CVE-2025-38103: Linux HID Bug Fixed; Azure Linux Attestation Explained

The Linux kernel bug tracked as CVE‑2025‑38103 — described upstream as “HID: usbhid: Eliminate recurrent out‑of‑bounds bug in usbhid_parse()” — has been fixed in the kernel stable trees, and Microsoft’s Security Response Center (MSRC) has published a product‑level attestation that Azure Linux includes the implicated open‑source component and is therefore potentially affected. That MSRC line is authoritative for Azure Linux, but it is deliberately scoped: it is an inventory attestation, not a global guarantee that no other Microsoft product can contain the same vulnerable code. Administrators should treat Azure Linux as a confirmed remediation priority while simultaneously validating other Microsoft‑supplied Linux kernels and images in their environments.

Background / Overview​

CVE‑2025‑38103 is an in‑kernel bug in the HID (Human Interface Device) subsystem. The upstream fix reworks the HID descriptor handling so that the kernel only attempts to parse the mandatory report descriptor and removes an unsafe loop that could index past the end of the reported descriptor array. The correction updates the HID descriptor structure usage, validates descriptor lengths / counts, and replaces the problematic iterator with a direct, bounds‑checked access to the mandatory report descriptor — eliminating the out‑of‑bounds fault path. The Linux kernel CVE team documented the affected files and the fixes applied into multiple stable releases.
Why this matters: the bug is a local kernel correctness issue in drivers/usb and drivers/hid code (examples include drivers/hid/usbhid/hid-core.c and drivers/usb/gadget/function/f_hid.c). In practice this type of flaw can cause kernel faults or denial‑of‑service when malformed HID descriptors are processed (for example, from USB devices, USB passthrough from guests, or gadget endpoints). Distribution vendors recorded the patch in their security advisories and assigned CVSS scores that vary by tracker; many downstream maintainers classified it as a correctness/availability risk and integrated the upstream fix into stable branches.

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Technical anatomy: what the fix changed and where​

What went wrong​

• The usbhid parsing path treated HID class descriptor data with an assumption about an array layout and iterated in a way that could index past allocated descriptor memory when bNumDescriptors / bLength fields were unexpected.
• The buggy pattern was present in multiple HID‑related code paths, meaning several drivers could trigger the same out‑of‑bounds read or fault when presented with malformed descriptors.
• A practical trigger path is local USB device interaction — either physical devices plugged into a host, guest‑to‑host device passthrough in virtualized deployments, or gadget/device modes that present descriptors to the host stack.

The upstream remediation​

• Upstream replaced the unsafe loop and direct indexing with code that:
• updates struct hid_descriptor usage to reflect mandatory vs optional parts per USB HID 1.11,
• validates bLength and bNumDescriptors before indexing,
• uses direct access to the mandatory report descriptor (rpt_desc) instead of desc[0],
• issues a warning when optional/unsupported HID class descriptors are present rather than blindly iterating.
• The kernel CVE announcement lists the exact fixed commits and the stable kernel trees where the fix was applied; affected/stable versions and backport guidance are included there.

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Microsoft’s public statement — what it actually says​

On its CVE product mapping pages MSRC uses a short FAQ line for many Linux kernel CVEs: “Azure Linux includes this open‑source library and is therefore potentially affected by this vulnerability.” Microsoft additionally documented that it began publishing machine‑readable CSAF/VEX attestations in October 2025 and that it will update CVE mappings if additional Microsoft products are identified that include the same upstream component. That wording is a product‑level inventory attestation — it confirms that Microsoft inspected Azure Linux build artifacts and found the implicated upstream code. It does not, by itself, assert exclusivity across all Microsoft products and images.
Two important readings of that phrase:

• As guidance to Azure Linux customers: authoritative and actionable — if you run Azure Linux, treat the product as in‑scope and apply Microsoft’s updates or follow recommended mitigations.
• As a statement about other Microsoft products: absence of an attestation is not evidence of absence — other Microsoft artifacts that kernels (WSL2 kernels, Azure Marketplace images, AKS node images, appliance images, or custom kernel builds distributed by Microsoft) must be inspected individually until Microsoft publishes an explicit Not Affected attestation or includes them in VEX mappings.

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Is Azure Linux the only Microsoft product that could be affected?​

Short answer: No — Azure Linux is the only Microsoft product Microsoft has publicly attested (so far) to include the vulnerable component for CVE‑2025‑38103, but that attestation is product‑scoped, not exclusive. Any Microsoft artifact that ships a Linux kernel built from an upstream source branch that predates the fix — and whose kernel configuration enables the HID/usbhid code paths — could carry the vulnerability until verified. Examples include:

• Azure Marketplace VM images and curated appliance images that include a vendor kernel.
• Windows Subsystem for Linux (WSL2) kernels Microsoft builds and distributes to Windows clients; those kernels often include a subset of HID/USB support for hardware/windowing features.
• AKS node images or managed container host kernels that Microsoft or its partners publish.
• Specialized Azure service kernels or VM SKUs built from vendor or Microsoft kernel trees.
• Third‑party or partner images published via Microsoft channels (Marketplace publishers sometimes ship kernels or kernel modules in images).

Operationally: treat each Microsoft‑distributed kernel artifact as an artifact to be verified until Microsoft’s VEX/CSAF inventory explicitly lists it as Not Affected.
Cross‑checks with independent sources (NVD, linux‑cve‑announce, distribution advisories) confirm the technical fix and the kernel files affected; distro trackers and the OSV/Ubuntu entries also map the fix into Azure‑branded kernels (linux‑azure), showing Azure Linux kernels were updated in response to the upstream patch. Those independent confirmations validate both the technical nature of the bug and Microsoft’s product‑scoped attestation.

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Practical detection and verification steps for administrators​

If you run Microsoft‑distributed kernels, follow a clear, evidence‑based discovery process. Below are the recommended steps in priority order.

1. Inventory all Microsoft‑supplied Linux artifacts​

• Enumerate Azure VMs, Marketplace images, AKS node pools, Azure appliance images, and any WSL2 kernel files you deploy.
• Capture full product names and exact kernel package versions (package names like linux‑azure, linux‑azure‑6.5, or kernel package release numbers). Vendor names alone are insufficient.

2. Determine kernel version and build provenance​

Run on each host:

• uname -a
• cat /proc/version
• zgrep CONFIG_HID /boot/config-$(uname -r) (or check /proc/config.gz if available)
• modinfo usbhid (if present as a module)
• If the kernel version is one of the upstream ranges introduced/fixed in the linux‑cve‑announce note, then treat it as potentially affected until you confirm the specific commit/backport status. The linux CVE announcement lists the exact fixed commits and affected stable versions for CVE‑2025‑38103.

3. Check for presence of affected source components in distributed binary packages​

• For Debian/Ubuntu: use apt/dpkg to find linux‑azure package versions; compare against OSV / Ubuntu OSV JSON mappings for the CVE which list affected binary versions.
• For RPM systems: query rpm -qa | grep kernel and inspect changelogs from vendor advisories (SUSE/Oracle/Red Hat often publish the list of CVEs applied to each kernel update).

4. Inspect kernel modules and configs​

• If HID code is built as a module, a quick check is: lsmod | grep usbhid and modinfo usbhid.
• If CONFIG_HID or CONFIG_USB_HID are disabled at build time, the presence of the source in upstream does not mean the binary exposes the vulnerable path — but you must verify the actual binary configuration. Use /boot/config-$(uname -r) or packaged kernel config artifacts.

5. Check Microsoft VEX/CSAF and product advisories​

• Review Microsoft’s MSRC CVE mapping for the CVE and the product‑level VEX/CSAF attestation to confirm which Microsoft product families are labelled “Potentially Affected” or “Not Affected.” Microsoft has committed to updating mappings as additional inventories complete; absence of a product name is not proof of absence.

6. Monitor for kernel OOPS / dmesg signatures​

• Watch dmesg and journalctl -k for OOPS traces that reference HID, usbhid, hid-core.c, f_hid, or related call stacks. Preserve crash dumps (kdump) for forensic analysis if you see repeating oops events possibly tied to USB device operations.

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Short‑term mitigations when immediate patching is not feasible​

• Block or blacklist the module: If your workload does not require usbhid/usbhid functionality, and the driver is modular, blacklist it to prevent module loads across reboots (e.g., create /etc/modprobe.d/blacklist‑usbhid.conf with “blacklist usbhid”).
• Restrict USB device access: Enforce endpoint control policies, prevent non‑trusted device attachment, and restrict guest passthrough of host USB devices in virtualized environments.
• Harden virtualization device passthrough: Disable unattended USB passthrough features in hypervisor VM config unless explicitly required; restrict which guests can use device redirection.
• Use vendor livepatch with caution: Confirm whether your vendor’s livepatch service includes the specific commit for this HID fix; many livepatch systems do not cover all kernel changes. Always confirm via vendor documentation before relying on livepatch as the sole mitigation.
• Accelerate patch windows for exposed hosts: Prioritize patching and reboot scheduling for VMs, appliances and endpoints in high‑risk contexts such as multi‑tenant hosts or shared developer images.

These mitigations reduce attack surface or prevent the code path from running, but the only reliable long‑term fix is to install vendor patched kernels (or verified backports) and reboot into them. Distribution and cloud vendors published kernel updates that include the upstream fix; review vendor advisories and apply their packages.

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Detection, monitoring and incident response guidance​

• Add SIEM rules to correlate:
• new kernel OOPS or panic events referencing HID/usbhid,
• repeated host reboots soon after USB device events,
• suspicious guest device passthrough flows on hosts.
• Preserve crash artifacts: enable kdump and capture relevant vmcore files for post‑mortem analysis.
• Forensic timeline: correlate USB plug/unplug logs, udev messages, hypervisor passthrough operations, and kernel crash timestamps.
• Threat hunting: look for patterns where local unprivileged users or containerized workloads trigger HID device activity immediately before kernel stability incidents; this may indicate accidental triggers or intentional tests of the vulnerable path.

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Critical analysis: strengths t’s attestation approach​

Microsoft’s phased CSAF/VEX rollout — starting with Azure Linux and promising to expand product attestations as inventories complete — is an important transparency advancement for enterprise defenders. Publishing machine‑readable VEX files lets automation ingest product‑component mappings and prioritize remediation at scale; Azure Linux being explicitly attested provides a clear, actionable target for many cloud‑native customers. That is a real strength.
However, there are operational and security riskattestation line too broadly:

• Attestation vs Exhaustive Scan: MSRC’s “Azure Linux includes … and is therefore potentially affected” is not a statement that other Microsoft images or kernels were scanned and found clean. Large vendors ship thousands of artifact variants; an attestation for one product is not a proof that another product with a different kernel build/config is safe. Treat lack of attestation as “unverified” rather than “not affected.”
• Artifact‑specific exposure: Kernel inclusion is dependent on three independent variables — upstream commit range, kernel configuration (CONFIG_* options), and vendor backports — so product naming alone does not fully determine exposure. Two kernels from the same vendor can differ substantially in what drivers are compiled in or backported. This complicates automated triage if users rely solely on product names.
• Timing and inventory lag: Phased rollouts introduce a window where a product may be vulnerable but not yet attested. Attackers who monitor public CVE records could focus on un‑attested artifacts distributed by the same vendor. Defenders must therefore supplement vendor attestations with their own artifact verification.
• Automation hazards: Secu ingest VEX/CSAF as a prioritized data source but not as the only signal. Blindly treating “not listed” as clean in automated patching or compliance checks could leave vulnerable kernels undiscovered.

In short: Microsoft’s approach improves transparency and provides automation-friendly data for Azure Linus must combine it with artifact‑level inspection and vendor changelog verification to achieve complete coverage.

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Recommended prioritized action plan (for system operators)​

• Immediately identify any Azure Linux instances and schedule/verify kernel updates for those hosts as MSRC’s attestation explicitly maps Azure Linux to the vulnerable component. Treat Azure Linux as a high‑priority remediation target.
• Inventory all other Microsoft‑supplied kernels (WSL2, Marketplace images, AKS node images, appliance images). For each artifact, verify:
• kernel version (uname -r),
• presence of HID/usbhid in the build config,
• vendor advisory mapping for CVE‑2025‑38103 or applied backported commits.
• Apply vendor patches or backports where available; where not immediately availablemodule blacklist, USB access controls, disable passthrough).
• Update SIEM/EDR detection rules to watch for OOPS traces mentioning HID/usbhid and preserve crash dumps for analysis.
• Track Microsoft’s VEX/CSAF feed and CVE mappings for updates — Microsoft will update product mappings if additional affected products are discovered, so automation should re‑check the VEX/CSAF inventory regularly.
• Communicate to stakeholders that attestation is not a universal safety statement — provide teams with concrete artifact‑level guidance and checklists so images distributed internally or from Marketplace publishers are verified before deployment.

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Final takeaways​

• CVE‑2025‑38103 is a Linux kernel HID/usbhid out‑of‑bounds correctness bug fixed upstream; multiple distros and kernel families have received the patch, and the technical changes are documented in the kernel CVE announcement and NVD.
• Microsoft’s public line — “Azure Linux includes this open‑source library and is therefore potentially affected” — is an important product‑scoped attestation and should be acted upon by Azure Linux customers immediately; it is not a blanket statement that other Microsoft products cannot include the same vulnerable code. Administrators must verify every Microsoft‑distributed kernel artifact they run.
• Defenders should combine Microsoft’s VEX/CSAF feed with direct artifact inventory, kernel config checks, module presence checks, vendor advisories, and detection rules to achieve reliable coverage.
• The most reliable long‑term remediation is installing vendor‑supplied patched kernels (or verified backports) and rebooting into them; short‑term mitigations (module blacklisting, USB policy enforcement, disabling passthrough) reduce exposure but do not replace patching.

If you manage Azure Linux or any Microsoft‑published Linux images, prioritize patch windows for those images, verify kernel configs across your fleet, and treat un‑attested Microsoft artifacts as unknown until you confirm their provenance and patch status. The MSRC attestation is valuable — use it — but do not let it substitute for artifact‑level verification and good operational hygiene.

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Source: MSRC Security Update Guide - Microsoft Security Response Center