CVE-2025-38226: Vivid Kernel Driver Risk in Azure Linux and Microsoft Artifacts

CVE-2025-38226 is a Linux-kernel vulnerability in the Virtual Video Test Driver (vivid) that can cause a vmalloc out‑of‑bounds write; Microsoft has publicly attested that Azure Linux (the Azure Linux distribution formerly known as CBL-Mariner) includes the affected upstream component, but that attestation applies only to that product family so far — it is not proof that Azure Linux is the only Microsoft product that could possibly contain the vulnerable code. In short: Microsoft has confirmed Azure Linux as an affected Microsoft artifact for this CVE, but absence of attestation for other Microsoft SKUs is absence of attestation, not evidence the code is not present elsewhere. Read on for a technical breakdown, practical detection and mitigation steps, and operational guidance for organizations that run Microsoft-delivered Linux artifacts in their environments.

Background / Overview​

The issue tracked as CVE-2025-38226 was found in the Linux kernel’s test driver code for the virtual video device known as vivid. The bug shows up as a KASAN (KernelAddressSANitizer) report: a vmalloc out‑of‑bounds write originating in the tpg (test pattern generator) routines used by vivid. Upstream kernel maintainers accepted patches that change the sizing logic in the composing/fill routines; those fixes were backported into multiple stable kernel trees.
Two important technical facts about this CVE matter for risk assessment:

• The vulnerable code resides under drivers/media/test-drivers/vivid — the virtual video test driver intended for kernel testing, not a production camera driver. It is a test module that can be built as a module or compiled into a kernel, controlled by standard kernel configuration options such as CONFIG_VIDEO_VIVID and related MEDIA_TEST_SUPPORT flags.
• The attack surface is primarily local: an attacker needs the ability to write to a device node associated with vivid (for example /dev/video0 if the vivid module is loaded and a device node exists). When exploitable, the bug can corrupt kernel memory (out‑of‑bounds write), potentially allowing privilege escalation or system compromise depending on the kernel build and surrounding mitigations.

Different vulnerability databases have published differing CVSS vectors and scores for the issue, reflecting nuance about required privileges and the practical difficulty of exploitation; the National Vulnerability Database characterizes the impact as high in its scoring, while some distributor advisories report a lower operational severity due to the local-only attack vector and the fact vivid is commonly disabled in production kernels.

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Why Microsoft’s Azure Linux attestation matters — and why it’s limited​

What Microsoft published​

Microsoft began publishing machine-readable CSAF/VEX attestations for third‑party CVEs as part of a transparency initiative. When Microsoft’s security team examines an externally assigned CVE that concerns an upstream open-source component, they may publish an attestation indicating the vulnerability status for a Microsoft product. For CVE‑2025‑38226, Microsoft’s public attestation explicitly covers Azure Linux, stating that Azure Linux includes the implicated upstream component and describing their stance on updates and transparency.

What that attestation means (and does not mean)​

• It is authoritative for Azure Linux artifacts. If you run Microsoft-published Azure Linux images, the VEX/CSAF attestation is the vendor-provided statement that helps you automate risk decisions for those specific images.
• It is not an exhaustive inventory across all Microsoft products. Microsoft has said it’s starting VEX/CSAF with Azure Linux and will expand attestations over time. That phased rollout means other Microsoft images, kernels, or Linux‑based artifacts may not yet have an attestation — that absence does not prove the vulnerable code is not present.
• Different build artifacts can and do differ. Microsoft ships many independently built artifacts: Azure Linux images, curated Marketplace images, container base images, VM images tied to specific VM SKUs, the WSL2 kernel, and more. Each artifact is produced with potentially different kernel versions, configuration flags, and module sets. Presence of upstream source code in one artifact doesn’t automatically imply it’s present in every artifact.

In practice: treat Microsoft’s attestation as a clear signal for Azure Linux images, and treat other Microsoft-delivered Linux artifacts as unverified until Microsoft either attests them or you validate them locally.

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What vivid is and why it shows up (or doesn’t) in product builds​

The vivid driver is a virtual video test device used by kernel developers to exercise the V4L2 (Video4Linux2) stack and related media subsystems. It is typically controlled by kernel configuration variables:

• CONFIG_VIDEO_VIVID — enable the vivid driver (module or built‑in)
• CONFIG_MEDIA_TEST_SUPPORT — enables test drivers and helpers used for media testing
• CONFIG_VIDEO_DEV and CONFIG_VIDEO_V4L2 — core video/V4L2 support

Because those options are configurable at build time, whether a given kernel image contains vivid depends entirely on the build configuration and the modules that the vendor installs with that kernel. Production-focused distributions and cloud vendors commonly disable test drivers in their default kernel builds, but that is not universal — some images keep broader module support for compatibility or testing reasons.

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Can other Microsoft products include vivid?​

Short answer: yes, they technically could — but whether any specific product actually includes vivid must be verified artifact‑by‑artifact.
Why yes:

• Microsoft produces multiple Linux artifacts (WSL kernel builds, Azure Linux, Marketplace images, container base images, custom kernels for certain VM SKUs). Any of those artifacts can include or exclude vivid based on build-time configuration.
• The device shows up as a kernel module or built-in driver; inclusion is not a function of the upstream CVE assignment alone but of the kernel configuration bundled with each artifact.

Why verification is required:

• Microsoft’s current VEX/CSAF rollout began with Azure Linux. The presence of a VEX attestation for Azure Linux confirms inclusion there; the absence of attestations for other products means Microsoft either did not find the component in those artifacts or has not completed mapping yet.
• External scanners or automated vulnerability tools may flag CVE‑2025‑38226 for Microsoft artifacts based on upstream metadata; those flags can be false positives unless confirmed by artifact-level inspection or vendor attestation.

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Practical detection: how to check whether a specific Microsoft-delivered Linux artifact contains vivid​

If you operate systems that use Microsoft-published artifacts (for example: Azure Marketplace images, container images from Microsoft Container Registry, WSL2 kernels on Windows, or Azure Linux VMs), follow these steps to confirm whether vivid is present.

On a running Linux system (VM, container that runs full systemd, WSL, etc.)​

• Check kernel config for the running kernel:
• zgrep 'CONFIG_VIDEO_VIVID' /proc/config.gz
• grep -i 'VIDEO_VIVID' /boot/config-$(uname -r)
  These commands reveal whether vivid was compiled into the running kernel and in what form (y = built-in, m = module).
• Look for modules and devices:
• lsmod | grep -i vivid
• modinfo vivid (returns module info if present in module tree)
• find /lib/modules/$(uname -r) -type f -iname 'vivid'
• ls -l /dev/video* (if a vivid device is present, device nodes may exist)
• Inspect driver presence in module tree:
• For installed kernels, check /lib/modules/$(uname -r)/kernel/drivers/media/test-drivers/ for vivid files.
• If you find /dev/video* and suspect vivid:
• udevadm info -q all -n /dev/video0
• dmesg | grep -i vivid

For offline images or container base images​

• Extract the image or mount it and inspect the kernel modules directory and any /boot/config* files included in the image.
• Example container test command:
• docker run --rm -it --entrypoint /bin/sh microsoft-image:tag
• Inside: grep -i vivid /boot/config-$(uname -r) || ls /lib/modules//kernel/drivers/media/test-drivers/vivid

WSL2 kernel checks​

• WSL2’s kernel source is published by Microsoft; you can inspect the WSL2 kernel config files in the WSL2-Linux-Kernel repository (the Microsoft/config-wsl configs). If the WSL2 build config does not enable CONFIG_VIDEO_VIVID, the shipped kernel likely does not include vivid. Note: custom user-provided WSL kernels can differ.

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Recommended remediation and mitigation steps​

If you determine vivid is present in a Microsoft artifact you manage, follow these prioritized steps:

• Patch promptly — Update the kernel or image to a vendor-released version that includes the official fix. Upstream kernel maintainers published fixes and stable-tree backports; distributors have issued corrected kernels for affected series. Prefer full kernel updates over cherry-picking single commits unless you have a disciplined kernel development/test process.
• Audit and restrict access to device nodes — If /dev/video* nodes are present on multi-user systems, restrict write access to those nodes using udev rules, filesystem permissions, or container isolation. The vulnerability requires local write access; controlling that access reduces practical exploitable surface.
• Unload/blacklist the vivid module where feasible — If the module is not required, unload it and blacklist it from loading:
• rmmod vivid
• echo "blacklist vivid" > /etc/modprobe.d/blacklist-vivid.conf
  Note: blacklisting is a mitigation, not a substitute for patching, because the issue can be present in built-in kernels where modules cannot be unloaded.
• Harden images — For container and VM base images, rebuild minimal images that exclude test drivers and unnecessary kernel modules. For Azure Marketplace or curated images, validate the contained kernel config as part of your CI image validation.
• Test before rollout — Kernel patches can change behavior. Test the updated kernel under representative workloads before cloudwide or enterprise rollout, especially for images used by production VMs or critical workloads.
• Monitor vendor advisories and VEX/CSAF feeds — Subscribe to Microsoft’s CSAF/VEX feed for Azure Linux and MSRC updates, and monitor distribution advisories (Debian, Ubuntu, Red Hat, etc.) for their patched kernel builds. Microsoft has committed to updating CVE/VEX entries if additional affected Microsoft products are found.

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Detection and threat hunting: what to look for in logs and telemetry​

Because successful exploitation would likely cause kernel memory corruption, detection signals are noisy but actionable:

• KASAN or kernel oops logs that reference vivid or tpg_fill_plane_buffer functions.
• Unexpected kernel oops, panics, or crashes on systems that recently handled video device operations.
• Processes with handles to /dev/video* performing unusual patterns (frequent writes with large buffers).
• Process names associated with vivid in test installations (some vivid threads may create process names like vivid-000-vid-c).

Deploy EDR/YARA-like rules to flag kernel messages containing vivid-related function names and to alert on new device nodes under /dev/video* on systems where they are unexpected.

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Operational considerations for Microsoft customers and defenders​

• Treat Azure Linux’s attestation as authoritative for Azure Linux images and apply vendor-supplied patches per Microsoft guidance. The published VEX/CSAF files are intended to enable automation and reduce false positives for that product family.
• For other Microsoft artifacts, don’t assume immunity. Images and kernels shipped under different product names may have different build-time options. Where Microsoft has not attested an artifact, perform local verification or reach out to Microsoft Security Response Center through official channels for clarification.
• Inventory matters. Maintain an accurate inventory of kernel artifacts in your environment — including Marketplace images, container base images used by CI/CD, Marketplace Marketplace partner images, and WSL kernels in developer machines — since each is an independently built artifact.
• Container risk differs from VM risk. A container image that contains kernel modules on disk is less likely to be directly exploitable in a hosted container runtime, since containers share the host kernel. The real risk is whether the host kernel (not the userland container image) loads the vulnerable module or has the vulnerable code compiled in. Always validate host kernels and hypervisor-provided kernels rather than relying solely on container filesystem contents.

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Supply-chain and governance recommendations​

This CVE highlights a broader supply-chain reality for enterprises consuming vendor-supplied open-source components:

• Demand artifact-level attestations. Vendor attestations like CSAF/VEX are useful only when they cover the artifacts you consume. Encourage vendors to expand attestations beyond a single product family to cover kernels shipped in other delivery channels.
• Automate artifact validation. Integrate checks in image-building pipelines that:
• Verify kernel config flags (reject images with test drivers enabled when not required).
• Scan module trees for known vulnerable modules.
• Validate image manifests against vendor VEX/CSAF attestations.
• Use least-privilege on device resources. Device nodes that allow direct device writes should have strict ACLs and be assigned only to controlled service accounts or containers with a clear operational need.

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Why scoring and severity can vary across sources​

You will see different CVSS scores and severity labels for the same CVE across distributors. That’s normal:

• Upstream databases compute scores based on the theoretical impact; some distributors adjust severity considering the typical deployment context (for instance, “test driver not enabled in production kernels”).
• The exploitability vector for vivid is local; most classifications therefore reduce the attack vector component, but the potential impact of a kernel memory corruption can still be severe (privilege escalation, arbitrary code execution), which raises the overall impact scoring.

When planning remediation, weigh both the theoretical impact and your environment-specific exposure (presence of device nodes, remote access policies, user privileges).

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What Microsoft’s statement means for your risk decisions​

Microsoft’s public response — that Azure Linux has been attested and that Microsoft will update CVE/VEX entries if additional products are identified — should be interpreted as:

• A clear signal to prioritize patching and validation for Azure Linux artifacts in your environment.
• A reminder that inventory and verification are still the customer’s responsibility for other Microsoft artifacts until Microsoft publishes additional attestations.
• An operational encouragement to use Microsoft’s CSAF/VEX data for automation where possible, while retaining independent verification steps for mission‑critical artifacts.

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Step-by-step checklist for administrators (actionable)​

• Identify all Microsoft-supplied Linux artifacts in your environment:
• Azure Linux images, Marketplace images, container base images from Microsoft registries, WSL2 kernels on developer machines, custom images from Microsoft partners.
• For each artifact, run artifact-level checks:
• Check /boot/config-$(uname -r) or the artifact’s kernel config for CONFIG_VIDEO_VIVID.
• Inspect /lib/modules/.../kernel/drivers/media/test-drivers/ for vivid files.
• Check running systems for loaded vivid module (lsmod, modinfo, /dev/video*).
• If vivid is present:
• Schedule patching to the vendor’s fixed kernel release.
• If immediate patching is not possible, unload and blacklist the module where feasible and restrict access to device nodes.
• Document compensating controls and planned remediation windows.
• If vivid is not present:
• Record the verification and periodic re-scan schedule; include checks in CI for future image builds.
• Subscribe to Microsoft’s CSAF/VEX feeds and upstream distro advisories for ongoing updates.

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Strengths and limitations of the current ecosystem response​

Strengths:

• Upstream kernel maintainers fixed the bug and backported fixes to stable branches; patches are publicly available for distribution maintainers.
• Microsoft’s adoption of CSAF/VEX and its early attestation for Azure Linux is a positive step toward machine-readable, vendor-backed signals that reduce false positives and operational confusion.
• Multiple Linux distributors and cloud vendors likewise published advisories and updated kernels.

Limitations / Risks:

• Vendor attestations are only as useful as their scope. A phased rollout that begins with one product family leaves a window where other artifacts remain unverified.
• Large vendors ship hundreds of independently built artifacts; achieving full mapping is a nontrivial engineering task and can lag real-time disclosure.
• Automated vulnerability scanners that operate purely on CVE-to-package mappings may generate noisy findings for environments that do not actually contain the vulnerable component.

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Final assessment and recommendations​

• Microsoft’s public statement that Azure Linux includes the impacted upstream component is authoritative for Azure Linux artifacts; treat it as a high-confidence alert for those images and patch accordingly.
• Do not assume other Microsoft products are free of the vulnerable code simply because Microsoft’s attestation currently names only Azure Linux. Absence of mention is not proof of absence.
• Operationally, prioritize:
• Inventory and artifact verification,
• Prompt kernel updates where vivid is present,
• Device-node access control and module blacklisting as short‑term compensating controls,
• Automation using VEX/CSAF where available but with local verification gates for high‑value artifacts.
• For organizations with large fleets or heavy reliance on Microsoft-delivered images, request clarifying attestations from vendors for specific SKUs and integrate attestations into automated risk workflows.

CVE‑2025‑38226 is a reminder that even test drivers matter in supply-chain security: small, often‑ignored kernel components can become high-impact issues when they end up in production artifacts. The combination of vendor attestations (CSAF/VEX), artifact-level verification, and disciplined patch management is the practical path to minimizing window-of-exposure for this class of vulnerability.

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Conclusion
Azure Linux is the only Microsoft product Microsoft has publicly attested so far to include the vulnerable vivid component for CVE‑2025‑38226, and that attestation should drive immediate remediation for Azure Linux consumers. However, because Microsoft ships many independent kernel artifacts, customers must verify the presence or absence of vivid in other Microsoft-provided images and kernels rather than assuming they are unaffected. Use the detection commands and operational checklist above to confirm exposure in your environment, apply vendor-supplied fixes promptly, and adopt artifact-level verification and vendor attestation ingestion as standard parts of your vulnerability management lifecycle.

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Source: MSRC Security Update Guide - Microsoft Security Response Center