Microsoft’s brief advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” captures an important operational fact — Microsoft has inventory‑checked and attested Azure Linux for the HID s32ton issue tracked as CVE‑2025‑38556 — but it does not, and cannot, be read as a categorical guarantee that no other Microsoft product includes the same vulnerable code.
CVE‑2025‑38556 is a Linux kernel vulnerability in the HID core where the function s32ton could be invoked with a 0‑bit width and trigger a shift‑out‑of‑bounds condition. The bug was discovered by syzbot fuzzing and addressed upstream by hardening s32ton so it returns a reasonable value instead of performing an invalid conversion. The fix has been merged into the stable kernel branches and has been propagated by multiple distributors. Microsoft’s MSRC entry for the CVE states that Azure Linux (the Microsoft‑maintained Linux distribution lineage used for Azure images) “includes this open‑source library and is therefore potentially affected.” Microsoft also noted it began publishing machine‑readable CSAF/VEX attestations in October 2025 and committed to updating CVE mappings if additional Microsoft products are identified as carriers of the same open‑source component. That language is an attestation of inventory scope — a declaration about what Microsoft has already checked — not an engineered proof that every other Microsoft SKU is free from the component.
Two important clarifications follow from that wording:
However, the procedural nature of the attestation introduces risk if readers misinterpret the wording as a universal exclusion. The wording “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate — but it is an inventory declaration for that product family alone. The pragmatic reality is that any Microsoft product that ships or runs a Linux kernel built from upstream trees predating the fix could also carry the vulnerable code until checked. Security teams must therefore treat the attestation as a positive signal for Azure Linux and as a reminder to perform artifact‑level inventories elsewhere.
Operationally, the most common gap I see in customer responses to vendor attestations is assuming the attestation covers all vendor artifacts. To avoid that mistake:
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background
CVE‑2025‑38556 is a Linux kernel vulnerability in the HID core where the function s32ton could be invoked with a 0‑bit width and trigger a shift‑out‑of‑bounds condition. The bug was discovered by syzbot fuzzing and addressed upstream by hardening s32ton so it returns a reasonable value instead of performing an invalid conversion. The fix has been merged into the stable kernel branches and has been propagated by multiple distributors. Microsoft’s MSRC entry for the CVE states that Azure Linux (the Microsoft‑maintained Linux distribution lineage used for Azure images) “includes this open‑source library and is therefore potentially affected.” Microsoft also noted it began publishing machine‑readable CSAF/VEX attestations in October 2025 and committed to updating CVE mappings if additional Microsoft products are identified as carriers of the same open‑source component. That language is an attestation of inventory scope — a declaration about what Microsoft has already checked — not an engineered proof that every other Microsoft SKU is free from the component. What CVE‑2025‑38556 actually is
Technical summary
- The defect is in the Linux kernel HID core (Human Interface Device subsystem).
- The vulnerable code path allows conversion of a 32‑bit quantity to a 0‑bit width (an illegal shift), producing an out‑of‑bounds read or kernel oops under certain device‑report layouts.
- The symptom is a robustness/availability failure (kernel crash or oops) rather than a straightforward remote code execution vector.
- Upstream maintainers repaired the routine by validating the bit width or returning a safe result when conversion to 0 bits is attempted.
Scope and distribution mapping
Major distributors and trackers (NVD, Ubuntu, Debian, Amazon Linux, Red Hat advisories, and vendor databases) have cataloged the issue and mapped fixed package/kernel versions. The vulnerability affects multiple kernel series depending on the inclusion of the vulnerable HID code, and distributors have backported or released patched kernel packages as appropriate. This makes the patch path straightforward for most Linux users: update to a kernel version that contains the upstream commits.What Microsoft said — and how to read it
Microsoft’s short advisory sentence — that Azure Linux “includes this open‑source library and is therefore potentially affected” — is a product‑level attestation arising from Microsoft’s CSAF/VEX rollout (the company began publishing these machine‑readable attestations in October 2025). That attestation confirms Microsoft performed an inventory for Azure Linux and found the implicated upstream code inside that product family; it further commits to updating the CVE/VEX entry if further Microsoft products are discovered to ship the same component.Two important clarifications follow from that wording:
- Attestation = authoritative for the named product only. The VEX/CSAF entry is an authoritative, machine‑readable signal for Azure Linux customers; treat it as the vendor‑proven fact for that product family.
- Absence of attestations ≠ proof of absence. Microsoft’s phased VEX rollout means other products may simply not yet have been inventory‑checked or attested. The company explicitly said it will update the CVE mapping if additional Microsoft products are identified as carriers. Therefore, you cannot use the lack of a second attestation to conclude other Microsoft artifacts are safe.
Is Azure Linux the only Microsoft product that includes the vulnerable library?
Short answer (journalistic bottom line)
No — Azure Linux is the only Microsoft product Microsoft has publicly attested to include the vulnerable HID code for CVE‑2025‑38556 so far, but that is an inventory statement, not a guarantee that other Microsoft products do not include the same kernel code. Any Microsoft product or artifact that ships or runs a Linux kernel build could include the vulnerable HID code until that artifact is verified or patched.Why that nuance matters (technical reasoning)
- Kernel code is a per‑artifact property. Whether the vulnerable HID code is present depends on the kernel version, configuration, and which modules were compiled in. A single codebase can produce many different kernel binaries with different drivers enabled or disabled. Microsoft ships multiple, distinct Linux kernel artifacts across its product lines.
- Multiple Microsoft surfaces run or ship kernels. Examples:
- Windows Subsystem for Linux (WSL2) ships a Microsoft‑built Linux kernel and updates it independently of Windows releases.
- Azure VM images, marketplace images, and AKS node images may use Azure‑tuned kernels or vendor kernels.
- Azure Linux (CBL‑Mariner lineage) is one Microsoft product family; others (WSL kernels, marketplace appliances, container images, curated agent binaries) are separate artifacts that require their own inventory.
- Static & embedded binaries complicate remediation. Some userland components or agents are statically compiled (for example, Go static builds) and embed runtime code that will not be fixed by updating the system kernel package. While CVE‑2025‑38556 is a kernel HID issue and therefore primarily addressed by updating kernels, this general point matters for other CVEs: per‑artifact verification is required.
Plausible Microsoft artifacts beyond Azure Linux to check
- WSL2 kernel binaries distributed by Microsoft to Windows users.
- Azure‑tuned kernels used for platform VMs and host images (linux‑azure).
- Azure Marketplace VM images and partner appliances (built by Microsoft or third‑parties).
- AKS node images and managed node pools.
- Any Microsoft‑distributed appliance, container image, or virtual appliance that includes a Linux kernel binary.
Verifying presence and assessing exposure — practical checklist
The following steps will let operations and security teams confirm whether a Microsoft product or artifact in their estate includes the vulnerable HID code and what remediation is required.- Inventory and identify all Linux kernel images you run:
- Check WSL2 kernel version on Windows hosts (wsl --status and /proc/version inside WSL).
- List kernels used by Azure VM images and AKS node pools.
- Identify any marketplace or partner VM images and containers in use.
- Determine kernel versions and module set:
- On a running host, inspect uname -r and compare to distributor advisories mapping fixed versions. For container or image artifacts, extract the kernel package metadata or SBOM where available.
- Cross‑check vendor/distro advisories:
- Use NVD, Ubuntu, Debian, Red Hat, Amazon Linux advisories to find which kernel releases include the fix. These trackers show the fixed kernel commits and package versions.
- Patch or replace:
- Apply the vendor/repository kernel updates that include the upstream fix.
- For managed cloud images, select updated platform images or request updated marketplace images from vendors.
- For immutable artifacts:
- Rebuild images that embed kernel trees or static kernels; redeploy patched images.
- Automate detection:
- Integrate Microsoft’s CSAF/VEX feed into your vulnerability management pipeline to get deterministic product‑level signals for Azure Linux and future Microsoft attestation expansions.
Why Microsoft’s CSAF/VEX rollout is a meaningful improvement — and its limits
Strengths (what Microsoft’s approach gains customers)
- Machine‑readable attestations remove ambiguity about which named product families Microsoft has inspected and whether they ship an implicated component.
- Deterministic automation: Security teams can use VEX/CSAF to feed automated triage workflows so that Azure Linux customers receive a clear, actionable signal.
- Transparency commitment: Microsoft publicly committed to expand these attestations and update CVE mappings as inventory work finds more affected artifacts, which is a measurable operational improvement in supply‑chain vulnerability communication.
Limitations and risks (what VEX/CSAF does not solve)
- Phased rollout = partial coverage. Starting with Azure Linux is pragmatic but means many Microsoft SKUs remain un‑attested until inventory work completes. Absence of an attestation is not a finding of safety.
- Per‑artifact complexity. The problem is not just “which product name” but “which build artifact.” Kernel config variability and static linking mean attested products do not exhaust the surface.
- Third‑party and marketplace images. Partner images and Marketplace appliances often ship with third‑party build choices and are the responsibility of their publishers; Microsoft’s attestation does not retroactively cover them unless explicitly stated.
- Operational blind spots. Long‑tail embedded systems, edge images, and custom images may never be included in VEX attestations and require internal SBOM and artifact verification to cover.
Cross‑verification — independent sources confirm the technical facts
Multiple independent vulnerability databases document the same description and remediation mapping for CVE‑2025‑38556. NVD summarizes the issue and links to upstream kernel commits; Debian and Ubuntu trackers show which kernel packages and releases are vulnerable or fixed; vendor advisories (Amazon Linux, Oracle Linux, Red Hat) published their own mappings and patch guidance. These independent records corroborate the core technical claim that the defect is a HID‑core robustness issue addressed by kernel patches. If an operator needs to be absolutely certain whether a given Microsoft image contains the vulnerable code, the authoritative path is simple: check the artifact’s kernel version against distributor advisory tables or request the product’s SBOM/VEX attestation. Microsoft has given Azure Linux customers that deterministic signal already; for anything else, perform the artifact validation described above.Recommended mitigation and operational playbook
- Priority 1 — Patch Azure Linux hosts immediately. If you run Azure Linux images, treat Microsoft’s attestation as definitive evidence to prioritize remediation. Use the patched kernel packages or updated Azure images Microsoft provides.
- Priority 2 — Inventory Microsoft artifacts in your estate. Identify WSL2 hosts, Azure VM SKUs, AKS node pools, Marketplace images, and any Microsoft‑distributed appliances. Validate their kernel versions.
- Priority 3 — Automate VEX consumption. Add Microsoft’s CSAF/VEX feed to your vulnerability management pipeline so that when Microsoft adds further product attestations the signals flow into your triage automation immediately.
- Priority 4 — Harden runtime controls. Where live patching or rapid kernel upgrades are infeasible, deploy compensating controls: limit device exposure to untrusted USB/HID devices, apply kernel lockdown policies where applicable, and isolate workloads that rely on untrusted device input streams.
- Priority 5 — For images or VMs you don’t control, request vendor updates. For Marketplace or partner images, ask the image publisher to confirm whether they ship fixed kernels or to provide an updated image.
- Gather a complete list of all hosts/images (including WSL instances) that run Linux kernels in your environment.
- Match kernel versions to distributor advisories (NVD, Ubuntu, Debian, vendor advisories).
- Patch or replace imagery with updated kernel packages that include the upstream fix.
- Where patches cannot be applied immediately, implement isolation and input filtering for HID devices.
- Subscribe to Microsoft’s CSAF/VEX feed and to your distro vendor advisories for ongoing alerts.
Critical analysis: strengths, gaps, and operational recommendations
Microsoft’s move to publish CSAF/VEX attestations is a positive, measurable step toward transparent supply‑chain vulnerability communications. The practice gives Azure Linux customers a fast, machine‑friendly answer about product scope and enables automated prioritization. That’s the tangible benefit here: deterministic, vendor‑authorized mappings reduce ambiguity during triage.However, the procedural nature of the attestation introduces risk if readers misinterpret the wording as a universal exclusion. The wording “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate — but it is an inventory declaration for that product family alone. The pragmatic reality is that any Microsoft product that ships or runs a Linux kernel built from upstream trees predating the fix could also carry the vulnerable code until checked. Security teams must therefore treat the attestation as a positive signal for Azure Linux and as a reminder to perform artifact‑level inventories elsewhere.
Operationally, the most common gap I see in customer responses to vendor attestations is assuming the attestation covers all vendor artifacts. To avoid that mistake:
- Treat every distinct image or binary as a separate inventory item.
- Demand SBOMs (or VEX responses) for third‑party marketplace images.
- Integrate VEX/CSAF feeds into your SIEM/CMDB so attestations automatically tag assets for follow‑up.
Unverifiable claims and cautionary flags
- Any claim that “only Azure Linux is affected” is not verifiable without Microsoft completing a company‑wide inventory and publishing attestations for every product. Microsoft’s public commitment to update CVE asset mappings means this is a dynamic list and may change as more artifacts are checked. Treat any one‑line vendor statement about a product as scoped unless the vendor explicitly declares comprehensive coverage across all its SKUs.
- Kernel commit metadata and fixed‑version mapping exist in upstream kernel trees and distribution advisories, but access to specific git.kernel.org commits can be inconsistent depending on mirror policies. Rely on distributor advisories (Ubuntu, Debian, Amazon Linux, Red Hat) to match package names and versions rather than attempting to deduce patch status solely from a commit hash.
Conclusion
Microsoft’s advisory for CVE‑2025‑38556 and the Azure Linux CSAF/VEX attestation is useful, transparent, and actionable for Azure Linux customers — it confirms a verified inventory result and should be used to prioritize remediation for those images. However, the attestation does not imply exclusivity: Azure Linux is the only Microsoft product Microsoft has attested to include the affected HID code so far, but other Microsoft artifacts that ship or run Linux kernels remain plausible carriers until they are inventory‑checked or independently verified. Security teams should treat the VEX statement as a strong signal for Azure Linux, perform per‑artifact verification across WSL2, Azure VM images, AKS nodes, marketplace images, and any Microsoft‑supplied appliances, and automate VEX/CSAF consumption into their triage workflows to catch future attestation updates.Source: MSRC Security Update Guide - Microsoft Security Response Center
