CVE-2025-38495: Azure Linux HID Code and CSAF VEX Attestations

Microsoft’s short advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate — but it is a product‑scoped attestation, not proof that Azure Linux is the only Microsoft product that could carry the vulnerable HID kernel code.

Background / Overview​

CVE‑2025‑38495 is an upstream Linux kernel bug in the HID (Human Interface Device) core: a report‑buffer accounting bug where code that assumed an implicit report ID byte could end up with a buffer that was one byte too small, reducing an eight‑byte guarantee to seven and causing out‑of‑bounds behavior in some device report paths. Upstream kernel maintainers committed fixes into the stable kernel trees and multiple downstream distributions have mapped those fixes into their packaged kernels. Microsoft’s Security Response Center (MSRC) entry for this CVE contains a short, explicit line that has drawn attention: it says Microsoft has found the implicated open‑source component inside the Azure Linux product family and therefore lists Azure Linux as “potentially affected.” The company also states it began publishing machine‑readable CSAF/VEX attestations in a phased rollout and will update the CVE mapping if additional Microsoft products are found to carry the same component. That is a deliberate transparency and inventorying message. The practical question security teams ask after reading that Microsoft sentence is straightforward: does that sentence mean only Azure Linux is affected across Microsoft’s product portfolio? The short, actionable answer is: No — Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the vulnerable HID code, but that attestation is not a guarantee of exclusivity.

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Why Microsoft worded it this way (CSAF/VEX context)​

Microsoft is rolling out machine‑readable vulnerability attestations (CSAF/VEX) as a way to tell customers, in an automated format, which of Microsoft’s product artifacts include a given upstream open‑source component and whether Microsoft has validated that artifact as affected, fixed, or not affected.

• This is a product‑level inventory attestation: it documents what Microsoft has inspected and validated for a named product family (Azure Linux).
• The attestation is authoritative for the named product: customers running Azure Linux images should treat Microsoft’s VEX/CSAF output as the canonical signal and follow Microsoft’s remediation guidance for those images.
• The attestation is phased, not exhaustive: Microsoft explicitly committed to expand the attestations as it inventories additional Microsoft artifacts. Absence of a product in Microsoft’s VEX feed is absence of attestation, not evidence that the product is safe.

This model is valuable because it provides deterministic data feeds for automation. It is also conservative: vendors typically start with a bounded product set they can inventory reliably (Azure Linux is a natural first candidate because Microsoft builds, packages and publishes those images). However, the real world of large vendors is messy: they publish many different artifacts, kernels, and images, and any of those could, in principle, contain the same upstream code until proven otherwise by inventory or inspection.

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What “includes this open‑source library” actually means technically​

When MSRC says a Microsoft product “includes this open‑source library” for a kernel CVE, the statement denotes that:

• Microsoft inspected the specific kernel binary, package, or image that makes up that product family.
• The inspected artifact contains the upstream source code, object, or compiled module that corresponds to the vulnerability (in this case the HID core code path described by CVE‑2025‑38495).
• Microsoft therefore treats that product family as in‑scope for remediation until a fixed artifact is published and the VEX/CSAF entry is updated.

Crucially, the presence or absence of a kernel bug is an artifact‑level property: it depends on the kernel version, which commits were included, and the kernel configuration flags (CONFIG_*) used when the binary was built. Two kernel binaries built from the same upstream tree can differ widely in module inclusion and in which drivers are compiled in or omitted. That makes per‑artifact verification necessary.

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Are other Microsoft products likely to include the same vulnerable HID code?​

Short answer: yes — it is plausible that other Microsoft artifacts could include the vulnerable code; Microsoft has only attested Azure Linux so far. The practical reasons:

• Microsoft ships multiple distinct Linux kernel artifacts:
• Azure Linux (the attested product family).
• linux‑azure / Azure‑tuned kernels used for some VM SKUs and Marketplace images.
• WSL2 (Windows Subsystem for Linux) — Microsoft publishes and ships a Microsoft‑built Linux kernel for WSL2; that kernel receives independent updates and may include a wide range of drivers. If the WSL2 kernel build predates the upstream fix or contains the driver in question, WSL2 instances could be affected.
• CBL‑Mariner lineage and other Microsoft Linux artifacts used internally or in various managed services (AKS nodes, host images).
• Azure Marketplace images, partner appliances, and container/VM images that Microsoft distributes or curates may include kernels or binaries that carry the affected code.
• Presence depends on kernel version and CONFIG flags: e.g., a cloud‑focused x86 kernel may omit certain HID drivers present in SoC or desktop kernels, and vice versa. That means some Microsoft artifacts will be unaffected simply because the driver or code path is not compiled into that kernel; others may be affected.
• Microsoft’s VEX rollout is phased: the company’s statement that Azure Linux “includes this open‑source library and is therefore potentially affected” is the result of the Azure Linux inventory. Microsoft committed to update the CVE/VEX entry if other Microsoft products are found to ship the component, which implies more attestations may follow.

In short: Azure Linux is currently the only Microsoft product publicly attested to include the HID code for CVE‑2025‑38495, but other Microsoft‑distributed kernels (WSL2, linux‑azure images, Marketplace images, AKS node images) remain plausible carriers until Microsoft either attests them or you inspect the artifact yourself.

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Independent confirmation and downstream evidence​

Multiple independent sources confirm the technical nature of the vulnerability and show broad downstream vendor activity mapping the fix into packaged kernels:

• The NVD entry for CVE‑2025‑38495 reproduces the upstream summary and the vulnerability description.
• Major Linux distributions (Ubuntu, SUSE, Oracle, Amazon Linux, etc. have published advisories or package mapping entries for the CVE and the upstream kernel commits; these distributors have shipped patched kernels in downstream channels.
• Upstream kernel commits and stable‑tree fixes are available in the kernel.org stable commit history; downstream trackers reference those commits as the authoritative fixpoint for packaging.

These independent confirmations matter because they let operations teams correlate the specific upstream commit IDs with distribution kernel versions and artifacts, which is necessary when verifying whether a particular image or host is fixed.

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Practical checklist: how to verify whether a Microsoft artifact you run is affected​

Operators should not rely on the presence or absence of a single VEX attestation to conclude safety. Instead use this artifact‑level checklist.

• Identify the Microsoft artifacts you run or inherit:
• Azure VM images (specify image SKU and publisher).
• Marketplace images and appliances.
• AKS node images or node pools.
• Windows hosts running WSL2 (note: WSL2 has a Microsoft‑shipped kernel).
• Any Microsoft‑provided or partner‑provided appliance or container image used in your environment.
• For each running host or image, determine the kernel version and build:
• On the host: run uname -r and inspect /boot/config‑$(uname -r) or /proc/config.gz to check kernel config options.
• For offline images, mount the image and inspect the kernel binary and the embedded config.
• Map the kernel version / commit to upstream fixes:
• Look up the upstream commit(s) that fixed CVE‑2025‑38495 in kernel stable trees.
• Check your kernel package changelog for those commit IDs or the CVE string. Distribution advisories map commit IDs to package updates.
• Confirm whether the HID subsystem code is present or compiled in:
• Search the kernel config for HID drivers or for the specific code paths carved up by the upstream patch.
• If the driver is compiled as a module, check whether that module exists under /lib/modules/$(uname -r)/ and whether it is loaded.
• If the artifact is Microsoft‑published (WSL2, linux‑azure, Azure Linux), check Microsoft’s CSAF/VEX feed for that CVE to see whether Microsoft has published attestations or remediations for that artifact. If not present, treat it as unverified and follow steps 1–4.
• Prioritize remediation for artifacts Microsoft has attested as affected (Azure Linux) and for artifacts you confirm to be vulnerable by version/config mapping. Apply vendor fixes or update to fixed kernel packages and plan reboots as required.

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Recommended remediation and mitigation steps​

• For Azure Linux customers: follow Microsoft’s remediation guidance, ingest Microsoft’s CSAF/VEX attestation for the CVE and apply the updated Azure Linux image/kernel releases Microsoft publishes. Microsoft’s attestation is authoritative for Azure Linux and should be prioritized.
• For WSL2 users: check the WSL2 kernel version in your Windows environment or any custom WSL kernel you deploy; update WSL via Microsoft’s documented update flow or replace custom kernels with patched builds where necessary.
• For Marketplace images, AKS nodes, and partner appliances: verify the kernel version and kernel config in each image and apply vendor updates or rebuild images with patched kernels.
• If immediate kernel update is impractical: apply compensating controls to reduce exploitation likelihood where possible:
• Restrict access to local device interfaces that must be used to trigger HID inputs.
• Limit untrusted user or container access to device nodes (/dev/hidraw*, USB passthrough).
• Tighten host access controls and monitoring for anomalous activity that exercises HID device paths.

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Strengths and risks of Microsoft’s CSAF/VEX‑first approach​

Strengths

• Authoritative automation signal: a VEX/CSAF attestation is machine‑readable and simplifies automated triage for customers who run the attested product. Azure Linux customers benefit immediately from that signal.
• Pragmatic rollout: starting with a bounded product family (Azure Linux) lets Microsoft produce high‑quality attestations quickly rather than delivering low‑quality, incomplete coverage.

Risks / Limitations

• Perception risk: some operators may misread the attestation as a blanket statement covering all Microsoft artifacts. That is incorrect — absence of a product in the VEX feed means it has not yet been attested, not that it is safe.
• Artifact heterogeneity: Microsoft ships many kernels and images. Each artifact must be checked separately because kernel builds vary by version and configuration. Relying solely on VEX results without artifact inspection risks leaving unverified, potentially vulnerable artifacts in production.
• Operational coverage: VEX simplifies automation for products the vendor attests but does not eliminate the need for customers to perform local inventory and verification across their estate (especially for mixed‑source images and third‑party Marketplace offerings).

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What cannot be verified from public statements (cautionary notes)​

• It is not possible from Microsoft’s single Azure Linux attestation to prove that no other Microsoft artifact contains the vulnerable HID code. Until Microsoft publishes attestations for additional artifacts, or until you inspect each artifact, the status of other Microsoft products is unknown. This is a factual limitation of phased attestations and of supply‑chain coverage.
• Public vulnerability databases and distro advisories can show where the upstream fix landed and which vendor kernel packages have been updated, but determining whether a specific Microsoft image in your environment is vulnerable requires the artifact‑level checks described above.

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Final takeaways for IT and security teams​

• Treat Microsoft’s Azure Linux attestation as an authoritative, actionable signal: prioritize patching Azure Linux images and kernels Microsoft has listed as affected.
• Do not assume the attestation implies exclusivity: verify other Microsoft‑supplied artifacts used in your estate (WSL2, linux‑azure kernels, AKS node images, Marketplace images). Each is an independent artifact that may or may not contain the vulnerability depending on kernel version and configuration.
• Use the practical checklist above to confirm exposure for each artifact and follow vendor fixes where available; if immediate updates are impossible, implement compensating controls to reduce exposure.
• Appreciate that Microsoft’s move to publish CSAF/VEX attestations increases transparency and automation for plaintiffs and defenders alike, but the rollout is phased — expect more product attestations over time and maintain artifact‑level verification as a standard practice.

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CVE‑2025‑38495 is a kernel robustness bug with a straightforward upstream fix; Microsoft’s MSRC attestation that Azure Linux includes the implicated code is correct and useful for Azure Linux customers. It is not, by itself, a statement that only Azure Linux could be affected. Any Microsoft artifact that ships or runs a Linux kernel build should be inventoried and verified until Microsoft’s VEX/CSAF feed or your own inspection proves the artifact is not vulnerable.

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Source: MSRC Security Update Guide - Microsoft Security Response Center