Azure Linux Attestations and CVE-2025-38470: What You Need to Know

Microsoft’s brief MSRC attestation that “Azure Linux includes this open‑source library and is therefore potentially affected” is a factual, product‑scoped inventory statement — but it does not mean Azure Linux is categorically the only Microsoft product that could include the vulnerable Linux kernel code for CVE‑2025‑38470.

Background / Overview​

CVE‑2025‑38470 is a Linux kernel defect in the 802.1Q VLAN handling code described as “net: vlan: fix VLAN 0 refcount imbalance of toggling filtering during runtime.” The bug arises when the kernel’s vlan subsystem and the netdevice rx‑vlan‑filter feature interact in certain sequences (for example, toggling rx‑vlan‑filter while a bonded interface or team is brought up/down). The practical problems reported by upstream and distributors include memory leaks and, in some sequences, null‑pointer dereferences or BUG_ON triggers when VLAN 0’s reference counts are mishandled. The widely reported CVSS framing is availability‑first (example vendor mappings show a 5.5 score, AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H). Technically, the kernel historically auto‑adds or auto‑removes VLAN 0 for devices when rx‑vlan‑filter is enabled; if rx‑vlan‑filter is toggled while a device is running, the code path that should account for whether VLAN 0 was automatically added can miss a refcount increment or decrement. That mismatch can lead to freeing a structure that is later referenced or to leaking VLAN information. Upstream fixes record an explicit flag (or bookkeeping) that marks whether VLAN 0 was added automatically on NETDEV_UP; that flag governs deletion on NETDEV_DOWN and prevents the refcount imbalance. Multiple distribution trackers (Debian, Oracle, Amazon Linux) and the OSV/NVD ecosystem ingested the upstream commit and list fixed package versions or backports.

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What Microsoft actually published — how to read the MSRC wording​

Microsoft’s Security Response Center (MSRC) entries for many Linux CVEs since October 2025 commonly include a short FAQ line that reads, effectively: “Azure Linux includes this open‑source library and is therefore potentially affected.” That wording is a machine‑readable VEX/CSAF attestation indicating Microsoft has completed an artifact inventory for the Azure Linux product family and has found the upstream component in that product’s builds. The attestation is therefore authoritative for Azure Linux images that Microsoft has inspected. Important interpretive points:

• Product‑scoped attestation: The MSRC line is an inventory result for a named Microsoft product family (Azure Linux). It is not a universal statement about all Microsoft products or images.
• Phased rollout: Microsoft began publishing CSAF/VEX attestations with Azure Linux as an initial scope and has said it will update CVE records if additional Microsoft products are discovered to ship the same component. Treat the attestation as the result of a completed inventory pass for that product, not as evidence that other SKUs are free of the code.
• Operational value: For customers running Azure Linux images, Microsoft’s attestation is a clear, actionable signal to treat those images as in‑scope and to apply updates when Microsoft or the distribution publishes a patch.

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Short, direct answer​

• Azure Linux is the only Microsoft product that Microsoft has publicly attested to include the vulnerable VLAN/802.1Q code for CVE‑2025‑38470 at the time of the advisory.
• That attestation does not prove exclusivity. Other Microsoft‑distributed Linux artifacts (for example, kernel builds used in other Azure images, WSL2 kernels, linux‑azure builds, Marketplace images, AKS node images or vendor appliances) could include the same upstream code depending on their kernel version and build configuration. Absence of a published Microsoft VEX/CSAF attestation for a product equals “not yet verified,” not “not affected.”

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Why the distinction matters (technical and operational nuance)​

A vendor‑published VEX/CSAF feed answers a narrow but crucial question: “Has the vendor confirmed that Product X contains Component Y?” It does not automatically answer whether the vendor’s other product artifacts also contain Component Y — because:

• Kernel composition varies by artifact. Whether a given Microsoft image contains the 802.1Q code depends on:
• The kernel version shipped.
• Kernel configuration (which CONFIG_8021Q or related options were enabled).
• Whether the vendor shipped a custom, backported, or third‑party kernel for that product.
• Backports complicate version arithmetic. Vendors frequently backport security fixes into older stable kernel trees or apply vendor patches; determining whether a particular build is vulnerable requires checking commit IDs or package changelogs — not only the numeric kernel version.
• Out‑of‑tree or OEM modules. Some images include out‑of‑tree modules or vendor driver packages which have their own release cadence and may or may not have been updated.

Because of these factors, a conservative operations posture treats the Azure Linux attestation as a firm “yes” for Azure Linux and treats every other Microsoft image as “verify me” until either (a) Microsoft publishes an attestation for it or (b) the operator performs artifact‑level checks.

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How to verify whether your Microsoft artifact is affected — practical checklist​

Operators need deterministic ways to decide whether the vulnerable code appears in a given artifact. The steps below are practical, repeatable, and prioritized.

1. Identify the artifact​

• For cloud VMs, identify the image (publisher, offer, sku, version) you deployed.
• For AKS/VM scale sets or Marketplace appliances, confirm the exact image name and kernel package.
• For WSL2, determine the kernel shipped to WSL instances (check the WSL kernel package or the kernel version inside the WSL distro).

2. Check the running kernel and modules​

• Run: uname -r to get the running kernel string.
• Check kernel config (if available): zcat /proc/config.gz | egrep 'CONFIG_8021Q|CONFIG_VLAN' or inspect the packaged config file in /boot/config-$(uname -r).
• List modules: lsmod | egrep '8021q|vlan' and modinfo 8021q to inspect module version and file path.

3. Inspect package changelogs / vendor advisories​

• Look up the kernel package changelog for entries referencing CVE‑2025‑38470 or the upstream stable commit ID. Upstream CVE pages and distro trackers (Debian, Oracle, Amazon Linux, etc. list the stable commit IDs and the package advisories that include fixes; match those to your distro’s package versions.

4. Search kernel logs for characteristic traces​

• Search dmesg/journal for BUG/OOPS or messages referencing unregister_vlan_dev, vlan_info, or other VLAN-related stack traces:
• sudo journalctl -k | egrep -i 'unregister_vlan_dev|vlan_info|vlan_vid_add|rx-vlan-filter'
• KASAN builds will reveal use‑after‑free traces; preserve vmcore if you see a panic.

5. For images that you cannot inspect directly​

• If you run managed images (Marketplace appliances, vendor images), contact the image publisher or Microsoft support and request a VEX/CSAF attestation or confirmation whether the vulnerable commit is present. Microsoft has committed to updating the CVE/VEX records if more products are found to ship the component.

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Recommended remediation and operational priorities​

• Patch and reboot affected hosts once the vendor/distro publishes a fixed kernel package (upstream fixes are small and backportable). Prioritize hypervisors, cloud-hosted control plane nodes, and network appliances that perform L2 bridging.
• If immediate patching is impossible:
• Restrict who may toggle rx‑vlan‑filter or manipulate bridge/VLAN state — limit to trusted operators.
• Avoid workflows that programmatically toggle vlan filtering on production interfaces while interfaces are being added/removed.
• Isolate hosts that run bridging/MST to reduce blast radius.
• Verify fixes by reproducing the triggering sequences in a test environment (for example: create bond interfaces, toggle rx‑vlan‑filter, bring devices up/down and check for dmesg oopses). After patching, validate that those reproductions no longer produce WARN/BUG traces.
• Integrate Microsoft’s CSAF/VEX feed into your vulnerability‑management automation and subscribe to your distro vendor security trackers so you receive package advisories as soon as they are published.

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Detection, hunting and forensic guidance​

• Kernel races and refcount imbalances typically reveal themselves through kernel WARN_ON, BUG_ON, oops traces, or KASAN logs. Centralize kernel logs (journalctl -k) and enable crash capture (kdump/vmcore) so you retain evidence across reboots.
• Monitor for these key signatures:
• unregister_vlan_dev BUG traces or null‑pointer oops originating from net/8021q/vlan.c.
• Unexpected memory leaks or “unreferenced object” KASAN output referencing VLAN structures.
• Correlation between orchestration events (container or VM lifecycle that manipulates bridges) and kernel oops in bridging code.

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Risk analysis: strengths of the upstream response and remaining caveats​

Strengths

• The upstream fix for CVE‑2025‑38470 is surgical and minimal — marking whether VLAN 0 was auto‑added and using that flag to decide deletion is a low‑risk change that is easy to backport to stable kernel branches. That trait accelerates distribution-level updates. Distribution and vendor trackers (Debian, Oracle, Amazon Linux) already map the fix to package advisories.
• The vulnerability’s attack model is primarily local/administrative: it requires operations that manipulate netdev state and rx‑vlan‑filter, so remote unauthenticated exploitation is not the usual scenario. Vendors classify the defect as Availability‑first rather than confidentiality/integrity.

Residual risks

• Vendor and OEM lag: Appliances, OEM kernels, and out‑of‑tree modules can remain unpatched much longer than mainstream distro kernels. If you rely on third‑party images or vendor kernels (for example, specialized network appliances or old Marketplace images), expect longer remediation timelines.
• Operational blast radius: Even though exploitation is local, the practical impact on shared infrastructure (hypervisors, NFV platforms) is high: a kernel oops on a hypervisor can disturb many tenants.
• Detection gaps: Kernel oops traces are ephemeral; capture and retention policies must be improved to analyze incidents and attribute root cause.

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Concrete recommendations for WindowsForum readers (engineers, SREs, security teams)​

• Treat Microsoft’s Azure Linux attestation as a definitive “yes” for Azure Linux. Patch Azure Linux images as updates are published.
• Inventory every Microsoft image you run. That includes linux‑azure kernels, WSL2 kernel builds, Marketplace images, AKS node images and any Azure Marketplace appliances. For each artifact:
• Verify kernel version and config.
• Check whether the 802.1Q code or related modules are built in.
• Consult the MSRC VEX/CSAF feed and your distro’s security tracker for explicit mappings.
• Automate the verification process. Use configuration management or host agents to collect uname -r, /boot/config-*, modinfo outputs, and package changelogs, and compare them against the upstream stable commit IDs listed in CVE trackers.
• Prioritize by exposure and service impact. Hypervisors, edge appliances, and hosts performing bridging or network virtualization should be highest priority for patching and verification.
• When in doubt, ask the image publisher or Microsoft support for VEX/CSAF status for that image — Microsoft has committed to updating CVE attestations when additional products are identified.

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Final assessment and practical takeaways​

• Microsoft’s statement that “Azure Linux includes the open‑source library and is therefore potentially affected” is accurate and valuable: it is an explicit VEX/CSAF attestation for Azure Linux and gives operators a deterministic input for automation and triage.
• That attestation is product‑scoped, not a technical proof of exclusivity across all Microsoft artifacts. Absence of attestation for another Microsoft product is not proof that the product does not include the vulnerable kernel code; it simply means Microsoft has not yet published a completed inventory for that product.
• Operators must therefore (a) patch attested Azure Linux images promptly, (b) inventory and verify other Microsoft images and kernel artifacts used in their estate, and (c) prioritize patching and operational mitigations for hosts that run bridging or L2 virtual switching. Use vendor package advisories and upstream stable commit IDs as the authoritative mapping for whether a kernel build has the fix.

In short: treat Microsoft’s Azure Linux attestation as an authoritative and actionable “yes” for that product, but keep treating other Microsoft‑distributed Linux artifacts as “verify me” until you either obtain a published VEX/CSAF attestation for them or you confirm the presence (or absence) of the fix by inspecting the artifact itself.

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Source: MSRC Security Update Guide - Microsoft Security Response Center