The Linux kernel patch that closed a net/mdiobus flaw assigned CVE-2025-38110 has drawn renewed attention to how large vendors — Microsoft included — publish product-level attestations for open-source components and what those attestations actually mean for operators running other Microsoft-supplied kernel artifacts. The technical fix is straightforward: add address validation before Clause 45 (C45) MDIO read/write operations so the kernel refuses out-of-range PHY addresses that could otherwise let mdio-tools trigger an out‑of‑bounds read or write into the mdiobus statistics array. The operational takeaway is less tidy: Microsoft’s public advisory confirms that
Azure Linux images were inspected and are
potentially affected, but that single attestation is not a guarantee no other Microsoft product contains the same vulnerable code. Administrators must treat the MSRC statement as a confirmed inventory for Azure Linux — and as an invitation to perform their own inventory across WSL, Marketplace images, and other Microsoft-provided kernel artifacts.
Background
The vulnerability arises in the Linux kernel’s MDIO subsystem — the code that exposes PHY (physical layer) management interfaces to userland through ioctl operations and supports both Clause 22 and Clause 45 MDIO access methods. Historically, kernels use a fixed upper bound for PHY addresses (commonly 32 entries, controlled by PHY_MAX_ADDR), but the MDIO ioctl interface accepts an address parameter from userland. In the reported defect, the kernel did not verify that the ioctl-supplied mdio address was within the intended 0–31 range before indexing the stats array or performing C45 read/write. That mismatch created a potential for out‑of‑bounds read or write operations into kernel memory used for mdiobus statistics — a correctness error that can lead to memory corruption, information disclosure, or system instability.
Multiple independent Linux distribution and vulnerability trackers ingested the upstream fix and mapped it to downstream advisories, confirming the same root cause and remedial change. Vendor advisories and kernel maintainers describe the fix as an explicit bounds check added prior to C45 read/write calls so that any mdio address that exceeds PHY_MAX_ADDR is rejected rather than used to index internal arrays.
What the fix changed — technical summary
- The kernel now performs address verification before executing Clause 45 MDIO read/write operations issued via ioctl.
- The verification enforces the intended PHY address range (the 5‑bit address space commonly limited by PHY_MAX_ADDR) and rejects invalid addresses rather than allowing them to index internal statistics arrays.
- The change does not alter normal MDIO functionality for valid addresses; it simply prevents misuse of the ioctl interface to reference memory outside the stats array.
- As part of the hardening, the fix may exclude out-of-range accesses from statistics collection, trading off an informational counter for safety — an acceptable and common kernel hardening practice.
Why this matters technically
A malformed ioctl from userland (for example, a crafted mdio-tools invocation) can supply an arbitrary mdio address. If the kernel blindly uses that value as an index into a fixed-size stats array, a large index may cause reads from or writes to memory beyond the array’s bounds. Even if the immediate read/write fails in practice, the attempt can expose kernel memory or corrupt data structures — classic vectors for privilege escalation, information leaks, or denial-of-service via crashes. The added pre-check eliminates that edge case by validating the address before any indexing occurs.
Scope and likely impact
- This is a local, low-complexity vulnerability: a user or process with the ability to issue MDIO ioctl calls (often local administrators, privileged tooling, or malicious code running on the host) can exercise the flaw.
- Exploitation requires access to MDIO-capable networking hardware or virtualized interfaces that expose MDIO operations to userspace tooling. Not all systems expose or permit MDIO ioctls to untrusted processes.
- The primary practical impact is on systems that compile and ship the vulnerable net/mdiobus code and expose MDIO ioctl entry points to userland. Many commodity server and embedded NICs provide PHY management interfaces, so the exposure is real for some deployment classes.
- Kernel maintainers and distro vendors classified the issue as high/important in several advisories — reflecting potential for serious impact if an adversary has local access and the MDIO interface is reachable.
Microsoft’s advisory: what it says and what it does not say
Microsoft’s Security Response Center (MSRC) entry for the CVE states that
Azure Linux includes the implicated open‑source library and is therefore “potentially affected,” and that Microsoft began publishing machine‑readable CSAF/VEX attestations in October 2025. The advisory adds that Microsoft will update the CVE entry if additional products are identified as carriers.
That language is precise and deliberate:
- It is an inventory attestation: Microsoft inspected the Azure Linux distribution lineage and found the specific kernel component that contained the vulnerability. As a result, Azure Linux customers have a clear, actionable signal that images in that product family require attention or mitigation.
- It is not an exhaustive statement that no other Microsoft product ships the same vulnerable code. Microsoft explicitly reserves the right to update the mapping if additional products are found to include the component.
- The public CSAF/VEX rollout is a phased inventory approach: large vendors commonly publish attestations for individual product families as they complete internal scans and bill-of-materials reconciliation.
Practical interpretation
The straightforward operational interpretation is:
- Treat Microsoft’s attestation for Azure Linux as authoritative: if you run Azure Linux images, you should assume the code was present and follow vendor remediation guidance.
- Do not assume other Microsoft products are unaffected simply because they are not listed. The absence of an attestation is not proof of absence; it is an artifact of a phased inventory and attestation rollout.
- Expect additional product mappings to appear over time as Microsoft’s inventory work continues. The advisory explicitly says it will update the CVE if more products are identified.
Is Azure Linux the only Microsoft product that could be affected?
Short answer: No — Azure Linux is the only Microsoft product Microsoft has publicly attested to include the vulnerable library at the time of the advisory. That is different from saying Azure Linux is the only Microsoft product that includes the code.
Longer explanation:
- Microsoft publishes multiple kernel artifacts and images that may carry upstream Linux kernel code: the Azure Linux distribution and kernels, the WSL2 kernel builds that Microsoft maintains and publishes, Marketplace images, and various kernel images used to back managed services and appliances.
- If any of those artifacts were built from a kernel tree that includes the unchanged net/mdiobus code in the vulnerable commit range, they could be carriers of the same defect until a vendor‑applied fix or backport is installed.
- Publicly visible Microsoft kernel trees — for example, the WSL2 kernel source that Microsoft publishes — show that Microsoft does maintain and publish substantial Linux kernel sources and derived artifacts. That makes it plausible that other Microsoft artifacts could contain the component in question.
- Microsoft’s attestation model is product-scoped: they begin with the product family they have the best inventory for (Azure Linux). Additional products will be mapped and attested as inventory work completes.
In operational terms: Azure Linux customers should act immediately; operators using other Microsoft-supplied kernels (WSL, Marketplace images, certain Azure VM images) should perform their own inventories and treat un‑attested artifacts as “not yet confirmed” rather than “confirmed clean.”
How to determine whether your Microsoft-supplied artifacts are affected
Administrators should adopt a layered, evidence-based approach to determine exposure across Microsoft-supplied artifacts:
- Inventory your Microsoft-derived kernel artifacts.
- Enumerate kernel versions and build provenance for all Microsoft-supplied images you run (Azure Linux VMs, Marketplace VM images, WSL kernels, AKS node images, linux-azure builds).
- Capture exact kernel release numbers and vendor package IDs rather than relying on generic product names.
- Map kernel versions to upstream patches.
- Identify the upstream commit(s) that fixed the net/mdiobus issue and determine whether the kernel version in your artifact includes that commit (directly or via a backport).
- If your kernel predates the fix, treat it as potentially vulnerable until the vendor backports the patch.
- Use binary/component scanning.
- Where possible, scan kernel modules and kernels for strings, symbols, or file-level indicators that match the vulnerable code or the fixed commit. This is faster than building from source but less definitive.
- Leverage SBOMs, CSAF, and VEX attestations where available. Microsoft is publishing CSAF/VEX attestations for Azure Linux and expanding that program.
- Confirm via vendor advisories.
- Monitor both Microsoft’s MSRC advisories and the downstream distro advisories for the vendors that supply kernels used in your environment.
- When Microsoft updates the CVE to list further affected products, use that as confirmation for those products.
- Apply mitigations and updates.
- If you find an affected artifact, apply the vendor-supplied kernel update or backport as soon as it is available and tested.
- For environments where an immediate patch is not possible, consider temporary mitigations described below.
Recommended mitigations for operators
If you confirm a vulnerable kernel or cannot conclusively determine the status, apply a defense-in-depth set of mitigations:
- Patch promptly: install vendor-issued kernel updates or apply the upstream patch via a backport where vendor support permits.
- Limit attack surface: restrict which users, processes, and containers can access MDIO ioctl interfaces. If MDIO operations are only necessary for a small administrative class of tools, restrict ioctl access using filesystem permissions, namespaces, or policy tools.
- Use kernel hardening: enable KASLR, Kernel Page-Table Isolation (if applicable for the workload), and other runtime defenses that make exploitation of memory corruption harder.
- Monitor for suspicious MDIO activity: log and alert on invocations of mdio-tools and other utilities that perform MDIO ioctls from non-standard contexts.
- Apply least privilege in onboarding images and containers: avoid shipping unnecessary management utilities that expose low-level hardware ioctl surfaces to broad container runtimes.
- Isolate management functions: perform PHY management tasks on dedicated, hardened management hosts rather than on multi-tenant general-purpose hosts.
- Test vendor attestations: when Microsoft publishes a CSAF/VEX attestation for a product you use, incorporate that VEX into your automated triage pipeline and cross-check it with your inventory.
Practical recommendations for Microsoft customers (Azure, WSL, Marketplace users)
- Azure Linux customers
- Treat Microsoft’s attestation as a definitive indicator that Azure Linux images were inspected and may be affected. Apply the provided kernel updates or follow Microsoft’s recommended remediation timeline.
- For managed Azure services that rely on Azure Linux images, consult the service bulletin and apply updates via the service console or follow the provider’s remediation plan.
- WSL2 users
- Check the WSL2 kernel build version on your systems. Microsoft maintains WSL2 kernel source trees and publishes kernel artifacts; determine whether the particular WSL kernel includes the vulnerable commit and update to a patched kernel where required.
- For distributed developer desktops, roll out WSL kernel updates through your typical desktop update channels.
- Marketplace image consumers
- Inspect Marketplace images’ kernel versions and build provenance. Marketplace images are often quick to receive security updates, but you must validate the image SKU and timestamp.
- Enterprise inventories
- Integrate VEX/CSAF outputs into your SBOM and vulnerability management system. Microsoft’s phased attestation program should be treated as an evolving data source — integrate updates automatically and re-run triage when new attestations arrive.
Risk analysis — what defenders should worry about and what they should not
What to worry about
- Local attackers with low-barrier ability to call MDIO ioctls on affected systems can attempt to induce out-of-bounds memory accesses.
- Systems that expose PHY management to multi-tenant workloads or containers create a broader risk footprint.
- Delayed patching is the most common operational failure that turns a medium‑risk kernel correctness bug into a significant real-world incident.
What not to overstate
- This is not a remote code execution worm‑style flaw that can be triggered across the network without local privileges. The exploit vector requires MDIO ioctl access or the ability to run code locally that issues such ioctls.
- Not every server or VM is exposed: many cloud images and virtual NICs do not provide direct MDIO ioctl access in a way that userland can exploit.
Unverifiable or uncertain areas
- Whether a given Microsoft product not yet listed by MSRC contains the vulnerable code often cannot be verified remotely without vendor-provided SBOM or VEX information. Operators should treat such cases with conservative assumptions and inventory evidence.
- There is sometimes ambiguity in downstream CVE numbering and mapping (some vendors listed the issue under CVE-2025-38110 while others referenced a sibling identifier); mapping vendor advisories to the exact upstream commit is the most reliable way to determine exposure.
Why vendor attestations matter — CSAF/VEX in practice
Microsoft’s rollout of CSAF/VEX attestations changes operational workflows by providing machine‑readable statements about which products were inspected and what the vendor found. The benefits are tangible:
- Automated triage: SOCs and vulnerability management systems can automatically determine that an attested product is affected and trigger remediation playbooks.
- Reduced noise: users can avoid unnecessary escalations when a product is attested as not shipping the vulnerable component.
- Auditability: attestations provide a machine‑readable audit trail for what the vendor inspected and when.
However, attestations are only as useful as the inventory completeness behind them. A phased VEX program is better than no VEX program, but defenders must account for the time gap between attestations and the possibility that other artifacts — especially vendor-supplied kernels in different product families — remain unchecked until Microsoft completes its inventory work.
Checklist for IT and security teams (actionable next steps)
- Inventory: capture kernel versions for all Microsoft-supplied artifacts (Azure Linux images, WSL kernels, Marketplace images, linux-azure).
- Map: map your kernel versions to the upstream commit range that fixed the net/mdiobus C45 bounds-check issue.
- Patch: apply vendor-supplied kernel updates where available; schedule emergency patching for affected production hosts.
- Restrict: reduce MDIO ioctl exposure by restricting tooling and access to management hosts.
- Monitor: log mdio-tools usage and set alerts for MDIO ioctl invocations from unexpected contexts.
- Integrate: ingest Microsoft’s CSAF/VEX attestations into your vulnerability management pipeline and re-triage assets when new attestations appear.
- Validate: for critical workloads, perform targeted testing (in staging) to ensure the patched kernel eliminates the vulnerable ioctl path.
Conclusion
CVE-2025-38110 (net/mdiobus Clause 45 bounds-check issue) is a classic kernel correctness vulnerability that was fixed upstream by adding a simple but essential address verification step before C45 read/write operations. Microsoft’s MSRC advisory and CSAF/VEX attestation for
Azure Linux is an important, actionable signal: Azure Linux artifacts were inspected and may be affected. However, this attestation is product-scoped and phased — it does not prove other Microsoft kernel artifacts are free of the code in question. Administrators must therefore combine vendor attestations with their own inventory, version mapping, and mitigation steps. Patch where possible, harden MDIO access, and treat un‑attested Microsoft artifacts as “not yet confirmed” until proven otherwise. The combination of vendor automation (CSAF/VEX) and conservative operator practices will be the most effective defense against this class of kernel memory-corruption exposures.
Source: MSRC
Security Update Guide - Microsoft Security Response Center