CVE-2025-38073: Azure Linux Attestations and Microsoft Product Scope

Microsoft’s short public advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is correct as a product‑level statement — but it is not a categorical guarantee that no other Microsoft product can include the same vulnerable Linux kernel code.

Background / Overview​

CVE‑2025‑38073 is a Linux kernel race condition that arises when the kernel’s block layer changes the blocksize (set_blocksize) concurrently with readers on the same device. The bug can let the kernel submit IO with a NULL block device reference and crash the host under the right interleaving of operations. Upstream kernel maintainers issued targeted stable-tree patches to truncate the page cache, enforce ordering around i_blksize changes, and take the appropriate locks during updates. The vulnerability was cataloged publicly on 18 June 2025 and is tracked in NVD and multiple vendor advisories; its practical impact is primarily availability (kernel crash / Oops) rather than an immediate remote code execution vector, and the exploitability profile is local and timing‑dependent. Upstream fixes landed in multiple stable kernel branches and distributors published backports for the kernel branches they ship. The Open Source Vulnerabilities (OSV) tracker and distribution advisories show the ranges and the stable fixes that operators should target. Why this matters operationally: the kernel is a shared, privileged component. Any product or image that ships a kernel built from an upstream commit range that predates the fixes — and that includes the affected block code in the kernel build — may be vulnerable. Identifying exposure therefore requires two things: (1) knowing which kernel commits / versions are vulnerable, and (2) knowing which Microsoft artifact you actually run and which kernel binary that artifact contains.

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What Microsoft actually published and what that wording means​

Microsoft’s public guidance for CVE‑2025‑38073 (and a growing set of third‑party CVE mappings) follows a pragmatic, product‑scoped disclosure model: the company began publishing machine‑readable CSAF/VEX attestations in October 2025 and has started the rollout with the Azure Linux distribution family. For the CVE in question Microsoft’s advisory explicitly states that Azure Linux includes the upstream component and is therefore potentially affected, and Microsoft pledged to update the CVE and its VEX/CSAF attestations if additional Microsoft products are found to ship the same upstream code. Put plainly:

• The sentence about Azure Linux is an attestation of inventory — Microsoft inspected that product family, found the upstream kernel component, and published a machine‑readable status for it.
• The statement does not assert that other Microsoft products are definitely clean. Absence of a VEX/CSAF attestation for a different Microsoft product is absence of evidence, not evidence of absence. Microsoft’s VEX program is phased and product‑by‑product.

This distinction matters for defenders and automation: the VEX/CSAF file for Azure Linux is authoritative for Azure Linux images and will let automation quickly triage, but customers running other Microsoft artifacts must still verify those artifacts specifically until Microsoft publishes attestations for them.

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

Short answer: No — not necessarily. Azure Linux is the only Microsoft product that Microsoft has publicly attested (so far) as including the affected upstream kernel component for CVE‑2025‑38073; however, other Microsoft artifacts could also include the same kernel code depending on the kernel version and build configuration those artifacts use.
Why that answer is the correct operational reading:

• Microsoft ships multiple, distinct Linux kernel artifacts and Linux‑based images that are produced in separate build pipelines: the Azure Linux distribution lineage (formerly CBL‑Mariner), WSL2 kernel builds used by Windows Subsystem for Linux, linux‑azure kernels or tuned kernels used for some Azure VM families, and curated Marketplace images / container host kernels. Each artifact is a different build and may include or omit kernel subsystems based on configuration flags (CONFIG_*) and packaging choices. If any of those artifacts used an upstream commit range that included the vulnerable code and compiled it in, they are potential carriers.
• Microsoft’s VEX/CSAF rollout explicitly started with Azure Linux as the first target. The company’s blog explains the phased rollout approach and why starting with a single product family is a pragmatic way to publish accurate, machine‑readable attestations. Microsoft has said it will expand attestations to additional products as inventory work completes. That is a process statement, not a security guarantee.

Concretely: Azure Linux is the only Microsoft product explicitly named in Microsoft’s VEX/CSAF attestation for this class of third‑party CVEs so far; the attestation is authoritative for Azure Linux only. Any other Microsoft‑shipped kernel or image must be checked independently or else waited on until Microsoft publishes a formal VEX attestation for it.

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Examples of Microsoft artifacts you should check​

• Azure Linux (CBL‑Mariner lineage / azl) — explicitly attested; treat as in‑scope and apply Microsoft patches as available.
• Windows Subsystem for Linux (WSL2) kernel — Microsoft maintains and ships a WSL2 kernel (open source on GitHub); if your Windows host uses a WSL kernel build that predates the fix or a custom kernel built from vulnerable versions, WSL instances may be vulnerable. Check the WSL kernel release/tag and update via the Microsoft update channels or by using the updated kernel from the microsoft/WSL2‑Linux‑Kernel repo.
• linux‑azure / Azure VM kernel builds and Marketplace images — certain VM images and Azure host artifacts carry Microsoft‑built kernel binaries; each image is an independent artifact and must be inventoried.
• Azure Kubernetes Service (AKS) node images and curated container host images — images may use a particular host OS/kernel; if the node OS is built from an affected kernel branch, nodes may be impacted.
• Partner Marketplace appliances and third‑party images published in Azure Marketplace — partners sometimes ship appliances with older kernels; these are not automatically covered by Microsoft’s product attestations unless the partner is included in the VEX mapping.

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How to verify whether a given Microsoft artifact in your estate is vulnerable​

1. Identify the artifact and the kernel it runs.
   • For a Linux VM: run uname -r and confirm the package/kernel package version your image uses.
   • For WSL2 on Windows: check the WSL kernel version with uname -r inside the WSL distro or examine the WSL kernel tag shipped by your Windows package. Microsoft publishes the WSL kernel source and release tags on GitHub.
2. Map the kernel version to the upstream vulnerable ranges and stable fixes.
   • OSV, NVD, and vendor advisories list affected version ranges and the fixed stable releases for CVE‑2025‑38073; use those authoritative ranges to decide if your kernel version is vulnerable or already contains the patch.
3. Check kernel configuration and modules.
   • Some kernel subsystems can be compiled out or built as modules. If the block code in question was not built into the kernel for that artifact, the artifact may not be vulnerable even if the kernel tree had the commit. Query /boot/config-$(uname -r) or the kernel config available in the image to confirm CONFIG_* flags relevant to the block layer. (Common commands: uname -r; zcat /proc/config.gz; grep -i block /boot/config-$(uname -r).
4. Consult Microsoft’s VEX/CSAF data and product attestations.
   • Where Microsoft has published CSAF/VEX attestations for a product, those attestations are authoritative for the mapped artifacts. Use Microsoft’s machine‑readable VEX data to speed triage for Azure Linux and any other Microsoft products that become attested later.
5. When in doubt, assume exposure for any artifact that ships a kernel built from an affected version and schedule patching or constrained mitigation until the artifact is confirmed fixed.

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Step‑by‑step remediation guidance (prioritized)​

1. Patch and reboot Azure Linux images immediately where Microsoft has marked the product as affected and a fix is available; apply the vendor kernel update and reboot hosts to ensure the updated kernel is running. Microsoft’s VEX/CSAF and the Linux distribution advisories show Azure Linux status and updates.
2. Inventory WSL2 hosts and update:
   • If WSL2 uses a Microsoft‑supplied kernel release, ensure Windows Update / WSL updates have been applied.
   • If users run custom WSL kernels, rebuild or obtain kernel binaries from patched upstream stable branches or Microsoft’s WSL kernel tree.
3. Patch Azure VM images and AKS nodes where the node image kernel falls in the vulnerable ranges. For managed node pools, consult your AKS update guidance for node image updates or node replacement workflows.
4. For appliances and Marketplace images, contact the image publisher if they are not Microsoft and request a security update; if the appliance runs on top of Microsoft host kernels, coordinate with Azure support for host patching or image replacement.
5. If immediate patching is not possible:
   • Restrict local access to hosts that expose block device management operations.
   • Isolate high‑risk multi‑tenant workloads and apply additional monitoring for kernel Oops and block IO errors.
   • Maintain logs and trace evidence for any kernel Oops to support incident response.
6. Automate inventory and VEX ingestion:
   • Integrate Microsoft’s CSAF/VEX feeds into your vulnerability management tooling where possible. That will speed triage for Azure Linux and, when Microsoft adds attestations for additional products, remove guesswork.

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Technical and programmatic strengths — what Microsoft’s approach does well​

• Machine‑readable VEX/CSAF attestations reduce noise and speed automation. By publishing attestations for Azure Linux first, Microsoft provides deterministic signals that automation can consume to triage which assets are impacted without manual mapping. This is a real operational win for organizations that adopt the VEX files.
• Upstream and distributor fixes exist and have been mapped. The kernel community and major distributors have merged and backported the fix; OSV/NVD and vendor advisories list the fixed stable branches and package advisories. That means tactical remediation is already available for updated kernel binaries.

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Risks, limitations, and things defenders must watch for​

• Phased attestations leave an interval of “unknown” for other Microsoft artifacts. The VEX rollout is product‑by‑product; until Microsoft publishes attestations for WSL, linux‑azure, Marketplace images, and other artifacts, those areas remain procedural blind spots that must be checked by operators. Absence of a VEX entry is not proof of absence.
• Different build pipelines → different exposures. A kernel feature can be present in one build and absent in another. That means you cannot infer safety solely from vendor statements that reference a single product; you must map the actual artifact binary your estate runs.
• Complex supply chains in Marketplace and partner appliances. Marketplace images and third‑party appliances may be built by partners and may not be covered by Microsoft’s attestations; these must be inventoried separately.
• Local, timing‑dependent vector still matters in multi‑tenant environments. Even though remote‑exploitation is not the primary scenario, multi‑tenant platforms and services where untrusted tenants can trigger block operations are higher risk; operators should err on the side of remediation and isolation.

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Practical checklist for WindowsForum readers and operators​

• Prioritize patching of Azure Linux images where Microsoft attests the product as affected.
• Inventory every place you run a Microsoft‑shipped kernel: WSL installations, Azure VM images, AKS nodes, Marketplace images, and any custom images derived from Microsoft artifacts.
• Map kernel versions to upstream affected ranges (use NVD/OSV or vendor advisories) and apply vendor or upstream fixes.
• If you find an unpatched Microsoft artifact in your estate and cannot patch immediately, triage with isolation, access controls, and enhanced monitoring for kernel Oops or block device errors.
• Subscribe to Microsoft’s CSAF/VEX feed for Azure Linux and watch for additional attestations expanding to other products.

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Conclusion​

Microsoft’s public advisory and VEX/CSAF attestations are a significant step toward clearer, machine‑readable vulnerability mapping for complex third‑party components. For CVE‑2025‑38073 the company has attested that Azure Linux includes the vulnerable upstream kernel component and is therefore potentially affected — that attestation is authoritative and actionable for Azure Linux customers. However, the attestation’s product scope is deliberately narrow and phased. It does not prove that other Microsoft artifacts (notably the WSL2 kernel builds, linux‑azure kernels, AKS images, or Marketplace appliances) are free of the same upstream kernel code. Those artifacts should be inventoried and validated against the upstream affected ranges and vendor fixes; defenders should assume possible exposure until a specific artifact is shown to be patched or attested as Not Affected.
Actions to take now: patch Azure Linux promptly, inventory and verify other Microsoft kernel artifacts in your environment (including WSL), map kernel versions to the upstream fixed releases, and ingest Microsoft’s VEX/CSAF outputs as they arrive to reduce triage time. The transparency improvements are meaningful; operational diligence is still required to ensure your entire estate — not just the product families Microsoft has started with — is covered.

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Source: MSRC Security Update Guide - Microsoft Security Response Center