CVE-2025-37800 Explained: Azure Linux Attestations and Kernel Race

Microsoft’s brief MSRC entry on CVE-2025-37800 names Azure Linux as a product that “includes this open‑source library and is therefore potentially affected,” but that product‑level attestation is exactly that — an authoritative inventory statement for Azure Linux, not a technical guarantee that no other Microsoft product can carry the vulnerable code.

Background / Overview​

CVE‑2025‑37800 was assigned to a Linux‑kernel race condition that can produce a NULL pointer dereference in the kernel driver core, specifically inside the dev_uevent() path. The defect manifests when a userspace reader accesses the device’s "uevent" attribute at the same time another thread is unbinding the device from its driver; a timing window can let dev->driver change from a valid pointer to NULL and later be dereferenced, causing a kernel oops or crash. The upstream fix is a small, surgical synchronization change: use READ_ONCE() to fetch the pointer, use WRITE_ONCE() when setting it, and acquire the bus drivers klist lock to keep the driver instance from disappearing while being accessed.
This type of bug is a robustness and availability issue: it is classified as a local, low‑privilege vector that can produce denial‑of‑service by crashing the kernel (medium severity in many databases). Multiple vendors and vulnerability databases tracked the CVE and published patches or advisory mappings after the upstream kernel patch was merged.

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What the MSRC entry actually says — and why that matters​

Microsoft’s public wording for this and similar Linux upstream CVEs has followed a consistent pattern in 2025 and 2026: when Microsoft’s product inventory contains an upstream open‑source component, MSRC publishes a product‑scoped attestation such as “Azure Linux includes this open‑source library and is therefore potentially affected.” That statement confirms Microsoft has found the upstream code in Azure Linux images and is treating Azure Linux as a confirmed carrier for operational guidance. Microsoft also committed to publish machine‑readable VEX/CSAF attestations and to update those attestations if other Microsoft products are later identified as carriers.
That phrasing is carefully limited on purpose. The presence of upstream Linux code in one Microsoft product does not imply exclusivity; Microsoft ships multiple independent Linux artifacts (Azure Linux kernels, WSL2 kernel builds, Azure VM kernel images, and other internal kernels used to support Azure services), and each artifact is a separate technical unit whose contents depend on the kernel version, configuration, and applied backports. Treat MSRC’s attestation as: definitive yes for Azure Linux; not a definitive no for other Microsoft artifacts until they are either explicitly attested as Not Affected or inventory‑checked and published as affected.

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Technical deep dive: why dev_uevent() was vulnerable​

The bug in plain language​

The Linux kernel exposes device attributes through sysfs; one of those attributes is uevent, which userspace can read to learn dynamic device properties. The dev_uevent() path constructs environment variables for uevents and at one point reads dev->driver to include DRIVER=... if a device is bound to a driver.
If one kernel thread is reading dev->driver inside dev_uevent() while another thread is in the middle of unbinding the device and sets dev->driver = NULL, a race can occur: the reader checks if (dev->driver) and then, before it uses the pointer, the unbinder flips it to NULL. The reader then dereferences a NULL pointer. The fix centered on two safety measures:

• Use READ_ONCE() when reading dev->driver to avoid torn reads and make the read atomic with respect to concurrent writes.
• Take the bus' drivers klist lock to prevent a driver instance from being removed while the reader is accessing it.
• Use WRITE_ONCE() on writes to dev->driver to ensure atomic store semantics.

Those changes eliminate the TOCTOU (time‑of‑check/time‑of‑use) window that allowed the crash. The patches are small and were backported across several stable kernel branches.

Impact surface and exploitability​

• Attack vector: local. An unprivileged user who can trigger reads of the uevent attribute (or orchestrate concurrent unbinds) may be able to trigger a kernel oops. That makes this primarily an availability/DoS issue, not a remote code execution flaw.
• Privilege requirements: low — the vulnerability was rated with low privilege required in many databases, because uevent attributes are often readable by unprivileged processes.
• Likelihood of remote exploitation: low without local access. Many cloud scenarios where users get shell access or run code inside VMs or containers make local exploitation realistic.
• Known exploitation in the wild: as of the last consolidated advisories there were no widespread reports of active exploitation; the principal risk is accidental or deliberate DoS on hosts that use affected kernel builds.

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Which products are affected — the inventory question​

What Microsoft has said​

For CVE‑2025‑37800 Microsoft’s publicly visible position at the time of the advisory was to list Azure Linux as the Microsoft product that “includes this open‑source library and is therefore potentially affected.” That mapping appears in Microsoft’s product‑scoped vulnerability attestations and the VEX/CSAF files that Microsoft has begun publishing for Azure Linux. Microsoft explicitly pledged to update the CVE/VEX entry if additional Microsoft products are later identified as carriers.

Does that mean Azure Linux is the only Microsoft product affected?​

No — not necessarily. The practical distinction is this:

• Azure Linux = confirmed carrier (Microsoft checked and attested that the vulnerable upstream code exists in that product).
• Other Microsoft products = un‑attested at that moment. Absence of an attestation is not proof of absence. Other Microsoft kernel artifacts — particularly the publicly maintained WSL2 kernel builds and the various linux-azure / linux-azure-6.x kernels used for Azure VMs — are independent builds that can include the same upstream file or change depending on version and config. Whether any particular Microsoft artifact is vulnerable depends on the kernel version and whether the vulnerable commit range is present or has been backported/fixed. Multiple WindowsForum analyses have emphasized this point and recommended inventory checks for each artifact.

Independent vulnerability databases and vendor advisories mapped the CVE to upstream kernels and to distributions (Debian, Ubuntu, Red Hat, SUSE, etc.), reinforcing that the bug is upstream Linux kernel code and can appear in any distribution or product that ships a kernel containing the affected commit range. That means any Microsoft product that ships a Linux kernel — not only Azure Linux — could be in scope until inventoried.

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How operators should verify exposure in Microsoft artifacts​

If you run Microsoft-managed Linux artifacts (Azure VM images, Azure Marketplace images, WSL kernels, or Azure Linux), do not assume Azure Linux is the only concern. Follow these verification steps:

• Identify the kernel build used by the artifact (kernel version + vendor string).
• On a running system: uname -a and check the exact package build ID (for distro packages).
• Compare the kernel version and commit range to the upstream CVE fix commits.
• The upstream commits and stable backports are recorded in kernel.org commit logs tied to the CVE; compare the version tags to see if your build is older than the patched release.
• Inspect the kernel package metadata or vendor VEX/CSAF files.
• For Microsoft’s Azure Linux, MSRC/VEX/CSAF files are published and identify affected azl kernel packages; other Microsoft products may not yet be attested but may be updated later.
• If in doubt, test for the presence of the patched lines in the kernel source used to build your artifact (or check vendor patch lists).
• If you maintain your own kernel builds (e.g., custom WSL kernel), ensure the code includes the READ_ONCE/WRITE_ONCE and klist locking changes.
• Apply vendor patches d kernel release where available.
• Distributions and kernel vendors published fixes and backports; apply the appropriate kernel update for your distro/build.

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Practical mitigation and remediation guidance​

• For cloud images and VMs: apply vendor kernel updates or switch to a patched kernel image provided by your cloud vendor. Microsoft has published Azure Linux updates and will extend VEX/CSAF mappings as they inventory other artifacts.
• For WSL2 users: check the WSL2 kernel build being used. Microsoft maintains a public WSL2 kernel repository; if you use a custom WSL kernel, rebuild it from patched sources. If you rely on the distribution’s kernel inside WSL, update the distro packages.
• For container hosts and multi‑tenant systems: prioritise patching hosts where unprivileged users can execute code or where untrusted containers run, since the attack vector is local.
• Short‑term hardening: while the fix is small and safe, there is no easy configuration toggle that removes the vulnerable code without losing device driver functionality; the practical course is to apply the kernel update. Use host isolation and monitoring to detect abnormal kernel oops events until patches are in place.

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Supply‑chain transparency and Microsoft’s VEX/CSAF approach​

Microsoft’s October 2025 announcement formalised a phased rollout of machine‑readable VEX/CSAF attestations starting with Azure Linux to improve customer visibility into which third‑party CVEs affroducts. That initiative is aimed at reducing uncertainty — for example, distinguishing which Azure Linux kernel builds include an upstream change — but it is intentionally gradual. Microsoft’s approach is “crawl, walk, run”: start with one product family, validate the data model, then expand the catalog of attestations to other products and services. That means you should expect Azure Linux to be first and other Microsoft artifacts to follow as inventory checks complete.
Why the phased approach matters: mapping every upstream CVE to every Microsoft artifact is a significant engineering and supply‑chain task. Each product/feature may use different kernel versions and have different backports, so accurate attestations require build‑level checking. Microsoft’s public statement commits to update CVE/VEX records when more artifacts are found to include the vulnerable code — which is exactly the responsible, incremental path.

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Critical analysis — strengths, gaps, and residual risks​

Strengths​

• Microsoft’s public atnux is an operationally useful, authoritative confirmation for customers that Azure Linux images include the implicated upstream code. That allows Azure Linux customers to act immediately, prioritise patches, and reduce false positives in vulnerability scanning.
• The upstream fix is small, well understood, and backported by kernel maintainers; stable kernel updates are available for many branches, making remediation straightforward where vendors and distributors push updates.
• Microsoft’s VEX/CSAF commitment is a structural improvement in supply‑chain transparency: machine‑readable attestations help automate triage and reduce the "unknown" zone for customers.

Gaps & risks​

• Microsoft’s attestation approach, by its nature, initially creates a two‑tier status: attested (Azure Linux) vs not yet attested (everything else). Operators who interpret “not attested” as “not affected” will be at risk. The gap is procedural, not technical, and requires operators to perform artifact‑level inventory checks.
• Multiple Microsoft kernel artifacts exist (WSL2, linux‑azure, custom Azure VM kernels, device images). Any of those could contain the affected upstream code depending on their version and applied patches. Until Microsoft expands its VEX catalog, customers must assume potential exposure and validate.
• For large, heterogeneous environments, patching kernels across images and appliances is operationally complex. Patching schedules, uptime SLAs, and compatibility testing create friction that can delay remediation and increase risk windows.

Unverifiable or changing elements (caution)​

• Whether every Microsoft product that contains a Linux kernel is already patched cannot be answered definitively from public records at a single point in time. Microsoft pledged to update attestations as they discover additional carriers; customers should therefore rely on artifact checks plus vendor advisories rather than a single MSRC entry to determine final exposure. This is a time‑sensitive fact and must be verified at the time of your audit.

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Practical checklist for WindowsForum readers (actionable steps)​

• Inventory:
• List every Microsoft artifact in your environment that ships or depends on a Linux kernel (Azure VM images, Azure Marketplace images, Azure Linux, WSL2 distributions, appliances).
• Record kernel version and package build IDs for each artifact.
• Compare:
• Map those kernel versions to the kernel.org commits or to vendor advisory lists that reference CVE‑2025‑37800. The kernel commits associated with the fix are referenced in stable patch notes and vulnerability databases.
• Patch:
• Apply vendor‑supplied kernel updates where available.
• For custom kernels (including WSL custom builds), merge the upstream fix lines (READ_ONCE/WRITE_ONCE + klist locking) and rebuild.
• Monitor:
• Watch for kernel oops or panic events in logs (dmesg, journalctl).
• Use host integrity checks and behavioral detection to spot unexplained reboots or oops events.
• Communicate:
• For multi‑tenant and cloud operators, publicise the patch windows and mitigations to tenants/customers; they may need to update guest kernels or their workloads accordingly.

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Why this matters beyond a single CVE​

CVE‑2025‑37800 illustrates a recurring reality of modern software supply chains: Microsoft and other large platforms increasingly ship or integrate upstream open‑source components. Publishing machine‑readable VEX/CSAF attestations for those components is a meaningful improvement in transparency, but it does not replace the need for per‑artifact verification and standard vulnerability management disciplines.
For customers, the practical takeaway is simple:

• Treat Microsoft’s MSRC product attestations as authoritative for the products listed and as a starting point for broader artifact verification.
• Do not assume an absence of attestation equals absence of exposure.
• Maintain the capability to perform the three‑step pattern of inventory → compare → remediate for any vendor‑provided artifact that implements open‑source components.

Microsoft’s commitment to extend VEX/CSAF mapping beyond Azure Linux will reduce the operational burden over time, but until that mapping is exhaustive, the responsibility for artifact‑level verification remains with operators and their security teams.

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Conclusion​

CVE‑2025‑37800 is a technical, locally exploitable Linux kernel race condition that can cause a NULL pointer dereference in dev_uevent(). Upstream fixes are available and have been integrated into stable kernels; vulnerability databases and vendors documented the issue and issued advisories. Microsoft’s public mapping confirms Azure Linux as a product that includes the implicated upstream code and is therefore potentially affected, and Microsoft has committed to expand VEX/CSAF attestations if other Microsoft products are found to ship the same code. That MSRC wording is a product‑scoped inventory attestation and should be read exactly that way — authoritative for Azure Linux, but not a categorical statement that no other Microsoft product can include the vulnerable code. Operators should treat other Microsoft artifacts as un‑attested and verify kernel versions and patch status for each artifact under their control.
The immediate operational advice is clear: inventory your Microsoft Linux artifacts, compare their kernel versions to the upstream patch commits, and apply vendor or upstream fixes promptly to close the availability risk posed by this bug.

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Source: MSRC Security Update Guide - Microsoft Security Response Center