded:
• lsmod | grep j1939
• modinfo j1939
• Inspect debug logs or kernel traces when unregister fails:
• dmesg | grep -i 'unregister_netdevice|vcan0|j1939'
• For WSL2: on the Windows host run:
• wsl --status ; inside WSL run uname -r and inspect /proc/config.gz or /boot/config-<version> if present.
• Compare kernel version and distribution package to the fixed versions published by distributors (Debian/Ubuntu/NVD mappings) and to the upstream kernel commit references if you maintain custom kernel builds.

Priority guidance: If your inventory shows Azure Linux images that match the vulnerable kernel range, patch or reimage those first — Microsoft has already attested Azure Linux as affected. Next, prioritize any Microsoft‑supplied images in production (Marketplace images, AKS node images, and linux‑azure kernels) and any large fleet WSL deployments running older WSL2 kernels. Finally, inspect any custom or partner images you run in Azure that may have been built from older upstream kernel trees.

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Recommended remediation and mitigation​

• Patch: apply the kernel update or vendor-provided package that includes the upstream fix. This is the definitive remediation path. Distribution trackers list the fixed kernel package versions; apply the appropriate patch for your distro/release.
• Short‑term mitigation for systems you cannot patch immediately:
• Avoid dynamically tearing down J1939 or virtual CAN devices in ways that would trigger unregister paths, where feasible.
• For hosts running user workloads that create/destroy many virtual CAN instances, consider temporary workload throttling or orchestration changes until patching is complete.
• Monitor kernel logs for unregister_netdevice hangs and escalate those hosts for immediate remediation.
• For WSL2: ensure Windows hosts have the latest WSL kernel updates via wsl --update; if you manage a fleet, include WSL kernel checks in your image validation pipeline.
• If you run Marketplace or third‑party images in Azure, confirm the image publisher and kernel provenance. Microsoft’s attestation covers Microsoft‑published Azure Linux images; it may not cover third‑party Marketplace images — and those publishers must be consulted for their own advisories.

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Strengths and risks of Microsoft’s attestation approach​

Microsoft’s recent adoption of machine‑readable CSAF/VEX attestations for product‑level inventory is beneficial: it provides automation-friendly signals that tell operators which Microsoft‑published artifacts the vendor has audited and mapped to a CVE. That clarity materially reduces triage friction for the products Microsoft has covered. Azure Linux being explicitly named gives Azure administrators a clear, prioritized remediation target.
However, the approach carries operational risks if misinterpreted:

• Risk of false reassurance: Treating the initial attestation as an exhaustive statement that no other Microsoft product is affected would be a mistake. Inventories are phased; many Microsoft artifacts may not have been inspected yet. The vendor explicitly says it will update CVE records if additional products are identified — an admission that the first attestation is limited in scope by design.
• Supply‑chain complexity: Microsoft does not control every kernel or image that runs in Azure — third‑party Marketplace images, partner appliances, or user-provided images may carry vulnerable kernels independently. Those artifacts must be inventoried separately.
• Timing mismatch: vendor attestations and distribution package updates do not always coincide with large fleet patch cycles. In practice, operations teams must do both vendor‑driven patching and their own verification runs to close exposure windows.

These limitations mean the safest operational posture is assume Azure Linux is affected and verify everything else, rather than assuming Microsoft’s attestation implies the rest of the vendor’s estate is safe.

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When Microsoft updates the CVE mapping — what to expect​

Microsoft’s explicit promise to update the CVE entry and VEX/CSAF attestations if additional products are identified provides a mechanism for later clarity. When Microsoft expands its attestations, the VEX files will typically enumerate whether a product is “Known Affected,” “Not Affected,” “Under Investigation,” or “Fixed.” Those machine‑readable attestations are useful for automated inventory systems and can be integrated into vulnerability management pipelines. Until such attestations appear, however, do local verification.
A cautionary note: absence of a VEX attestation for a Microsoft product at a point in time is not a guarantee of safety — it simply means Microsoft has not completed the inventory for that product yet. Security teams must continue to verify and protect their own artifacts, and not rely solely on the vendor’s attestation cadence.

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Conclusion — recommended action checklist​

• Treat Azure Linux images as confirmed in‑scope and apply Microsoft’s recommended kernel updates or reimages without delay. Azure Linux is the one Microsoft product Microsoft has attested as containing the vulnerable j1939 code at the time of the advisory.
• Inventory all Microsoft‑supplied and third‑party images you run (WSL2, linux‑azure kernels, Marketplace images, AKS node images, container base images). Use the verification runbook above to confirm whether j1939 appears in the kernel or module set for each artifact.
• Patch or reimage affected systems according to distributor/advisor guidance (Debian/Ubuntu and upstream kernel stable trees list fixed versions). Prioritize production fleet nodes and any hosts showing unregister_netdevice-related log entries.
• Monitor Microsoft’s VEX/CSAF feed and the MSRC update guide for any expansion of product attestations; when Microsoft updates the CVE mapping to include other products, treat that as an authoritative addition to the product inventory. Until then, perform artifact‑level checks for everything Microsoft published and for third‑party images you run.

In short: Microsoft’s public attestation that Azure Linux includes the affected open‑source code is authoritative and actionable for Azure Linux customers, but it is not a blanket assurance that no other Microsoft product could contain the same vulnerable code. Verification and patching remain the final arbiter of risk for every Microsoft or third‑party artifact in your environment.

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Source: MSRC Security Update Guide - Microsoft Security Response Center