Microsoft’s concise attestation that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate — but it is a scoped inventory statement, not proof that no other Microsoft product can or does contain the same vulnerable kernel code.
CVE‑2025‑40003 is a Linux kernel use‑after‑free (UAF) defect rooted in the mscc ocelot driver’s handling of a cyclic delayed work item. The bug arises when the driver attempts to cancel and destroy the workqueue while a delayed work item can still be rescheduled by its timer; the workqueue destruction sequence then races with a later timer expiry that requeues the work, producing kernel warnings and potential instability. Upstream patches replace the unsafe cancelation pattern with synchronous disable/cancel semantics so that delayed work cannot be resubmitted after the queue is torn down. Vendors and distributions have ingested the upstream fix and published advisories mapping the upstream commit into vendor kernels and packages. Ubuntu, Debian/OSV, and other distribution trackers list the issue and outline the remediation steps (kernel update or vendor backport). These trackers characterize the impact primarily as availability and stability (kernel warnings, oopses, KASAN traces), not as an immediately wormable remote code execution (RCE) vector. Microsoft’s public messaging for several recent open‑source CVEs has followed the same pattern: publish product‑level attestations using machine‑readable CSAF/VEX formats (starting with Azure Linux), and update those attestations if inventory work discovers additional Microsoft artifacts that ship the affected upstream code. That programmatic transparency improves automation for customers, but it also changes the semantics of vendor statements — they describe what Microsoft has validated so far, not the exhaustive state of every Microsoft build and image.
Why that distinction matters:
Key quick checklist (copyable)
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
CVE‑2025‑40003 is a Linux kernel use‑after‑free (UAF) defect rooted in the mscc ocelot driver’s handling of a cyclic delayed work item. The bug arises when the driver attempts to cancel and destroy the workqueue while a delayed work item can still be rescheduled by its timer; the workqueue destruction sequence then races with a later timer expiry that requeues the work, producing kernel warnings and potential instability. Upstream patches replace the unsafe cancelation pattern with synchronous disable/cancel semantics so that delayed work cannot be resubmitted after the queue is torn down. Vendors and distributions have ingested the upstream fix and published advisories mapping the upstream commit into vendor kernels and packages. Ubuntu, Debian/OSV, and other distribution trackers list the issue and outline the remediation steps (kernel update or vendor backport). These trackers characterize the impact primarily as availability and stability (kernel warnings, oopses, KASAN traces), not as an immediately wormable remote code execution (RCE) vector. Microsoft’s public messaging for several recent open‑source CVEs has followed the same pattern: publish product‑level attestations using machine‑readable CSAF/VEX formats (starting with Azure Linux), and update those attestations if inventory work discovers additional Microsoft artifacts that ship the affected upstream code. That programmatic transparency improves automation for customers, but it also changes the semantics of vendor statements — they describe what Microsoft has validated so far, not the exhaustive state of every Microsoft build and image. What Microsoft actually said — and what that means
The wording: “includes the open‑source library and is therefore potentially affected”
Microsoft’s MSRC entry for the CVE maps the upstream component to Azure Linux and states that the distro “includes this open‑source library and is therefore potentially affected.” Read practically, this is a product‑scoped attestation: Microsoft completed an inventory for Azure Linux and found the relevant kernel code there, so Azure Linux images are in scope for remediation and are being tracked via Microsoft’s published VEX/CSAF files. It is not an exclusive technical guarantee that other Microsoft artifacts cannot contain the same code.Why vendors publish VEX/CSAF per product
VEX (Vulnerability Exploitability eXchange) is a machine‑readable technique for telling customers which product artifacts are known affected, not affected, fixed, or under investigation. Microsoft’s initial rollout prioritized Azure Linux (formerly CBL‑Mariner) as the first product family for which they would publish VEX attestations; they’ve said they will extend this to other products as inventories are completed. That phased rollout is procedural — it reflects where the vendor has finished mapping components into product artifacts, not a claim that unlisted products are safe.Technical anatomy of the bug (brief)
- Component: Linux kernel networking driver for Microsemi (MSCC) Ocelot family (net: mscc: ocelot).
- Fault class: Use‑after‑free / workqueue timer rescheduling race. The driver calls cancel_delayed_work during teardown, but the delayed work’s timer may still be pending and can requeue the work after the workqueue was destroyed, producing queue failures and potential data structure use‑after‑free traces.
- Fix pattern upstream: Use synchronous disable/cancel primitives and ensure a delayed work item cannot be requeued after the workqueue is destroyed (for example, replace cancel_delayed_work with disable_delayed_work_sync where appropriate and guard rescheduling paths).
- Impact: local kernel instability and warnings; practical threat model is availability and reliability. No public proof of an easy remote exploit to gain code execution or escalation has been published at disclosure.
Is Azure Linux the only Microsoft product that includes the library and is therefore potentially affected?
Short answer: No — not necessarily. Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the implicated upstream code for this CVE, and that attestation is authoritative for Azure Linux images. However, other Microsoft products and artifacts that ship or run Linux kernels could also include the same upstream driver and therefore be affected depending on kernel version and build configuration. Absence of a product name in Microsoft’s VEX/CSAF is absence of attestation, not proof of absence.Why that distinction matters:
- Microsoft ships multiple Linux kernel artifacts (Azure Linux images, linux‑azure kernels used for some VM families, the WSL2 kernel distributed for Windows Subsystem for Linux, container/Marketplace images, and internal CBL‑Mariner derivatives). Each artifact is a separately built kernel binary with its own CONFIG_* options and module composition. Presence of the ocelot driver is a per‑artifact, build‑time property.
- A product‑level VEX/CSAF mapping tells you the vendor inspected Product X and found Component Y. It does not say they inspected every other product yet. Microsoft has explicitly said it will update CVE/VEX entries when it discovers additional Microsoft products that ship the same upstream component.
Practical examples of Microsoft artifacts that should be validated
The following Microsoft artifacts could be carriers depending on kernel version and configuration; treat each as an independent verification task rather than assuming they are safe because they aren’t listed yet:- Windows Subsystem for Linux (WSL2) kernel builds: Microsoft publishes and maintains a WSL2 kernel tree. If that kernel build includes the ocelot driver or the same upstream commit range, it could be affected. Verify the kernel version and build configuration in your WSL2 kernel image.
- linux‑azure (Azure platform kernel) and kernel packages used in Azure Marketplace images: these are separately maintained kernel artifacts; they may include different drivers and options than Azure Linux images.
- CBL‑Mariner lineage images or other Microsoft container host images that predate or parallel Azure Linux releases: these historically share lineage and could include the same kernel subsystems.
- Custom images and Marketplace appliances published by Microsoft partners or Microsoft engineering teams: verify kernel content and module lists on those images.
Verifying whether a specific Microsoft artifact includes the vulnerable component
To determine whether a given image or kernel is actually carrying the ocelot driver code that was fixed in CVE‑2025‑40003, operators must check two concrete things:- Kernel version and upstream commit range shipped in the product — does the shipped kernel include the vulnerable upstream commit range?
- Kernel configuration / module presence — was the relevant driver built into the kernel (CONFIG_*) or provided as a module that is shipped and loaded?
- Inspect the kernel version and changelog in the image:
- On a running Linux host: uname -a; check package metadata (rpm -qi kernel or dpkg -s linux-image-....
- Check vendor kernel changelogs for backports or applied patches that already include the fix.
- Check module list for ocelot or mscc drivers:
- lsmod | grep ocelot
- grep -r "ocelot" /lib/modules/$(uname -r) || true
- Inspect kernel config:
- zcat /proc/config.gz | grep -i OCEL or the specific driver name if /proc/config.gz is available.
- Look for CONFIG_MSCC*OCELOT (or the driver’s exact CONFIG** symbol) in the kernel config file packaged with the vendor kernel.
- For container images:
- Extract or run the image and inspect /lib/modules and /boot for kernel artifacts.
- Check image package metadata and vendor SBOM or VEX/CSAF if available.
- For WSL2:
- Check the WSL2 kernel binary shipped with Windows (or in the WSL2 repo) to see whether the ocelot driver is built or present.
Operational recommendations and mitigation checklist
Operators and security teams should take a structured, prioritized approach:- Immediate priority: If you run Azure Linux images, treat Microsoft’s VEX/CSAF attestation and the vendor kernel updates as authoritative and apply the vendor patches per Microsoft guidance now. Azure Linux is explicitly attested and therefore in scope.
- Short term (0–72 hours):
- Inventory all Microsoft‑supplied Linux artifacts in your estate: AKS node images, Marketplace images, WSL2 instances, linux‑azure kernel images in VMs, container base layers, and any Microsoft containers you run. Produce a list of kernel versions and image identifiers.
- Scan those images for presence of the ocelot/mscc driver (module names, symbols, sources). If you find the driver present, escalate to patching or mitigations.
- Integrate Microsoft’s CSAF/VEX feeds into your triage pipelines so that future attestations for other Microsoft products are automatically consumed.
- Where patching is delayed, apply containment controls: restrict access to affected hosts, reduce exposure of affected workloads, and limit administrative operations that can manipulate hardware queues if those operations can trigger the driver.
- For WSL2 endpoints, where kernel changes require Windows update cadence, track Microsoft’s WSL kernel releases and apply Windows or WSL updates when vendor patches are delivered.
- Medium term (weeks):
- Build or refine automation to extract kernel configs and module inventories from any new or updated images during CI/CD or image registry scans.
- Ensure image/SBOM transparency: request or produce component inventories for marketplace and vendor images before deploying to production.
- Validate vendor backports: when a vendor publishes a fixed kernel package, confirm the vendor changelog (or distribution advisory) actually includes the upstream commit or the equivalent backport.
- Hunting and detection guidance:
- Look for kernel WARN_ON, oops, or KASAN traces mentioning workqueue queueing failures and ocelot_check_stats_work logging patterns in kernel logs.
- Correlate spurious device or network driver errors on nodes running Microsoft images with recent kernel updates or teardown flows.
Strengths of Microsoft’s approach — and notable benefits for customers
- Machine‑readable VEX/CSAF: Microsoft’s publication of VEX attestations for Azure Linux is a practical step to reduce noisy telemetry and false positives in enterprise triage. When a vendor provides a deterministic mapping between CVE and product artifact, automation can make faster, more accurate decisions about patch prioritization and exposure.
- Scoped, auditable inventory statements: Attestations that say “this product includes component X” are a useful, auditable record for defenders and compliance teams; they allow immediate prioritization for the named product family.
- Commitment to update: Microsoft has openly stated it will broaden attestations as inventories for other product families complete, which is a transparent stance compared with previous vendor practices that were less granular.
Risks, potential misinterpretations, and operational hazards
- Misreading attestation as exclusivity: Treating a product‑level statement as a universal exclusion is the largest operational risk. If teams assume “only Azure Linux” is affected without verifying other kernels, they may leave exposures unpatched. The correct posture is: act quickly on Microsoft’s attested products and concurrently verify other artifacts in your environment.
- Overreliance on VEX rollout timing: Vendors often roll out product‑by‑product attestations. The inventory process can lag, and absence of an attestation can simply mean Microsoft hasn’t completed that product’s mapping yet, not that the product is safe.
- Backport and packaging variance: Vendor kernels may include upstream fixes or backports that alter how exposure manifests. A given kernel version string is not enough; check vendor changelogs and the presence/absence of the specific upstream fix. Failure to validate backport inclusion can lead to false sense of security.
A short, practical triage playbook (ranked steps)
- Patch attested products first: apply the vendor updates for Azure Linux immediately. Microsoft’s attestation makes these the canonical highest‑priority artifacts.
- Inventory and inspect: list every Microsoft‑supplied Linux image and kernel binary in your estate and check for presence of the ocelot/mscc driver. Use automation to extract /lib/modules, kernel configs and package metadata.
- Harden and contain: restrict access, reduce network exposure, and avoid operations that might repeatedly teardown the driver or queue the problematic work until patched.
- Integrate VEX/CSAF: consume Microsoft’s CSAF/VEX feeds into your patch management and triage tooling so you receive deterministic signals as Microsoft expands attestations.
- Validate fixes: once you install vendor kernel updates, verify that the kernel changelog or package metadata contains the upstream fix commit or backport note.
Flags on unverifiable claims
Any public statement that a given Microsoft product is the “only” possible carrier of the vulnerable code is unverifiable without exhaustive inventory of every Microsoft artifact and their build configurations. Microsoft’s own messaging clarifies that the Azure Linux attestation is a product‑scope statement and that additional mappings will be published if other products are found to include the upstream component. Treat claims about exclusivity with caution and require artifact‑level verification for non‑attested products.Conclusion
Microsoft’s public advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is a precise, actionable attestation that should be treated as authoritative for Azure Linux customers and automated triage systems. However, it is not a universal guarantee that no other Microsoft product ever contained or currently contains the same upstream kernel code. The correct operational response is twofold: patch Azure Linux images now as Microsoft instructs, and immediately inventory and verify all other Microsoft‑supplied kernel artifacts in your environment (WSL2 kernels, linux‑azure, Marketplace images, container base layers) for the presence of the ocelot/mscc driver and the upstream fix. Rely on Microsoft’s VEX/CSAF feeds for automation, but do not substitute them for artifact‑level inspection where rapid, enterprise‑specific risk decisions are required.Key quick checklist (copyable)
- Patch Azure Linux images immediately.
- Inventory all Microsoft Linux artifacts (AKS node images, Marketplace VMs, WSL2 installations).
- Scan images for ocelot/mscc driver presence (lsmod / lib/modules / kernel config).
- Integrate Microsoft’s CSAF/VEX feeds into triage automation.
- Validate vendor backport or kernel changelogs before marking hosts remediated.
Source: MSRC Security Update Guide - Microsoft Security Response Center
