CVE-2025-39850: Linux VXLAN Null Pointer Fix in Azure Linux

The Linux kernel vulnerability tracked as CVE-2025-39850 — a NULL-pointer dereference in the VXLAN implementation when handling FDB nexthop objects under the vxlan "proxy" option — has been fixed upstream, and Microsoft’s public attestation currently lists Azure Linux as a product that “includes this open‑source library and is therefore potentially affected.”

Background / Overview​

VXLAN (Virtual Extensible LAN) is a widely used kernel networking feature for overlay networks in cloud, virtualization, and container platforms. The specific defect in CVE-2025-39850 occurs in the vxlan codepaths that synthesize ARP or IPv6 Neighbor responses when the device is configured with the proxy option. In some FDB (Forwarding Database) configurations the code assumed an FDB entry pointed to a concrete remote destination; that assumption breaks when the FDB entry references a nexthop group instead of a single remote destination, producing a kernel NULL‑pointer dereference (NPD) when the code dereferences a missing remote pointer. The vulnerability was documented in public vulnerability databases and fixed in the upstream kernel by adding the necessary existence checks before dereference. Microsoft’s Security Response Center (MSRC) entry for this CVE includes an attestation stating that Azure Linux (Microsoft’s Azure-branded Linux distribution and related kernel artifacts) includes the open-source library and is therefore potentially affected — and adds that Microsoft will update the CVE entry and machine-readable attestations (CSAF/VEX) if impact to additional Microsoft products is identified. That is Microsoft’s published, authoritative inventory statement for the products it has validated so far.

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What CVE-2025-39850 actually is — technical summary​

• The defect appears when a VXLAN device is configured with the proxy option and is expected to suppress ARP/NS for on‑behalf replies.
• When a neighbor entry’s MAC is associated with an FDB nexthop group rather than a concrete remote destination, the vxlan code can try to dereference the missing remote pointer and crash (NULL-pointer dereference).
• The practical symptom is a kernel NPD or OOPS that can cause process- or host-level crashes — an availability/denial-of-service outcome rather than an immediate remote code execution or privilege escalation primitive.
• Upstream maintainers fixed the issue by verifying the existence of the remote destination before dereferencing and by earlier-dropping of packets that cannot be resolved safely. Public CVE trackers (NVD, DEBIAN security tracker, OSV, Tenable) document the bug details and reproduce steps.

This is a classic kernel robustness defect: a missing NULL check or incorrect object‑lifetime assumption that turns an edge-case control flow into a host crash. The attack surface is primarily local or tenant-adjacent (an actor able to create or manipulate relevant network objects or inject crafted packets), but in multi‑tenant clouds that surface can be meaningful because a kernel crash on a host can impact many tenants and services.

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Microsoft’s attestation: what it says — and what it does not say​

Microsoft’s public statement for CVEs of third‑party open‑source components increasingly uses machine‑readable CSAF/VEX attestations. Those attestations are product‑scoped: they declare which Microsoft product artifacts the company has validated as shipping the implicated open‑source component, and they state the current status (e.g., known affected, fixed, under investigation). For CVE-2025-39850 Microsoft has explicitly published that Azure Linux includes the relevant kernel component and is therefore potentially affected, and has pledged to update the CVE mapping if additional Microsoft products are later found to include the same vulnerable code.
Key takeaways from Microsoft’s wording (practical interpretation)

• Authoritative for Azure Linux: Microsoft’s attestation is an authoritative, machine‑readable signal that specific Azure Linux builds ship a kernel that, at the time of inventory, included the vulnerable vxlan implementation.
• Not a universal absence claim: Absence of other Microsoft product names in the attestation is not definitive proof that those other products are unaffected. Microsoft’s wording explicitly leaves room to add other products later as inventories complete.
• Operational intent: Microsoft is following a common large‑vendor practice: publish the product set it has validated first (to give customers deterministic automation inputs) and expand the mapping as more internal SKUs are checked.

That means the MSRC line is not a reassurance that “only Azure Linux” could be affected — it is a precise statement that Azure Linux has been checked and flagged accordingly, with a commitment to widen the mapping should further Microsoft artifacts be discovered to ship the same kernel code.

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Is Azure Linux the only Microsoft product that could include the vulnerable code?​

Short answer: No — Azure Linux is not necessarily the only Microsoft product that could include the vulnerable vxlan code, and Microsoft’s attestation intentionally does not claim exclusivity. The attestation indicates the product Microsoft has validated so far; other Microsoft artifacts may yet be found to carry the same kernel code depending on build pipelines, packaging choices, and reuse of kernel trees. Multiple independent analyses of Microsoft’s attestation model arrive at the same operational conclusion: treat Azure Linux’s status as authoritative for that product only, and perform host‑level inventory and verification across other Microsoft-supplied images and kernels in your estate.
Why Azure Linux being named does not prove exclusivity (detailed reasons)

• Build-time variability: Microsoft produces multiple Linux-related artifacts (linux-azure/kernel-azure packages, Marketplace VM images, AKS node images, container base images, the WSL2 kernel distributed with Windows, and various internal images). Each artifact is an independent build and may or may not include the same kernel configuration and module set.
• Statically linked or embedded binaries: Some images include prebuilt static binaries or kernel modules that are not updated automatically when a distribution package is updated; inventories must explicitly check for those baked-in artifacts.
• Marketplace and partner images: Marketplace or partner images can include different kernels and third‑party components that Microsoft does not maintain directly; these images can lag in patching cadence.
• Vendor backports and forked kernels: Cloud vendors and OEMs sometimes backport fixes or maintain forked kernel trees with different patchsets and timelines. Presence or absence of a fix is a per-kernel-package, per-image property.

In short: Azure Linux is the product Microsoft has verified; other Microsoft products that ship Linux kernels or kernel modules (or that rely on underlying kernels) may be in scope until positively inventoried as not affected or already fixed.

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Which Microsoft artifacts you should check in your environment​

If your environment uses Microsoft services, images, or tooling that touch Linux kernels or network overlays, verify exposure across these artifacts:

• Azure VM images (publisher images and custom Marketplace images).
• linux-azure / kernel-azure packages used in Azure guest images and in some Marketplace offerings.
• AKS (Azure Kubernetes Service) node images and node pools (check node OS image and kernel version).
• Windows Subsystem for Linux (WSL2) default kernel or any custom WSL kernel builds distributed by Microsoft or used internally.
• Azure Marketplace appliance images or partner images that may bundle kernels.
• Containers or VM images hosted in Azure that were built from Azure Linux or Microsoft-provided build pipelines.
• Any on‑prem or hybrid systems that consume Microsoft-delivered kernels or kernel packages (for example, internal VHDs, golden images, or automation pipelines using Microsoft artifacts).

Do not assume that an attestation naming Azure Linux automatically covers these other items. Each image or package requires explicit verification. This is precisely why Microsoft’s VEX/CSAF approach publishes per-product attestations and why it promises to expand them when additional affected products are discovered.

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How to verify whether you are affected — practical checklist​

Follow these prioritized, deterministic checks. Each step produces verifiable evidence you can use for triage and patching decisions.

• Inventory images and artifacts
• List all Azure VM images, AKS node images, and Marketplace images in use.
• Identify the kernel package (uname -r) and the package name (linux-image, kernel, linux-azure, etc. for each host.
• Confirm whether the running kernel or packaged kernel includes the vxlan implementation and whether the fix is present
• On a host, run: uname -r
• Check whether vxlan is compiled in or available as a module: lsmod | grep vxlan and modinfo vxlan
• Inspect the package changelog: apt changelog linux-image-$(uname -r) or rpm -q --changelog kernel-$(uname -r)
• Search the kernel package changelog / distro advisory for the upstream commit or CVE mapping (package maintainers typically reference the kernel commit IDs that implement the fix). Public trackers and distro advisories (Debian, SUSE, SLES, Ubuntu) have already mapped the CVE to their packages in many cases.
• Detect runtime indicators
• Inspect kernel logs for NPD/OOPS traces referencing vxlan_xmit, arp_reduce, neigh_reduce or explicit NULL pointer dereference stacks: journalctl -k | grep -iE 'vxlan|arp_reduce|neigh_reduce|vxlan_xmit'
• If you observe repeated kernel crashes or OOPSes that match the public repro stacks, prioritize patching.
• Check Azure Linux VEX/CSAF attestation
• Pull the MSRC / VEX/CSAF attestation for this CVE to see the exact Azure Linux product versions mapped as Known Affected or Fixed. Microsoft’s attestation is machine‑readable and intended for automation; incorporate it into your vulnerability ingestion pipeline.
• For containers and images
• Inspect container base images for kernel modules or tools that may include vxlan-related code. Note: containers typically rely on the host kernel; however, some specialized appliances and images bundle kernel modules into initramfs or custom kernels — check image builds and SBOMs where available.
• WSL2 and developer images
• If you or your developers use WSL2, check the running WSL kernel version and any custom WSL kernels in CI tooling or developer VM images.

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Mitigation, remediation and verification steps​

• Apply vendor-supplied kernel updates. The definitive remediation is to run a kernel that contains the upstream patch that checks for the remote destination before dereferencing. Distributors have mapped the upstream fix into their kernel packages; update and reboot into the patched kernel. Public advisories and SLES/enterprise vendor pages list fixed package versions where applicable.
• If you cannot patch immediately:
• Restrict control-plane and management-plane access to VXLAN configuration and FDB manipulation APIs to trusted operators only.
• Avoid enabling the vxlan proxy option on hosts that must remain unpatched or that run untrusted workloads.
• In environments with multi‑tenant workloads, consider moving tenants or isolating high‑risk workloads to patched hosts.
• Validate remediation:
• After patching and reboot, reproduce previously observable NPD/OOPS traces in a safe test environment (if you had such reproductions) and verify the crash no longer occurs.
• Confirm kernel package changelogs reference the specific upstream commit IDs associated with the fix.
• Monitor kernel logs for recurrence and validate that OOPS traces referencing vxlan_xmit or {arp,neigh}_reduce no longer appear.

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Operational risk and prioritization guidance​

• Priority high: hosts that:
• Run VXLAN with the proxy option enabled, AND
• Host multi‑tenant workloads, edge/virtual networking appliances, or control-plane tools that can manipulate FDB/nexthop entries.
• Priority medium: single-tenant VMs that use VXLAN but are not exposed to untrusted users.
• Priority lower: desktop workstations and lab systems that do not run VXLAN or where the vxlan module is not loaded.

The primary practical threat here is availability — kernel crashes that disrupt services. In cloud environments, a kernel OOPS on a host can create cascading service outages or complicate incident response. There is no authoritative, publicly disclosed exploit chain demonstrating remote code execution from this specific defect at the time of disclosure; nevertheless, the availability risk alone is operationally significant in cloud and multi‑tenant contexts.

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Strengths of the upstream and vendor response — and residual risks​

Strengths

• The upstream fix is small, targeted, and low‑risk: it adds necessary existence checks and drops packets earlier where appropriate, making backporting straightforward.
• Multiple distribution trackers and vulnerability databases have mapped the fix into vendor packages, enabling patch automation and packaging updates across major distros.
• Microsoft’s CSAF/VEX attestation practice provides machine‑readable mappings that help automation and deterministic triage for Azure Linux customers.

Residual risks

• Long‑tail exposures: Marketplace images, third‑party appliances, custom golden images, and embedded or vendor‑backported kernels can lag upstream fixes — these often require separate vendor coordination.
• Inventory gaps: Absence of a public attestation for a product does not equal absence of vulnerable code. Microsoft’s wording explicitly reserves the right to expand product mappings as more inventories are completed. Treat attested products as positively identified, and other products as unknown until verified.
• Detection noise: Kernel OOPS traces can be transient and may be lost on reboot; collect logs and vmcores immediately when investigating crashes.

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Recommended communication to stakeholders (concise brief for execs/ops)​

• What happened: A kernel VXLAN robustness bug (CVE‑2025‑39850) could cause a host crash in some VXLAN proxy configurations.
• Who Microsoft named: Microsoft has publicly attested Azure Linux as a product that includes the affected open‑source kernel component and is therefore potentially affected; Microsoft will update the CVE mapping if additional Microsoft products are identified. This attestation is authoritative for the product it covers but not an exhaustive inventory of all Microsoft artifacts.
• Immediate ask to IT/ops:
• Inventory all Microsoft-supplied images and kernels in use (Azure VMs, AKS nodes, Marketplace images, WSL kernels).
• Patch kernels whose package changelogs or vendor advisories map to CVE‑2025‑39850 or include the upstream commits that fix vxlan NULL-pointer dereferences.
• Isolate or rehost unpatched multi‑tenant workloads until the host kernel is patched and validated.
• Outcome target: All production hosts with VXLAN proxy enabled and exposed to untrusted inputs either patched and rebooted or isolated within 7 business days (adjust per business risk).

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Conclusion​

Microsoft’s public statement that Azure Linux includes the open‑source library and is therefore potentially affected is an important, machine‑readable attestation that helps Azure customers automate triage and remediation. However, that statement is deliberately scoped: it identifies the product Microsoft has validated so far — not the entire universe of Microsoft artifacts. Operators must therefore treat the attestation as authoritative for Azure Linux and proactively inventory other Microsoft‑provided images, kernels, and marketplace artifacts in their estates to determine whether they too include the vulnerable vxlan paths.
The technical fix is straightforward, available upstream and in many distributor packages; the operational challenge lies in inventorying kernels, validating package mappings, updating and rebooting hosts, and mitigating long‑tail and third‑party images that can lag patches. Prioritize patching for hosts that use VXLAN proxy functionality and expose networking control to less‑trusted actors; where patching cannot be immediate, restrict access to VXLAN control-plane operations and isolate vulnerable hosts until they can be updated and validated.

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Source: MSRC Security Update Guide - Microsoft Security Response Center