The short answer is:
No — Azure Linux is not necessarily the only Microsoft product that could include the vulnerable component, but it is the only Microsoft product Microsoft has
publicly attested as including the affected code for this CVE at the time of the advisory; absence of an attestation does not prove absence of exposure and operators must treat un-attested Microsoft artifacts as
potentially affected until verified.
Background / Overview
CVE‑2025‑39748 is a Linux kernel eBPF verifier bug summarized as “bpf: Forget ranges when refining tnum after JSET.” The vulnerability was reported by automated tooling (Syzbot) and fixed upstream by changing how the verifier refines type/number (tnum) ranges after a JSET instruction so that it does not retain inconsistent range information that can arise on unreachable paths. The practical impact is an availability issue (kernel warnings / OOPS / potential kernel instability) and the attack vector is local (an attacker or unprivileged process would need the ability to load specially crafted eBPF programs). This advisory triggered vendor and distribution notices and prompt backports; major Linux distributors have catalogued the CVE and linked the upstream fix in their security trackers. The common operational guidance is to apply the vendor-supplied kernel update and, where immediate patching isn't possible, restrict unprivileged BPF usage and tighten capability grants for BPF program loading.
Why the Microsoft wording matters — parsing the MSRC statement
Microsoft’s MSRC product pages and their newer CSAF/VEX attestations use a precise, machine-readable vocabulary when describing which
products include a given third‑party component. When Microsoft states that “Azure Linux includes this open‑source library and is therefore potentially affected,” that is an attestation of
inventory completeness for the Azure Linux family — not a categorical negative for every other Microsoft SKU. Microsoft has also stated it will update CVE entries if additional internal products are identified as carriers and will publish CSAF/VEX attestations as it finishes component mapping across its product portfolio. That policy behavior has been documented in their recent advisories and VEX attestations.
Put simply:
- Microsoft’s attestation that Azure Linux includes an affected component is authoritative for Azure Linux images.
- Microsoft’s statement that it will update the CVE if more Microsoft products are found to ship the component means the current product list reflects work completed so far, not a comprehensive, final denial for all Microsoft product lines.
This is an important nuance. Customers who only read the short sentence and stop there can be misled into thinking other Microsoft products are safe. The correct operational interpretation is: treat the MSRC attestation as an authoritative inventory datapoint for Azure Linux,
and actively verify any other Microsoft images, containers, agents, or appliances you run.
The technical reality: why product attestations don't prove global absence
There are several concrete technical reasons why the presence (or absence) of a vulnerable component in one product does not automatically map to all other artifacts from the same vendor:
- Binary/static linking vs. system packages: A component (kernel code, a library, or a Go runtime) can be present because a binary was statically linked into an image or packaged inside a vendor-supplied artifact. A package manager query for a system package (apt/dpkg/rpm) may not reveal such embedded instances.
- Many artifacts, many build systems: Large vendors publish a wide range of artifacts — images, curated container runtimes, marketplace appliances, agent installers, WSL distributions, VM images, and more. Each artifact’s build pipeline and dependency list can differ. One image (Azure Linux) may have been analyzed and attested; others may not yet have completed that inventory pass.
- Third‑party and marketplace images: Marketplace images and partner-provided artifacts can include vulnerable libraries even if the vendor’s curated images do not. Those artifacts often escape centralized inventory checks and therefore remain a plausible exposure vector until inspected.
- Container and image layering: An Azure-hosted container or VM might include a user-supplied layer built from older base images or third-party wheels; the vulnerable code can ride in via those layers even if the top-level Microsoft image has been patched.
Because of these realities, a product-level VEX/CSAF attestation is a
powerful automation signal, but it is not a universal guarantee across every binary or artifact bearing a vendor label.
What the upstream evidence says about CVE‑2025‑39748
Vendor trackers and national databases describe CVE‑2025‑39748 consistently: it is a correctness/robustness fix in the Linux kernel’s BPF verifier related to refining tnum ranges after a JSET, caught by Syzbot, and classified as medium-severity with an availability impact. Distributors such as Ubuntu, SUSE, Debian and cloud provider advisories cataloged the CVE and pushed kernel updates that include the upstream fix. Key technical points verified across independent trackers:
- The bug arises because the verifier’s branch analysis can produce inconsistent range information on an unreachable verification path after a JSET instruction; the practical symptom is a kernel warning or, in some cases, instability.
- The exploitability profile is local — an attacker must be able to load or trigger the specific BPF program pattern. Whether that’s possible without privileges depends on the host policy for unprivileged BPF. Many distributions default to more secure configurations, but permissive developer images and some containers can allow unprivileged BPF loads.
- The upstream fix is conservative: it changes how the verifier treats the range refinement after JSET (for example, forgetting ranges when narrowing tnums after JSET), avoiding risky broad rewrites while restoring correctness. Distributors have backported this change into stable kernel branches.
Practical implications for Windows / Microsoft customers
Many Microsoft customers run Linux images or Linux-based agents (for monitoring, telemetry, container orchestration) across Azure and on-premises. The operational question is: where should you look for exposure, and how do you verify?
High-priority surfaces to inspect first:
- Azure Linux images — Microsoft has attested this product; treat the VEX/CSAF output as authoritative for those images and apply any Microsoft-published fixes or updated images.
- Curated Azure container images and ML runtimes (Azure Machine Learning curated environments, Azure Container Registry official images): these can and do include libraries and kernels independently of the base image; verify the runtime artifacts used by your workloads.
- Marketplace and partner images — inspect these artifacts explicitly because they may not be covered by Microsoft’s attestations.
- WSL kernels and agent bundles — any Microsoft-distributed Linux kernel image used in WSL or agent installers should be verified for the kernel version and CVE backport status.
- Custom images and snapshots — images created from older snapshots or community images can carry vulnerable kernels or embedded binaries.
Operational checks and tools (concise):
- Check the kernel version on suspect hosts:
- uname -r
- Inspect package changelogs for the kernel package and confirm the upstream commit or CVE mapping from your distro’s security tracker.
- Look for BPF-related log noise and verifier warnings:
- journalctl -k | grep -i bpf
- dmesg | grep -i verifier
- Verify unprivileged BPF policy:
- sysctl kernel.unprivileged_bpf_disabled
- If unprivileged BPF is permitted and you cannot patch immediately, set kernel.unprivileged_bpf_disabled = 1 as a temporary mitigation.
- Use bpftool to inspect loaded programs and attachments (run in maintenance windows):
- bpftool prog show
- bpftool map show
- For images and containers: scan your images by unpacking them and checking installed kernel packages or embedded binaries; for compiled artifacts, confirm the build toolchain and rebuild with a patched kernel/toolchain if required.
Apply vendor kernel updates as the definitive remediation for the kernel-level CVE; distributions have made fixes available in their stable branches and security trackers. Confirm your distribution’s advisory and kernel package changelog reference CVE‑2025‑39748 or the upstream fix commit before declaring remediation complete.
Steps to verify Microsoft-sourced artifacts (recommended playbook)
- Inventory Microsoft artifacts you run (images, VMs, WSL kernels, curated containers, agents). Include marketplace images and partner images.
- For each artifact, determine the kernel version and the package changelog or build provenance; map the kernel package to upstream stable commit IDs or the distro advisory that lists CVE‑2025‑39748. Use vendor security trackers for the authoritative mapping.
- If an artifact is a container image, unpack and inspect package layers or compile-time toolchain identifiers; rebuilt containers with patched base images if necessary.
- For Azure Linux instances, use Microsoft’s published CSAF/VEX attestation for the product to automate detection; treat that attestation as authoritative for Azure Linux but still verify at runtime for concrete assurance.
- Apply patches and reboot hosts in a staged manner (pilot → broader ring → production), prioritizing hosts that are multi‑tenant, run untrusted workloads, or host BPF-heavy observability/packet-processing agents.
Strengths in Microsoft’s approach — what they did right
- Machine-readable attestation commitment: Microsoft’s move to publish CSAF/VEX attestations for product inventories is a pragmatic win for automation and transparency. Operators can query those attestations to make fast, policy-driven decisions for Azure Linux and other attested products.
- Clarity about scope and the update process: Microsoft explicitly states it will update CVE entries as it discovers additional product artifacts, which is better than silence or vague “not affected” claims that can be misunderstood. That commitment reduces uncertainty for customers who prioritize relying on vendor attestations.
- Actionable guidance: For Azure Linux customers the attestation is an operational green light to patch or to consume Microsoft-published fixed images; for all other Microsoft artifacts the messaging prompts customers to verify rather than to assume safety.
Risks, limitations and open questions
- Attestation lag: Large organizations publish attestations per product family over time. The absence of a product in Microsoft’s VEX/CSAF feed is often a reflection of inventory work still in progress, not proof of absence. Assume possible exposure until verified.
- Embedded/third‑party artifacts: Marketplace images, partner appliances, or statically linked binaries inside Microsoft-supplied artifacts may not be covered by a product-level attestation and therefore remain a blind spot unless explicitly inspected.
- Operational drift: Customers that build custom images based on Microsoft images may reintroduce older kernels or third-party components; a vendor attestation does not propagate automatically down the entire supply chain.
- Local attack vector nuance: While the CVE is ranked medium and is local in nature, many operational environments enable BPF loading for performance and observability. Those environments — particularly shared developer hosts and multi-tenant systems — can have meaningful operational exposure even though the bug is not a remote RCE.
- Verification burden: The practical burden falls to operators: to achieve high confidence they must combine vendor attestations with artifact inspection and runtime verification (kernel logs, bpftool, sysctl settings).
Recommended mitigations and operational checklist
- Immediate (if you run Azure Linux): Follow Microsoft’s attestation and apply the Microsoft-supplied image or kernel update for Azure Linux; confirm the VEX/CSAF attestation shows the fixed status for the product.
- Immediate (if you run other Microsoft images or third‑party images on Azure): Don’t assume safe — inventory artifacts and verify kernel packages or rebuild images with patched base images. Treat Microsoft’s published attestation as one input among many.
- Universal mitigations while patching:
- Set kernel.unprivileged_bpf_disabled = 1 where appropriate to block unprivileged BPF loads.
- Ensure CAP_BPF and CAP_SYS_ADMIN capabilities are only held by trusted service accounts.
- Audit container and CI runners for permissions that would allow BPF program loads.
- Monitoring and detection:
- Alert on unusual kernel verifier warnings and BPF-related OOPS messages in kernel logs.
- Monitor bpftool outputs and /proc/kallsyms for unexpected BPF trampolines or load patterns if the CVE in question touches the trampoline lifecycle. (For CVE‑2025‑39748 the primary symptom is verifier-range warnings rather than trampoline leaks, but general BPF telemetry is valuable.
- Patch validation: After upgrading kernels, validate by reproducing representative eBPF workloads or the BPF selftests that previously triggered the issue; absence of the previously observed verifier warnings and OOPS messages is a practical validation.
How to interpret vendor statements going forward
Vendor-provided CSAF/VEX attestations are a major step forward for supply-chain transparency. They allow automation and consistent policy evaluation at scale. But they are also
incremental: vendors will release attestations by product family as inventory work completes. Operational teams must therefore treat attestations as a
starting point for automated controls — and combine them with artifact inspection, image scanning, and runtime telemetry to reach operational assurance.
When a vendor states “we will update the CVE if additional products are identified,” that is a commitment to expand the attestation — not an absolute denial of presence in other products. Use vendor attestations to prioritize response (start with the attested product), but do not stop inventory work for non-attested product images you actually run.
Conclusion
CVE‑2025‑39748 is a medium‑severity Linux kernel BPF verifier bug with a local/availability attack vector. Microsoft’s MSRC attestation that
Azure Linux includes the relevant component is authoritative for Azure Linux images and is a useful, machine-readable signal to automate remediation for that product family. However,
Azure Linux is not the only Microsoft artifact that could include the vulnerable component — it is simply the only product Microsoft has publicly attested as including it so far. Absence of attestation is not proof of absence; operators must verify other Microsoft images, curated runtimes, marketplace appliances, and any statically linked binaries they run.
Action items for organizations:
- Treat the MSRC/Azure Linux attestation as an authoritative, prioritized remediation signal for Azure Linux images and apply vendor-supplied updates.
- Inventory and inspect other Microsoft-supplied or Microsoft-hosted artifacts you run; assume possible exposure until you can confirm otherwise.
- Apply vendor kernel updates from your distribution (Ubuntu, Debian, SUSE, Amazon Linux, etc. and use mitigations (disable unprivileged BPF, tighten CAP_BPF/CAP_SYS_ADMIN) while you complete rollouts.
The combination of vendor attestations, distribution security trackers, and runtime telemetry gives the highest confidence. Use all three: trust the attestation where it exists; verify artifacts where it does not; and patch or harden runtime policies to close the window of exposure.
Source: MSRC
Security Update Guide - Microsoft Security Response Center