Azure Linux CVE-2025-38403: Understanding Microsoft Attestations and Cross Product Risk

Microsoft’s short FAQ answer — that “Azure Linux includes this open‑source library and is therefore potentially affected” — is accurate as a product‑level attestation, but it does not mean Azure Linux is the only Microsoft product that could include the vulnerable code. Microsoft’s published VEX/CSAF attestations started with Azure Linux in October 2025 and explicitly commit to updating CVE records if additional Microsoft products are discovered to ship the same upstream component; until those inventories complete, other Microsoft artifacts remain unknown rather than proven safe.

Background / Overview​

CVE‑2025‑38403 is a Linux‑kernel vulnerability titled “vsock/vmci: Clear the vmci transport packet properly when initializing it.” The bug is a correctness issue in the vsock/vmci transport code where the vmci_transport_packet was not fully cleared during initialization, allowing uninitialised data to remain in the structure. Upstream maintainers addressed the problem by zeroing the structure (memset) in vmci_transport_packet_init before populating its fields. Public vulnerability trackers and multiple distribution advisories record the fix and the package updates that include it. Why the CVE matters operationally: the vulnerability is a kernel‑level correctness defect in a virtualization/VM socket transport path (vsock/vmci). Its practical exploitability is generally assessed as local and context‑dependent — remote unauthenticated exploitation is not the common scenario — but in multi‑tenant cloud or shared infrastructure a latent kernel bug can still lead to host instability, information leakage, or privilege escalations depending on environment and available attack surface. Distributor trackers (for example Amazon Linux advisories) scored the issue with non‑trivial severity in their contexts and listed fixed package versions, underscoring that mainstream distros and cloud images have treated it as meaningful to patch.

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What Microsoft actually said — the nuance that matters​

Microsoft’s Security Response Center (MSRC) pages for a set of kernel CVEs, including CVE‑2025‑38403, include a short FAQ response that has been repeated across many CVE entries: Azure Linux (Microsoft’s curated Linux distribution family) includes the implicated open‑source component and is therefore potentially affected, and Microsoft noted it began publishing CSAF/VEX attestations in October 2025 and will update CVE entries if impact to additional Microsoft products is identified. That is a clear product‑level inventory statement and a procedural commitment — not an exclusionary blanket claim about all Microsoft products. Two practical takeaways from that wording:

• Authoritative for Azure Linux: Microsoft has completed an inventory for the Azure Linux artifacts it examined and found the upstream component — that attestation is authoritative for those images and should be acted upon by Azure Linux customers.
• Not proof of exclusivity: Microsoft’s statement does not mean other Microsoft products (WSL2 kernels, linux‑azure kernels, Marketplace images, AKS node images, curated appliances, etc. do not include the same code. Absence of a VEX/CSAF attestation for another product is not the same as evidence that product is unaffected. Microsoft explicitly said it would update the CVE/VEX mapping if additional products are identified, which implicitly acknowledges that their initial inventory was scoped.

For security teams the operational translation is simple: treat Azure Linux as confirmed in‑scope and patch it per vendor guidance; treat other Microsoft artifacts as unknown until Microsoft publishes an attestation for them or you perform artifact‑level verification.

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Cross‑checking the claim: independent verification​

Responsible reporting and triage requires corroborating vendor statements with independent sources. For CVE‑2025‑38403 that means checking upstream kernel commits, distribution advisories, and public vulnerability trackers.

• Upstream kernel commits and multiple distribution trackers document the fix: Debian’s tracker and the NVD/OSV listings show the memset change and list package versions where the fix is delivered. Those entries confirm the technical fix and timeline.
• Commercial cloud distro trackers (Amazon Linux Security advisories) and vendor advisories show the CVE mapped into cloud images, with vendor‑specific severity and fixed‑version mappings, demonstrating that mainstream cloud vendors treat the issue as actionable for their images. Amazon’s ALAS pages list CVE‑2025‑38403 with a CVSS context in their environment and mapped fixed kernel builds.
• NVD has a record for CVE‑2025‑38403 that mirrors the upstream description and references upstream commits; NVD’s record is a neutral cross‑reference for the technical description.

Put together, these independent sources validate both the technical nature of the fix and the reality that multiple distributions and cloud vendors have taken steps to map and patch the vulnerability. They also show that Microsoft’s product‑level attestation for Azure Linux is consistent with the broader ecosystem’s treatment of the CVE.

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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 so far, but that does not prove exclusivity. Microsoft’s VEX/CSAF rollout began with Azure Linux in October 2025 and is deliberately phased; that is why many MSRC CVE pages repeat the same FAQ text: they are reporting results for the Azure Linux inventory and promising to expand the mapping as they complete inventories of other product families. Until Microsoft attests other SKUs, each Microsoft artifact must be considered an independent case that requires verification. Why the inventory needs to be per‑artifact:

• Kernel inclusion is a build‑time property. Whether a given binary includes a particular source or driver depends on the kernel version, backports, and the kernel CONFIG_* flags used when building that kernel. Two Microsoft kernels built at different times or for different product families may include different subsets of upstream code.
• Microsoft ships multiple Linux and Linux‑adjacent artifacts that are not identical: the WSL2 kernel shipped with Windows, the linux‑azure kernel packages used in some VM SKUs, Azure Marketplace images, AKS node images, and the Azure Linux images are all distinct build artifacts. Any of those could contain the same vsock/vmci code depending on the build configuration and upstream commit range.
• VEX/CSAF publishing is phased by design. Large vendors commonly start official machine‑readable attestations with a single product family, then add more products once their inventories complete. The presence of an attestation for Azure Linux therefore reflects completion for that product family, not completion for the entire vendor portfolio.

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How to verify whether your Microsoft artifact is affected (practical steps)​

Inventory and artifact inspection are the only way to prove whether a particular image or kernel binary contains the vulnerable code. Use the following practical checks — these are intentionally artifact‑level and fast to run.

• Identify the artifact and kernel build
• On a running image: uname -r to get the kernel release string.
• If you have the image file: mount it or boot a test VM to query the kernel package version and build metadata.
• Inspect kernel configuration (determines whether vsock/vmci was built in or as a module)
• If /proc/config.gz exists: zcat /proc/config.gz | egrep 'VSOCK|VMCI|VMWARE|VIRTIO'
• If no config is present, check packaged config or build metadata (SBOM, build logs).
• Check loaded modules and built‑in symbols
• lsmod | egrep 'vsock|vmci|vmci_transport'
• grep -R "vmci_transport" /lib/modules/$(uname -r) 2>/dev/null
• Map kernel package to upstream commits
• Use distribution changelogs and vendor advisory mapping to match package versions to upstream stable commit IDs (this is more reliable than a bare kernel version when vendors backport fixes).
• Vendors typically list commit IDs or DLA/ALAS/ELSA advisories mapping to upstream commits; cross‑reference these to confirm whether the memset fix is present.
• If you can’t verify quickly, apply mitigations (see next section) and treat the artifact as “unknown / potentially affected” until proved otherwise.

Audit and inventory automation suggestions:

• Ingest vendor VEX/CSAF feeds (Azure Linux attestations are already published) into your vulnerability management platform.
• Maintain an SBOM / build metadata record for all Microsoft‑supplied images and kernels in your estate.
• Script the kernel config and module checks across fleets for rapid triage.

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Short‑term mitigations and remediation options​

If you find Azure Linux images that match Microsoft’s attestation, apply the vendor patch or updated kernel package as soon as operationally reasonable. For other Microsoft artifacts where you cannot immediately confirm impact, consider these compensations:

• If the vsock module is built as a module and not required by your workloads, unload and blacklist it (modprobe -r vsock; echo "blacklist vsock" > /etc/modprobe.d/blacklist‑vsock.conf) — but only do this after validating there are no dependent subsystems (containers, guest hypervisors or telemetry features) that need it.
• Restrict access to local VM socket interfaces and isolate workloads that could trigger the vulnerable code path. Limit privileged container capabilities, and enforce least privilege for user namespaces.
• Harden kernel attack surface: disable unneeded virtualization hooks, tighten cgroups and seccomp policies, and limit untrusted code execution on hosts that run tenant workloads. Centralize kernel logs (journalctl -k) and enable crash capturing (kdump) to capture KASAN or oops traces if the issue triggers.
• Plan and communicate reboot windows. Kernel patches require reboots; prioritize images marked by Microsoft’s VEX as affected (Azure Linux) and then address other artifacts based on your inventory results.

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Longer‑term recommendations for cloud and enterprise operators​

• Adopt product‑level VEX/CSAF ingestion and mapping in your vulnerability management pipeline. Microsoft’s VEX rollout is designed for machine consumption and will expand over time; consuming it reduces manual triage load and clarifies vendor attestations.
• Maintain per‑artifact SBOMs and build metadata. Kernel inclusion is an artifact‑level fact; SBOM or build metadata makes it trivial to prove presence or absence of an upstream source file across images.
• Treat vendor attestations as authoritative for the named product(s) and as signposts for where to start remediation. However, don’t replace artifact verification with vendor attestation absence — vendors are doing phased rollouts and attestations will lag inventory completion.
• Build an automated fleet script to check kernel configs and module presence and to compare installed kernel package changelogs against upstream commit IDs; this will let you triage hundreds or thousands of images quickly.

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Risk analysis: strengths, caveats, and potential pitfalls​

Strengths of Microsoft’s approach

• Machine‑readable signals: Publishing CSAF/VEX in a machine‑readable format for Azure Linux is a significant improvement for transparency and automation; it gives defenders a high‑confidence, vendor‑issued signal to kick off remediation.
• Pragmatic phasing: Starting with Azure Linux lets Microsoft validate processes and release stable attestations rather than publishing incomplete or noisy lists.

Caveats and residual risks

• Attestation lag & incompleteness: A single‑product attestation cannot prove that other Microsoft artifacts are safe. Treat the Microsoft FAQ sentence as a scoped inventory result rather than a company‑wide exclusion.
• Operational friction for kernel patching: Kernel fixes require reboots and careful scheduling. If upgrade windows are long (regulatory or stability constraints), the exposure window is extended. Prioritization and phased rollouts must reflect both risk and operational constraints.
• Artifact diversity in cloud estates: Microsoft produces many Linux artifacts (WSL kernels, linux‑azure kernels, Marketplace images). Each must be verified independently and may require separate remediation paths.

Potential pitfalls to avoid

• Assuming “no attestation = no problem.” That is the single most dangerous operational mistake here. Absence of a VEX entry for a product equals absence of attestation, not proof of the code’s absence. Inventory first, then act.
• Trying to avoid patching by forcibly unloading modules without confirming dependencies. In some environments the vsock or related modules may be used by guest tooling or monitoring; incorrect removal can break functionality.

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Conclusion — clear, practical answer​

Microsoft’s MSRC pages and VEX/CSAF program have explicitly attested that Azure Linux includes the open‑source component implicated by CVE‑2025‑38403 and is therefore potentially affected. That attestation is authoritative and actionable for Azure Linux customers. However, Microsoft has not claimed that Azure Linux is the only Microsoft product that could carry the code; in fact the vendor’s wording and phased VEX roll‑out indicate the opposite: other Microsoft artifacts remain subject to inventory and verification until Microsoft either attests them or you confirm impact via artifact inspection. In practical terms:

• If you run Azure Linux images: consider them confirmed carriers and prioritize the patch and reboot cycle according to Microsoft guidance.
• If you run other Microsoft images (WSL2, linux‑azure, Marketplace images, AKS nodes, appliances): do not assume they are safe — perform the artifact checks described above or wait for Microsoft to publish VEX attestations for those products.
• Use vendor VEX/CSAF feeds, SBOMs, and automated artifact inspection to reduce uncertainty going forward.

This answer synthesizes Microsoft’s published attestation language and independent cross‑checks (upstream commits, Debian/NVD/ALAS trackers) so you can both understand the scope of Microsoft’s statement and take concrete steps to verify and remediate exposure across mixed estates.

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Source: MSRC Security Update Guide - Microsoft Security Response Center