CVE-2025-38643: Linux Wireless Lock Bug Impacts Azure Linux and More

CVE-2025-38643 is a recently disclosed Linux kernel vulnerability in the wireless subsystem — a missing lock in cfg80211_check_and_end_cac — and while Microsoft’s advisory names Azure Linux explicitly, that does not mean Azure Linux is the only Microsoft product that could include the affected open‑source library or be exposed by this bug.

Background / Overview​

The vulnerability tracked as CVE‑2025‑38643 is a concurrency/locking bug in the Linux kernel’s cfg80211 wireless configuration API. In short, a worker path (cfg80211_propagate_cac_done_wk can call into code that expects the wiphy mutex to be held while it does not actually take that lock. That mismatch can trigger kernel warnings and inconsistent state when certain wireless operations — examples include DFS (dynamic frequency selection) and CAC (channel availability check) flows exercised by hostapd or driver/mac80211 changes — occur.
This type of defect is a synchronization issue rather than a classic memory‑corruption vulnerability. Public trackers and distro advisories describe it as causing kernel WARN_ON messages and potential reliability problems; real‑world impact centers on availability and stability (kernel warnings, spurious reboots or device misbehavior) rather than documented remote code execution or immediate privilege escalation. Distributions and cloud vendors have mapped the CVE to kernel commits and started packaging fixes into stable kernels and cloud‑provider kernel images.

The technical short version​

• Root cause: a missing wiphy mutex acquisition in a cfg80211 worker path.
• Symptom: kernel WARN messages originating from net/wireless/chan.c when particular cfg80211 paths are invoked.
• Practical impact: reliability/availability issues, noisy logs, and potential denial‑of‑service in multi‑tenant or automated environments; exploitation to produce more severe outcomes (RCE/LPE) would require additional, unrelated flaws and currently has not been shown in the wild.
• Fix: upstream kernel patch adds the missing lock / balances get/put semantics; fix is small and low‑risk to backport.

Who ships or uses cfg80211?​

cfg80211 is the canonical Linux kernel wireless configuration layer; it is present in upstream kernels and is enabled/disabled at build time through kernel config (CONFIG_CFG80211) and as a module in many distributions. Any Microsoft product or image that ships a Linux kernel configured with cfg80211 enabled — or that delivers a kernel package derived from upstream trees that contains the vulnerable code — could be affected.
Microsoft itself maintains and ships several Linux kernels, images, and packages for Azure and other offerings (for example, the linux‑azure kernel used in Azure VMs and the Azure Linux distribution). Those Microsoft‑maintained kernel packages have historically included cfg80211 fixes and wireless subsystem changes in their changelogs, and Microsoft’s public advisory explicitly calls out Azure Linux as a distribution that includes the open‑source library in question and is being tracked for impact. That statement is the vendor’s assessment at disclosure time and is the authority for initial customer guidance.
However, Microsoft is not the only shipper of Linux kernels or kernel modules running on Microsoft infrastructure: many Azure Marketplace images, partner‑provided images, and third‑party Linux distributions offered in Azure (for example, Canonical's Ubuntu images, Red Hat and SUSE images, and vendor images) include cfg80211 based on their kernel configuration. From an attack surface perspective, any VM or service running a Linux kernel that contains the vulnerable cfg80211 code path could show the same symptoms until patched.

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Is Azure Linux the only Microsoft product that includes cfg80211?​

Short answer: No — Azure Linux is not necessarily the only Microsoft product that could include cfg80211 or be impacted. Azure Linux is the most visible Microsoft product that the vendor has explicitly acknowledged in its advisory, but any Microsoft product or offering that ships, runs, or manages a Linux kernel with cfg80211 enabled may contain the vulnerable code.
Why that distinction matters:

• Microsoft maintains multiple Linux kernel artifacts used across Azure and related offerings. The kernel trees and packages used for Azure VM host images, marketplace images, and certain first‑party services are not a single monolithic product; they are multiple artifacts that may be rebuilt, reconfigured, or repackaged for different surface areas.
• linux‑azure / Azure kernel packages — the kernel packages Microsoft publishes for Azure VMs and for some Ubuntu flavours used in Azure include cfg80211 fixes in their changelogs and are maintained in the azure kernel track. If cfg80211 is present in those kernels, those images are in scope.
• Third‑party Linux images on Azure (Ubuntu, RHEL, SUSE, community images) are not Microsoft‑authored, yet they run on Microsoft infrastructure and may include the vulnerable subsystem — responsibility for fixes falls to the distro vendors, but Azure customers often use Microsoft’s marketplace or Microsoft‑curated images that pull in those kernels.
• Windows Subsystem for Linux (WSL) runs a Microsoft‑provided Linux kernel inside Windows. WSL’s kernel tree is published and modifiable; by default it is tailored for the WSL environment and large classes of hardware drivers (including typical physical Wi‑Fi device drivers) are not exposed in the same way as on a physical Linux host. That said, WSL kernels are buildable/configurable and users who enable CFG80211 or load wireless modules into a custom WSL kernel build could bring the cfg80211 code into scope on their Windows hosts.
• Other Microsoft projects or services that embed or publish Linux images or kernels (for example, Azure Marketplace images and Azure Kubernetes Service node images) may also include cfg80211 depending on the particular image and kernel configuration.

In other words, while Azure Linux is the Microsoft product explicitly named in the advisory, it is not logically the only possible Microsoft surface that might contain the vulnerable cfg80211 code — any Microsoft image, kernel package, or product that carries a Linux kernel with cfg80211 enabled should be treated as potentially affected until verified.

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Evidence and verification (what we can and cannot confirm)​

• The kernel defect and the upstream code changes that fix the missing lock are present in upstream kernel commits and are tracked by the common vulnerability databases and Linux distro security trackers.
• Microsoft’s public advisory specifically mentions Azure Linux and describes Microsoft’s commitment to keep the distro up to date; Microsoft also stated that if additional Microsoft products are impacted they will update the CVE record / advisory.
• Distribution trackers (Ubuntu, Debian, Amazon Linux, RHEL trackers) show mappings of CVE‑2025‑38643 into kernel updates and package changelogs that include cfg80211 fixes; those mappings corroborate the vulnerability description and the availability of patched kernel packages for various vendors.
• WSL’s Linux kernel source is published by Microsoft and is configurable — community documentation shows how to enable cfg80211 in a WSL custom kernel. That demonstrates possibility rather than default exposure: by default WSL kernels are built for the WSL use case and may not expose typical Wi‑Fi driver stacks; users who intentionally enable wireless modules or use custom kernels may change that.

What can’t be conclusively proven at disclosure time without further vendor confirmation:

• A comprehensive, product‑by‑product inventory of every Microsoft product and image that includes cfg80211 (Microsoft has stated it will update the CVE if more products are affected). Customers and operators should not assume only Azure Linux is affected — the vendor’s advisory leaves open the possibility of additional impact as teams finish inventory and confirm builds.
• Whether specific managed Azure service internal hosts (backend infrastructure that customers do not directly manage) include the vulnerable subsystem. For most managed services, Microsoft will publish whether internal images or managed host fleets are in scope.

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Strengths in the response and what operators should trust​

• The technical fix upstream is small and narrowly scoped: adding the required lock or balancing the get/put semantics in the cfg80211 worker path. That kind of minimal patch tends to be low‑risk from a regression standpoint.
• Multiple independent trackers — distro security pages, cloud vendor advisories, and upstream kernel commit logs — corroborate the root cause and the presence of the fix in stable kernel trees.
• Microsoft’s stated process (keeping Azure Linux up to date and publishing CSAF/VEX advisories in its transparency pipeline) is a positive signal for enterprise customers who rely on Microsoft to manage kernel and distro security for Azure images.

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Risks, caveats and operational realities​

1. Backport lag and image fragmentation
   • Even though upstream fixes are small, backporting and packaging those fixes into long‑lived or vendor‑forked kernels (embedded appliances, custom cloud images, third‑party marketplace images) can take time. That delay is the primary operational exposure risk.
   • Microsoft customers commonly run a mix of first‑party Azure Linux images, third‑party distro images, and custom images — patch status will vary across that fleet.
2. Surface‑area ambiguity
   • The presence of cfg80211 depends on kernel config. Some kernels (cloud‑optimized or WSL tailored kernels) do not expose full driver stacks; others (desktop or general‑purpose kernels) do. Operators must verify kernel configs or module lists to confirm exposure.
3. Realistic exploitation model
   • This CVE is primarily a reliability/locking bug that produces WARN_ON logs and may cause instability. It is not, as disclosed, an immediately weaponizable remote RCE. However, noisy WARNs and operator distraction can be amplified as part of multi‑step attacks; defensive teams should not treat it as harmless.
   • The most likely attacker value is causing denial‑of‑service or using the condition as a building block in a more complex exploit chain — those uses are speculative and require additional conditions.
4. Cloud/managed services complexity
   • Azure customers may assume that Microsoft will patch everything containerized or host‑level; however, cloud providers and independent distributions share responsibilities. When images are built by third parties but hosted in Azure Marketplace, the image builder (not Microsoft) is responsible for distributing distro kernel updates — yet customers tend to treat Marketplace images as single‑vendor artifacts. This muddles operational responsibility during incident response.

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Concrete detection and remediation steps for administrators​

Short checklist (immediate):

1. Inventory kernels and modules
   • For every Linux VM or host you manage in Azure or elsewhere: run uname -r and check kernel package names (e.g., linux-azure, generic, etc..
   • Check whether cfg80211 is present as a module: lsmod | grep cfg80211 or grep CONFIG_CFG80211 in /boot/config-$(uname -r).
2. Check vendor advisories and update channels
   • For Azure Linux images: follow Microsoft’s kernel/kernel‑package update advisories and apply any published Azure Linux kernel packages.
   • For third‑party distro images (Ubuntu, Debian, RHEL, SUSE) running on Azure: use the distro vendor’s security tracker to find the CVE→package mapping and install the patched kernel packages they provide.
3. Patch and reboot
   • Kernel fixes require installing the updated kernel package and rebooting into it. Plan staged reboots for production workloads and verify in a pilot pool first.
4. WSL specific guidance
   • If you run WSL with the default Microsoft kernel, the exposure is unlikely for typical WSL configurations because many hardware drivers are not present by default.
   • If you use a custom WSL kernel or have enabled USB or wireless support to WSL, ensure your custom kernel build does not enable vulnerable cfg80211 code paths, or rebuild against a patched upstream base.
5. AKS / node pools and Marketplace images
   • For AKS or ASH clusters using Microsoft‑provided images or Azure Linux node images: update node pools to patched images and perform rolling node replacements.
   • For unmanaged VMs created from marketplace images, check the image maintainer and apply distro updates as directed.
6. Temporary mitigations when patching is delayed
   • Reduce the exposure of untrusted workloads that can touch sysfs or wireless debug interfaces.
   • Blacklist or unload cfg80211 where feasible on systems that do not need Wi‑Fi support (note: blacklisting may break hardware that requires the module).
7. Monitoring and hunting
   • Add alerts for kernel WARN_ON strings that match the cfg80211 callsites; correlate with module lists and recent config changes.
   • After patching and rebooting, monitor kernel logs for recurrence of the previously observed WARN messages.

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Operational playbook for Azure customers (prioritized)​

1. Identify images and node types
   • Map which VMs, scale sets, AKS node pools, and marketplace images in your subscription are running Azure Linux, linux‑azure kernels, or other Linux distributions.
2. Prioritize exposure by business impact
   • High priority: production multi‑tenant hosts, build servers, CI runners, and any guest environments where untrusted users can write to sysfs or load kernel modules.
   • Medium priority: single‑tenant application servers with limited local access.
   • Low priority: tightly controlled infrastructure where only administrators have local access and wireless drivers are not present.
3. Patch/update schedule
   • Pilot: patch a canary set of machines and verify there are no regressions.
   • Rollout: perform phased kernel updates with scheduled reboots.
   • Verify: confirm patched kernels are running and the WARN_ON signature no longer appears in kernel logs.
4. Vendor engagement
   • For proprietary or OEM images where Microsoft is not the kernel vendor, contact the image publisher (Canonical/RHEL/SUSE or OEM) for timelines on backports.

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Why Microsoft's transparency matters — and where to push for clarity​

Microsoft’s advisory practice — naming Azure Linux and committing to CSAF/VEX publication — is helpful for enterprise reliance and automation. Publishing clear CSAF or VEX data for Azure Linux allows automated response tooling to decide whether particular images are affected, whether container images or node pools need patching, and which KBs or kernel package versions fix the issue.
Operators should push Microsoft and image publishers to provide:

• Exact kernel package versions or build commits that include the fix for each image SKU.
• A product‑level list of Microsoft artifacts (Azure Linux releases, linux‑azure versions, WSL kernel versions) that were scanned and their remediation status.
• Guidance for customers who run custom images built atop Azure Linux or who use mixed distro images in the same estate.

The vendor’s statement that it will update the CVE if additional products are affected is correct practice — but until that mapping is published, administrators must assume a wider surface than the single distribution and confirm their own inventories.

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Critical analysis — strengths, residual risk, and recommended posture​

Strengths

• The upstream fix is small and surgical, reducing the likelihood of regressions.
• Major distro and cloud vendors have mechanisms to map the CVE into packaged kernel updates; many already list the CVE and track fixes.
• Microsoft’s operational commitment to keep Azure Linux updated and to publish CSAF/VEX provides a pathway for transparent remediation.

Residual risk

• Backport and image drift: vendor‑forked kernels and embedded/third‑party images create a long tail of unpatched systems.
• Assumed coverage gaps: customers often assume that an Azure or Marketplace image is fully managed by Microsoft; in reality, the maintenance boundary may differ across images.
• Detection noise: kernel WARN messages can be noisy and may be ignored unless teams tune monitoring to the exact callsites; that delay in detection increases time‑to‑remediation risk.

Recommended posture

1. Treat this CVE as a kernel‑level reliability risk that requires patching and reboots.
2. Prioritize inventory and confirmation rather than blind trust in a single vendor statement.
3. Retain conservative segmentation and least privilege on hosts that could host untrusted workloads or where users can influence kernel interfaces.
4. Use vendor advisories to map CVE→package→image; do not rely solely on the CVE name string to decide remediation coverage.

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Practical examples — commands and checks (quick reference)​

1. Identify running kernel:
   • uname -a
2. Confirm whether cfg80211 is present as a module:
   • lsmod | grep cfg80211
3. Check kernel config for your running kernel (where available):
   • grep CONFIG_CFG80211 /boot/config-$(uname -r)
4. Search kernel logs for the signature warning:
   • journalctl -k | grep -i 'wdev_chandef|cfg80211_propagate_cac_done_wk|cfg80211'
5. On Azure, map image SKUs to kernel packages:
   • Use your deployment inventory to list VM images, then consult the image publisher’s security advisories for the exact kernel package updates.

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Closing assessment and customer action plan​

CVE‑2025‑38643 is a real Linux kernel defect in cfg80211 with a straightforward upstream fix. Microsoft has specifically acknowledged Azure Linux as an image that includes the relevant open‑source library and is tracking remediation; however, Azure Linux is not the only possible Microsoft surface where cfg80211 may exist. Any Microsoft‑published kernel artifact (linux‑azure, Azure image kernels) or customer images that enable cfg80211 are in scope until confirmed otherwise.
Action plan for IT teams:

1. Inventory all Linux kernels in your Microsoft environment (including Azure VM images, AKS node pools, and any WSL custom kernels).
2. Cross‑reference the kernel package/build with vendor/distro advisories and apply patched kernel packages promptly.
3. Reboot hosts into patched kernels during controlled maintenance windows.
4. Tune monitoring for kernel WARN signatures and verify remediation.
5. Where images are vendor‑provided (Marketplace/third‑party), coordinate with the image maintainer to confirm the patch timeline.

Proactive patch management and a clear inventory are the decisive controls here: don’t rely on a single line in an advisory — verify your kernels, apply the updates, and treat the issue as a stability/availability risk that is straightforward to neutralize with timely kernel updates and rollouts.

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Source: MSRC Security Update Guide - Microsoft Security Response Center