CVE-2025-39751: ALSA Ca0132 Patch in Linux Kernel and Azure Linux Attestation

The Linux kernel patch labeled as CVE‑2025‑39751 fixed a small but real buffer‑overflow risk in the ALSA hda/ca0132 driver’s add_tuning_control function — and Microsoft’s public advisory that “Azure Linux includes this open‑source library and is therefore potentially affected” is accurate for the product it names, but it is not a categorical guarantee that no other Microsoft product can contain the same vulnerable code.

Background / Overview​

The technical root cause is straightforward: the ALSA hda/ca0132 code used an unsafe sprintf against a 44‑byte buffer in add_tuning_control, which could overflow if string inputs were too long. Upstream maintainers replaced the sprintf with snprintf (and related bounds‑checking) to ensure the buffer cannot be exceeded. The upstream commit history and distribution advisories document the change and map where the fix was applied.
Microsoft’s Security Response Center (MSRC) used the firm, narrow-language approach it has adopted since rolling out machine‑readable VEX/CSAF attestations: when Microsoft inspected its Azure Linux distribution (the company’s Linux distro) it found the implicated ALSA component and therefore listed Azure Linux as a product that “includes this open‑source library and is therefore potentially affected.” That attestation is useful and actionable for Azure Linux customers, but it is a product‑scoped inventory statement — not an exclusivity guarantee. Microsoft has also published guidance about VEX/CSAF and the phased rollwhich explains the intent behind these targeted mapping statements.
Two additional facts change how defenders must treat CVE‑2025‑39751 today. First, the upstream fixes are visible in kernel trees and distribution advisories, so vendors and distros issued backports and kernel updates. Second, the CVE identifier itself was later marked as rejected/withdrawn by the responsible CVE authority in the upstream kernel‑CVE communication channels, and multiple vulnerability aggregators reflect that status. That rejection does not erase the upstream patch or the underlying code correction — it typically means the CVE entry was considered a duplicate, misfiled, or otherwise not appropriate as a unique CVE ID after review. Nevertheless, the practical engineering result remains unchanged: the code was corrected upstream and distributions applied fixes.

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What Microsoft’s Azure Linux attestation actually means​

Product‑scoped attestation, not global exclusion​

Microsoft’s short statement — that Azure Linux “includes this open‑source library and is therefore potentially affected” — does three things:

• Confirms Microsoft inspected that particular product family and found the upstream component in the artifacts that ship as Azurure Linux customers to treat the product as in‑scope for remediation and to apply the vendor’s kernel updates.
• Leaves open the possibility that other Microsoft artifacts may or may not include the same component — those require separate attestations or per‑artifact inspection.

This distinction is crucial in supply‑chain and vulnerability triage: attesting one product as affected reduces noise for that product and lets customers automate decisions (patch Azure Linux now), but it does not let organisations assume other Microsoft images, kernels, or appliances are unaffected.

Why Microsoft starts with Azure Linux​

Microsoft’s phased VEX/CSAF rollout intentionally began with a single product family — Azure Linux — to get the attestation pipeline, validation, and automation right before expanding to additional product families. The VEX model is intentionally selective: it states the presence/absence/fixed status for each product only after inventory verification or source/packaging inspection. That rollout design explains why many CVE entries list Azure Linux early while other Microsoft SKUs may remain un‑mapped until the company completes further artifact checks.

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Is Azure Linux the only Microsoft product that includes the affected ALSA code?​

Short answer: No — not necessarily. Operationally precise answer: Azure Linux is the only Microsoft product Microsoft has publicly attested to include the ALSA hda/ca0132 code for CVE‑2025‑39751, but that attestation is product‑scoped and not an assurance that other Microsoft artifacts are free of the code. Absence of a public VEX/CSAF mapping for a given Microsoft product is absence of attestation, not proof of absence.

How a kernel component appears (or not) in a product​

Whether any Microsoft product includes this ALSA driver depends on a sequence of build‑time decisions and packaging choices:

• Kernel source and commit range: the presence of a function or driver is determined by the kernel version/commit used to build the kernel shipped in the product. The upstream fix and its commit range determine which kernel versions are vulnerable and which receive backports.
• Kernel configuration (CONFIG_* flags): drivers like the Creative CA0132 codec are governed by Kconfig options (for example, CONFIG_SND_HDA_CODEC_CA0132 and related DSP options). If a kernel build disables the feature, the driver never exists in that kernel binary. If it is compiled as a module, it will only load if hardware is present or modprobe is invoked. (cateee.net)
• Distribution or product packaging: vendors can choose to include or strip out modules, build monolithic kernels, or vendorize drivers — packaging choices change the final delivered artifact.
• Image variants and SKU differences: Microsoft ships multiple Linux artifacts (Azure VM images, container host images, kernel packages, WSL kernels, marketplace images). Each artifact’s kernel build must be individually checked.

Because these decisions are made per artifact, the only definitive way to know whether a particular Microsoft product includes the vulnerable code is (A) Microsoft publishing a VEX/CSAF attestation for that product, or (B) performing a per‑artifact binary/source inspection for the presence of the driver (e.g., checking kernel config, module lists, or /lib/modules for snd‑hda‑codec‑ca0132).

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Which Microsoft artifacts are plausible carriers — and how to check them​

Azure Linux is the obvious confirmed carrier because Microsoft inspected it and said so. The following Microsoft artifacts are plausible carriers and deserve verification:

• Azure Marketplace VM images or Microsoft‑provided Linux images (which may use kernels with ALSA enabled).
• linux‑azure kernels or Microsoft‑provided kernel packages used in some cloud images.
• Container host images maintained by Microsoft that include full kernel stacks, if those images ship kernels with ALSA enabled.
• WSL2 kernels shipped with Windows (Windows Subsystem for Linux uses a Microsoft‑built Linux kernel). Whether WSL kernels include specific ALSA HDA drivers depends entirely on the WSL2 kernel configuration used for that release. Historically, many WSL kernels minimized hardware support for sound, but WSL kernel options evolve; you must inspect the specific kernel config for the WSL build you run.

How to check a given artifact (practical checklist)

• If the host is running a Microsoft kernel: check the running kernel and config.
• uname -a to get kernel version.
• If available: zgrep CONFIG_SND_HDA_CODEC_CA0132 /proc/config.gz or grep CONFIG_SND_HDA_CODEC_CA0132 /boot/config-$(uname -r).
• If you cannot access the running kernel config: inspect /lib/modules/$(uname -r) for snd‑hda‑codec‑ca0132. Example:
• ls /lib/modules/$(uname -r)/kernel/sound || lsmod | grep snd_hda
• For offline image inspection (VM image, WSL kernel file, or marketplace image):
• Extract or mount the image and inspect /boot/config-* and /lib/modules.
• For kernels built and published by Microsoft (e.g., WSL kernel sources or Azure kernel packages), compare vendor release notes and the kernel commit tags to the upstream kernel commit where the fix was applied.
• Track Microsoft’s VEX/CSAF feed for newly published attestations for specific product SKUs. Microsoft has committed to expanding mappings beyond Azure Linux over time.

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Timeline and current status (important operational note)​

• Upstream: the ALSA hda/ca0132 fix (replace sprintf with snprintf and add bounds checks) landed in kernel trees and was backported by distributions. Distribution advisories and vendor CVE pages trace the commits to packages.
• Microsoft: Microsoft publicly attested that Azure Linux included the implicated open‑source library and would publish VEX/CSAF attestations and update the mapping if additional Microsoft products were found to include the same component. That is consistent with Microsoft’s phased transparency approach.
• CVE status: after initial publication, the CVE record for CVE‑2025‑39751 was later marked REJECTED by the Linux CVE announcement channels and reflected as withdrawn by multiple third‑party aggregators. A rejected CVE typically means the identifier was determined to be a duplicate, misassigned, or otherwise inappropriate, but it does not negate the underlying code correction or the need for customers to apply patches where the code existed. Always verify upstream commit IDs and distribution advisories rather than relyingce/absence of a CVE number.

Caveat: because CVE allocation and reconciliation processes sometimes change the metadata (merge or reject IDs), defenders must rely on the canonical technical artifacts (kernel commits, distribution advisory changelogs, vendor patches) for accurate remediation mapping.

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Practical remediation and mitigation guidance​

If you are responsible for affected systems, follow a measured, artifact‑centric approach:

• For Azure Linux customers:
• Treat Microsoft’s attestation as authoritative for Azure Linux. Apply Microsoft’s published kernel updates for Azure Linux promptly when Microsoft releases them, or follow Microsoft’s patch guidance in the VEX/CSAF files.
• If you run multi‑tenant workloads, apply the usual minimization controls for device nodes (restrict access to /dev/snd/*, avoid passing host audio devices into containers).
• Validate patched kernels by verifying kernel version and presence of the upstream commit IDs or by confirming the vendor advisory lists a fix.
• For operators using other Microsoft artifacts (WSL kernels, Marketplace images, linux‑azure kernels, managed node images):
• Do not assume absence. Perform the artifact checklist above to confirm whether the ALSA driver exists in the binary you run.
• If a Microsoft product does include the driver and the vendor has not yet published an attestation or patch, apply containment measures: restrict access to sound device nodes, block module auto‑load where feasible, and isolate untrusted workloads from host hardware access until the patch is available.
• Track vendor advisories and the VEX/CSAF feed for updated attestations and patch availability.
• General defensive practices:
• Add a simple automation check to your image‑build pipes with the relevant kernel config flags or modules, so you can quickly identify images that contain voice, audio, or HDA Codec drivers.
• For fleet management systems, annotate images and nodepools with the kernel features they include; this makes rapid triage easier the next time a kernel‑component CVE appears.

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Risk analysis: strengths and gaps in Microsoft’s approach​

What Microsoft did well

• Targeted, auditable attestation: By naming Azure Linux specifically, Microsoft gave a narrow, high‑signal alert customers can act on immediately. That reduces hunting for false positives and supports automation.
• Transparent tooling (VEX/CSAF): Publishing machine‑readable attestations is a material improvement for customers and security vendors; it enables automated triage and scaled patching decisions across complex environments.

Potential risks and limitations

• Attestation ≠ exhaustiveness: A single‑product attestation can be misread as a broad “only this product” guarantee; defenders who interpret it that way risk overlooking other Microsoft artifacts that may carry the same code. Numerous examples show Azure Linux being attested first while others remain un‑mapped pending inventory checks.
• CVE metadata churn can confuse defenders: the eventual rejection of CVE‑2025‑39751 shows that CVE lifecycle metadata can change after publication. Attack surface decisions should be driven by technical artifacts (commits, vendor advisories) and product attestations, not solely by CVE IDs.
• Visibility gap for closed artifacts: Not all Microsoft artifacts are fully transparent to external parties. For images or appliance binaries where Microsoft does not publish kernel sources or configs, defenders must estations or on internal image inspection to confirm exposure.

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When “not yet attested” becomes “not affected”: an operational rubric​

To convert uncertainty into action, apply this rubric:

• If Microsoft has attested a product as “includes the component”: treat the product as affected and patch per Microsoft guidance. (Azure Linux in this case.)
• If Microsoft has published a VEX/CSAF status of “Not Affected” for a product: you can accept that statement for that product and move on.
• If there is no Microsoft attestation for a product you run:
• Verify locally (kernel config, modules, /boot/config, /lib/modules).
• If verification is infeasible, treat the product as under investigation and apply containment (device node restrictions, module load controls) until the product is either attested Not Affected or a patch is available.
• Automate these checks for all Microsoft images in your estate so attestation gaps are visible and actionable.

This rubric maps directly to Microsoft’s VEX model and helps keep noise low while ensuring customers don’t misinterpret a single product attestation as universal immunity.

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Unverifiable claims and caveats​

• It is impossible, from public records alone, to assert with absolute certainty whether every Microsoft product other than Azure Linux ever included the specific ALSA code — without either Microsoft publishing a product‑scoped VEX/CSAF attestation or customers performing binary/config inspection of each artifact. Treat any blanket claim that “no other Microsoft product is affected” as speculative until verified.
• The rejection of the CVE identifier changes the tracking label but does not obviate tream fixes where the code exists. Always map the patch commit or fix diff to the kernel packages your systems run rather than relying only on the presence/absence of a CVE number.

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Conclusion and recommended next steps​

Microsoft’s statement that Azure Linux includes the open‑source library referenced by the advisory is accurate for the product it names and is the correct immediate signal for Azure Linux customers to patch. However, Azure Linux being named does not prove exclusivity. Other Microsoft artifacts — WSL kernels, linux‑azure kernels, Marketplace images, and container host images — are plausible carriers depending on their kernel build choices and configuration, and each must be verified on its own merits.
Practical action items for defenders:

• If you run Azure Linux: apply Microsoft’s published kernel updates now and validate the update.
• If you run other Microsoft images or WSL: inspect kernel configs/modules for the snd‑hda‑codec‑ca0132 driver or track Microsoft VEX/CSAF attestations as they are published.
• Add automation to your build and deployment pipelines that flags kernel config options and loaded modules so future kernel component CVEs can be triaged fast.
• Do not rely solely on a CVE identifier’s presence; reconcile the upstream commit IDs and vendor advisories to your kernel package versions for precise remediation mapping.

In short: Azure Linux is the product Microsoft publicly attested as containing the affected ALSA code for this advisory — that attestation matters and should be acted on — but it is not a technical proof that no other Microsoft product can or does include the vulnerable code. Per‑artifact verification or Microsoft’s expanded VEX/CSAF attestations are the only reliable ways to move from “potentially affected” to “definitively affected” or “definitively not affected.”

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Source: MSRC Security Update Guide - Microsoft Security Response Center