CVE-2025-37921: Linux VXLAN vnifilter Locking Bug and Azure Attestation

The recently assigned CVE-2025-37921 patches a locking bug in the Linux kernel’s VXLAN vnifilter code that could leave the Forwarding Database (FDB) in an inconsistent state when a Virtual Network Identifier (VNI) is deleted. Microsoft’s public wording on the CVE names Azure Linux as a product that “includes this open‑source library and is therefore potentially affected,” but that statement is a product‑scoped attestation — it confirms Azure Linux as a known carrier of the upstream component for this CVE, not a categorical guarantee that no other Microsoft product contains the same vulnerable code.

Background / Overview​

VXLAN (Virtual eXtensible LAN) is an overlay networking technology widely used in cloud, virtualization, and container networking. The kernel implements VXLAN in multiple modes; one such mode, vnifilter, integrates VXLAN handling with the Linux bridge/Netlink VNI APIs. The vulnerability fixed by CVE‑2025‑37921 is a concurrency/locking correctness problem: when a VNI is deleted from a VXLAN device operating in vnifilter mode, the FDB entry associated with the configured default remote may be deleted without holding the required hash lock. That unlocked deletion triggers kernel lockdep warnings and can create a race that may lead to inconsistent FDB state and undefined behavior under concurrent operations.
Technically, the bug arises in drivers/net/vxlan/vxlan_vnifilter.c and is blamed on an earlier commit that introduced wrapper helpers for FDB lookup and inadvertently missed taking the hash lock on deletion. Upstream maintainers fixed the defect by acquiring the hash lock before performing the deletion and releasing it afterward; multiple stable kernel commits and downstream distribution advisories list the relevant patch set. The issue is reproducible using a simple ip/bridge command sequence that adds and then deletes a VNI on a VXLAN interface configured with a default remote.
Across public trackers the vulnerability is described consistently: it was assigned CVE‑2025‑37921, published on May 20, 2025, and carries a moderate-to-high impact rating depending on the scoring authority. Upstream and major distributions have released patches or incorporated the fix into kernel updates.

---

What Microsoft actually said — parsing the MSRC wording​

Microsoft’s short advisory language that you quoted — “Azure Linux includes this open‑source library and is therefore potentially affected” — is an operational inventory attestation. It means Microsoft inspected the Azure Linux product family (their Azure‑branded Linux distribution and kernel artifacts) and confirmed the presence of the upstream VXLAN code implicated by this CVE. Microsoft also published a roadmap for machine‑readable attestations (CSAF/VEX) beux and said it will update the CVE record if additional Microsoft products are identified as carriers. That policy and phrasing are deliberate: they give immediate, authoritative guidance for Azure Linux customers while allowing Microsoft to expand attestations over time.
It is important to read that sentence precisely. It does three things:
as a known carrier where the upstream component was found.

• Declares Azure Linux images “potentially affected” for the purposes of triage and remediation.
• Commits to expanding machine‑readable mappings (CSAF/VEX) to additional Microsoft products when inventory work finds them.

It does not say “only Azure Linux” or “no other Microsoft product contains this code.” The phrasing is therefore operationally useful but not exclusive.

---

Short, direct answer to the user’s question​

No — Azure Linux is not necessarily the only Microsoft product that includes the open‑source library referenced by CVE‑2025‑37921. Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the implicated upstream VXLAN vnifilter code for this CVE; absence of a public attestation for other Microsoft products is absence of evidence, not evidence of absence. Other MicrosofL kernel builds, linux-azure kernel images used in certain Azure services, Marketplace images, Azure Kubernetes Service node images, and any bespoke kernel builds Microsoft distributes—could include the same upstream files depending on kernel version and build configuration. Microsoft has stated it will update the CVE/VEX mapping if additional products are found to be affected.

---

Why the echnical and operational reasons)​

Several technical realities explain why a single product attestation cannot be interpreted as a global guarantee across all Microsoft artifacts:

• Kernel composition is a per‑artifact property. The exact kernel version, commit set, and build-time configuration determine which upstream changes are present. Two Microsoft artifacts can ship very different kernels even if both are branded “Azure” artifacts.
• Drivers and features can be compiled in or out. The presence of a source file in the Lnot automatically mean it is enabled in a particular kernel build; distribution packagers choose CONFIG_ flags and module packaging per SKU.
• Microsoft distributes multiple Linux artifacts. The WSL2 kernel, linux‑azure kernels, Azure Marketplace images, and AKS node images are distinct products or artifacts that undergo separate build/test pipelines; each must be inventory‑checked individually to determine exposure.
• VEX/CSAF rollout is phased. Microsoft began publishing machine‑readable VEX attestations in October 2025 starting with Azure Linux; more product attestations will follow as Microsoft completes inventory work. Until those attestations exist, defenders must either rely on vendor attestations for stated products or perform their own artifact inspections.

The practical result: treat Microsoft’s attestation about Azure Linux as authoritative for that product, but do not assume other Microsoft kernels or images are unaffected unless they are explicitly attested or verified locally.

---

How to verify whether a Microsoft artifact you run is affected​

If you operate Microsoft‑distributed artifacts (images, kernels, or services), do not simply assume “not listed = safe.” Use the following verification steps to determine exposure to CVE‑2025‑37921:

• Inventory the artifacts you run.
• List all kernels and images you run that originated from Microsoft (WSL2 kernel downloads, linux-azure packages, Marketplace images, AKS node images, etc.).
• Inspect kernel version and configuration.
• Check uname -r and compare kernel version to the upstream stable releases that contain the fix.
• If you can access the kernel config (for example /proc/config.gz or the distribution's config package), look for CONFIG_VXLAN and whether the vnifilter bits or modules are present.
• Look for the vulnerable source/module.
• Confirm whether drivers/net/vxlan/vxlan_vnifilter.c (or the compiled module) appears in the artifact’s kernel image or module list.
• Consult vendor VEX/CSAF outputs.
• Consume Microsoft’s published VEX/CSAF inventory feeds for Azure Linux and wait for further attestations. Treat those feeds as canonical for artifacts they cover.
• Apply patched kernels or vendor updates.
• If an artifact is confirmed affected, apply the vendor’s kernel/security update and reboot where required.

These steps are deliberately conservative: they shift verification from assumption to observable evidence. Where you cannot easily inspect an image (for example a managed cloud service with opaque internals), rely on vendor communications and VEX/CSAF attestations, and treat “under investigation” as a trigger for follow-up with the vendor.

---

What the public trackers and distributions say about the technical risk​

Upstream and downstream trackers describe CVE‑2025‑37921 as a locking correctness issue that generates lockdep warnings and could produce inconsistent FDB state in concurrent scenarios. The Linux kernel CVE team and distribution maintainers recommend updating to a patched kernel; stable commits and distribution advisories list the relevant fixes. The NVD entry describes the issue and is currently undergoing reanalysis; distribution advisories (Ubuntu, Debian, Oracle, SUSE, Amazon Linux, Red Hat downstream pages) have published their own advisories or mapping and list patched packages or guidance.
Severity and exploitability context:

• Multiple downstream trackers assign a moderate-to-high impact score; for example, Ubuntu’s advisory lists a high CVSS assessment for certain contexts, while Amazon’s ALAS labels it as Important with a CVSS v3 base near 7.0–7.8 depending on authority. This reflects that while the bug is a concurrency/locking defect (not a straight memory-corruption remote RCE), the state inconsistency can cause kernel warnings, availability issues, or other undefined behavior in multi-tenant or automation-heavy networking stacks.

In pcorrectness bug that can destabilize kernel networking state in certain operational scenarios, and therefore distributors have classified it as worth patching promptly. There is no widespread public exploit code linking it to remote code execution at the time of disclosure; nevertheless, production networks and cloud images should be patched to remove the risk of instability and to satisfy compliance scanning.

---

Practical mitigation and triage guidance for operators​

• Prioritize patching Azure Linux images immediately if you run them. Microsoft’s attestation names Azure Linux as a known carrier and—critically—Microsoft has committed to publishing VEX/CSAF attestations that make it easier to automate triage for Azure Linux customers. Patch cadence for Azure Linux will reflect Microsoft’s kernel update schedule for that product.
• Inventory other Microsoft artifacts (WSL2 kernels, linux-azure kernels in Azure image catalogs or Marketplace images, AKS node images). If you rely on Microsoft-supplied kernel images for on‑prem or cloud workloads, treat them as unverified until an explicit VEX/CSAF attestation or vendor advisory covers them.
• For environments that cannot immediately update the kernel:
• Isolate management planes and control plane interfaces from untrusted networks.
• Limit the ability for unprivileged users to manipulate bridge/VXLAN configuration (rtnetlink) on multi‑tenant hosts.
• Add monitoring for kernel lockdep warnings and bridge/VNI manipulation logs; an uptick in the specific lockdep messages cited in upstream notes is a red flag.
• Use configuration-based mitigations where feasible:
• If VXLAN vnifilter is not required, consider disabling the feature or building kernels without the module in highly locked-down or appliance environments.
• For conensure host network namespaces are controlled and that orchestration systems do not inadvertently expose netlink actions to untrusted containers.
• Track vendor bulletins and VEX feeds. Microsoft’s machine‑readable VEX rollout is intended to reduce uncertainty for Azure Linux; subscribe to your vendors’ security advisory feeds and to VEX/CSAF outputs where available so your automation can reduce manual work.

---

How this constraint affects security teams and scanners​

Vulnerability scanners and SBOM/VEX consumers must adopt an operational posture that recognizes the difference between a single‑product attestation and universal absence:

• Do not suppress or ignore a vulnerability solely because it is not listed under a given vendor’s product set. Instead, cross‑check the artifact at rest (kernel image, module list, config) or ask the vendor for product‑specific attestations.
• Treat Microsoft’s Azure Linux VEX output as authoritative for Azure Linux images. When other Microsoft artifacts are later attested, ingest those VEX triage. Until then, flag Microsoft artifacts with unknown status for human review or artifact inspection.

This approach reduces both false positives (where vendor attestations indicate not affected) and false negatives (where absence of an attestation is mistakenly treated as safety).

---

Why vendors publish product-scoped attestations first​

From a vendor operational standpoint, publishing attestations for a single product family first is a pragmatic path:

• It lets the vendor validate the VEX/CSAF pipeline on a known, well-instrumented product.
• It provides immediate remediation guidance for a subset of customers while the vendor completes inventory across hundreds or thousands of digital artifacts.
• It reduces the time between upstream disclosure and actionable guidance for at least one product family.

Microsoft’s VEX rollout followed that pattern: Azure Linux was chosen as the initial product for machine‑readable attestations, and Microsoft committed to expand coverage as inventory work continues. That is a reasonable tradeoff between immediate transparency and operational feasibility. However, it leaves a window where other Micr un‑attested but still possibly affected, so defenders must not equate “not yet attested” with “not affected.”

---

Risk summary and final assessment​

• The 37921 is a correctness/locking defect in the VXLAN vnifilter code that can be triggered by deleting a VNI when a default remote is configured. It produces lockdep warnings and can result in inconsistent FDB state under concurrency. Upstream and distributions classify it as significant enough to warrant patching.
• Microsoft’s advisory that Azure Linux includes the implicated open‑source library and is therefore potentially affected is an authoritative, product‑scoped attestation for Azure Linux. It does not constitute a guarantee that all Microsoft products are free of the vulnerable code. In short: Azure Linux = confirmed/attested; other Microsoft artifacts = unverified until attested or inspected.
• Operational action: patch Azure Linux immediately; inventory and verify other Microsoft artifacts; subscribe to vendor VEX/CSAF feeds and to distribution advisories; enforce network and operational mitigations while updates are staged.

---

Practical checklist for WindowsForum readers (quick reference)​

• If you run Azure Linux images:
• Apply vendor kernel updates as soon as they are available and reboot affected hosts. Treat Microsoft’s attestation as authoritative for Azure Linux.
• If you run WSL2, Marketplace images, AKS node pools, or other Microsoft-supplied kernels:
• Inspect kernel versions and configs or seek a VEX/CSAF attestation from Microsoft for that specific artifact.
• If you can’t verify, treat the artifact as requiring follow-up and prioritize images used in multi‑tenant or externally exposed roles.
• If you operate network virtualization or multi‑tenant hosts where VNIs are created/removed automatically:
• Monitor for lockdep warnings and network instability.
• Enforce strict privilege separation on who can configure bridging and netlink operations.
• For compliance teams:
• Record the vendor’s VEX/CSAF attestation in your remediation tickets where applicable and use that as proof of product scope for Azure Linux.

---

Conclusion​

CVE‑2025‑37921 is a genuine kernel correctness bug in VXLAN vnifilter code that distributions and downstream vendors have patched; the technical fix is straightforward but important in environments that manipulate VNIs at scale. Microsoft’s public statement that “Azure Linux includes this open‑source library and is therefore potentially affected” is an accurate, narrow attestation that helps Azure Linux customers prioritize updates, but it is not a blanket statement that other Microsoft products are unaffected. Security teams should treat Azure Linux as a confirmed remediation target, and treat other Microsoft artifacts as unverified until Microsoft publishes additional VEX/CSAF attestations or until those artifacts are inspected directly. The safest course: patch known carriers, inventory and verify other artifacts, and subscribe to vendor attestations so future disclosures automatically shrink uncertainty.

---

Source: MSRC Security Update Guide - Microsoft Security Response Center