Understanding CVE-2025-39713: Azure Linux Attestation vs Global Risk

The recently assigned CVE‑2025‑39713 is a kernel‑level TOCTOU (time‑of‑check/time‑of‑use) race in the Linux media driver rainshadow‑cec that can lead to a buffer overflow in the interrupt handler; Microsoft’s public advisory for this CVE names Azure Linux as a product that “includes this open‑source library and is therefore potentially affected,” but that phrasing is a product‑scoped attestation — not a technical proof that no other Microsoft product or artifact can contain the same vulnerable code.

Background / Overview​

CVE‑2025‑39713 was assigned for a defect inside drivers/media/cec/usb/rainshadow/rainshadow‑cec.c: the interrupt handler rain_interrupt() performed a check of rain->buf_len before acquiring the associated spinlock, creating a race with the work handler that updates buf_len. Multiple simultaneous interrupts could read buf_len as not full, both proceed to write, and increment buf_len past DATA_SIZE, enabling a kernel buffer overflow and memory corruption. Upstream maintainers fixed this by moving the spin_lock before the buffer‑full check and ensuring the lock is released on overflow paths.
Independent vulnerability databases and distribution trackers document the issue and list affected kernel trees and fixed versions. Public vulnerability trackers and advisory aggregators classify the flaw as a local TOCTOU race that can cause kernel memory corruption and denial of service, and they report patches merged into multiple stable kernel branches.
At the same time MSRC (Microsoft Security Response Center) published a product attestation for this CVE noting that Azure Linux includes this open‑source library and is therefore potentially affected and promised that, if other Microsoft products are identified as carriers, the CVE/VEX entry will be updated. That wording — and Microsoft’s broader rollout of machine‑readable VEX/CSAF attestations for Azure Linux — is important context for how Microsoft communicates exposure to embedded open‑source components.

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What Microsoft’s wording actually means​

Product‑scoped attestation, not global inventory​

When MSRC says “Azure Linux includes this open‑source library and is therefore potentially affected,” they are making a deliberate, product‑level statement: they have inventory‑checked the Azure Linux distribution and found the upstream component (in this case, the kernel driver) present in that product family. This is an authoritative attestation for the named product — but it is not an exhaustive, across‑the‑entire‑company search showing the component is absent from every Microsoft artifact (images, kernels shipped inside other products, WSL kernels, Marketplace images, security agents, appliance images, etc.). Treat the Azure Linux listing as a confirmed hit for that product, and treat the absence of other product entries as absence of attestation, not proof of absence.

Why Microsoft takes this approach​

Microsoft has begun publishing VEX/CSAF attestations to provide precise, machine‑readable signals about which of its products include which third‑party components and whether those products are affected, fixed, or not affected. This phased approach started with Azure Linux because it’s a clear, self‑contained distribution Microsoft controls end‑to‑end; it’s operationally feasible to inventory and publish a VEX for that family first, then expand to more products over time. Microsoft’s messaging around CVEs and Azure Linux is therefore both a transparency improvement and an operationally practical step rather than a guarantee of exclusivity.

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So — is Azure Linux the only Microsoft product that could include rainshadow‑cec?​

Short answer: No — Azure Linux is the only Microsoft product Microsoft has publicly attested to include this component so far, but that does not mean other Microsoft products cannot or do not include the same vulnerable code. The attestation is a scoped inventory result. Until Microsoft performs the same inventory and publishes a VEX entry proving absence or presence for other products, operators should assume other Microsoft artifacts that ship a Linux kernel could plausibly contain the driver.
Why this nuance matters:

• Kernel drivers are upstream components; whether a given Microsoft image or product contains them depends on the exact kernel version, configuration, and modules included at build time. Two images from the same vendor can have different kernels and therefore different exposures.
• Microsoft publishes Azure Linux VEX entries because that product is centrally built and controlled; other Microsoft products (for example, WSL2, certain Azure Marketplace images, embedded Linux used in appliances or edge devices, or third‑party images distributed via Microsoft channels) may use different kernel builds or configurations and therefore may or may not include the same driver.
• There is historical precedent: Microsoft’s previous attestation language for other kernel CVEs named Azure Linux as an affected product while subsequent investigations showed kernel artifacts under Microsoft control (for example a WSL2 kernel source tree or other Microsoft‑packaged kernels) also contained the upstream component; Microsoft then updated CVE/VEX mappings as inventory work completed. That precedent underlines that an initial Azure Linux attestation is not an exclusive claim.

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Evidence and corroboration (what public sources say)​

• The NVD entry and multiple vulnerability trackers describe the defect, its root cause (TOCTOU race in rain_interrupt()), and the upstream fix. These sources show the vulnerability is a kernel media driver issue and provide the technical basis for patching and risk assessment.
• Distribution trackers (Debian security tracker, advisory aggregators, and vendor advisories like Amazon Linux ALAS) list affected kernel versions and the fixed package releases; these independent sources validate the vulnerability’s presence across mainstream kernel branches and document which distribution releases received backported fixes. That helps operators map exposure to actual kernel package versions in use.
• Microsoft’s public posture — publishing CSAF/VEX attestations and explicitly naming Azure Linux as a confirmed carrier for some third‑party CVEs — is documented in Microsoft’s transparency blog and is visible in Microsoft’s CVE/VEX pages. Microsoft has stated it will update CVE/VEX entries if additional Microsoft products are found to include the component. That corporate policy is material to how you interpret the Azure Linux statement.
• Industry discussion and independent analysis (community write‑ups, vendor advisories, and security forums) consistently interpret MSRC’s wording as product‑scoped and recommend local verification for any other Microsoft artifacts or images your organization runs. That consensus is a practical operational insight for defenders.

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Practical risk analysis for Microsoft customers​

• Attack surface and exploitability
• CVE‑2025‑39713 is a local TOCTOU kernel race; it requires local code execution or a way to induce the driver to handle rapid concurrent interrupts from an untrusted source. In practice, this typically results in denial of service (kernel crash) or memory corruption; there are no confirmed public exploits for privilege escalation as of the current advisories, but kernel memory corruption in interrupt context is inherently dangerous and merits remediation.
• Which Microsoft assets are likely to be affected beyond Azure Linux?
• Any Microsoft product that ships a Linux kernel built with the media/rainshadow‑cec code path compiled in (either built‑in or as a module) could be affected. Plausible places to check include:
• WSL2's custom kernel builds (if your environment uses a Microsoft‑supplied WSL kernel binary).
• Azure Marketplace VM images (non‑Azure Linux images shipped by Microsoft or partner images that reuse Microsoft kernels).
• Security and management agents for Linuxules in their delivered kernel images (rare, but possible for specialized products).
• Edge or appliance images Microsoft supplies for Azure Stack/other offerings that use Linux.
• Likelihood depends on build choices
• Kernel configuration flags determine whether a driver exists in the final image. A component present in upstream source may be omitted by a kernel build through Kconfig. Thus, identical upstream code can exist in one product and be absent in another. This is why Microsoft’s product‑only attestation is meaningful and why operators need artifact‑level checks.

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What defenders should do now — step‑by‑step checklist​

Below are practical, prioritized actions to verify exposure and mitigate risk across environments running Microsoft artifacts or third‑party Linux images.

• Treat the Azure Linux attestation as a confirmed hit for that product
• If you run Azure Linux images, schedule kernel updates immediately to the vendor‑supplied patched kernel packages documented by your distribution or by Microsoft’s VEX/CSAF attestations. Distribution trackers list the patched versions and package names to install.
• Inventory kernel versions and module lists across Microsoft‑supplied artifacts
• On each host, collect:
• uname -a
• rpm -q kernel or dpkg -l | grep linux-image (depending on distro)
• lsmod and modinfo for potentially related module names
• Search /lib/modules/$(uname -r) and /boot for references to rainshadow or rainshadow‑cec (grep -R 'rainshadow' /lib/modules /boot /usr/src).
• If a module or driver symbol is present in the built kernel image (built‑in), the above checks will show it; if present as a module, modinfo will return metadata. These checks are fast and should be automated in your fleet scanner. (Examples shown as diagnostic suggestions; adapt to your management tooling.)
• For WSL2 and other Microsoft custom kernels
• If you use the Microsoft‑supplied WSL kernel binary, query the kernel version (uname) inside WSL, and, if you have host access, compare the WSL kernel source or the WSL kernel binary info against the fixed upstream commits. If your WSL kernel contains the driver, update the WSL kernel or apply Microsoft’s WSL update when available. Historically, components shipped in WSL kernels have been the subject of separate CVE coverage and were updated when Microsoft identified overlap.
• Check Azure Marketplace images and any Microsoft‑provided VM images
• If you run Marketplace images published by Microsoft or partners, treat each image as an independent artifact — image family names do not guarantee identical kernel contents. For critical workloads, validate the running kernel as above, using automated audit scripts before and after image deployment.
• Use machine‑readable VEX/CSAF feeds
• Microsoft has started publishing VEX/CSAF attestations for Azure Linux and intends to expand this capability. Ingest these attestations into your vulnerability management tooling to map Microsoft’s product attestation state to your inventory automatically. However, note that VEX coverage is currently phased; absence of an attestation for a given Microsoft product is not proof that a product is not affected.
• Patch promptly and test
• Apply the official upstream or distribution‑provided kernel fixes. Distribution trackers show which kernel package versions include the backport. Where patching is disruptive, evaluate compensating controls (isolate affected hosts, restrict local access, apply kernel livepatching if available from your vendor), but prioritize a full kernel update as the durable fix.
• Monitor for updates from Microsoft
• Microsoft has said it will update CVE/VEX entries if additional Microsoft products are identified as carriers. Monitor MSRC advisories and your existing vendor feeds for any updated attestations that expand the list of affected Microsoft products. Because MSRC publishes machine‑readable files over time, automating ingestion is the most reliable approach to stay current.

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How to verify presence of the vulnerable component (concrete checks)​

• Quick module search (run as root):
• grep -R --line‑number "rainshadow" /lib/modules/$(uname -r) /boot || true
• modinfo rainshadow || true
• If the driver is built into the kernel (not a module):
• zcat /proc/config.gz | grep -i rainshadow || grep -i CEC /boot/config-$(uname -r) || true
• Look for related CONFIG options that indicate whether the media/cec USB driver family is enabled.
• If you have image sources:
• Search kernel source trees you ship inside products (for example WSL2 kernel source snapshots used in your environment) for the file path drivers/media/cec/usb/rainshadow.
• Use vulnerability scanners that map CVE → kernel symbols and search vendor images automatically (many enterprise scanners can flag kernel module presence by signature). If your tooling cannot search inside custom images, extract and inspect the kernel or boot image where feasible.

Note: These are practical, commonly used checks. If you need to automate fleet‑wide detection, integrate these checks into your configuration management or EDR/asset inventory tooling.

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Why this matters to organizations running Microsoft‑provided Linux artifacts​

• False reassurance risk: Relying on the absence of a VEX/CSAF entry for a Microsoft product as evidence of absence is risky. The Azure Linux attestation is useful and authoritative for that product, but it is a desk‑level inventory statement, not a universal guarantee.
• Patch timing and blast radius: Kernel fixes need to be applied at the host / image level. If a Microsoft product (other than Azure Linux) happens to ship a kernel containing the vulnerable driver, a fleet‑wide update will be necessary there as well. If you run heterogeneous images — some Microsoft‑supplied, some vendor‑supplied — ensure you check each artifact individually.
• Attack complexity vs. impact: This TOCTOU race is a local exploit class; it usually requires local unprivileged code execution or a way to trigger the driver repeatedly. However, kernel memory corruption in interrupt context can cause reliability failures and potentially open other attack paths; organizations with shared hosting, multi‑tenant workloads, or that grant many internal users code execution privileges should prioritize remediation.

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Strengths and limitations of Microsoft’s current transparency approach​

Strengths​

• Machine‑readable VEX/CSAF: Microsoft’s shift to publishing CSAF/VEX for Azure Linux improves automation, reduces noisy alerts for customers, and gives a clear inventory statement for that distribution. This is a tangible step forward in supplier transparency and is immediately useful for customers who run Azure Linux.
• Deliberate phased rollout: Beginning with a single, well‑scoped product avoids early mistakes and allows Microsoft to fine‑tune processes before expanding VEX coverage across many product lines.

Limitations and risks​

• Scope‑limited attestations: VEX/CSAF entries only attest to the products they cover. Until Microsoft expands coverage, customers must perform artifact‑level verification for any other Microsoft‑provided images they run. This creates an operational gap that defenders must address proactively.
• Timing of updates: Inventory and attestation are manual and time‑consuming for large vendors. There may be a lag between the discovery of vulnerable components in other Microsoft artifacts and the update of CVE/VEX records.
• Potential for complacency: Customers may misread Microsoft’s Azure Linux statement as a universal “only Azure Linux is affected” claim. That interpretation is incorrect and could lead to unpatched exposures in other artifacts. Security teams must avoid this trap.

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Final recommendations (operational priorities)​

• If you run Azure Linux: apply the vendor‑supplied kernel update promptly according to the distribution advisory.
• Inventory every Linux kernel you run that originated from Microsoft (WSL2 binaries, Marketplace images, appliance images) and run the concrete checks above to detect the rainshadow driver or its symbols.
• Ingest Microsoft’s CSAF/VEX feeds and your distribution advisories into your vulnerability management pipeline, but treat VEX as an authoritative product feed only — continue artifact‑level verification as a parallel control.
• For environments where patching kernels is operationally difficult, implement compensating controls: restrict local code execution, enforce stronger host‑isolation, and prioritize host segmentation until the kernel update is deployed.
• Monitor MSRC and your distribution tracker feeds for updates expanding the list of Microsoft products that include the component; update your remediation plans accordingly.

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Conclusion​

CVE‑2025‑39713 is a real kernel TOCTOU race in the rainshadow‑cec driver with an upstream fix already merged and distribution patches rolling out. Microsoft’s public statement that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate but limited in scope: it’s an attestation about Azure Linux, not a universal claim that no other Microsoft product can include the same vulnerable code. Defenders must therefore combine Microsoft’s VEX/CSAF signals with artifact‑level inventory and verification across any Microsoft‑supplied kernels or images they run. In short: treat the Azure Linux attestation as authoritative for that product, but continue to verify and patch all Linux artifacts you operate, including WSL kernels and Marketplace images, until Microsoft’s broader attestation coverage (or your own verification) proves them clean.

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Source: MSRC Security Update Guide - Microsoft Security Response Center