CVE-2025-38468: Azure Linux Attestation and WSL Patch Guidance

Microsoft’s MSRC advisory for CVE-2025-38468 confirms that the vulnerable code — a Linux kernel traffic‑control bug in net/sched where htb_lookup_leaf can hit a BUG_ON when presented with an empty rbtree — is present in the Azure Linux product family, and Microsoft says it has begun publishing machine‑readable CSAF/VEX attestations (October 2025) and will update the CVE if additional Microsoft products are found to ship the same upstream component.

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Background / Overview​

CVE‑2025‑38468 is a stability/availability bug in the Linux kernel’s HTB traffic‑control scheduler. In short, a sequence involving htb dequeueing, a netem qdisc invoking a child qdisc that drops the packet (blackhole), and backlog bookkeeping can leave an hprio rbtree root pointer set to NULL. When htb_lookup_leaf later runs it hits a BUG_ON which can crash the kernel — an availability/denial‑of‑service condition rather than a confidentiality or privilege‑escalation issue. Upstream maintainers fixed the code by returning NULL in the failing code path rather than triggering a BUG_ON, which preserves the expected error flow and avoids the panic. The vulnerability is cataloged across major vulnerability trackers and vendor advisories (NVD, Debian, Ubuntu, various distro security pages), and kernels in multiple distributions received patches or backports on different timetables. That means the presence — and exposure — of this bug depends on the kernel binary and configuration actually deployed in your environment, not merely the fact that a vendor’s source tree contains the code.

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What Microsoft actually said — and what it did not​

Microsoft’s public FAQ language for this and other Linux CVEs follows a clear pattern: it identifies the Microsoft product (or product family) that Microsoft’s internal inventory and build‑provenance checks have validated as shipping the implicated upstream component, and it publishes a CSAF/VEX attestation listing that product. For CVE‑2025‑38468 Microsoft named Azure Linux as the product it has confirmed “includes this open‑source library and is therefore potentially affected.” Microsoft also stated it began publishing CSAF/VEX attestations in October 2025 and pledged to update CVE entries if additional Microsoft products are identified as affected. That is accurate — and intentionally conservative. What the MSRC text does not do is make a blanket claim that no other Microsoft product contains the same Linux kernel component. The wording is an inventory attestation: this is what we have checked and mapped so far. It is not an exhaustive denial that other Microsoft SKUs or services might include the same upstream code. That distinction matters for defensive planning.

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Short answer to the user’s question​

• Strictly literal, as a public attestation: Microsoft has publicly attested that Azure Linux includes the library and is therefore potentially affected; it has not publicly attested that any other Microsoft product includes it.
• Practical/technical answer: No — Azure Linux is not necessarily the only Microsoft product that could include the vulnerable Linux kernel code. Any Microsoft product or service that ships, distributes, or runs a Linux kernel build (or supplies Azure images that contain a kernel) deserves verification — most notably Windows Subsystem for Linux (WSL/WSL2) and various Azure VM and image offerings. Microsoft ships and maintains Linux kernels in multiple places; those kernels may include the same net/sched code and therefore must be checked on a per‑product, per‑build basis.

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Why Azure Linux was named first (and why that’s sensible)​

Microsoft’s initial VEX/CSAF rollout started with Azure Linux artefacts. Azure customers consume Microsoft‑produced and Microsoft‑curated VM images (and in some cases Azure‑tuned kernel packages). By starting with Azure Linux, Microsoft provided the clearest, highest‑value attestation first: an inventory signal that directly benefits cloud customers running those images. That’s a practical disclosure strategy and consistent with how vendors roll out machine‑readable attestations: validate the most directly impacted products and expand the mapping as internal inventories are completed. The important takeaway for operators is that Microsoft’s attestation is authoritative for the Azure Linux family, but not proof that other Microsoft products are unaffected.

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Microsoft products and artifacts that should be checked (concrete list)​

Any Microsoft artifact that ships or distributes a Linux kernel binary — or supplies VM images that embed a kernel — is in scope for verification. The most obvious ones to check are:

• Windows Subsystem for Linux (WSL/WSL2) — Microsoft ships a Microsoft‑built Linux kernel and hosts the kernel source and releases publicly. If the WSL kernel version running on a machine predates the upstream fix, that WSL instance could be exposed. Microsoft maintains WSL kernel releases and distributes the kernel via Windows Update and the WSL update flow.
• Azure virtual machine images and Azure‑tuned kernels — many Azure Marketplace images use distribution vendor kernels (Ubuntu, RHEL, etc., but Azure also offers Azure‑tuned kernels and image families that Microsoft helps maintain. An Azure VM instance running an unpatched kernel (whether provided by the image vendor or Microsoft) may be vulnerable. Check the kernel package and image attestations for each image you run.
• Managed node images (AKS, Azure Scale Sets, marketplace appliances) — if the node image or appliance includes a Linux kernel binary, that kernel must be inventoried and verified.
• Any Microsoft‑published appliance or product that bundles Linux (for example, some edge or cloud appliances) — treat these case‑by‑case.

Note: Microsoft container base images do not include the host kernel binary; container images run on a host kernel. A vulnerable host kernel therefore still presents a risk to container workloads even if the container image itself is only userland.

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How to determine whether a given system or Microsoft product is affected — runbook​

The vulnerability is in the kernel binary. You can determine exposure quickly with a short, deterministic checklist. These are safe, low‑complexity steps you can hand to engineers or run as part of automated checks.

1) Inventory the kernels you actually run​

• Linux VMs: collect uname -r and distribution package information from every VM.
• uname -r
• cat /etc/os-release
• For Debian/Ubuntu: dpkg -l | grep linux-image; apt policy linux-image-$(uname -r)
• For RHEL/CentOS: rpm -q --whatprovides kernel; rpm -q --changelog kernel | grep CVE‑2025‑38468
• WSL2 clients:
• On the WSL side: uname -r
• On the Windows host: wsl --status (shows kernel version and whether Microsoft provided kernel is in use) and wsl --update to fetch Microsoft’s kernel updates.
• AKS and other managed node pools:
• For AKS, list node pool images and SSH into a node to run uname -r and package checks (kubectl get nodes; kubectl describe node <node> and then ssh into the node or use the cloud provider‑specific tooling).
• Azure Marketplace images:
• For each image, check the published image metadata in the Azure portal and the kernel package deployed inside the running VM.

2) Confirm whether the kernel version includes the upstream fix​

• Check upstream/fixed kernel releases and the stable backport(s). If your kernel version is older than the fixed release for that stable series, assume it’s vulnerable until the vendor’s advisory or package shows a backport.
• Use vendor advisories (Ubuntu, Debian, RHEL, SUSE, Amazon Linux) to map kernel package versions to the fix. For example, Ubuntu and Debian trackers list CVE entries and associated fixed package versions; if your package version ≥ fixed version you are patched.

3) Verify kernel configuration / module presence​

• The kernel source tree may contain a driver or qdisc, but a product is only exposed if the running kernel was configured to build that feature into the image or as a loadable module.
• zcat /proc/config.gz | grep -i 'HTB|NET_SCH' (to find CONFIG_NET_SCH_HTB or related flags)
• lsmod | grep sch_htb
• modinfo sch_htb
  If the HTB qdisc (sch_htb) is not built into the running kernel and not loadable in your environment, the specific code path may not be provoked — but because HTB is commonly built into many kernels and is part of core net/sched code, you should not assume absence without a config check.

4) Look for vendor VEX/CSAF attestations​

• Microsoft’s MSRC update‑guide pages include FAQ attestations (the Azure Linux attestation you quoted). Use vendor CSAF/VEX artifacts where available to validate which product artifacts the vendor has scanned. Microsoft has indicated it will expand those attestations over time; a missing attestation does not mean “guaranteed safe.”

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Remediation and mitigation guidance​

• Update the kernel where possible
• The correct remediation is to install a kernel package that contains the upstream fix (or an equivalent vendor backport) and reboot the system to run the patched kernel. For distribution kernels, apply the vendor’s security updates (apt/yum/zypper‑based updates).
• For WSL: run wsl --update on the Windows host or install the Microsoft provided WSL kernel update package; alternatively, users who maintain a custom WSL kernel should rebuild it from Microsoft’s WSL kernel sources patched with the fix. Microsoft publishes WSL kernel releases and the WSL kernel source publicly.
• If you cannot immediately patch, reduce attack surface
• Limit use of user‑controllable qdisc operations on hosts where local users can create netem/blackhole qdiscs (this bug requires local qdisc manipulation to trigger).
• Harden access control on systems where unprivileged users may be able to attach qdiscs or modify network settings (the exploit scenario involves local interaction).
• For cloud environments, prefer updated marketplace images and patch baselines for node pools and autoscale groups.
• Verify post‑patch by checking kernel version and package changelogs
• After updating, confirm uname -r, package version, and vendor changelog to ensure the fix is included.
• If you rely on custom or vendor kernels (for example, Azure‑tuned kernels), confirm with the vendor’s advisory and your image builder outputs that the fix is present.
• Build automated inventory checks
• Add automated checks for vulnerable kernel versions into your CI/CD and monitoring pipelines: collect kernel release strings, check against a curated list of fixed minimum versions, and alert owners of nodes that fall below the threshold.

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Technical caveats and nuance (what defenders often miss)​

• Presence in source ≠ exposure in binary. The kernel source tree contains many files and optional features. A product may include the relevant file in its source tree while producing kernel binaries that do not enable the offending code path. Always validate the running kernel config or module list.
• Upstream fix vs. vendor backport timing. Upstream commits landed on a specific kernel branch and then were backported into multiple stable releases. Different distributions and vendors backported the fix at different times. A kernel line such as 5.X may have the fix applied in a later patch release for that line; check vendor changelogs.
• Local vs. remote exploitation. This CVE is an availability/stability issue that is not remotely exploitable in its documented form — it requires local ioctl/qdisc operations to reach the failing code path — but local breakage on critical hosts is a real operational risk. Prioritize patching heavily used multi‑tenant or privileged host classes (build servers, CI runners, shared cloud VMs, developer laptops in regulated environments).

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Practical checklist for Windows and Azure admins (copy‑pasteable)​

• Azure VMs (Linux):
• SSH into each image type: uname -r
• For Debian/Ubuntu: apt update && apt list --upgradable | grep linux-image
• If using Azure image families, consult image metadata and update images via patch‑management (Azure Update Manager) and redeploy node pools.
• WSL2 clients:
• On Windows host: wsl --status
• Inside WSL: uname -r
• To update: wsl --update and restart WSL (wsl --shutdown)
• AKS nodes:
• kubectl get nodes -o wide
• ssh to a sample node (or use cloud provider tooling) and run uname -r
• If nodes are unpatched, recreate node pools from patched images or apply vendor updates and roll the pool.
• Verify the fix:
• After applying updates, confirm kernel package changelogs list the CVE or upstream commit; confirm uname -r corresponds to patched release.

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Risk summary and final analysis​

• Microsoft’s public statement that Azure Linux “includes this open‑source library and is therefore potentially affected” is a precise inventory attestation — it is accurate and useful for Azure customers because it confirms Microsoft scanned and mapped that product family. However, the statement is not a technical guarantee that no other Microsoft product ships the same kernel component. Microsoft’s VEX/CSAF rollout is ongoing, and Microsoft has explicitly said it will update CVE entries if additional products are discovered to ship the component.
• From a defensive perspective, any Microsoft product that ships a Linux kernel binary (Windows Subsystem for Linux, Azure images and kernels, AKS node images, appliances) is a candidate for validation. WSL2 is a confirmed Microsoft‑distributed kernel that historically has had CVEs and has its own release cadence; it must be checked.
• Operational impact is real but local: CVE‑2025‑38468 is an availability/stability bug exploitable with locally executed tc/qdisc operations that manipulate netem/blackhole in the scenario described in the upstream report. For multi‑tenant or developer resources where local untrusted users exist, prioritize patching; for tightly controlled single‑tenant hosts, follow standard kernel update procedures and risk‑assess accordingly.

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Recommendations (priority actions)​

• Treat Microsoft’s MSRC Azure Linux attestation as an authoritative signal for Azure Linux images and patch those immediately if they are in scope.
• Inventory all Microsoft‑provided or Microsoft‑modified kernels in your environment (WSL, Azure images, AKS node pools). Verify kernel versions and configs; patch or replace images where necessary.
• Implement short‑term mitigations for hosts that cannot be patched immediately: restrict unprivileged qdisc operations, isolate development hosts, and throttle access to systems where local users can create netem/blackhole qdiscs.
• Add this CVE and the kernel package mapping to your patch management playbooks. Automate detection of kernel versions and vendor package versions in your asset inventory.
• Watch MSRC/CSAF/VEX updates. Microsoft will expand attestations as inventory checks complete; add MSRC VEX feeds to your vulnerability ingestion so you get notified if Microsoft later identifies additional affected products.

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Conclusion​

Microsoft’s MSRC attestation that Azure Linux includes the open‑source component implicated by CVE‑2025‑38468 is accurate and valuable for Azure customers, but it should not be read as a universal exclusion for other Microsoft products. Any Microsoft product that ships or runs a Linux kernel — most prominently WSL2 and Azure VM images / node images — should be treated as potentially affected until you verify the kernel binary and configuration include the upstream fix or equivalent vendor backport. The pragmatic defensive play is simple: inventory kernels, check versions and configs, apply vendor patches or Microsoft’s WSL updates, and add VEX/CSAF attestations into your vulnerability automation so you know when vendors expand their inventory mapping.

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Source: MSRC Security Update Guide - Microsoft Security Response Center