Understanding CVE-2025-38262: Azure Linux Attestation and Microsoft VEX CSAF Limitations

Microsoft’s concise advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is correct for the product Microsoft has inventory‑checked — but it is a product‑scoped attestation, not proof that no other Microsoft product could include the same vulnerable uartlite code.

Background / Overview​

CVE‑2025‑38262 is a Linux kernel robustness bug in the uartlite serial driver where a race during probe/registration can allow a code path to call uart_add_one_port while the uart driver’s state is not yet initialized, producing a NULL pointer dereference and a kernel panic (availability impact). The flaw was documented in canonical vulnerability databases and distribution advisories and has been patched in the upstream kernel stable trees. Technically, the race occurs when two UARTlite device instances probe concurrently. One probe thread may call uart_register_driver, which allocates and assigns memory to the driver’s uart_state, while a second probe bypasses driver registration and reaches ulite_assign → uart_add_one_port expecting the driver state to exist. That mismatch can dereference a NULL pointer and crash the kernel; the practical mitigation is to ensure the uart driver is registered at init so probe ordering cannot hit the race. Distribution and cloud vendors treated the CVE as a medium‑severity local availability issue (CVSS ~5.5) and shipped backports or rebuilds of affected kernel packages. Why this matters operationally: kernel crashes on shared infrastructure are a real risk for stability and multi‑tenant cloud services. The defect is not a demonstrated remote code‑execution worm‑vector; its primary impact is denial‑of‑service on hosts that load the uartlite driver or expose the affected hardware paths. Multiple mainstream distros and cloud vendors published advisories and fixed kernel packages after the upstream patch.

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What Microsoft actually said — parsing the MSRC wording​

Microsoft’s public advisory text for the CVE includes the phrase you quoted: that Azure Linux includes this open‑source library and is therefore potentially affected. That statement is paired with Microsoft’s public commitment to publish machine‑readable CSAF/VEX attestations (the VEX pilot began in October 2025) and to update the CVE/VEX data if additional Microsoft products are later identified as carriers of the same open‑source component. Two immediate readings of that message:

• It is an authoritative confirmation for Azure Linux: Microsoft has performed an inventory of the Azure Linux product family and found the upstream uartlite component inside the builds it examined. That product is therefore a confirmed in‑scope carrier for triage and patching.
• It is not an exclusive claim that no other Microsoft product contains the component. Microsoft explicitly framed this as a phased, product‑by‑product attestation process and said it will update the advisory and VEX records if more Microsoft products are identified. Absence of a VEX attestation for another Microsoft SKU therefore means “not yet attested,” not “not affected.”

That distinction — attested versus exclusive — is the core of the operational question. Large vendors ship many distinct artifacts (VM images, kernel builds, WSL kernels, Marketplace images, curated container base images and appliances). Whether any given Microsoft artifact includes an upstream kernel driver is a per‑artifact, build‑time property driven by kernel version, CONFIG_* choices and backports. Microsoft’s VEX pilot with Azure Linux gives defenders a deterministic signal for that product only; it does not replace artifact‑level validation across an entire estate.

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Is Azure Linux the only Microsoft product that includes uartlite (short answer)​

No — not necessarily. Azure Linux is the only Microsoft product Microsoft has publicly attested as including the uartlite component for CVE‑2025‑38262 at the time Microsoft published the VEX/CSAF data. But that attestation does not prove other Microsoft artifacts are free of the driver.
Put more precisely:

• Definitive: Microsoft confirmed Azure Linux images contain the implicated open‑source library and therefore prioritized Azure Linux in the VEX/CSAF mapping.
• Non‑definitive: Microsoft has not — in a single short advisory — claimed to have exhaustively scanned every Microsoft kernel artifact, so other Microsoft‑distributed Linux kernels, WSL2 kernels, linux‑azure variants used for some VM SKUs, Azure Marketplace images, AKS node images or partner appliances could still ship the same upstream files depending on their kernel version and build configuration. Microsoft has said it will update the CVE if additional products are identified.

This is consistent with how VEX/CSAF is intended to work: a vendor publishes per‑product attestations that are machine‑readable, deterministic and scoped. A positive attestation is actionable for the named product; conversely, a missing attestation is an unknown that must be verified through SBOMs or artifact inspection.

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Why absence of an attestation ≠ absence of the vulnerable code​

From a technical perspective, whether a kernel artifact contains the uartlite driver is determined by three independent factors:

• Kernel version / upstream commits: the presence or absence of the upstream fix depends on which stable upstream release or backport was used in that build. If the build predates the fix or lacks the backport, it may be vulnerable.
• Kernel configuration (CONFIG_* options): uartlite is controlled by a Kconfig symbol (CONFIG_SERIAL_UARTLITE), and the driver can be built‑in or compiled as a module (module name: uartlite). Different Microsoft kernel artifacts may enable or disable that symbol.
• Packaging/build choices and vendor backports: vendors sometimes apply their own backports or patches; a product can be safe if the vendor has already applied the upstream fix, even if the kernel version superficially looks old. Conversely, a product can be exposed if the vendor used an upstream branch that lacked the fix.

For these reasons, a product‑level VEX attestation for Azure Linux is useful and accurate for Azure Linux customers, but it is not a technical shortcut for validating all other Microsoft artifacts.

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How to verify whether a Microsoft‑supplied artifact you run contains uartlite​

Administrators should not rely solely on vendor attestations when they run mixed estates. Use artifact‑level checks to determine exposure. The steps below are practical, short, and apply to cloud VMs, on‑prem hosts running Microsoft images, and WSL2 kernels.

• Check the running kernel version:
• uname -r
• Check whether the uartlite kernel module is present or loaded:
• modinfo uartlite
• lsmod | grep uartlite
• Inspect the kernel config used to build the running kernel to see whether CONFIG_SERIAL_UARTLITE is enabled:
• zgrep CONFIG_SERIAL_UARTLITE /proc/config.gz
• or check /boot/config-$(uname -r) for CONFIG_SERIAL_UARTLITE.
• Examine package changelogs or distro advisories that map CVE‑2025‑38262 to the kernel package versions you run:
• On Debian/Ubuntu: apt changelog linux-image-$(uname -r) | grep -i 38262
• On RPM systems: rpm -q --changelog kernel | grep -i 38262
• For WSL2: if you use the Microsoft‑shipped WSL2 kernel, confirm the kernel version and review the WSL kernel repo/config for the symbols included; for custom WSL kernels inspect the image you built. If /proc/config.gz is present inside WSL, use the same zgrep approach above.
• If you maintain custom VM images, Marketplace appliances or container base images, extract the kernel image and check its config and module set before declaring the image safe.

These commands map to standard kernel verification practices and are directly relevant for the uartlite driver because the Kconfig symbol is CONFIG_SERIAL_UARTLITE and the module name is uartlite per the kernel driver database.

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Practical guidance for Azure Linux customers (and why the VEX attestation matters)​

If you run Azure Linux images (Microsoft’s curated cloud Linux distro), Microsoft’s VEX/CSAF attestation is authoritative: treat Azure Linux as in‑scope and follow Microsoft’s patch guidance. The VEX file gives a machine‑readable “Known Affected / Fixed / Under Investigation / Not Affected” status for Azure Linux that you can ingest into automation and ticketing systems. Microsoft began this VEX pilot in October 2025 and used Azure Linux as the initial product family for which it would publish attestations. Recommended immediate steps for Azure Linux instances:

• Apply Microsoft’s kernel updates or Azure Linux image updates as soon as feasible.
• Reboot affected hosts after the kernel package or image is updated to ensure the fixed kernel is running.
• In container hosts and VM fleets, prioritize image rebuilds and redeployments through your CI/CD pipeline so new instances are launched with patched images.

The VEX/CSAF attestation simplifies this triage by giving you a clear, vendor‑published answer for the Azure Linux family that your automation can use to prioritize patch windows.

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What administrators running other Microsoft artifacts should do​

If you run other Microsoft‑distributed artifacts — for example:

• WSL2 shipped kernel (Windows Subsystem for Linux)
• linux‑azure or azure‑tuned kernels used in some VM SKUs
• AKS node images or curated VM Marketplace images published by Microsoft
• Partner appliances or Marketplace VMs repackaged by Microsoft

— do not assume safety because Azure Linux was attested. Until Microsoft publishes a VEX attestation for those specific artifacts, treat them as unknown and perform the artifact checks outlined earlier. Microsoft has committed to updating the CVE and VEX records if additional Microsoft products are discovered to ship the same component; that update would be the authoritative sign that a product is in‑scope. Operational checklist for other Microsoft artifacts:

• Inventory all Microsoft images and kernel artifacts you use and map them to their build provenance.
• Run the kernel config and module checks shown earlier against each running artifact.
• Reconcile your inventory against Microsoft’s CSAF/VEX feed as Microsoft expands attestations.
• If your environment cannot tolerate uncertain images, treat unknown artifacts as high priority for verification and patching.

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Strengths and risks of Microsoft’s VEX/CSAF approach​

Strengths

• Deterministic, machine‑readable attestations reduce noise and false positives for automation and large estates. The pilot with Azure Linux gives defenders a concrete signal they can trust for that product family.
• Phased rollout is pragmatic: Microsoft can validate inventory and quality‑check attestations for one product before scaling across many SKUs, reducing errors in published VEX data.
• Commitment to update: Microsoft has pledged to update CVE/VEX records if additional products are identified, which preserves the ability to expand coverage over time.

Risks and limitations

• Temporary “unknowns” across the vendor footprint: until attestations cover all artifacts, customers must still perform artifact‑level verification for Microsoft products beyond Azure Linux. Absence of an attestation is not evidence of absence.
• Operational overhead: organizations must reconcile SBOMs, VEX feeds and live artifact checks — this is nontrivial for mixed estates with many custom images and partner appliances.
• Potential for misunderstandings: concise vendor wording can be misread as exclusivity; security teams must interpret VEX/CSAF content with the explicit product scope in mind.

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Cross‑checking and verification (how the conclusion is validated)​

This analysis cross‑checked the CVE technical description and severity against multiple independent sources (NVD, Ubuntu, Debian/Red Hat trackers and cloud vendor advisories), confirming the underlying defect and the typical remediation approaches (kernel package updates / image rebuilds). Microsoft’s process statement about publishing machine‑readable CSAF and VEX attestations — and the October 2025 Azure Linux pilot — is published on the MSRC blog, which explains the phased approach and why a single product appears in VEX before the rest of the vendor portfolio is fully mapped. That public statement is the authoritative explanation for why Azure Linux would be named first without implying exclusivity. Independent operational explainers (community posts and vendor security trackers) repeatedly emphasize the same point: a VEX attestation is authoritative for the named product but does not preclude other artifacts from being carriers — those must be validated individually. This is a consistent reading across vendor blogs, NVD entries and distro advisories.

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Final guidance — concise operational takeaways​

• Azure Linux: treat Microsoft’s attestation as authoritative — patch Azure Linux images immediately per Microsoft guidance.
• Other Microsoft artifacts (WSL2, linux‑azure, Marketplace images, AKS nodes): do not assume safety; verify each artifact using kernel config/module checks and package changelogs.
• Use Microsoft’s CSAF/VEX feed as it expands to automate triage, but keep artifact‑level verification as a mandatory guardrail until VEX coverage is complete.

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The short, direct answer to your initial question is this: Azure Linux is the only Microsoft product Microsoft has publicly attested to include the uartlite library for CVE‑2025‑38262; however, that attestation is product‑scoped and not an exclusive proof that other Microsoft products cannot include the same vulnerable code. Treat Azure Linux as a confirmed hit and verify all other Microsoft artifacts individually until Microsoft’s VEX feed or your own artifact inspection proves otherwise.

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Source: MSRC Security Update Guide - Microsoft Security Response Center